CN110784989A - Free grounding film, circuit board and preparation method of free grounding film - Google Patents

Abstract

The embodiment of the invention provides a free grounding film, a circuit board and a preparation method of the free grounding film, wherein the free grounding film comprises a first conductor layer, a conductive adhesive layer, a second conductor layer and an adhesive film layer which are sequentially stacked, and one surface of the second conductor layer, which is close to the adhesive film layer, is provided with a conductive protrusion; when the free grounding film is used for grounding the printed circuit board, the electromagnetic shielding film is arranged on the printed circuit board, the free grounding film is pressed with the electromagnetic shielding film through the adhesive film layer, the conductive protrusions penetrate through the adhesive film layer and the insulating layer of the electromagnetic shielding film and are electrically connected with the shielding layer of the electromagnetic shielding film, so that interference charges accumulated on the shielding layer are effectively led out, interference sources formed by accumulation of the interference charges are avoided, and the integrity of signal transmission is effectively guaranteed.

Description

Free grounding film, circuit board and preparation method of free grounding film

Technical Field

The invention relates to the field of electronics, in particular to a free grounding film, a circuit board and a preparation method of the free grounding film.

Background

With the rapid development of the electronic industry, electronic products are further miniaturized, light-weighted and densely assembled, and the development of flexible circuit boards is greatly promoted, so that the integration of element devices and wire connection is realized. The flexible circuit board can be widely applied to industries such as mobile phones, liquid crystal display, communication, aerospace and the like.

Under the push of the international market, the functional flexible printed circuit board is dominant in the flexible printed circuit board market, and an important index for evaluating the performance of the functional flexible printed circuit board is Electromagnetic Shielding (EMI Shielding for short). With the integration of the functions of communication equipment such as mobile phones, the internal components thereof are rapidly high-frequency and high-speed. For example: besides the original audio transmission function, the camera function has become a necessary function, and WLAN (Wireless Local Area network), GPS (Global Positioning System) and internet function have become popular, and the integration of the sensing component in the future makes the trend of rapid high-frequency and high-speed of the component unavoidable. Problems of electromagnetic interference inside and outside the device, signal attenuation during transmission, insertion loss, and jitter caused by high-frequency and high-speed driving are becoming serious.

At present, the free grounding film commonly used for the existing circuit board generally comprises a conductor layer and a conductive adhesive layer, wherein the conductor layer is in contact conduction with the shielding layer of the electromagnetic shielding film through the conductive adhesive layer, so that when the printed circuit board is applied to electronic equipment, the printed circuit board can be electrically connected with the shell of the electronic equipment through the free grounding film, and interference charges of the electromagnetic shielding film are led out. However, in the process of implementing the invention, the inventor finds that the prior art has at least the following problems: at high temperature, because the conductive adhesive layer expands, the conductive particles which originally contact with each other in the conductive adhesive layer are pulled apart or the conductive particles which originally contact with the electromagnetic shielding film are pulled apart, so that the connection failure of the free grounding film and the electromagnetic shielding film is caused, and the interference charges accumulated on the electromagnetic shielding film can not be effectively led out, thereby forming an interference source and influencing the signal transmission of the circuit board.

Disclosure of Invention

The invention aims to provide a free grounding film, a circuit board and a preparation method of the free grounding film, which can realize reliable connection of the free grounding film and an electromagnetic shielding film so as to ensure that interference charges in the electromagnetic shielding film can be led out and ensure the integrity of signal transmission.

In order to solve the technical problem, the invention provides a free grounding film, which comprises a first conductor layer, a conductive adhesive layer, a second conductor layer and an adhesive film layer which are sequentially stacked, wherein a conductive protrusion is arranged on one surface, close to the adhesive film layer, of the second conductor layer;

when the free grounding film is used for grounding a printed circuit board, an electromagnetic shielding film is arranged on the printed circuit board and comprises a shielding layer and an insulating layer which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the conductive protrusions penetrate through the adhesive film layer and the insulating layer and are electrically connected with the shielding layer.

As an improvement of the above scheme, the first conductor layer is provided with a first through hole penetrating through the upper and lower surfaces of the first conductor layer, the second conductor layer is provided with a second through hole penetrating through the upper and lower surfaces of the second conductor layer, and the conductive adhesive layer is provided with a third through hole penetrating through the upper and lower surfaces of the conductive adhesive layer;

the conductive bumps are correspondingly formed on the outer sides of the second through holes; the conductive protrusion is formed by cooling and solidifying when the fusible metal flows to the outer side of the second through hole from the first through hole through the third through hole; and the outer side of the second through hole is far away from the conductive adhesive layer.

As an improvement of the above scheme, the first conductor layer is provided with a first through hole penetrating through the upper and lower surfaces thereof, and the second conductor layer is provided with a second through hole penetrating through the upper and lower surfaces thereof;

the conductive bumps are correspondingly formed on the outer sides of the second through holes; the conductive protrusion is formed by cooling and solidifying the fusible metal from the first through hole and the hot melt adhesive of the conductive adhesive layer when flowing to the outer side of the second through hole; and the outer side of the second through hole is far away from the conductive adhesive layer.

As an improvement of the scheme, the surface of the conductive bump is provided with convex conductor particles.

As an improvement of the above scheme, the adhesive film layer includes an adhesive layer containing conductive particles; or the adhesive film layer comprises an adhesion layer without conductive particles.

As an improvement of the above scheme, the free grounding film further includes an anti-oxidation layer, and the anti-oxidation layer is disposed on a surface of the first conductor layer away from the conductive adhesive layer.

As an improvement of the scheme, the free grounding film further comprises a strippable protective film layer, and the strippable protective film layer is arranged on one surface, away from the second conductor layer, of the adhesive film layer.

As an improvement of the above aspect, the free grounding film further includes a third conductor layer, the third conductor layer is disposed on the surface of the second conductor layer on which the conductive bumps are formed, and the third conductor layer covers the conductive bumps to form bumps.

As an improvement of the above, the surface of the protrusion is provided with convex conductive particles.

In order to solve the same technical problem, the invention also provides a circuit board, which comprises a printed circuit board and an electromagnetic shielding film arranged on the printed circuit board, wherein the circuit board further comprises the free grounding film in any scheme of the embodiment;

the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the conductive protrusions penetrate through the adhesive film layer and the insulating layer and are electrically connected with the shielding layer;

or, the electromagnetic shielding film comprises a shielding layer and an insulating layer which are stacked, the free grounding film is pressed with the electromagnetic shielding film through the adhesive film layer, and the protruding part pierces the adhesive film layer and the insulating layer and is electrically connected with the shielding layer.

In order to solve the same technical problem, the present invention further provides a method for preparing a free grounding film, which is suitable for preparing the free grounding film of the first embodiment, and comprises the steps of:

forming a first conductor layer, forming a conductive adhesive layer on one surface of the first conductor layer, and forming a second conductor layer on one surface of the conductive adhesive layer, which is far away from the first conductor layer;

forming a conductive bump on one surface of the second conductor layer, which is far away from the conductive adhesive layer;

forming an adhesive film layer on one surface of the second conductor layer on which the conductive bumps are formed; when the free grounding film is used for grounding the printed circuit board, an electromagnetic shielding film is arranged on the printed circuit board and comprises a shielding layer and an insulating layer which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the conductive protrusions penetrate through the adhesive film layer and the insulating layer and are electrically connected with the shielding layer.

