JP2001119130A - Method for bonding electrical contact - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for bonding an electric contact, which can replace conventional soldering and nonconformity due to the soldering be eliminated. SOLUTION: When two electrical contacts are bonded electrically with each other, a conductive adhesive agent 30 in which a high conductive metallic ultra- fine particle of about 5 nm in particle size is mixed and dispersed, for example, in an adhesive liquid polymer material 44, is interposed between the electrical contacts, and it is fixed temporarily and baked thereafter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for mounting a chip constituting an electronic component or a semiconductor device on a substrate or the like. The present invention relates to a method for joining electrical contacts for electrically joining electrodes (contacts).

[0002]

2. Description of the Related Art For example, electrical bonding between electrical bonding bumps arranged on the surface of a chip constituting an electronic component or a semiconductor device and electrodes provided at positions corresponding to these bumps on a substrate is required. , A solder composed of tin and lead (hereinafter referred to as Sn-
Micro soldering using Pb solder or solder) is widely used. According to this type of soldering method, generally, a bonding strength of about 39.2 MPa is ensured, an electric resistivity of the solder bulk is about 17 μΩcm, and a melting temperature is about 180 ° C. And a well-balanced bonding characteristic can be easily obtained.

FIG. 3 shows a QFC (quad flapper) by a conventional general reflow soldering using this kind of solder.
1 shows an example of a surface mounting process of mounting IC packages of the (t package) type on both sides of a printed wiring board. First, as shown in FIG. 3A, a solder paste 12a is printed at a predetermined position on the surface (upper surface) of the printed wiring board 10, and as shown in FIG. 3B, the solder paste 12a is sandwiched between the solder pastes 12a. The adhesive 14 is applied to the predetermined position. Then, as shown in FIG. 3C, the respective leads 22a are pressed against the respective solder pastes 12a, and the IC
The package 20a is mounted on the surface of the printed wiring board 10, and the adhesive 14 is dried and cured.

Next, as shown in FIG. 3D, after the printed wiring board 10 is inverted, a solder paste 12b is printed at a predetermined position on the back surface (upper surface) of the printed wiring board 10, and FIG. e) While pressing each lead 22b against each solder paste 12b, as shown in FIG.
0b is attached to the back of the printed wiring board 10, and then
For example, by heating to 200 to 300 ° C., the solder pastes 12a and 12b are melted and solidified.

Here, an IC package 20a is mounted on one surface (front surface) of the printed wiring board 10 using an adhesive 14 in consideration of the melting property of the solder.

[0006]

However, in recent years,
For example, electrical bonding bumps provided on the surface of a chip that constitutes a semiconductor device, and electrodes provided on a substrate are becoming finer and narrower. Accordingly, a conventional bonding method using micro soldering has been developed. Then, the following drawbacks have been strongly regarded as problems.

[0007] If the interface has good wettability, an interaction layer of a solid solution, an intermetallic compound, or the like is formed between the material to be joined and the molten solder, which causes deterioration of mechanical and electrical properties. In the case of a combination having significantly different soldering properties due to the difference in the material of the members to be joined, the joining operation and conditions become complicated. Due to severe erosion of the material to be joined by the solder, the composition of the solder and the underlying treatment of the material to be joined may be troublesome. Table 1 shows the problems of the joining method using the micro soldering with emphasis on the phenomenon, generation mechanism, and countermeasures.

[0008]

[Table 1] The failure phenomena shown in Table 1 are inevitably caused by the interaction with the material of the member to be joined when the Sn-Pb solder is used. Due to basic physics and chemistry phenomena. Here, the defect phenomena No. in Table 1 above was used. The formation mechanism of the intermetallic compound and the erosion of the material to be joined by the solder of the defect phenomenon No. will be examined.

FIG. 4 shows an equilibrium diagram of the Cu—Sn system. As is clear from FIG. 4, copper (Cu) and Sn-Pb
Considering the solder contact, it can be seen that various equilibrium phases can exist depending on the respective composition ratios of Cu-Sn. In particular,
In each case of 24% Cu and 44% Cu, the intermetallic compounds Cu ₃ Sn and Cu ₆ Sn ₅ are stably present, both of which are considered to have properties close to ceramic, and are hard and brittle, and have high electrical Since a portion having a resistivity is formed, the joint becomes extremely intense.

On the other hand, FIG. 5 shows the rate at which various metal atoms dissolve in the molten 60% Sn-40% Pb solder. The relationship in FIG. 5 is represented by the following equation (1).

(Equation 1)

As apparent from FIG. 5, when comparing each element at a constant temperature, tin dissolves fastest, gold and silver dissolve relatively quickly, and other elements dissolve relatively slowly. You can see that On the other hand, in relation to the temperature, each element shows an exponential function type of the equation (1), and the dissolution rate sharply increases as the temperature increases.

The disadvantages shown in Table 1 are all Sn-Pb
Since there is a physical-chemical interaction between the solder and the member to be joined, there is no way to eliminate them 100%. Therefore, in order to fundamentally avoid these problems, it is necessary to employ a joining means based on a completely different principle instead of the joining method using the conventional Sn-Pb solder.

