JP2014175523A - Printed wiring board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a satisfactory soldered state by melting a cream solder which is applied on a land of a printed wiring board, earlier than a solder ball of a BGA component when soldering the BGA component to the printed wiring board.SOLUTION: A cream solder 7 to be applied onto a land 6 of a printed wiring board 5 is formed in a frustum shape which is inclined down toward a side of the land 6 around a portion to which a solder ball 4 of a BGA component 1 is abutted. Therefor, a gap between a number of cream solders 7 formed densely is widened, hot air is easily passed therebetween, and hot air is easily blown to the cream solders 7. A surface area of a vertical face 7b to which hot air is blown well is increased. Further, a cubic volume of the cream solder 7 is reduced in comparison with that of a conventional cream solder having a rectangular cross section, and heat capacity is reduced. Therefore, the cream solder 7 of reduced heat capacity receives much more heat and is molten earlier than the solder ball 4.

Description

  The present invention relates to a printed wiring board for mounting a BGA component.

  As a semiconductor device for surface mounting, there is a BGA (Ball Grid Array) component. This is a structure in which pads (electrodes) are provided in a lattice shape on one surface of a package in which electronic components such as LSIs are sealed, and solder balls for connection are provided on these pads. In the printed wiring board on which this BGA component is mounted, cream solder is applied onto a land (electrode) for soldering the solder ball of the BGA component, and the solder ball is applied to the cream on the land. It is placed on a printed wiring board so as to match the solder, heated with hot air in a reflow oven to melt cream solder and solder balls, and solder the BGA component to the printed wiring board.

JP 2002-158439 A

  In the soldering by the reflow method, when the cream solder is melted before the solder balls, a good soldering state can be obtained. This is because when the cream solder is melted first, the melted cream solder wets the surface of the solder ball of the BGA component, thereby removing and activating the oxide film and dirt on the surface of the solder ball. It is supposed to be.

In the printed wiring board disclosed in Patent Document 1, the applied cream solder has a rectangular cross-sectional shape. For this reason, when a BGA component is mounted on a printed wiring board (the solder ball is in contact with the upper surface of the cream solder), the gap formed between the solder ball and the cream solder is narrow, and hot air is generated in the gap. It is hard to enter. Further, when the cross-sectional shape of the cream solder is rectangular and the height is the same, the volume is larger than that of other cross-sectional shapes such as a trapezoidal shape, and thus the heat capacity is large. In such cream solder, the temperature rise when heated by hot air is slow, and it takes time for the central portion in contact with the solder ball to reach the melting temperature. On the contrary, the solder balls are easily hit by hot air and melt at a relatively early time. Therefore, solder balls often start to melt before the cream solder melts, and a good soldering state may not be obtained.
In particular, when the cream solder is lead-free (lead-free) solder and the solder ball is leaded solder, the solder ball having a lower melting point than the lead-free cream solder is more likely to melt first, A good soldering state cannot be obtained.

  The present invention has been made in view of the above circumstances. The purpose of the present invention is to make cream solder lead-free solder, and even if the solder balls of BGA parts are leaded solder, cream solder is more preferable than solder balls. An object of the present invention is to provide a printed wiring board that can be melted first.

  In the invention of claim 1, the shape of the cream solder applied to the land of the printed wiring is a frustum shape in which the peripheral portion of the portion to which the solder ball is applied is inclined downward toward the land side. The gaps between the cream solders are widened, and hot air can easily pass between the cream solders. Further, the volume of the cream solder is reduced and the heat capacity is reduced. In this way, cream solder with a smaller heat capacity can receive more heat (hot air), so even if the solder balls are lead-free solder and the cream solder is lead-free solder, It can be melted faster than the solder balls. For this reason, the previously melted cream solder wets the surface of the solder ball that has not yet melted, and after removing the oxide film and dirt on the surface of the solder ball, the solder ball melts and fuses with the solder of the cream solder. As a result, a good soldering state can be obtained.

  The invention according to claim 2 is such that the descending inclined portion of the peripheral portion of the portion to which the solder ball is applied in the cream solder is formed in an arc concave shape. If it does in this way, since the surface area of the downward inclination part of cream solder becomes larger, the amount of heat which cream solder receives from hot air can be increased more.

  According to the invention of claim 3, when the solder ball is made of leaded solder containing lead and the cream solder is made of lead-free solder, the portion (upper surface) to which the solder ball is applied is applied to the solder solder applied to the land. It is a spherical concave surface that accepts.

  In this way, the gap between the solder ball and the upper surface of the frustum-shaped cream solder is narrower or there is no gap, so it is difficult for hot air to hit the part of the solder ball that has entered the spherical concave surface, or hot air Since it does not hit, the melting time of the solder ball is delayed. On the other hand, it is difficult for hot air to hit the spherical concave surface, or hot air does not hit the spherical concave surface, but in the case of cream solder, the volume is small, so the upper surface was made spherical concave for the melting time The increase in the amount of heat received by increasing the heat receiving area by tilting the peripheral portion of the upper surface downward has a greater effect than the decrease in the amount of heat received by. For this reason, if the upper surface of the cream solder is made into a spherical concave surface, the melting point of the solder ball can be delayed, so that the certainty about melting the cream solder earlier is increased.