As an improvement of the above scheme, the first conductor layer is provided with a first through hole penetrating through the upper and lower surfaces of the first conductor layer, the second conductor layer is provided with a second through hole penetrating through the upper and lower surfaces of the second conductor layer, and the conductive adhesive layer is provided with a third through hole penetrating through the upper and lower surfaces of the conductive adhesive layer;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

cooling and solidifying the fusible metal after flowing from the first through hole to the outer side of the second through hole through the third through hole, so as to form a conductive bump on one surface of the second conductor layer away from the conductive adhesive layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

As an improvement of the above scheme, the conductive adhesive layer is a hot melt adhesive layer; the first conductor layer is provided with a first through hole penetrating through the upper surface and the lower surface of the first conductor layer, and the second conductor layer is provided with a second through hole penetrating through the upper surface and the lower surface of the second conductor layer;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

a fusible metal is arranged at the first through hole, the fusible metal flows to the second through hole from the first through hole at a preset temperature, and the conductive adhesive layer forms hot melt adhesive to flow to the second through hole in the flowing process of the fusible metal;

cooling the fusible metal and the hot melt adhesive which flow to the outer side of the second through hole, so that a conductive bump is formed on one surface, away from the conductive adhesive layer, of the second conductor layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

As an improvement of the above scheme, before forming an adhesive film layer on a surface of the second conductor layer on which the conductive bump is formed, the method further includes:

forming conductor particles on the outer surface of the conductive bump by one or more processes of physical roughening, chemical plating, physical vapor deposition, chemical vapor deposition, evaporation plating, sputtering plating, electroplating and hybrid plating.

As an improvement of the above scheme, the forming of the adhesive film layer on the surface of the second conductor layer on which the conductive bump is formed specifically includes:

forming an adhesive film layer on a release film, and then transferring the adhesive film layer to the surface of the second conductor layer, on which the conductive bumps are formed, in a pressing manner; or the like, or, alternatively,

and directly forming an adhesive film layer on one surface of the second conductor layer on which the conductive bumps are formed.

In order to solve the same technical problem, the present invention further provides a method for preparing a free-grounding film, which is suitable for preparing the free-grounding film in the second embodiment, and comprises the steps of:

forming a first conductor layer, forming a conductive adhesive layer on one surface of the first conductor layer, and forming a second conductor layer on one surface of the conductive adhesive layer, which is far away from the first conductor layer;

forming a conductive bump on one surface of the second conductor layer, which is far away from the conductive adhesive layer;

forming a third conductor layer on one surface of the second conductor layer on which the conductive bumps are formed, and forming bumps at positions where the third conductor layer covers the conductive bumps;

forming a glue film layer on one surface of the third conductor layer, which is far away from the second conductor layer; when the free grounding film is used for grounding the printed circuit board, an electromagnetic shielding film is arranged on the printed circuit board and comprises a shielding layer and an insulating layer which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the protruding portion penetrates through the adhesive film layer and the insulating layer and is electrically connected with the shielding layer.

As an improvement of the above scheme, the first conductor layer is provided with a first through hole penetrating through the upper and lower surfaces of the first conductor layer, the second conductor layer is provided with a second through hole penetrating through the upper and lower surfaces of the second conductor layer, and the conductive adhesive layer is provided with a third through hole penetrating through the upper and lower surfaces of the conductive adhesive layer;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

arranging a fusible metal at the first through hole, enabling the fusible metal to flow from the first through hole to the second through hole through the third through hole at a preset temperature, and cooling and solidifying the fusible metal after the fusible metal flows to the outer side of the second through hole, so that a conductive bump is formed on one surface, away from the conductive adhesive layer, of the second conductor layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

As an improvement of the above scheme, the conductive adhesive layer is a hot melt adhesive layer; the first conductor layer is provided with a first through hole penetrating through the upper surface and the lower surface of the first conductor layer, and the second conductor layer is provided with a second through hole penetrating through the upper surface and the lower surface of the second conductor layer;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

a fusible metal is arranged at the first through hole, the fusible metal flows to the second through hole from the first through hole at a preset temperature, and the conductive adhesive layer forms hot melt adhesive to flow to the second through hole in the flowing process of the fusible metal;

cooling the fusible metal and the hot melt adhesive which flow to the outer side of the second through hole, so that a conductive bump is formed on one surface, away from the conductive adhesive layer, of the second conductor layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

Compared with the prior art, in the method for preparing the free grounding film, the circuit board and the free grounding film, which are provided by the embodiment of the invention, the conductive protrusions are arranged on the surface, close to the adhesive film layer, of the second conductor layer, so that when the free grounding film is laminated with the electromagnetic shielding film through the adhesive film layer, the conductive protrusions can sequentially pierce the adhesive film layer and the insulating layer of the electromagnetic shielding film and are connected with the shielding layer of the electromagnetic shielding film;

or, the third conductor layer is arranged on the surface of the second conductor layer on which the conductive protrusion is formed, so that the position of the third conductor layer covering the conductive protrusion corresponds to the protrusion, and the adhesive film layer is covered on the surface of the third conductor layer far away from the second conductor layer, so that when the free grounding film is laminated with the electromagnetic shielding film through the adhesive film layer, the protrusion can sequentially pierce the adhesive film layer and the insulating layer of the electromagnetic shielding film and is connected with the shielding layer of the electromagnetic shielding film, thereby effectively leading out the interference charges accumulated in the electromagnetic shielding film, further ensuring the grounding of the electromagnetic shielding film, and effectively avoiding the problem that the free grounding film is not connected with the electromagnetic shielding film to fail to lead out the interference charges rapidly due to the lamination of the free grounding film and the electromagnetic shielding film through the conductive adhesive layer at high temperature, thus ensuring the integrity of the signal transmission.

Drawings

Fig. 1 is a schematic structural diagram of a first free grounding film according to an embodiment of the present invention;

FIG. 2 is a schematic structural diagram of a second free-grounding film according to an embodiment of the present invention;

FIG. 3 is a schematic structural diagram of a third free-grounding film according to an embodiment of the present invention;

FIG. 4 is a schematic structural diagram of a first free-grounding film according to a second embodiment of the present invention;

FIG. 5 is a schematic structural diagram of a second free-grounding film provided in the second embodiment of the present invention;

FIG. 6 is a schematic structural diagram of a third free-grounding film provided in the second embodiment of the present invention;

FIG. 7 is a schematic structural diagram of a fourth free-grounding film provided in the second embodiment of the present invention;

FIG. 8 is a cross-sectional view of a free-grounding film provided by an embodiment of the present invention;

fig. 9 is a schematic structural diagram of a first circuit board according to a third embodiment of the present invention;

fig. 10 is a schematic structural diagram of a second circuit board according to a third embodiment of the present invention;

FIG. 11 is a flow chart of a method for fabricating a free-grounding film according to a fourth embodiment of the present invention;

fig. 12 is a flowchart of a method for preparing a free-grounding film according to a fourth embodiment of the present invention.