For this reason, a method using a conductive adhesive has been developed as one of the joining methods of bump-electrode and the like based on completely different joining means, which is free from the disadvantages of the conventional soldering joining method. The conductive adhesive includes particles such as silver and carbon as a conductive filler (conductive filler).
It is mixed and dispersed in an organic polymer having adhesiveness, and by using this, it is possible to simultaneously realize adhesiveness and conductivity. As a polymer material which is a main component of the conductive adhesive, an epoxy resin having excellent adhesiveness and durability is generally used.

However, a joining method using a conductive adhesive and a joining method using an Sn—Pb solder (soldering)
When compared with the above, there is a problem that the electrical resistivity of the connection part is much higher than that of the solder in the joining method using the conductive adhesive. Table 2 shows the electrical resistivity and adhesive strength of the conductive adhesive using silver and the conductive adhesive using nickel as the conductive filler.

[Table 2] From Table 2, it can be seen that even with the conductive adhesive using silver as the conductive filler, the level is one digit larger than that of the solder.

As described above, the cause of the increase in the electrical resistivity is mainly that the size of the metal particles that should be dispersed sufficiently uniformly is large, and that the uniformity of the dispersion is reduced due to aggregation in the polymer material. It is thought that. Therefore, there is a great need for realizing a conductive adhesive having an electrical resistivity comparable to that of solder and establishing a joining technique for electrical contacts such as bumps and electrodes using the same.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a method of joining electrical contacts which can be replaced with conventional soldering and eliminates all the problems peculiar to the soldering.

[0017]

According to the first aspect of the present invention, in electrically connecting two electrical contacts, ultrafine metal particles having good conductivity are contained inside a liquid polymer material having adhesiveness. A method of joining electrical contacts, characterized in that a conductive adhesive in which is mixed and dispersed is interposed between two electrical contacts, temporarily fixed, and fired.

By using this, ultrafine metal particles are uniformly mixed and dispersed without causing aggregation in the liquid polymer material, and the metal ultrafine particles are shrunk and hardened during the subsequent firing. By firmly contacting each other, it is possible to obtain an electrical resistivity substantially equal to that of the solder.

According to a second aspect of the present invention, there is provided the method for joining electrical contacts according to the first aspect, wherein the ultrafine metal particles are ultrafine silver particles produced by thermally decomposing an organic complex containing silver. It is. The ultrafine silver particles are produced, for example, by heating and purifying silver stearate in a nitrogen atmosphere at about 250 ° C. for 4 hours.

According to a third aspect of the present invention, there is provided the method for joining electrical contacts according to the second aspect, wherein the ultrafine silver particles have a cluster shape with a size of about 5 nm. Thereby, the ultrafine silver particles have an extremely small particle size, and the degree of uniform dispersion in the polymer material becomes extremely good.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. Figure 1 shows a QFC (quad flat pack)
1 shows a method for bonding electrical contacts according to one embodiment of the present invention applied to a surface mounting process of mounting an age) type IC package on both sides of a printed wiring board.

First, as shown in FIG. 1A, a conductive adhesive 3 is applied to a predetermined position on the surface (upper surface) of the printed wiring board 10.
0a is printed. Then, as shown in FIG.
The respective leads 22a of the C package 20a are attached to the respective conductive adhesives 30a and temporarily fixed, and thereafter, for example,
The conductive adhesive 30a is fired by heating to about 0 ° C.

Next, as shown in FIG. 1C, after the printed wiring board 10 is turned over, a conductive adhesive 30b is printed at a predetermined position on the back surface (upper surface) of the printed wiring board 10, and FIG. As shown in (d), each lead 22b of the IC package 20b is attached to each of the conductive adhesives 30b and temporarily fixed, and then heated to, for example, about 200 ° C., and the conductive adhesive 30
b is fired.

According to this embodiment, since no solder is used, the steps of applying and drying / curing the adhesive 14 in the conventional example shown in FIG. 3 can be omitted, and the steps can be simplified. it can.

FIG. 2A shows a state immediately after the conductive adhesive 30 is applied to the surface of the printed wiring board 10.
3 conceptually shows the state of the conductive adhesive 30 after firing.

The conductive adhesive 30 employs, as its conductive element, silver ultrafine particles 40 having good conductivity, for example, composed of simple silver, and the periphery thereof is covered with an alkyl chain shell 42. It is mixed and dispersed in a liquid polymer material 44 which is a mixed liquid of an agent and an organic solvent. The ultrafine silver particles (metal ultrafine particles) 40 have a size of about 5 nm and are in a very small cluster shape.

