JPH10303248A - Wiring board having solder bump, manufacture of the wiring board and metal mask for forming solder bump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metal mask for forming solder bump, by which product development and manufacture are easy, whose cost is low and which can suit ably form the solder bump. SOLUTION: Etching resist 22 formed of photosensitive epoxy resin whose thickness is 25 μm is adhered to the upper/lower faces of a stainless plate 21 whose thickness is 40 μm. Exposure and sensitization are executed and a hole whose diameter is 140 μm is given to etching resist 22 in a place where the hole 14 of the metal mask 13 is scheduled to be formed. Hydrofluoric sulfide solution is showered and sprayed on both sides of the stainless plate 21 provided with etching resist 22 at the pressure of 2 kg/cm<2> . Then, etching is executed for 13 minutes. The hole 14 whose vertical cross section with 160 μm of an opening end and 180 μm of the diameter of a central part is formed on the stainless plate 21. Then, the metal mask 13 is finished by removing etching resist 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wiring board having a solder bump for connecting a semiconductor element and a wiring board using the solder bump, a method of manufacturing the wiring board, and a metal mask for forming a solder bump. .

[0002]

2. Description of the Related Art Conventionally, when a semiconductor element (hereinafter, referred to as a flip chip) is mounted on a flip-chip mounting board (hereinafter, simply referred to as a wiring board) by a so-called flip-chip method, the following procedure is adopted. ing.

For example, bumps made of high-temperature solder are formed on flip-chip pads, bumps made of eutectic solder having a lower melting point than high-temperature solder are formed on pads of a wiring board, and the bumps are brought into contact with each other. And heating to melt only the eutectic solder.

Here, as one of the techniques for forming solder bumps on a wiring board, there is a solder paste printing method using a metal mask, which is performed in the following procedures (1) to (6). 1) First, a pad and a solder resist are provided on the surface of a base material to be a wiring board (see FIG. 10A).

2) Next, a metal mask is placed on the solder resist (FIG. 10B). 3) Next, perform squeegee printing using a metal mask,
The opening is filled with a solder paste (FIG. 10C). 4) Next, the metal mask is removed (FIG. 10D).

5) Next, the eutectic solder particles are melted in a reflow furnace, and then cooled to form solder bumps (FIG. 10E). 6) Next, the flux is removed, and a wiring board having solder bumps on the surface is completed (FIG. 10 (f)).

The following three types of metal masks are used for printing the above-mentioned solder paste depending on the manufacturing method. Etching mask As shown in FIG. 11A, a photosensitive etching resist is applied to a stainless steel plate, the etching resist at an opening is removed by exposure and development, and the stainless steel plate is etched with hydrofluoric acid or the like. Unnecessary portions are removed by etching, and then the etching resist is removed to complete the process.

Laser Drilling Mask As shown in FIG. 11B, a laser drilling mask is manufactured by drilling holes one by one on a stainless steel plate with a YAG laser. When using, use it upside down.

Electroformed (additive) mask As shown in FIG. 11 (c), the electroformed mask is formed by applying a photosensitive resin, removing the photosensitive resin except where it is desired to form an opening by exposure and development, and removing a stainless steel plate. Electrolytic Ni plating is performed thereon for a predetermined mask thickness, and the mask made of electrolytic Ni is removed from the stainless steel plate to complete the process.

[0010]

SUMMARY OF THE INVENTION
The metal mask has the following problems, and further improvement is required. The etching mask has an advantage that it can be easily manufactured, but has a problem that the transferability of the solder paste is poor as shown in FIG. In other words, the conventional etching mask has burrs near the center of the cross section of the opening, so when the solder paste is applied and the mask is separated from the substrate, the solder paste is caught by the burrs and is transferred to the substrate and transferred to the substrate. There is a problem that the amount of solder paste in the opening to be formed becomes small.

The laser drilling mask does not require the work of producing a film such as an etching resist or a photosensitive resin at the time of manufacture, and the opening has a tapered cross section.
This has the advantage that the solder paste is less likely to remain in the openings and the transferability to the substrate is good.However, since lasers are used for drilling individual holes, there is a problem that the cost increases when the number of holes is large. Was. In particular, when the mask opening diameter is reduced, as shown in FIG. 11B, the cross-sectional shape becomes a taper that spreads downward, so that the opening diameter at the upper part of the opening needs to be considerably reduced. The solder paste is filled from the small opening diameter. For this reason, there is a problem that the filling property of the solder paste is poor, the height of the solder bumps varies greatly, and the bonding property of the flip chip is reduced.

When the mask thickness is small, the dimensional accuracy is reduced due to the thermal expansion of the mask material due to heating at the time of drilling. Therefore, it is necessary to accurately manufacture a thin mask having a thickness of less than 70 μm, especially 50 μm or less. It is difficult. Further, since the inner wall of the mask opening is rough, there is a disadvantage that the filling of the solder paste into the opening is reduced.