Wherein, 1, a first conductor layer; 10. a first through hole; 2. a conductive adhesive layer; 20. a third through hole; 3. a second conductor layer; 30. a second through hole; 4. a glue film layer; 5. a conductive bump; 6. a conductive particle; 7. a third conductor layer; 70. a boss portion; 8. an oxidation-resistant layer; 9. an electromagnetic shielding film; 90. an insulating layer; 91. a shielding layer; 11. a printed wiring board.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example one

Referring to fig. 1, an embodiment of the invention provides a free grounding film, which includes a first conductor layer 1, a conductive adhesive layer 2, a second conductor layer 3, and an adhesive layer 4, which are sequentially stacked; a conductive bump 5 is arranged on one surface of the second conductor layer 3 close to the adhesive film layer 4;

referring to fig. 9, when the free grounding film is used for grounding a printed circuit board, an electromagnetic shielding film 9 is arranged on the printed circuit board 11, the electromagnetic shielding film 9 includes a shielding layer 91 and an insulating layer 90 which are stacked, the free grounding film is pressed with the electromagnetic shielding film 9 through the glue film layer 4, and the conductive protrusion 5 pierces through the glue film layer 4 and the insulating layer 90 and is electrically connected with the shielding layer 91. The material of the conductive bump 5 may be a pure metal, an alloy, or a conductive adhesive with good conductivity, which is not limited herein.

In this embodiment, the conductive bumps 5 are disposed on the surface of the second conductor layer 3 close to the glue film layer 4, so that in the process of pressing the free grounding film and the electromagnetic shielding film 9, the conductive bumps 5 can smoothly pierce through the glue film layer 4 and the insulating layer 90 and contact with the shielding layer 91, thereby effectively guiding out the interference charges accumulated in the electromagnetic shielding film, and further ensuring the integrity of signal transmission.

In this embodiment, preferably, the conductive bump 5 extends into the adhesive film layer 4, so that the conductive bump 5 can penetrate through the adhesive film layer 4 and the insulating layer 90 more easily during the pressing process. Alternatively, the conductive bump 5 may also pierce the adhesive layer 4, so that the conductive bump 5 can directly pierce the insulating layer 90 during the pressing process. Of course, the conductive bump 5 may not extend into the adhesive film layer 4 but be covered by the adhesive film layer 4.

Referring to fig. 2, preferably, the surface of the conductive bump 5 is provided with one or more conductive particles 6. The surface of the conductive protrusion 5 is provided with the conductor particles 6, so that the conductive protrusion 5 can smoothly pierce the adhesive film layer 4 and the insulating layer 90 of the electromagnetic shielding film 9 in the pressing process, and normal conduction of interference charges is guaranteed.

In the embodiment of the present invention, the height of the conductor particles 6 is preferably 20 μm to 100 μm, the thickness of the adhesive film layer 4 is preferably 0.1 μm to 80 μm, and the thickness of the insulating layer 90 is preferably 1 μm to 20 μm. By providing the height of the conductor particles 6 preferably in the range of 20 μm to 100 μm and the thickness of the glue film layer 4 preferably in the range of 0.1 μm to 80 μm, it is ensured that the conductor particles 6 can penetrate through the glue film layer 4 and the insulating layer 90 of the electromagnetic shielding film 9, thereby ensuring that the free-grounding film can conduct away the interfering charges accumulated on the electromagnetic shielding film 9. Preferably, the conductive particles 6 are distributed in a concentrated manner on the outwardly protruding positions of the surface of the conductive protrusions 5, so that the adhesive film 4 is easier to pierce. Of course, the non-convex portions of the surface of the conductive bump 5 may also have the conductive particles 6 distributed. In addition, the conductor particles 6 may also be distributed on other positions of the second conductor layer 3 near the glue film layer 4.

In the embodiment of the present invention, the conductor particles 6 may be located at a certain distance from the inner surface of the adhesive film layer 4 (i.e. the surface close to the second conductor layer 3), and may also be in contact with the inner surface of the adhesive film layer 4 or extend out of the outer surface of the adhesive film layer 4. In addition, the outer surface of the adhesive film layer 4 may be a flat surface without undulation, or may be an uneven surface with undulation.

In the embodiment of the present invention, it should be noted that the shape of the conductor particles 6 shown in the figures is only an example, and due to differences in process means and parameters, the conductor particles 6 may also be in other shapes such as clusters, ice-hanging shapes, stalactite shapes, and dendrites. The conductive particles 6 in the present invention are not limited to the shapes shown in the drawings and described above, and any conductive particles 6 having piercing and conductive functions are within the scope of the present invention.

In an embodiment of the present invention, the conductor particles 6 comprise one or more of metal particles, carbon nanotube particles and ferrite particles. Further, the metal particles include single metal particles and/or alloy particles; the single metal particles are made of any one of aluminum, titanium, zinc, iron, nickel, chromium, cobalt, copper, silver and gold, and the alloy particles are made of any two or more of aluminum, titanium, zinc, iron, nickel, chromium, cobalt, copper, silver and gold.

In the embodiment of the present invention, the forming process of the conductive bump 5 is as follows: the first conductor layer 1 is provided with a first through hole 10 penetrating through the upper and lower surfaces thereof, the second conductor layer 3 is provided with a second through hole 30 penetrating through the upper and lower surfaces thereof, and the conductive bump 5 is formed by cooling and solidifying a fusible metal when flowing from the first through hole 10 to the outer side of the second through hole 30 through the conductive adhesive layer 2.

Specifically, referring to fig. 3 and 8, in a preferred embodiment, the conductive bump 5 is formed by cooling and solidifying the fusible metal flowing from the first through hole 10 to the outside of the second through hole 30 through the conductive adhesive layer 2, which is represented by:

the conductive adhesive layer 2 is provided with a third through hole 20 penetrating through the upper surface and the lower surface of the conductive adhesive layer; the conductive bump 5 is correspondingly formed at the outer side of each second through hole 30;

the conductive bump 5 is formed by cooling and solidifying the fusible metal when flowing from the first through hole 10 to the outside of the second through hole 30 through the conductive adhesive layer 2, and is embodied as follows: the conductive bump 5 is formed by cooling and solidifying when the fusible metal flows from the first through hole 10 to the outer side of the second through hole 30 through the third through hole 20 of the conductive adhesive layer 2; wherein, the outer side of the second through hole 30 is far away from the conductive adhesive layer 2.

Preferably, the first through holes 10, the second through holes 30 and the third through holes 20 are aligned one by one, so that the fusible metal flows from the first through holes 10 to the outer sides of the corresponding second through holes 30. Alternatively, the first through hole 10, the second through hole 30 and the third through hole 20 may be in a misaligned state, for example, the first through hole 10 and the second through hole are aligned, and the second through hole and the third through hole are aligned, the fusible metal may also flow from the first through hole 10 to the outside of the second through hole 30, which is aligned or misaligned therewith, through the third through hole 20.

It should be noted that, in the embodiment of the present invention, the fusible metal flowing out of the corresponding through hole is divided into the following three cases: the first is that the fusible metal flows out almost completely from inside the corresponding through hole, and there is no residue inside the through hole; secondly, metal is remained or even filled in the through hole; thirdly, there is metal residue at the edge of the outer opening (i.e., the opening facing outward) of the first through-hole 10. Wherein, no metal remains in the via of fig. 3, but fig. 3 is only exemplary, and the above three cases are all within the protection scope of the present invention. It will be appreciated that reference is made to fig. 6 and 7, referred to below, in relation to fig. 3.