Here, the ultrafine silver particles 40 having a small size of about 5 nm, which are covered with an alkyl shell 42, are formed.
For example, a linear fatty acid silver salt prepared by saponifying myristic acid, stearic acid or oleic acid with sodium hydroxide and then reacting with silver nitrate (the number of carbon atoms in the alkyl chain = 14, 18, 18 ω) is obtained. , 25
It is manufactured by heating in a nitrogen atmosphere at about 0 ° C. for 4 hours and purifying. When such ultra-fine silver particles 40 having an extremely small particle size of 5 nm at the cluster level are supplied and dispersed in a liquid polymer material 44 in a state of being dissolved in an organic solvent such as cyclohexane, for example, It has extremely good dispersibility and does not aggregate with each other.
0 is uniformly mixed in the medium in a stable state. That is, when the polymer material 44 is in a liquid state, as shown in FIG. 2A, the silver ultrafine particles 40 are not in contact with each other,
It is uniformly dispersed in the polymer material 44.

As a main component of the adhesive of the polymer material 44, for example, a thermosetting resin such as an epoxy-based or phenol-based resin is used. This type of thermosetting resin is dried and hardened by, for example, heating and holding at a temperature of 170 ° C. or more, and causes a certain volume shrinkage at that time. On the other hand, it is known that the alkyl chain shell 42 covering the ultrafine silver particles 40 disappears by heating at about 200 ° C.

That is, as described above, when the conductive adhesive 30 is baked at 200 ° C., as shown in FIG. 2B, the alkyl chain shell 42 covering the periphery of the ultrafine silver particles 40 disappears, and furthermore, As the polymer material 44 shrinks and hardens, the silver ultrafine particles 40 come into direct contact with each other, and sintering occurs at the contact portion. As a result, the silver particles become electrically conductive as a whole, and at the same time, the polymer material 44 is cured to complete the bonding. Through the above process, the electrical resistivity of this conductive adhesive can be reduced to about 1/9 of the conventional conductive adhesive using silver as the conductive filler shown in Table 2, and the current 60% Sn-40% P
The value can be improved to almost the same level as the b solder.

This process is basically the same as that for imparting conductivity to a conductive resin other than the adhesive, a conductive rubber (elastomer), a conductive coating material, and the like. In addition, by using an epoxy-based or phenol-based synthetic resin as a main component of the adhesive and appropriately selecting the curing time of the synthetic resin, a bonding force of at least 20 to 40% of the solder is secured. The electrical characteristics and long-term durability of the solder can be made equal to or higher than the above-mentioned solder.

In general, it has been pointed out that the mechanical properties of the joint portion are relatively brittle in the joint by the adhesive as compared with the soldering joint, so that the toughness and the crack resistance are inferior as a problem. In this regard, modified (flexible or toughened) phenol-based, epoxy-based, acrylic-based, and polyimide-based adhesives have been put to practical use as structural adhesives for aircraft, for example. There is a product. Therefore, it is considered that this type of problem can be dealt with by imitating this as necessary.

In order to cause the denaturation of the epoxy adhesive, a necessary function can be obtained by dissolving a specific elastomer in a liquid resin and curing the liquid resin. For example, by adding and modifying an elastomer, the epoxy adhesive has a room temperature peel strength of 1.1 kN.
/ M from 5.5 kN / m to 5 kN / m. In addition, the polyimide-based adhesive has a temperature of 19.3MP
a and extremely high lap shear strength.

As described above, according to the method for joining electrical contacts of the present invention, unlike conventional soldering, tin-
Electrical and electronic components that can avoid the adverse effects caused by wet contact between the lead alloy melt and the surface of the member to be joined at high temperatures, and at the same time cover most of the advantages of soldering Can be joined. Therefore, instead of the conventional soldering, the aforementioned metal physics
It is possible to achieve a sound electrical connection that eliminates all the problems inherent to soldering (see Table 1) that are inevitably caused by chemical phenomena.

[0034]

As described above, according to the present invention,
It is possible to provide a method of joining electrical contacts which can be replaced with conventional soldering and eliminates all the problems peculiar to the soldering.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a method of joining electrical contacts according to an embodiment of the present invention in the order of steps.

FIG. 2A shows a state immediately after a conductive adhesive is applied,
(B) is a figure which shows notionally the state after baking of a conductive adhesive.

FIG. 3 is a diagram showing an example of a conventional method for joining electrical contacts in the order of steps.

FIG. 4 is a diagram showing a Cu—Sn binary phase diagram and a structure of a soldering interface with copper.

FIG. 5 is a graph showing the dissolution rate of each element in a molten 60% Sn-40% Pb solder of various metals.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Printed wiring board 20a, 20b IC package 22a, 22b Lead (contact) 30, 30a, 30b Conductive adhesive 40 Silver ultrafine particle (metal ultrafine particle) 42 Alkyl chain shell 44 Polymer material

Claims (3)

[Claims] When electrically connecting two electrical contacts, a conductive adhesive in which ultra-fine metal particles having good conductivity are mixed and dispersed inside a liquid polymer material having an adhesive property is used between the two electrical contacts. A method of joining electrical contacts, wherein the electrical contacts are temporarily fixed and fired. 2. The method according to claim 1, wherein the ultrafine metal particles are ultrafine silver particles produced by thermally decomposing an organic complex containing silver. 3. The ultrafine silver particles have a size of 5 nm.
3. The method for joining electrical contacts according to claim 2, wherein the electrical contacts have a degree of cluster shape.