In the electroformed mask, since the inner peripheral surface of the opening is stood perpendicular to the plate surface, even if the opening is small,
Although there is an advantage that both the filling property and the transfer property of the solder paste are excellent, there is a problem that the cost is high because the mask is formed by plating. Also, the material of the mask is limited to a material such as Ni that can be formed by plating, and Ni or the like is softer than stainless steel, so that there is a problem that the mask life is short. Further, since the mask strength is low, the mask plate thickness is 70
If the thickness is less than 50 μm, and if it is 50 μm or less, it is difficult to manufacture and handle. If the thickness is increased to avoid this, the solder volume filled in the opening will increase, and the solder bridge between adjacent bumps will increase. There is a problem that is easily generated. Further, as shown in FIG. 11C, since the side wall of the opening is substantially perpendicular to the plate surface, the solder paste adheres to the side wall of the opening, so that a desired amount of solder paste cannot be printed. However, since the amount of adhesion varies, the amount of solder paste sometimes varies.

Therefore, conventionally, the thickness and opening diameter of the metal mask to be used cannot be accurately calculated with respect to the height of the solder bump to be obtained. It is necessary to manufacture several types of masks, actually perform solder paste printing, and determine the mask specifications by trial and error, which has a major problem that product development and manufacturing are not easy.

According to the present invention, the specifications (plate thickness, opening diameter, etc.) of a metal mask can be calculated with high accuracy, and the mask can be easily manufactured.
An object of the present invention is to provide a metal mask for forming a solder bump, which is capable of suitably forming a solder bump at a low cost, and to provide a wiring board having the solder bump and a method for manufacturing the wiring board.

[0016]

According to the first aspect of the present invention, there is provided a metal mask for forming a solder bump, which is used when a solder paste is printed on a wiring board in order to form a solder bump. The metal mask used is characterized in that the vertical cross-sectional shape of the pattern hole corresponding to the solder bump to be formed is a substantially barrel shape in which the diameter at the center is larger than the diameter at the opening end.

In the present invention, since the vertical cross-sectional shape of the pattern hole of the metal mask is substantially barrel-shaped, the printing volume of the solder paste can be set accurately. That is, for example, in the conventional electroformed mask, the printed volume varies depending on the state of the solder paste attached to the side wall of the pattern hole. However, in the present invention, as shown in FIG. A fixed amount of solder paste stably adheres to the portion, and the printing volume is always the volume of a cylinder defined by the diameter of the opening end of the pattern hole (smaller than the center) and the thickness of the mask plate. Print volume. Further, since there is no burr at the center of the pattern hole side wall as in the conventional etching mask, the transferability is also improved from that point. Therefore, bumps having a small height variation and a uniform height can be formed. Therefore, the bonding property with an electronic component such as a flip chip is improved.

When filling a pattern hole of a metal mask with a solder paste by, for example, squeegee printing, as shown in FIG. 1B, the opening side wall and the mask are opened at the upper opening end of the pattern hole in the figure. Since the angle between the upper surface and the upper surface is acute, the solder paste can be filled more reliably than, for example, a conventional electroformed mask having a substantially right angle at the upper opening end. Therefore, also from this point, variation in bump height can be reduced.

Therefore, according to the present invention, the thickness of the metal mask and the diameter of the opening end suitable for obtaining the desired height of the solder bump can be accurately calculated, so that the troublesome work of determining the mask conditions is required. It becomes unnecessary, and product development and manufacturing become easy. Here, the printing volume by the solder paste printing method (the volume of the solder paste that fills the openings such as the inside of the pattern holes of the metal mask and the openings of the solder resist).
The calculation method of will be described. Incidentally, since a solder resist (for insulation) is usually formed on the surface of the substrate around the pad, the case where the solder resist is provided will be described as an example.

As shown in FIG. 2, when printing a solder paste, a solder resist is formed in advance around pads on a wiring board, and a metal mask is arranged on the solder resist. At this time, the metal mask is arranged so that the opening of the solder resist and the pattern hole of the metal mask communicate in the vertical direction in the drawing.

That is, since the solder paste is filled in the opening formed by the pattern hole and the opening, the volume of the opening becomes the volume of the solder paste to be filled. Further, since the ratio of the solder in the solder paste is determined, the volume of the solder bump, and thus the height of the solder bump determined from the volume, can be set by determining the volume of the opening.

For example, when the dimensions of each part are set as shown in FIG.
Is calculated by the following equation (1). Open end diameter of metal mask; thickness of DMMO metal mask; opening diameter of TMM solder resist; thickness of DSRO solder resist; TSR pad diameter; thickness of DP pad; TP VS = π (DSRO / 2) ² × TSR + π (DMMO / 2) ² × TMM-π (DP / 2) ² × TP (1) About half of the solder paste is solder (the rest is flux)
Therefore, the volume V of the solder bump is about half of the volume VS of the solder paste.