It should be noted that the melting temperature of the fusible metal is preferably 300-2000 ℃.

In the embodiment of the present invention, the through holes (i.e., the first through holes 10, the second through holes 30, and the third through holes 20) may be regularly or irregularly distributed on the corresponding structural layers (i.e., the first conductor layer 1, the second conductor layer 3, and the conductive adhesive layer 2); the through holes are regularly distributed on the corresponding structural layer, namely the through holes are the same in shape and are uniformly distributed on the corresponding structural layer; the through holes are irregularly distributed on the corresponding structure layer, that is, the through holes are irregularly distributed on the corresponding structure layer in different shapes. Preferably, the through holes have the same shape, and are uniformly distributed on the corresponding structural layer. In addition, the through hole may be a circular through hole, and may also be a through hole of any other shape, and fig. 8 of the present invention only illustrates that the through hole is a circular through hole, but any other shape of through hole is within the scope of the present invention.

In the embodiment of the present invention, in order to ensure that the conductive bump 5 can be formed by cooling and solidifying when the fusible metal flows from the first through hole 10 to the outside of the second through hole 30 through the third through hole 20, and at the same time, to ensure that the first conductor layer 1, the conductive adhesive layer 2, and the second conductor layer 3 are not easily broken, it is preferable that the cross-sectional area of the first through hole 10, the cross-sectional area of the second through hole 30, and the cross-sectional area of the third through hole 20 in this embodiment are 0.01 μm ²-1mm ²。

In addition, in order to ensure that enough of the conductive bumps 5 can be formed to ensure that the adhesive film layer 4 and the insulating layer 90 can be pierced by the conductive bumps 5,thereby connecting with the shielding layer 91 while ensuring that the first conductor layer 1, the conductive adhesive layer 2 and the second conductor layer 3 are not easily broken, and preferably, the number of the first through holes 10 in the first conductor layer 1 per square centimeter is 5 to 10 in this embodiment ⁶(ii) a The number of the second through holes 30 in the second conductor layer 3 per square centimeter is 5 to 10 ⁶(ii) a The number of the third through holes 20 in each square centimeter of the conductive adhesive layer 2 is 5-10 ⁶. Accordingly, the number of the conductive bumps 5 per square centimeter of the second conductor layer 3 is 5-10 ⁶。

In another preferred mode, the conductive bump 5 is formed by cooling and solidifying when the fusible metal flows from the first through hole 10 to the outside of the second through hole 30 through the conductive adhesive layer 2, which is embodied by:

a first through hole 10 penetrating through the upper and lower surfaces of the first conductor layer 1 is formed, and a second through hole 30 penetrating through the upper and lower surfaces of the second conductor layer 3 is formed; the conductive bump 5 is correspondingly formed at the outer side of each second through hole 30;

the conductive bump 5 is formed by cooling and solidifying the fusible metal from the first through hole 10 and the hot melt adhesive of the conductive adhesive layer when flowing to the outer side of the second through hole 30; wherein, the outer side of the second through hole 30 is far away from the conductive adhesive layer 2. Specifically, the conductive adhesive layer 2 is a hot melt adhesive layer, and since the temperature of the fusible metal flowing from the first through hole 10 is high, the fusible metal can perform hot melting on the conductive adhesive layer 2 on the flow path, so that the conductive adhesive layer 2 on the flow path is hot melted into hot melt adhesive and flows to the second through hole 30 together with the fusible metal.

The first through holes 10 and the second through holes 30 may be aligned one to one, or may be in a non-aligned state, and reference may be specifically made to the description of the first through holes 10 and the second through holes 30, which is not described herein again.

In the embodiment of the present invention, the thickness of the first conductor layer 1 and the thickness of the second conductor layer 3 are preferably 2 μm to 45 μm, so as to ensure that the first conductor layer 1 and the second conductor layer 3 are not easily broken and have good flexibility. In order to ensure that the first conductor layer 1 and the second conductor layer 3 have good electrical conductivity, the first conductor layer 1 and the second conductor layer 3 respectively include one or more of a metal conductor layer, a carbon nanotube conductor layer, a ferrite conductor layer, and a graphene conductor layer. Wherein the metal conductor layer comprises a single metal conductor layer and/or an alloy conductor layer; the single metal conductor layer is made of any one of aluminum, titanium, zinc, iron, nickel, chromium, cobalt, copper, silver and gold, and the alloy conductor layer is made of any two or more of aluminum, titanium, zinc, iron, nickel, chromium, cobalt, copper, silver and gold.

In the above embodiments, the first conductor layer 1 and the second conductor layer 3 may have a single-layer structure or a multi-layer structure. In addition, the first conductor layer 1 and the second conductor layer 3 may be arranged in a grid shape, a foam shape, etc. according to the actual production and application requirements.

In the above embodiment, both surfaces of the first conductor layer 1 may be flat or wavy, and a surface of the second conductor layer 3 away from the protrusion may be flat or wavy, which is not specifically limited herein.

In the above embodiments, in order to protect the adhesive film layer 4, the free grounding film in this embodiment further includes a peelable protective film layer (not shown), and the peelable protective film layer is disposed on a side of the adhesive film layer 4 away from the second conductor layer 3. The peelable protective film layer plays a role in protection so as to prevent the adhesive film layer 4 from being damaged before use, and therefore the free grounding film can be pressed with the electromagnetic shielding film 9 through the adhesive film layer 4. It should be noted that, when the free grounding film is laminated on the electromagnetic shielding film 9, the peelable protective film layer needs to be peeled off, and then the free grounding film is laminated with the electromagnetic shielding film 9 through the adhesive film layer 4.

In addition, the strippable protective film layer comprises a PPS film layer, a PEN film layer, a polyester film layer, a polyimide film layer, a film layer formed after epoxy resin ink is cured, a film layer formed after polyurethane ink is cured, a film layer formed after modified acrylic resin is cured or a film layer formed after polyimide resin is cured.

Referring to fig. 1 to 3, in order to protect the first conductor layer 1 to ensure that interference charges can be led out, in this embodiment, the free grounding film further includes an anti-oxidation layer 8, and the anti-oxidation layer 8 is disposed on a surface of the first conductor layer 1 away from the conductive adhesive layer 2.

In one preferable mode, the material of the oxidation preventing layer 8 is one of a metal material, ferrite, graphite, a carbon nanotube, graphene, and silver paste; wherein the metal material is any one of aluminum, titanium, zinc, iron, nickel, chromium, cobalt, copper, silver, gold and molybdenum; or the metal material is an alloy formed by two or more of aluminum, titanium, zinc, iron, nickel, chromium, cobalt, copper, silver, gold and molybdenum; in this case, the thickness of the oxidation preventing layer 8 is 0.01 μm to 5 μm, and the thickness of the oxidation preventing layer 8 is preferably 0.1 μm to 1 μm. In addition, the oxidation preventing layer 8 may be formed by one or a composite process of at least two of chemical plating, physical vapor deposition, chemical vapor deposition, evaporation plating, sputtering plating, and electroplating.