  Further, by changing the radius of the spherical concave surface, the size of the gap between the solder ball and the spherical concave surface of the cream solder can be adjusted. As a result, the size of the gap formed between the spherical concave surface of the cream solder and the solder ball can be adjusted, and consequently the amount of heat received by the solder ball can be adjusted, so the radius of the spherical concave surface can be adjusted according to the size of the solder ball. It is possible to adjust the melting time of the solder ball after melting the cream solder.

The schematic sectional drawing which shows the 1st Embodiment of this invention and shows the melting process of cream solder in order Bottom view of BGA parts Schematic partial cross-sectional view of BGA parts Schematic partial cross-sectional view of a printed wiring board Perspective view of cream solder applied to land Schematic cross-sectional view showing the cream solder application process FIG. 1 equivalent diagram showing a second embodiment of the present invention FIG. 1 equivalent view showing a third embodiment of the present invention Sectional drawing which shows an example of the process of forming the upper surface of cream solder into a spherical concave surface

Hereinafter, the present invention will be specifically described with reference to embodiments.
[First Embodiment]
1 to 6 show a first embodiment of the present invention. FIG. 2 shows the BGA component 1. The BGA component 1 is mainly composed of a package 2 in which an electronic component such as an LSI (not shown) is sealed, and pads (electrodes) 3 shown in FIG. Solder balls 4 are provided on the pads and serve as connection terminals. For the solder ball 4 in this embodiment, a well-known leaded solder is used. The electronic component is connected to the pad 3 by a bonding wire or the like.

  FIG. 4 shows a printed wiring board 5 on which the BGA component 1 is mounted. The printed wiring board 5 has a conductive path (not shown) printed on the surface or inside, and a land 6 connected to the conductive path is formed on the surface. Among the lands 6, required ones are arranged in a grid pattern at the same pitch intervals as the solder balls 4 of the BGA component 1 in order to mount the BGA component 1.

  Of the lands 6 of the printed wiring board 4, the cream solder 7 is provided on the lands 6 (hereinafter simply referred to as lands 6) on which the BGA component 1 is mounted. The cream solder 7 is obtained by adding flux to solder powder to have an appropriate viscosity, and the solder is made of, for example, lead-free solder (lead-free solder) not containing lead. There are various lead-free solders such as SnAgCu, SnCu, and SnZnBi.

  As shown in FIG. 5, the cream solder 7 has a truncated cone shape in which the periphery of the portion (upper surface) to which the solder ball 4 of the BGA component 1 is applied is linearly inclined downward toward the land 6 side. Is formed. As shown in FIG. 6, the cream solder 7 is formed by placing a metal mask 8 on the printed wiring board 5, placing a cream solder material 9 on the metal mask 8, and scratching it with a squeegee 10. In other words, the metal mask 8 has the same truncated cone-shaped hole 8 a as the cream solder 7, and when the squeegee 10 is scratched, the cream solder material 9 is embedded in the hole 8 a to form the cream solder 7. is there.

  In order to mount the BGA component 1 on the printed wiring board 5 in which the frustoconical cream solder 7 is formed on each land 6, the BGA component 1 is attached to each solder ball 4 as shown in FIG. It is placed on the printed wiring board 5 so as to match the cream solder 7 on the land 6 and put in a reflow furnace. In the reflow furnace, the solder balls 4 and the cream solder 7 are heated by hot air passing through the gap between the printed wiring board 5 and the BGA component 1.

  When heating in a reflow oven, the cream solder has been in the shape of a short column (with a rectangular cross section). Hot air is unlikely to enter between each other. In addition, the cream solder made of lead-free solder has a high melting point, and there is a possibility that the melting start time is delayed as compared with the solder ball.

  On the other hand, in this embodiment, the cream solder 7 has a truncated cone shape. For this reason, the outer side of the upper surface 7a (the base of the truncated cone) of the cream solder 7 is a conical surface 7b that is inclined downward toward the printed wiring board 5 side. The gap between each other becomes wide, and hot air easily enters between the cream solders 7. In addition, the surface area of the conical surface 7b is larger than the surface area of the circumferential side surface of the conventional cylindrical cream solder, and the amount of heat received increases, and the cream solder 7 of the present embodiment has a trapezoidal cross section. When the entire volume is smaller than that of a conventional columnar cream solder and the same amount of heat is received, the temperature rises faster than the columnar conventional cream solder.