On the other hand, the solder bump formed from the solder paste has a shape obtained by roughly cutting a sphere by surface tension at the time of melting, so that its volume V is calculated from the pad diameter DP and the bump height T in the following a) and b. ) Can be calculated separately. Therefore, from the pad diameter DP and the desired bump height T,
Thickness TMM of metal mask and diameter DMM of opening end of pattern hole
O can be calculated.

In the following calculation, the radius of the cut sphere constituting the bump shape is L, the radius of the sphere L and the bump height T
And t is the difference. a) As shown in FIG. 3A, when the bump is smaller than a hemisphere, L ² = r ² + t ² , and T = Lt, L = (r ² / T + T) / 2 (2) t = ( ^{r 2 / T-T) /} 2 ... (3)

[0025]

(Equation 1)

B) As shown in FIG. 3B, when the bump is larger than a hemisphere, L ² = r ² + t ² and T = L + t
From L = (r ² / T + T) / 2 (2 ′) t = (T−r ² / T) / 2 (3 ′)

[0027]

(Equation 2)

Therefore, the doubled solder volume V (2
Since V) is the print volume VS, the opening end diameter DMMO of the metal mask can be calculated from equation (4) or (4 ') and equation (1). According to a second aspect of the present invention, the pattern holes of the metal mask are formed by etching a metal plate for a metal mask from both surfaces thereof.

That is, in the present invention, as shown in FIG. 4, for example, an etching resist is provided on both sides of a metal plate, and etching is performed from both sides of the metal plate. In this state, the vertical cross-sectional shape of the pattern hole becomes substantially barrel-shaped, and as described in the above-mentioned claim 1, transferability, which was a disadvantage of the conventional etching mask (due to the burr at the center), is improved. Is done. Also, compared to conventional laser drilling masks and electroformed masks, their manufacture is easier and the cost can be greatly reduced to about one tenth.

In the present invention, since the diameter of the opening end of the pattern hole is larger than that of the conventional etching resist by performing over-etching, the diameter of the etching resist is desirably set smaller.
In the invention of the metal mask for forming a solder bump according to claim 3,
The thickness of the metal mask is 25 to 50 μm.

That is, the thickness is less than 70 μm, especially 25 to 25 μm, which is difficult to manufacture with a conventional electroformed mask or laser drilling mask.
By using a 50μm thick metal mask,
Small solder bumps can be precisely formed while suppressing variations in volume and height.

By reducing the thickness of the metal mask, the amount of solder paste remaining in the metal mask when the metal mask is separated from the wiring board is reduced, and there is no burr at the center of the opening due to over-etching of the metal mask. In addition, transferability is improved. Further, by reducing the plate thickness, the mask opening end diameter can be increased in order to obtain the same printing volume, so that the filling property of the solder paste into the pattern holes can be further improved. In addition, the volume of the solder paste to be printed is almost the same as the volume calculated from the thickness of the metal mask and the opening end diameter of the metal mask. Become. Therefore, variation in the height of the solder bumps is reduced.

The metal mask according to the first to third aspects is
It may be used alone, or may be pasted on a mesh to form a so-called combination mask. By using this combination mask, even a thin mask can be used.
Since it is straightened without distortion and adheres closely to the wiring board, it is difficult for paste dripping to occur during solder printing, and the occurrence of solder bridges due to this is reduced.

Further, in the method of manufacturing a metal mask according to any one of claims 1 to 3, the metal mask for a metal plate is etched from both surface sides to form a pattern hole of the metal mask. A method of manufacturing a metal mask for forming a solder bump, wherein the vertical cross-sectional shape has a step of forming a substantially barrel shape having a diameter at a central portion larger than a diameter at an opening end can be adopted.

In this manner, the mask can be easily manufactured by merely adjusting the etching time and the like in the conventional etching mask method. In the invention of the wiring board having the solder bump according to claim 4, the diameter of the pad of the wiring board is 150 μm or less, and the height of the solder bump formed on the pad is 70 μm or less on average, and the height varies. Is within ± 20% of the average value.

According to the present invention, the pad diameter of the wiring board is 15
Even in the case of a solder bump having a height of 0 μm or less and an average height of the solder bump of 70 μm or less, the variation in the height is as small as ± 20% of the average value. Get higher.

In particular, in the case where the pads of the wiring board are a pad group in which pads that require different amounts of solder to form solder bumps of the same height are mixed, a flip chip or the like is used. The bondability tends to be deteriorated, but since there is little variation in height, good bondability can be obtained.