In another preferred mode, the oxidation preventing layer 8 is made of a mixture of glue and conductive particles; wherein the volume ratio of the conductive particles to the glue is 5-80%; in this case, the thickness of the oxidation preventing layer 8 is preferably 0.1 μm to 5 μm. The oxidation preventing layer 8 may be formed by a process of curing after coating.

In the above embodiment, one of the structures of the adhesive film layer 4 is specifically represented as: the adhesive layer 4 includes an adhesive layer (not shown) containing conductive particles; therefore, the glue film layer 4 not only has an adhesion function to make the electromagnetic shielding film 9 and the free grounding film adhere tightly, but also the glue film layer 4 has a conductive function, and cooperates with the first conductor layer 1 and the second conductor layer 3 to rapidly guide out interfering electrons. The conductive particles of the adhesive film layer 4 may be mutually separated conductive particles or agglomerated large conductive particles; when the conductive particles are mutually separated, the area of electrical contact can be further increased, and the uniformity of the electrical contact is improved; and when the conductive particles are large agglomerated conductive particles, the piercing strength can be increased.

In the above embodiment, another structure of the adhesive film layer 4 is specifically as follows: the adhesive layer 4 includes an adhesive layer (not shown) containing no conductive particles; therefore, the adhesive film layer 4 has an adhesive effect, so that the electromagnetic shielding film 9 and the free grounding film are tightly adhered, the insertion loss of the circuit board in the using process is reduced, and the bending property of the circuit board is improved.

In the above embodiments, the adhesive film layer 4 is a pure adhesive film layer (i.e., a film layer formed by applying pure adhesive on the second conductor layer 3) or a conductive adhesive film layer (i.e., a film layer formed by applying conductive adhesive on the second conductor layer 3). Specifically, the material used for the adhesive film layer 4 is selected from the following materials: modified epoxy resins, acrylic resins, modified rubbers, and modified thermoplastic polyimides.

It should be noted that the adhesive film layer 4 may be a pure adhesive film layer or a conductive adhesive film layer, and may also be made of other materials, as long as it is ensured that the adhesive film layer 4 can be smoothly pierced by the conductive protrusion 5 in the pressing process.

In the above embodiment, the conductive adhesive layer 2 is disposed between the first conductor layer 1 and the second conductor layer 3, so that the flexibility of the free grounding film is further improved. .

In the above embodiments, the first conductive layer 1, the conductive adhesive layer 2 and the second conductive layer 3 may be porous or non-porous.

Example two

Referring to fig. 4, another free grounding film provided in the embodiments of the present invention includes a first conductor layer 1, a conductive adhesive layer 2, a second conductor layer 3, a third conductor layer 7, and an adhesive film layer 4; the first conductor layer 1, the conductive adhesive layer 2 and the second conductor layer 3 are sequentially stacked; a conductive bump 5 is arranged on one surface of the second conductor layer 3, which is far away from the conductive adhesive layer 2; the third conductor layer 7 is disposed on the one surface of the second conductor layer 3, and a protruding portion 70 is formed at a position of the third conductor layer 7 covering the conductive bump 5 (the relationship between the protruding portion 70 and the conductive bump 5 may be one-to-one, one-to-many, or one-to-many, etc., when the relationship between the protruding portion 70 and the conductive bump 5 is one-to-one, the shape and structure of the protruding portion 70 may correspond to or may be different from the shape and structure of the conductive bump 5); the adhesive film layer 4 is covered on one surface of the third conductor layer 7, which is far away from the second conductor layer 3;

referring to fig. 10, when the free grounding film is used for grounding a printed circuit board, an electromagnetic shielding film 9 is disposed on the printed circuit board 11, the electromagnetic shielding film 9 includes a shielding layer 91 and an insulating layer 90 which are stacked, the free grounding film is pressed with the electromagnetic shielding film 9 through the glue film layer 4, and the protruding portion 70 pierces through the glue film layer 4 and the insulating layer 90 and is electrically connected to the shielding layer 91, so that interference charges are normally led out under the cooperation of the first conductor layer 1, the conductive glue layer 2, the second conductor layer 3 and the third conductor layer 7.

In the embodiment of the present invention, the forming process of the conductive bump 5 is as follows: the first conductor layer 1 is provided with a first through hole 10 penetrating through the upper and lower surfaces thereof, the second conductor layer 3 is provided with a second through hole 30 penetrating through the upper and lower surfaces thereof, and the conductive bump 5 is formed by cooling and solidifying a fusible metal when flowing from the first through hole 10 to the outer side of the second through hole 30 through the conductive adhesive layer 2.

Specifically, referring to fig. 6, in a preferred embodiment, the conductive bump 5 is formed by cooling and solidifying when the fusible metal flows from the first through hole 10 to the outer side of the second through hole 30 through the conductive adhesive layer 2, specifically:

the conductive adhesive layer 2 is provided with a third through hole 20 penetrating through the upper surface and the lower surface of the conductive adhesive layer; the conductive bump 5 is correspondingly formed at the outer side of the second through hole 30.

The conductive bump 5 is formed by cooling and solidifying when the fusible metal flows from the first through hole 10 to the outer side of the second through hole 30 through the third through hole 20; wherein, the outer side of the second through hole 30 is far away from the conductive adhesive layer 2.

Preferably, the first through holes 10, the second through holes 30 and the third through holes 20 are aligned one by one, so that the fusible metal flows from the first through holes 10 to the outer sides of the corresponding second through holes 30. Alternatively, the first through hole 10, the second through hole 30 and the third through hole 20 may be in a misaligned state, for example, the first through hole 10 and the second through hole are aligned, and the second through hole and the third through hole are aligned, the fusible metal may also flow from the first through hole 10 to the outside of the second through hole 30, which is aligned or misaligned therewith.

Referring to fig. 7, in another preferred mode, the conductive bump 5 is formed by cooling and solidifying the fusible metal flowing from the first through hole 10 to the outside of the second through hole 30 through the conductive adhesive layer 2, which is specifically represented by:

a first through hole 10 penetrating through the upper and lower surfaces of the first conductor layer 1 is formed, and a second through hole 30 penetrating through the upper and lower surfaces of the second conductor layer 3 is formed; the conductive bump 5 is correspondingly formed at the outer side of the second through hole 30.

The conductive bump 5 is formed by cooling and solidifying the fusible metal from the first through hole 10 and the hot melt adhesive of the conductive adhesive layer 2 when flowing to the outer side of the second through hole 30; wherein, the outer side of the second through hole 30 is far away from the conductive adhesive layer. Specifically, since the temperature of the fusible metal flowing from the first through hole 10 is high, the fusible metal thermally fuses the conductive adhesive layer 2 on the flow path, so that the conductive adhesive layer 2 on the flow path is thermally fused into the thermal fuse and flows to the second through hole 30 together with the fusible metal.

The first through holes 10 and the second through holes 30 may be aligned one by one, or may be in an misaligned state.

In the present embodiment, the protrusion 70 extends into the adhesive film layer 4 or pierces the adhesive film layer 4. In addition, referring to fig. 5, in order to further secure the piercing ability of the projection 70, it is preferable that the surface of the projection 70 is provided with one or more conductive particles 6.