  As described above, in the cream solder 7 according to the present embodiment having a large amount of heat received and a small heat capacity, even if the contained solder powder is a lead-free solder having a high melting point, the temperature is quickly reached to the central portion in contact with the solder ball 4. To rise. For this reason, the cream solder 7 is melted earlier than the solder balls 4 made of leaded solder, and when melted, the solder balls 4 are embedded in the cream solder 7 and the entire BGA component 1 starts to sink (FIG. 1). (B)). Then, the melted cream solder 7 is wetted along the surface of the solder ball 4 to remove the dirt and oxide film on the surface of the solder ball 4 and activate (FIG. 1 (c)). Thereafter, the solder ball 4 is melted and fused and integrated with the solder of the melted cream solder 7. And finally, it cools and the pad 3 of the BGA component 1 and the land 6 of the printed wiring board 5 are soldered.

  As described above, in this embodiment, even if the solder ball 4 is leaded solder and the cream solder 7 is lead-free solder, the cream solder 7 is melted more quickly and the surface of the solder ball 4 is activated, Since the balls 4 are melted, a good soldering state can be obtained.

[Second practical form]
FIG. 7 shows a second embodiment of the present invention. This embodiment differs from the first embodiment described above in that the conical surface 7b of the cream solder 7 is formed in an arc concave shape.
In this way, since the area of the conical surface 7b becomes wider, the amount of heat received from the hot air increases, and the melting time of the cream solder 7 becomes earlier.

[Third practical form]
8 and 9 show a third embodiment of the present invention. This is the same as the second embodiment described above in that the conical surface 7b of the cream solder 7 has an arc concave shape, but is different from the second embodiment in that a shallow spherical concave surface into which the solder ball 4 enters the upper surface 7a. It is in that. In order to make the upper surface 7a into a spherical concave surface, for example, as shown in FIG. 9, after forming the frustoconical cream solder 7 with the metal mask 8, before removing the metal mask 8, the spherical mold 11 is used. The upper surface 7a of the cream solder 7 is pressed to form a spherical concave surface.

  When the upper surface 7a is thus made spherical concave, when the BGA component 1 is placed on the printed wiring board 5, the solder balls 4 enter the spherical concave surface of the upper surface 7a of the cream solder 7, as shown in FIG. Therefore, there is almost no gap between the solder ball 4 and the cream solder 7, and hot air is difficult to enter. For this reason, almost no heat is received from the portion of the solder ball 4 entering the spherical concave surface, the amount of heat received by the solder ball 4 is reduced, and the melting time is delayed.

  On the other hand, the cream solder 7 does not receive heat from the upper surface 7a, but even if the upper surface 7a is flat, the gap with the solder ball 4 is originally small and the influence of the hot air hardly enters. Further, since the volume is reduced due to the spherical concave surface, the delay of the melting time of the cream solder 7 is not as much as that of the solder balls 4.

Thus, in the third embodiment, the melting time of the cream solder 7 can surely be made earlier than the melting time of the solder balls 4.
In the third embodiment, the size of the gap formed between the solder ball 4 and the upper surface 7a of the cream solder 7 is changed by changing the radius of the spherical concave surface. Then, when the size of the gap formed between the spherical concave surface of the cream solder 7 and the solder ball 4 changes, the amount of hot air entering the gap can be changed somewhat. Since the amount of heat received can be adjusted, it is possible to adjust the melting time of the solder balls 4 after melting the cream solder 4 by changing the radius of the spherical concave surface according to the size of the solder balls 4 and the like. Become.

[Other Embodiments]
The present invention is not limited to the embodiment described above and shown in the drawings, and the following modifications or changes are possible.
The shape of the cream solder 7 is not limited to the truncated cone, and may be other shapes such as a truncated pyramid and a triangular truncated pyramid.
Both the solder ball 4 and the cream solder 7 may be lead-free solder, or both may be leaded solder.
What is necessary is just to determine suitably the angle of the inclined surface 7b of the cream solder 7 for adjustment of a heat receiving amount.
In the cream solder 7 in the second embodiment, that is, the inclined surface 7b is linear, the upper surface may be formed as a spherical concave surface.

  In the drawings, 1 is a BGA component, 3 is a pad, 4 is a solder ball, 5 is a printed wiring board, 6 is a land, and 7 is cream solder.

Claims (3)

A printed wiring board for mounting a BGA component in which pads are arranged in a grid pattern on one surface of an electronic component package and solder balls for connection are provided on the pads,
A land for connecting the solder balls of the BGA component; cream solder is applied to the lands; and the solder is applied to the lands by applying the solder balls of the BGA component to the cream solder. In printed wiring boards for soldering balls,
The printed wiring board according to claim 1, wherein the shape of the cream solder applied to the land is a frustum shape in which a peripheral portion of a portion to which the solder ball is applied is inclined downward toward the land side.   The printed wiring board according to claim 1, wherein, of the cream solder applied to the land, a descending inclined portion of a peripheral portion of the portion to which the solder ball is applied has an arc concave shape. The solder ball is made of leaded solder containing lead, the cream solder is made of lead-free solder not containing lead,
3. The printed wiring board according to claim 1, wherein a portion of the cream solder applied to the land to which the solder ball is applied is formed as a spherical concave surface that receives the solder ball. 4.

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