According to a fifth aspect of the present invention, there is provided a method of manufacturing a wiring board having solder bumps for connection to electronic components, the method comprising: A solder paste is printed on the pads of the wiring board using a mask, and the solder is melted by heating to form solder bumps.

According to the present invention, since the solder bumps are formed by using the metal masks of the first to third aspects, even when a small solder bump is formed, variations in the bump height can be reduced. Thereby, for example, the bonding property with the flip chip is improved.

In a method of manufacturing a wiring board having solder bumps for connection to electronic components, a solder paste is applied onto a pad of the wiring board by using the metal mask according to any one of claims 1 to 3. By printing and heating to melt the solder, the pad diameter is 150 μm or less and the height of the solder bumps formed on the pad is 70
μm or less, the height variation is ± 20% of the average value
A method for manufacturing a wiring board having solder bumps, wherein solder bumps falling within the range described above are formed.

[0041]

Next, an example (embodiment) of an embodiment of the present invention will be described. Here, an integrated circuit chip (hereinafter, referred to as a flip chip), which is a semiconductor element,
An example is a resin laminated board (flip chip mounting wiring board, hereinafter simply referred to as a wiring board) mounted face down.

In the present embodiment, a metal mask having a large number of pattern holes (through holes) corresponding to individual solder bumps, a method of manufacturing the same, and a wiring in which solder bumps are formed by a solder paste printing method using the metal mask. The substrate and its manufacturing method will be described.

A) First, the flip chip and the wiring board in this embodiment will be described. As shown in FIG. 5A, the flip chip 1 is an LSI device made of Si having an outer diameter of 12.5 × 18 mm and a plate thickness of 0.4 mm. On one side, element-side solder bumps 3
Are provided in a predetermined arrangement pattern.

That is, as shown in an enlarged manner in FIG.
A large number of pads (element-side pads) 4 having a pad diameter of 140 μm × thickness 1 μm are provided on the base material 2 (downward in the figure) in a predetermined arrangement pattern. A substantially hemispherical element-side solder bump 4 made of high-temperature solder of 95Pb-5Sn (melting point: 314 ° C.) is formed. The element-side pad 4 is formed by forming a Cu vapor deposition film 4b on a Cr vapor deposition film 4a.

On the other hand, as shown in FIG. 6 (a), the wiring board 6 on which the flip chip 1 is mounted is made of a Japanese Patent Application No. 8-76960.
No. 2 is the same as that shown in Example 1 of
A large number of substrate-side solder bumps 8 are provided in a predetermined arrangement pattern on one surface (on the joining side) of a plate-shaped base material 5 having a size of 5 × 25 mm and a thickness of about 1 mm.

That is, as shown in FIG.
On the base material 7 (upper in the figure), the pad diameter DP = 140 μ
Pad (substrate side pad) 9 with mx thickness TP = 15 µm
Are provided in a predetermined arrangement pattern, and 37Pb-63Sn having a height T = 60 μm is provided on the substrate-side pad 9 thereof.
The substrate-side solder bump 8 is formed by cutting a sphere made of eutectic solder having a melting point of 183 ° C.

The substrate-side pad 9 is made of Ni
Plating is applied to about 5 μm, and a solder resist 1 having a thickness TSR = 20 μm is formed around the substrate side pad 9.
1 is formed. The solder resist 11 is provided as an insulating layer for preventing the eutectic solder from adhering to other places when the eutectic solder is melted.
Diameter DSRO = 2 so as to surround the circumference of the
It has an opening 12 of 00 μm.

B) Next, a description will be given of a metal mask used for forming the substrate-side solder bumps 8 and a method of forming the metal mask, which are essential parts of the present embodiment. As shown in FIG. 7, an etching resist 22 made of a photosensitive epoxy resin having a thickness of 25 μm is stuck on both upper and lower surfaces of a stainless steel plate 21 having a thickness of 40 μm.

Next, by exposing and exposing, a hole having a diameter of 140 μm is formed in the etching resist 22 at a position where the through hole 14 of the metal mask 13 is to be formed. Next, a hydrofluoric acid solution is showered and sprayed on both sides of the stainless steel plate 21 provided with the etching resist 22 at a pressure of ² kg / cm ² , and etching is performed for 13 minutes.

As a result, a through-hole 14 having a substantially barrel-shaped vertical cross section with a diameter of the opening end of 160 μm and a diameter of the central portion of 180 μm is formed in the stainless steel plate 21. After that, the metal mask 13 of the present embodiment is completed by removing the etching resist 22.

In this embodiment, as described below, the board-side solder bumps 8 made of eutectic solder having a height of 60 μm are formed on the board-side pads 9 of the wiring board 6 by a solder paste printing method. , One board-side solder bump 8
Since there are 500 bumps and solder printing is performed simultaneously on nine products, the through holes 14 of the metal mask 13
It has 500 holes.