In the above embodiment, the free grounding film in this embodiment further includes a peelable protective film layer (not shown), and the peelable protective film layer is disposed on a side of the adhesive film layer 4 away from the second conductor layer 3. In addition, the free grounding film further comprises an anti-oxidation layer 8, and the anti-oxidation layer 8 is arranged on one surface, far away from the conductive adhesive layer 2, of the first conductor layer 1.

As a modification of the above embodiment, the adhesive layer 4 includes an adhesive layer containing conductive particles; alternatively, the adhesive layer 4 includes an adhesive layer containing no conductive particles.

In the above embodiment, the conductive adhesive layer 2 is disposed between the first conductor layer 1 and the second conductor layer 3, so that the flexibility of the free grounding film is further improved. It should be noted that, in the present embodiment, specific arrangement, specific shape, size, specific position, specific function, and the like of the structures of the first conductor layer 1, the conductive adhesive layer 2, the second conductor layer 3, the conductive protrusions 5, the conductive adhesive layer 2, the adhesive layer 4, the conductor particles 6, the anti-oxidation layer 8, the peelable protective film layer, and the like can refer to the related contents in the first embodiment, and are not described herein again.

EXAMPLE III

Referring to fig. 9 and 10, an embodiment of the present invention provides a circuit board, which includes a printed circuit board 11 and an electromagnetic shielding film 9 disposed on the printed circuit board 11.

Referring to fig. 9, the wiring board further includes the free grounding film of any of the first embodiments; the electromagnetic shielding film 9 comprises a shielding layer 91 and an insulating layer 90 which are stacked, the free grounding film is pressed with the electromagnetic shielding film 9 through the adhesive film layer 4, and the conductive protrusions 5 penetrate through the adhesive film layer 4 and the insulating layer 90 and are electrically connected with the shielding layer 91.

Or, referring to fig. 10, the wiring board further comprises the free grounding film of any of the second embodiments; the electromagnetic shielding film 9 comprises a shielding layer 91 and an insulating layer 90 which are stacked, the free grounding film is pressed with the electromagnetic shielding film 9 through the adhesive film layer 4, and the protruding portion 70 pierces the adhesive film layer 4 and the insulating layer 90 and is electrically connected with the shielding layer 91.

In this embodiment, reference may be made to the description of the above embodiments for implementation of the free grounding film, and details are not repeated here.

Preferably, the printed circuit board 11 is one of a flexible single-sided board, a flexible double-sided board, a flexible multilayer board, and a rigid-flex printed board.

In a specific implementation, when the wiring board is applied to an electronic device, the interference charges accumulated in the electromagnetic shielding film 9 can be conducted out by electrically connecting the free grounding film with a housing of the electronic device.

In the embodiment of the present invention, with the above structure, in the process of pressing the free grounding film and the electromagnetic shielding film 9, the conductive bumps 5 on the second conductor layer 3 are used to sequentially pierce the adhesive film layer 4 and the insulating layer 90, and are connected to the shielding layer 91; or the bulge part 70 on the third conductor layer 7 is utilized to sequentially pierce the adhesive film layer 4 and the insulating layer 90 and is connected with the shielding layer 91, so that interference charges accumulated in the electromagnetic shielding film are effectively led out, the grounding of the electromagnetic shielding film is further ensured, the problem that the interference charges cannot be rapidly led out due to the connection failure of the free grounding film and the electromagnetic shielding film because the free grounding film is pressed with the electromagnetic shielding film through the conductive adhesive layer at high temperature is effectively avoided, and the influence on the normal work of the printed circuit board caused by the accumulation of the interference charges is effectively avoided.

Example four

Referring to fig. 11, an embodiment of the present invention provides a method for preparing a free grounding film, which is suitable for preparing the free grounding film described in the first embodiment, and includes steps S11 to S13:

s11, forming a first conductor layer 1, forming a conductive adhesive layer 2 on one surface of the first conductor layer 1, and forming a second conductor layer 3 on one surface, far away from the first conductor layer 1, of the conductive adhesive layer 2.

And S12, forming a conductive bump 5 on one surface of the second conductor layer 3 away from the conductive adhesive layer 2.

S13, forming an adhesive film layer 4 on the surface of the second conductor layer 3 on which the conductive bumps 5 are formed; when the free grounding film is used for grounding a printed circuit board, an electromagnetic shielding film 9 is arranged on the printed circuit board 11, the electromagnetic shielding film 9 comprises a shielding layer 91 and an insulating layer 90 which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film 9 through the glue film layer 4, and the conductive protrusions 5 penetrate through the glue film layer 4 and the insulating layer 90 and are electrically connected with the shielding layer 91.

In this embodiment, the conductive bump 5 is preferably formed in a manner that: the first conductor layer 1 is provided with a first through hole 10 penetrating through the upper and lower surfaces thereof, the second conductor layer 3 is provided with a second through hole 30 penetrating through the upper and lower surfaces thereof, and the conductive bump 5 is formed by cooling and solidifying a fusible metal when flowing from the first through hole 10 to the outer side of the second through hole 30 through the conductive adhesive layer 2.

Specifically, referring to fig. 3, the conductive adhesive layer 2 is provided with a third through hole 20 penetrating through the upper and lower surfaces thereof;

the step S12 is specifically:

a fusible metal flows to the outer side of the second through hole 30 from the first through hole 10 through the third through hole 20 of the conductive adhesive layer 2 and is cooled and solidified, so that a conductive bump 5 is formed on one surface, away from the conductive adhesive layer 2, of the second conductor layer 3; wherein, the outer side of the second through hole 30 is far away from the conductive adhesive layer 2.

Or, the conductive adhesive layer 2 is a hot melt adhesive layer;

the step S12 is specifically:

arranging a fusible metal at the first through hole 10, and enabling the fusible metal to flow from the first through hole 10 to the second through hole 30 at a preset temperature, wherein the conductive adhesive layer 2 forms hot melt adhesive to flow to the second through hole 30 in the flowing process of the fusible metal;

the fusible metal and the hot melt adhesive flowing to the outside of the second through hole 30 are cooled, so that the conductive bump 5 is formed on the surface of the second conductor layer 3 away from the conductive adhesive layer 2.

It should be noted that the melting temperature of the fusible metal is preferably 300-2000 ℃. In addition, the conductive bump 5 may be formed by directly applying a conductive adhesive bump on the surface of the second conductor layer 3 away from the conductive adhesive layer 2 and solidifying the conductive bump 5, or by directly plating a metal dot on the surface of the second conductor layer 3 away from the conductive adhesive layer 2 to form the conductive bump 5, and the like, which is not limited herein.

In the above embodiment, preferably, after the step S12 and before the step S13, the preparation method further includes:

a plurality of conductor particles 6 are formed on the outer surface of the conductive bump 5 by one or more processes of physical roughening, electroless plating, physical vapor deposition, chemical vapor deposition, evaporation plating, sputtering plating, electroplating, and hybrid plating, see fig. 2.

In the foregoing embodiment, specifically, the step S13 is specifically:

forming an adhesive film layer 4 on a release film, and then transferring the adhesive film layer 4 to the surface of the second conductor layer 3 on which the conductive bump 5 is formed in a pressing manner; or the like, or, alternatively,

an adhesive film layer 4 is directly formed on the surface of the second conductor layer 3 on which the conductive bumps 5 are formed.