C) Next, a description will be given of a wiring board having solder bumps performed by using the metal mask 13 and a method of manufacturing the same. Solder particle size 15 as solder paste
A solder paste having eutectic solder particles of about 35 μm and a flux was used. As the flux, any of R, RMA and RA types and water-soluble flux can be used in ascending order of reducing power. Further, the content of the flux in the solder paste is about half by volume.

First, as shown in FIG. 8A, the wiring board 6 before the formation of the board-side solder bumps 8 is formed on the surface of a base material 7 which is a resin laminated board having a conductive portion (not shown) inside. , A substrate side pad 9 and a solder resist 11 are provided. The solder resist 11 is formed by a general photolithography technique at a location excluding the substrate-side pad 9 and its periphery. Specifically, a negative-type ultraviolet-curable epoxy resin adhesive is printed and dried on the surface of the base material 7, and ultraviolet light is applied to a portion where the solder resist 11 is to be formed, whereby the epoxy resin is cured. An unnecessary portion of the resin is dissolved and removed with a solution to form a solder resist 11.

Next, as shown in FIG. 8B, a metal mask 13 is placed on the solder resist 11. At this time, the mounting is performed so that each opening 12 of the solder resist 11 and each through hole 14 of the metal mask 23 coincide with each other. The thickness TMM of the metal mask 13 is as small as 40 μm, and the opening end diameter DMMO is set to 160 μm, which is considerably smaller than that of the conventional one, in order to form the board-side solder bump 8 having a low height.

Here, the dimensions of the substrate-side pad 9, the dimensions of the opening 12 of the solder resist 11, and the dimensions of the metal mask 1
The filling amount of the solder paste defined by the size of the through hole 3 will be described. As already described with reference to FIG. 2, the volume VS of the solder paste to be filled in the openings 12 formed by the openings 12 of the solder resist 11 and the through holes 14 of the metal mask 13 is calculated by the above equation (1). ) Can be calculated.

As described with reference to FIG. 3, the volume V of the solder bump can be calculated from the above equation (4) or (4 ′). Further, as described above, since the volume of the solder in the solder paste is about half, the volume VS of the solder paste is about twice the volume V of the solder bump.

Therefore, the dimensions (plate thickness, diameter) of the through-holes 14 of the metal mask 13 in accordance with the given dimensions of the opening 12 of the solder resist 11 and the size of the substrate-side pad 9 so that the volume VS is obtained. Set. This gives the volume VS
Is filled, the board-side solder bump 8 having a desired volume and height is obtained.

Next, as shown in FIG. 8C, a solder paste 17 composed of eutectic solder particles 17a having a solder particle diameter of 15 to 35 μm and a flux 17b is used as the solder paste 17, and the metal mask 13 is removed. And squeegee printing is performed. As a result, a predetermined volume V
The solder paste 17 corresponding to S is filled.

Next, as shown in FIG. 8D, the metal mask 13 is removed. Next, as shown in FIG.
Is filled in a far-infrared ray reflow furnace at a maximum temperature of 210 ° C. to melt the eutectic solder particles 17 a and then cool to form the board-side solder bumps 8.

Next, as shown in FIG. 8F, the flux 17b is removed to complete the wiring board 6 having the board-side solder bumps 8 on the surface. The calculated printed volume of the board-side solder bump per board-side pad is approximately 6.0 × 10
^-4 mm, which is a calculation in which a board-side solder bump having a height of 62 μm is formed.

As described above, in the present embodiment, the following effects are obtained by the above-described configuration. (1) The actual height of the board-side solder bumps 8 in this embodiment is 61 μm on average, 50 μm minimum and 70 μm maximum, with little variation and the bump height is almost as calculated. This is because the solder paste has a high viscosity, so that the solder paste adheres to the wall surface of the through hole 14 of the metal mask 13 and is not transferred, so that the amount of solder is slightly reduced. However, in this example, the through-hole 14 is substantially barrel-shaped and the diameter at the center thereof is large, so that the amount of adhesion is stable and the solder paste in a substantially cylindrical shape having a diameter equal to the opening end diameter of the through-hole 14 is used. This is because the transfer is stable.

Therefore, in this embodiment, the thickness of the metal mask 13 to be used and the through-hole diameter (opening end diameter) can be accurately calculated by the required height of the board-side solder bumps 8, so that a troublesome mask is required. There is no need to set conditions, and there is an advantage that product development and manufacturing become easy.

Further, since there is little variation in the bump height, the bonding property of the flip chip 1 is excellent. (2) In the conventional electroformed mask, the filling property is poor because the wall surface of the through hole is perpendicular to the mask plate surface and is not an acute angle as in the present embodiment. For this reason, it is impossible to form the board-side solder bump 8 having a small pad diameter and a small bump height like the metal mask 13 of this embodiment while reducing the variation in the bump height. Further, the thickness of the mask cannot be reduced due to insufficient strength of the material. In addition, even with a conventional laser drilling mask (laser mask), the filling of the solder paste is poor because the wall surface of the through hole is rough and the opening end diameter on the upper surface must be designed to be relatively small. For this reason,
A solder bump having a small height cannot be formed while reducing the variation in the bump height. In addition, since the dimensional accuracy is poor, the thickness of the mask cannot be reduced.

On the other hand, if the etching mask as in the present embodiment is used, the metal plate thickness can be reduced to a minimum of about 25 μm and the through hole diameter can be reduced to a minimum of 140 μm. For example, the opening diameter of the solder resist 11 is 160 μm,
1 thickness 20 μm, pad diameter 120 μm, pad thickness 1
When the thickness is 5 μm, the substrate-side solder bump 8 having a solder bump height of 50 ± 8 μm can be formed.

(3) In this embodiment, since the metal mask 13 can be formed by etching, the mask cost is lower than that of a laser drilling mask or an electroformed mask. (4) In this embodiment, since a stainless steel plate can be used as the material of the metal mask 13, the mask life is longer and the manufacturing cost can be reduced as compared with an electroformed mask made of Ni.

(5) In this embodiment, the mask manufacturing equipment is
It is exactly the same as a conventional etching mask, and mass production is easy because only etching conditions need to be changed. <Experimental Example> Next, an experimental example performed to confirm the effect of the present embodiment will be described.

In this experiment, solder bumps (substrate-side solder bumps) were formed using the metal mask of the above embodiment.
The variation in the height of the solder bump is measured. (Experimental conditions) As shown in FIGS. 9A and 9B, 21 non-via portion pads and 4 via portion pads are arranged (total 25 locations), solder resist opening diameter: 200 μm, solder resist thickness: 20 μm, board side pad diameter: 14
0 μm, board side pad thickness (underlayer Cu pad thickness): 1
Solder bumps were formed on a wiring board having a structure of 5 μm using the etching mask of the present example as shown in Table 1 below.

Here, the via portion pad refers to a pad having a surface having a depression in which a pad for forming a solder bump is formed also as a via for connecting to a lower wiring layer. The size of the recess is appropriately determined depending on the thickness of the insulating layer and the like. In this example, the depth is 40 μm and the diameter is 70 μm.
Is a substantially cylindrical concave portion. On the other hand, the non-via portion pad is a type of pad in which a pad for forming a solder bump does not serve as a via, and refers to a pad having a flat surface.

As comparative examples, an electroformed mask (plate thickness 70 μm, opening diameter 180 μm), a laser mask (plate thickness 70 μm, upper end opening diameter 160 μm, lower end opening diameter 1
70 μm) to form solder bumps in the same manner.
Note that solder bumps were formed on three wiring boards using the same type of mask.

Then, the height of the solder bump formed on each pad was measured by the following procedures. Measurement of "Thickness of Underlayer Cu Pad" Focus is first focused on the surface of the resin layer using a microscope. Next, the focal point is set on the upper part of the Cu pad, and the thickness of the underlying Cu pad is measured from the amount of vertical movement of the microscope lens barrel during this time.

A solder bump is formed by the method of the above embodiment. Measurement of “Thickness of base Cu pad + Solder bump height” First, focus is set on the surface of the resin layer using a microscope. Next, the focal point is adjusted to the top of the solder bump, and the “thickness of the underlying Cu pad + the height of the solder bump” is measured from the vertical movement amount of the microscope lens barrel during this time.

Calculation of “Solder Bump Height” The solder bump height is obtained by calculation from the above “thickness of base Cu pad + height of solder bump” −the thickness of base Cu pad. The results are shown in Table 1 below.

[0073]

[Table 1]

In Table 1, the calculated bump height is a calculated bump height calculated from the dimensions of a metal mask or the like. AVE indicates an average value, SD indicates a standard deviation, MAX indicates a maximum value, MIN indicates a minimum value, and R indicates a range (range).

In Table 1, it is shown that if the difference between the calculated bump height and the actually measured bump height is small, the size of the through hole of the mask can be determined by calculation. With the etching mask of this example, the measured value was 25 μm for the non-via portion and the via portion of each of the three wiring boards for the calculated bump height of 63 μm.
Total of the pads (75 places in total), average 61 μm
It was almost equal to. That is, according to the etching mask of this example, it can be seen that a solder bump having a height almost as calculated can be obtained.

On the other hand, with the electroformed mask, the calculated bump height 9
The actual measurement average is 64 μm, which is significantly different from 2 μm. Therefore, in order to obtain a bump having a desired height, it is necessary to perform not only calculations but also conditions for determining the dimensions of the mask through repeated trial manufacture. Similarly, with a laser mask,
67μm in actual measurement average for the calculated bump height of 87μm
Although not as large as when using an electroformed mask, the values were quite different. Therefore, a condition setting operation is required even with a laser mask.