It should be noted that the method for manufacturing the free grounding film provided in this embodiment is only one example of the method for manufacturing the free grounding film described in the first embodiment, and the free grounding film described in the first embodiment may also be manufactured by other manufacturing methods, for example, the conductive bump 5 may be formed on one surface of the second conductor layer 3, and then the first conductor layer 1 and the conductive adhesive layer 2 may be formed.

EXAMPLE five

Referring to fig. 12, an embodiment of the present invention provides a method for preparing a free grounding film, which is suitable for preparing the free grounding film described in the second embodiment, including steps S21 to S24:

s21, forming a first conductor layer 1, forming a conductive adhesive layer 2 on one surface of the first conductor layer 1, and forming a second conductor layer 3 on one surface, far away from the first conductor layer 1, of the conductive adhesive layer 2.

And S22, forming a conductive bump 5 on one surface of the second conductor layer 3 away from the conductive adhesive layer 2.

S23, forming a third conductor layer 7 on the surface of the second conductor layer 3 on which the conductive bumps 5 are formed, and forming bumps 70 at positions where the third conductor layer 7 covers the conductive bumps 5 (the relationship between the bumps 70 and the conductive bumps 5 may be one-to-one, one-to-many, or multiple-to-one).

S24, forming an adhesive film layer 4 on a surface of the third conductor layer 7 away from the second conductor layer 3; when the free grounding film is used for grounding a printed circuit board, an electromagnetic shielding film 9 is arranged on the printed circuit board 11, the electromagnetic shielding film 9 comprises a shielding layer 91 and an insulating layer 90 which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film 9 through the glue film layer 4, and the protruding portion 70 pierces through the glue film layer 4 and the insulating layer 90 and is electrically connected with the shielding layer 91.

In this embodiment, the conductive bump 5 is preferably formed in a manner that: the first conductor layer 1 is provided with a first through hole 10 penetrating through the upper and lower surfaces thereof, the second conductor layer 3 is provided with a second through hole 30 penetrating through the upper and lower surfaces thereof, and the conductive bump 5 is formed by cooling and solidifying a fusible metal when flowing from the first through hole 10 to the outer side of the second through hole 30 through the conductive adhesive layer 2.

Specifically, referring to fig. 6, the conductive adhesive layer 2 is provided with a third through hole 20 penetrating through the upper and lower surfaces thereof;

the step S22 is specifically:

arranging a fusible metal at the first through hole 10, enabling the fusible metal to flow from the first through hole 10 to the corresponding second through hole 30 through the third through hole 20 of the conductive adhesive layer 2 at a preset temperature, and cooling and solidifying the fusible metal after the fusible metal flows to the outer side of the second through hole 30, so as to form a conductive bump 5 on the surface, away from the conductive adhesive layer 2, of the second conductor layer 3; wherein, the outer side of the second through hole 30 is far away from the conductive adhesive layer 2.

Alternatively, referring to fig. 7, the conductive adhesive layer 2 is a hot melt adhesive layer;

the step S22 is specifically:

arranging a fusible metal at the first through hole 10, and enabling the fusible metal to flow from the first through hole 10 to the second through hole 30 at a preset temperature, wherein the conductive adhesive layer 2 forms hot melt adhesive to flow to the second through hole 30 in the flowing process of the fusible metal;

the fusible metal and the hot melt adhesive flowing to the outside of the second through hole 30 are cooled, so that the conductive bump 5 is formed on the surface of the second conductor layer 3 away from the conductive adhesive layer 2.

It should be noted that the melting temperature of the fusible metal is preferably 300-2000 ℃. In addition, the conductive bump 5 may be formed by directly applying a conductive adhesive bump on the surface of the second conductor layer 3 away from the conductive adhesive layer 2 and solidifying the conductive bump 5, or by directly plating a metal dot on the surface of the second conductor layer 3 away from the conductive adhesive layer 2 to form the conductive bump 5, and the like, which is not limited herein.

As a modification of the above, after the step S23 and before the step S24, the method further includes:

a plurality of conductor particles 6 are formed on the outer surface of the protrusion 70 by one or more processes of physical roughening, electroless plating, physical vapor deposition, chemical vapor deposition, evaporation plating, sputtering plating, electroplating, and hybrid plating, see fig. 5.

The method for manufacturing the conductor particles 6 may further include: in manufacturing the third conductor layer 7, the conductor particles 6 and the third conductor layer 7 may be an integral structure formed by a one-step molding process. In addition, when the third conductor layer 7 has a multilayer structure, the method for manufacturing the conductor particles 6 may further include: the third conductor layer 7 is formed by sequentially forming a layer by layer, and the conductor particles 6 are formed at corresponding positions of each layer when each layer is formed, so that the third conductor layer 7 with the conductor particles 6 is formed.

In this embodiment of the present invention, the step S24 specifically includes:

forming an adhesive film layer 4 on a release film, and then transferring the adhesive film layer 4 to one surface of the third conductor layer 7 away from the second conductor layer 3 in a pressing manner; or

And directly forming an adhesive film layer 4 on one surface of the third conductor layer 7 far away from the second conductor layer 3.

It should be noted that the method for manufacturing the free grounding film provided in this embodiment is only one example of the method for manufacturing the free grounding film described in the second embodiment, and the free grounding film described in the second embodiment may also be manufactured by other manufacturing methods, for example, the conductive bump 5 may be formed on one surface of the second conductor layer 3, and then the first conductor layer 1 and the conductive adhesive layer 2 may be stacked.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and substitutions can be made without departing from the technical principle of the present invention, and these modifications and substitutions should also be regarded as the protection scope of the present invention.

Claims (18)

1. A free grounding film is characterized by comprising a first conductor layer, a conductive adhesive layer, a second conductor layer and an adhesive film layer which are sequentially stacked, wherein a conductive bulge is arranged on one surface, close to the adhesive film layer, of the second conductor layer;

when the free grounding film is used for grounding a printed circuit board, an electromagnetic shielding film is arranged on the printed circuit board and comprises a shielding layer and an insulating layer which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the conductive protrusions penetrate through the adhesive film layer and the insulating layer and are electrically connected with the shielding layer.

2. The free-grounding film of claim 1, wherein the first conductor layer is provided with a first via hole penetrating through the upper and lower surfaces thereof, the second conductor layer is provided with a second via hole penetrating through the upper and lower surfaces thereof, and the conductive adhesive layer is provided with a third via hole penetrating through the upper and lower surfaces thereof;

the conductive bumps are correspondingly formed on the outer sides of the second through holes; the conductive protrusion is formed by cooling and solidifying when the fusible metal flows to the outer side of the second through hole from the first through hole through the third through hole; and the outer side of the second through hole is far away from the conductive adhesive layer.

3. The free-grounding film of claim 1, wherein the first conductor layer is provided with a first via hole penetrating upper and lower surfaces thereof, and the second conductor layer is provided with a second via hole penetrating upper and lower surfaces thereof;

the conductive bumps are correspondingly formed on the outer sides of the second through holes; the conductive protrusion is formed by cooling and solidifying the fusible metal from the first through hole and the hot melt adhesive of the conductive adhesive layer when flowing to the outer side of the second through hole; and the outer side of the second through hole is far away from the conductive adhesive layer.