Next, the variation in the height of each bump will be examined. Looking at the solder bumps (21 × 3) formed on the non-via pad portion, the etching mask of this example has an average of 61 μm, a maximum of 70 μm, and a minimum of 53 μm, and has a small variation (61 ± 9 μm = 61 μm ± 1).
About 5%). This indicates that the mask of this example has good transferability and fillability and can print a stable amount of solder paste.

On the other hand, in the electroformed mask, the variation is slightly large, with an average of 66 μm, a maximum of 77 μm, and a minimum of 55 μm (66 ± 11 μm = about 66 μm ± 17%).
This is probably because the amount of solder paste attached to the side wall of the through hole of the electroformed mask is not stable, and the amount of solder paste to be printed is not stable.

In the case of a laser mask, the average is 69 μm.
m, the maximum is 82 μm, and the minimum is 47 μm, and the variation is quite large (69 ± 22 μm = about 69 μm ± 32%). This is because, in the laser mask, the filling property is poor because the upper opening diameter is small or the inner wall of the opening is rough.
This may be because the amount of solder at each bump is not stable because the filling may not be sufficient.

Further, the solder bumps (4 × 3) formed on the via pads will be examined. In the etching mask of this example, the average is 61 μm, the maximum is 70 μm, and the minimum is 50 μm.
The variation is also small (61 ± 11 μm = 6).
About 1 μm ± 18%). This is because according to the mask of this example, the amount of solder paste adhering to the side wall of the through hole is stable.

On the other hand, in the electroformed mask, the average is 57 μm, the maximum is 67 μm, and the minimum is 45 μm, and the variation is slightly large (57 ± 12 μm = 57 μm ± 21%). This is considered to be because the amount of solder paste adhering to the side wall of the through hole is not stable.

In the case of a laser mask, the average is 57 μm.
m, the maximum is 71 μm, and the minimum is 40 μm, and the variation is large (57 ± 17 μm = about 57 μm ± 30%). Furthermore, comparing the bump height between the non-via portion pad and the via portion pad, the etching mask of this example shows that the average bump height is 61 μm in both the non-via portion and the via portion, and there is no difference in bump height.

This is because the mask of the present example has a good filling property of the solder paste, and the solder paste is sufficiently filled into the pad recess. Therefore, the mask between the via pad having the recess and the non-via pad having no recess is provided. It is considered that there was no difference in bump height. For this reason, in the mask of this example, even if statistics are obtained for all pads (25 × 3 each) in the non-via portion and the via portion, the average is 61 μm, the maximum is 70 μm, and the minimum is 50 μm.
m and the variation is small (61 ± 11 μm = 61 μm).
On the other hand, in the electroformed mask, the non-via portion has an average height of 66 μm, and the via portion has an average height of 57 μm, which is significantly different in height. This is because, in the electroformed mask, the filling property of the solder paste is not good, so that the solder recess cannot be sufficiently filled in the pad recess. As a result, the height of the solder bump is reduced by the amount of the recess. It is considered something. Looking at all the pads, the average is 64 μm, the maximum is 77 μm,
With a minimum of 45 μm, the variation is further increased (64 ±
19 μm = about 64 μm ± 30%). In the laser mask, the non-via portion averaged 69 μm, and the via portion averaged 5 μm.
7 μm, the difference is even larger. This is due to the poor filling of the solder paste with the laser mask,
It is considered that the solder bumps could not be sufficiently filled in the pad recesses, so that the height of the solder bumps was reduced accordingly. Further, when statistics are obtained for all the pads, there is an extremely large variation (67 ± 27 μm = about 67 μm ± 40%) with an average of 67 μm, a maximum of 82 μm, and a minimum of 40 μm.

As described above, in a wiring board having a large variation in the height of the solder bumps, there is a risk that a connection failure may occur when a flip chip is connected via the solder bumps. In addition, when the solder paste is not sufficiently filled in the pad recess in the via portion pad due to poor filling property, when the solder bump is formed by reflow, the air trapped in the recess causes voids in the solder bump. May be formed. In this case, on the contrary, when the apparent solder bump height is increased, or when the air inside the void escapes to the outside and the void is crushed, there is a risk of blowing out the molten solder and causing a problem. It is not preferable that the property is poor.

As described above, the height (volume) of the solder bump formed by the etching mask of the present embodiment is as calculated (designed).
have. Therefore, it is not necessary to perform a condition setting operation for repeatedly producing a mask. In addition, since the filling and transfer properties are good, a certain amount of solder paste can be printed, and solder bumps with small variations in height can be formed, so that when connecting to electronic components such as flip chips, it is stable and reliable. Can connect.

In particular, in this example, the height variation can be suppressed to 20% or less for a small bump having a size that is likely to vary in height. Furthermore, the mask of the present example is a pad group in which pads requiring different amounts of solder (solder paste) are mixed to form solder bumps of the same height (for example, specifically, a via portion pad and a non-via portion). It is preferably applied to a wiring board having both pads. This is because, since the filling property is good, a necessary amount of solder paste can be sufficiently filled, and a solder bump with small height variation can be formed regardless of the difference in the pad.

The present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes without departing from the gist of the present invention.

[0088]

As described above in detail, in the metal mask for forming a solder bump according to the first aspect of the invention, since the vertical cross-sectional shape of the pattern hole is substantially barrel-shaped, the metal mask is exfoliated with excellent filling properties. In this case, the transferability is excellent, so that a desired print volume can be precisely obtained. Therefore, variations in bump height can be reduced, and for example, the bonding property of a flip chip is improved.

Further, for the required height of the solder bump,
Since the thickness and the diameter of the through-hole of the metal mask to be used can be accurately calculated, the troublesome work of setting the mask conditions is not required, and the product development and manufacturing are easy, and the cost can be reduced. In the invention of the metal mask for forming a solder bump according to claim 2, the pattern hole of the metal mask is formed by etching a metal plate for the metal mask from both surfaces thereof. In comparison, the production is easy and the cost can be reduced.

According to the third aspect of the present invention, since the thickness of the metal mask is as thin as 25 to 50 μm, when a small solder bump is formed, the variation in volume and height is suppressed. Thus, a solder bump can be formed accurately. According to the invention of the wiring board having the solder bump according to claim 4, the diameter of the pad of the wiring board is 150.
μm or less, and even when the average height of the solder bumps is as small as 70 μm or less, the variation in the height is ± 2 of the average value.
Since it is small within the range of 0%, for example, the bonding property with the flip chip becomes high. In the method of manufacturing a wiring board having solder bumps according to the fifth aspect, the solder bumps are formed using the metal masks according to the first to third aspects. Can be reduced. Thereby, for example, the bonding property with the flip chip is improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating an operation of a metal mask according to the present invention.

FIG. 2 is an explanatory diagram showing a method for setting a volume of a solder paste to be filled.

FIG. 3 is an explanatory diagram showing a relationship between dimensions of each part of a solder bump.

FIG. 4 is an explanatory view showing a procedure for forming a metal mask by etching.

FIG. 5 shows a flip chip according to an embodiment,
(A) is a plan view thereof, and (b) is an enlarged sectional view showing the vicinity of an element-side solder bump.

FIGS. 6A and 6B show a wiring board according to an example, in which FIG. 6A is a plan view thereof, and FIG. 6B is an enlarged sectional view showing the vicinity of a substrate-side solder bump;

FIG. 7 is an explanatory view showing a procedure for forming a metal mask by etching according to the example.

FIG. 8 is an explanatory diagram illustrating a method of forming a board-side solder bump according to an example. FIG.

FIGS. 9A and 9B are plan views showing a wiring board before a solder bump is formed, and FIG. 9B is a cross-sectional view showing a solder bump and the vicinity thereof;

FIG. 10 is an explanatory diagram showing a conventional technique.

FIG. 11 is an explanatory diagram showing a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Flip chip (integrated circuit chip, semiconductor element) 2 ... Base material 3 ... Element side solder bump 4 ... Element side pad 6 ... Wiring board (flip chip mounting wiring board) 8 ... Board side solder bump 9 ... Board side pad

Claims (5)

[Claims] 1. A metal mask used for printing a solder paste on a wiring board in order to form a solder bump, wherein a vertical cross-sectional shape of a pattern hole corresponding to the solder bump to be formed is an open end. A metal mask for forming a solder bump, wherein the metal mask has a substantially barrel shape having a larger diameter at a central portion than a diameter. 2. The metal mask for forming a solder bump according to claim 1, wherein the pattern holes of the metal mask are formed by etching a metal plate for the metal mask from both surfaces thereof. . 3. The method according to claim 1, wherein the metal mask has a thickness of 25 to 50.
3. The metal mask for forming a solder bump according to claim 1, wherein the metal mask has a thickness of μm. 4. 4. The pad of the wiring board has a diameter of 150 μm or less, and the height of the solder bump formed on the pad is 70 μm or less on average, and the variation in the height is ± 20% of the average value. A wiring board having solder bumps, wherein 5. A method of manufacturing a wiring board having solder bumps for connection to an electronic component, wherein a solder paste is formed on a pad of the wiring board by using the metal mask according to claim 1. A method for manufacturing a wiring board having solder bumps, wherein the solder bumps are formed by printing, heating and melting the solder.