4. A free-grounding film as claimed in claim 1, wherein the surface of the conductive bump is provided with convex conductor particles.

5. The free grounding film of claim 1, wherein the glue layer comprises an adhesive layer containing conductive particles; or the adhesive film layer comprises an adhesion layer without conductive particles.

6. The free grounding film of any one of claims 1 to 5, wherein the free grounding film further comprises an oxidation preventing layer, and the oxidation preventing layer is disposed on a surface of the first conductor layer away from the conductive adhesive layer.

7. A free grounding film as claimed in any one of claims 1 to 5, further comprising a peelable protective film layer provided on the side of the adhesive film layer remote from the second conductor layer.

8. The free grounding film of any one of claims 1 to 5, further comprising a third conductor layer, wherein the third conductor layer is disposed on a surface of the second conductor layer on which the conductive bump is formed, and a bump is formed on a position of the third conductor layer covering the conductive bump.

9. A free grounding film as claimed in claim 8, wherein the surface of the protrusions is provided with convex conductor particles.

10. A wiring board comprising a printed wiring board and an electromagnetic shielding film provided on said printed wiring board, wherein said wiring board further comprises a free grounding film according to any one of claims 1 to 7; the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the conductive protrusions penetrate through the adhesive film layer and the insulating layer and are electrically connected with the shielding layer;

or, the wiring board further comprises the free grounding film of claim 8 or 9; the electromagnetic shielding film comprises a shielding layer and an insulating layer which are stacked, the free grounding film is pressed with the electromagnetic shielding film through the adhesive film layer, and the protruding portion pierces the adhesive film layer and the insulating layer and is electrically connected with the shielding layer.

11. A method for preparing a free-grounding film, suitable for preparing a free-grounding film according to any one of claims 1 to 7, comprising the steps of:

forming a first conductor layer, forming a conductive adhesive layer on one surface of the first conductor layer, and forming a second conductor layer on one surface of the conductive adhesive layer, which is far away from the first conductor layer;

forming a conductive bump on one surface of the second conductor layer, which is far away from the conductive adhesive layer;

forming an adhesive film layer on one surface of the second conductor layer on which the conductive bumps are formed; when the free grounding film is used for grounding the printed circuit board, an electromagnetic shielding film is arranged on the printed circuit board and comprises a shielding layer and an insulating layer which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the conductive protrusions penetrate through the adhesive film layer and the insulating layer and are electrically connected with the shielding layer.

12. The method for preparing a free grounding film as claimed in claim 11, wherein the first conductor layer is provided with a first through hole penetrating through the upper and lower surfaces thereof, the second conductor layer is provided with a second through hole penetrating through the upper and lower surfaces thereof, and the conductive adhesive layer is provided with a third through hole penetrating through the upper and lower surfaces thereof;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

cooling and solidifying the fusible metal after flowing from the first through hole to the outer side of the second through hole through the third through hole, so as to form a conductive bump on one surface of the second conductor layer away from the conductive adhesive layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

13. The method of making a free-grounding film of claim 11, wherein the conductive adhesive layer is a hot melt adhesive layer; the first conductor layer is provided with a first through hole penetrating through the upper surface and the lower surface of the first conductor layer, and the second conductor layer is provided with a second through hole penetrating through the upper surface and the lower surface of the second conductor layer;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

a fusible metal is arranged at the first through hole, the fusible metal flows to the second through hole from the first through hole at a preset temperature, and the conductive adhesive layer forms hot melt adhesive to flow to the second through hole in the flowing process of the fusible metal;

cooling the fusible metal and the hot melt adhesive which flow to the outer side of the second through hole, so that a conductive bump is formed on one surface, away from the conductive adhesive layer, of the second conductor layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

14. The method for preparing a free-grounding film as claimed in claim 11, wherein before forming the adhesive film layer on the surface of the second conductor layer on which the conductive bump is formed, the method further comprises:

forming conductor particles on the outer surface of the conductive bump by one or more processes of physical roughening, chemical plating, physical vapor deposition, chemical vapor deposition, evaporation plating, sputtering plating, electroplating and hybrid plating.

15. The method for preparing a free-grounding film as claimed in claim 11, wherein a glue film layer is formed on the surface of the second conductor layer on which the conductive bumps are formed, specifically:

forming an adhesive film layer on a release film, and transferring the adhesive film layer to the surface of the second conductor layer, on which the conductive bumps are formed, in a pressing manner; or the like, or, alternatively,

and forming an adhesive film layer on one surface of the second conductor layer on which the conductive protrusion is formed.

16. A method for preparing a free-grounding film, which is suitable for preparing the free-grounding film of claim 8 or 9, comprising the steps of:

forming a first conductor layer, forming a conductive adhesive layer on one surface of the first conductor layer, and forming a second conductor layer on one surface of the conductive adhesive layer, which is far away from the first conductor layer;

forming a conductive bump on one surface of the second conductor layer, which is far away from the conductive adhesive layer;

forming a third conductor layer on one surface of the second conductor layer on which the conductive bumps are formed, and forming bumps at positions where the third conductor layer covers the conductive bumps;

forming a glue film layer on one surface of the third conductor layer, which is far away from the second conductor layer; when the free grounding film is used for grounding the printed circuit board, an electromagnetic shielding film is arranged on the printed circuit board and comprises a shielding layer and an insulating layer which are arranged in a stacked mode, the free grounding film is in press fit with the electromagnetic shielding film through the adhesive film layer, and the protruding portion penetrates through the adhesive film layer and the insulating layer and is electrically connected with the shielding layer.

17. The method for preparing a free grounding film as claimed in claim 16, wherein the first conductor layer is provided with a first through hole penetrating its upper and lower surfaces, the second conductor layer is provided with a second through hole penetrating its upper and lower surfaces, and the conductive adhesive layer is provided with a third through hole penetrating its upper and lower surfaces;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

arranging a fusible metal at the first through hole, enabling the fusible metal to flow from the first through hole to the second through hole through the third through hole at a preset temperature, and cooling and solidifying the fusible metal after the fusible metal flows to the outer side of the second through hole, so that a conductive bump is formed on one surface, away from the conductive adhesive layer, of the second conductor layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

18. The method of making a free grounding film of claim 16, wherein the conductive adhesive layer is a hot melt adhesive layer; the first conductor layer is provided with a first through hole penetrating through the upper surface and the lower surface of the first conductor layer, and the second conductor layer is provided with a second through hole penetrating through the upper surface and the lower surface of the second conductor layer;

then form electrically conductive protruding on the second conductor layer keeps away from the one side of conductive adhesive layer, specifically do:

a fusible metal is arranged at the first through hole, the fusible metal flows to the second through hole from the first through hole at a preset temperature, and the conductive adhesive layer forms hot melt adhesive to flow to the second through hole in the flowing process of the fusible metal;

cooling the fusible metal and the hot melt adhesive which flow to the outer side of the second through hole, so that a conductive bump is formed on one surface, away from the conductive adhesive layer, of the second conductor layer; and the outer side of the second through hole is far away from the conductive adhesive layer.

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