JPH11112133A - Method for planarizing solder bump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent occurrence of positional shift by pressing a flat metallic retaining block exhibiting low solder wettability is pressed against a bump forming a pad of a printed wiring board and then heating the block at a temperature lower than the melting point of solder thereby keeping a constant spacing to a wiring board while deforming the bump plastically. SOLUTION: A pad 14 is formed on a printed wiring board 10 having upper surface being held on a flat holding block 12 at a grid position which is matched with the arrangement of the area array terminal of a bare chip or a flip-chip to be mounted. A solder bump 16 is formed on each pad 14. A retaining block 18 made of a metal having low solder wettability, e.g. a Cr-Cu alloy or titanium, has a coefficient of linear expansion approximate to that of the printed wiring board 10. A dam-like edge 20 flush with the flat surface of the flattened solder bump 16 is formed on the circumference of the retaining block 18. The retaining block 18 is set on the printed wiring board 10 and abutted against the top of the solder bump 16 which is then hot pressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for flattening a solder bump formed on a pad of a printed wiring board so as to make the height uniform.

[0002]

2. Description of the Related Art A bare chip in which the chip itself is mounted on a printed wiring board, a flip chip (FC) in which solder bumps are formed on the bare chip, and a ball grid array (BG) in which ball-shaped terminals are arranged in a lattice in a sealing case.
A) or a chip size package (CSP) smaller than or equal to or slightly larger than the chip size is known.

[0003] These packages (PKG) usually have terminals arranged in a plane, ie, area array terminals.
For example, cream solder or solder balls are supplied onto pads of a printed wiring board, and solder bumps of a predetermined size are formed on each pad by reflow heating. At this time, the solder on the pad is melted by reflow heating and becomes hemispherical due to surface tension. On the hemispherical solder bumps thus formed, an area array terminal such as a bare chip, FC, BGA, or CSP is placed and soldered by reflow heating.

[0004]

In this case, since the solder bumps formed on the pads are hemispherical, there is a problem that the terminals on the package side are easily displaced. For example, FC or BG
Since A and CSP usually have hemispherical terminals using solder balls, these hemispherical terminals are mounted on hemispherical solder bumps on the printed wiring board side, which tends to cause misalignment. . Although the metal film is exposed on the bare chip bonding pad, this pad is also slightly higher, so when it is placed on the hemispherical solder bump on the printed wiring board side, the edge of the pad hits the spherical surface of the solder bump Position easily.

On the other hand, Japanese Patent Application Laid-Open No. H8-17959 discloses that a solder ball is held in a blind hole formed in a metal plate which is not easily wetted by solder, and a wiring board is stacked thereon and heated to a solder melting temperature, and the solder ball is wired. A method for forming a solder bump to be bonded to a pad of a board is shown. Here also
It has been shown that the bottom of the blind hole formed in the metal plate is a flat surface, and the flatness of the tip surface of the solder bump is enhanced by this flat surface.

FIG. 7 is a view for explaining a method of forming a solder ball by this method, and FIG. 8 is a view for explaining a method of joining the solder ball to a wiring board. In these figures, reference numeral 1
Is a metal plate, which is made of a metal which is not easily wetted by solder, for example, titanium or stainless steel. A large number of blind holes 3 are formed on the upper surface of the metal plate 1 so as to match the grid positions of the pads of the wiring board 2.

A predetermined amount of cream solder is supplied to these blind holes 3 by screen printing or the like and heated, so that the cream solder is melted, formed into a ball shape by its surface tension, and solidified to form a solder ball 4. (FIG. 7). Then, the wiring board 2 is stacked on the metal plate 1 with a spacer 5 having an appropriate thickness interposed therebetween, and the solder ball 4 is heated by placing a weight 6 such that the wiring board 2 does not move when the solder ball 4 is melted. Melt (FIG. 8). Then, the molten solder is sucked into the pads 7 of the wiring board 2 by the surface tension, and the solder bumps 8 are formed.

However, since the volume of the cream solder used in this method generally decreases when it is melted (reflowed), the volume of the solder ball 4 formed here becomes smaller than the volume of the blind hole 3. For this reason, when the solder bump 8 is formed by this method, the volume of the solder ball 4, the volume and shape of the blind hole 3, the size of the pad 7, and the like are affected. The tip of the solder bump 8 is separated from the bottom of the blind hole 3 of the metal plate (1). Therefore, in this method, it is generally difficult to flatten the tip of the solder bump 8 to a uniform height.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and flattenes the tops of solder bumps formed on pads of a printed wiring board to reduce the displacement between the terminals on the package side and the solder bumps. It is an object of the present invention to provide a method of flattening a solder bump, which hardly occurs, and positions both of them with high precision so that a semiconductor package can be mounted.

[0010]

According to the present invention, there is provided a method of flattening a solder bump for flattening a top portion of a solder bump formed on a pad of a printed wiring board. Pressing the block against the solder bumps, heating the solder bumps to a temperature lower than the solder melting temperature and plastically deforming the solder bumps, thereby making the distance between the solder bumps and the printed wiring board constant, achieved by a solder bump flattening method. Is done.

That is, a flat metal plate which is hardly wetted by solder is pressed against a solder bump formed by a known method, heated at a temperature at which the solder does not melt, and the top of the solder bump is plastically deformed so as to be flattened. . Here, when the solder is a normal eutectic solder and has a melting point of about 183 ° C., heat to about 150 ° C. and press with a force of about 30 kg against the case where the number of solder bumps is about 1500 in total. For about 20 seconds. The reason for heating to about 150 ° C. is to lower the hardness of the solder and facilitate plastic deformation.

One of the holding block and the holding block for holding the printed wiring board is held so that the pressing direction is vertical, and the other is held so that the inclination with respect to the pressing direction can be changed. And the flatness of the solder bumps can be improved.

[0013]

FIG. 1 shows a solder bump according to an embodiment of the present invention before plastic deformation, and FIG. 2 shows a solder bump after plastic deformation. FIG. 3 is a diagram showing a printed wiring board made by this method.

In FIGS. 1 and 2, reference numeral 10 denotes a printed wiring board, which is mounted on and held by a holding block 12 having a flat upper surface. On the printed wiring board 10, pads 14 are formed at lattice positions corresponding to the arrangement of the area array terminals such as bare chips, FC, BGA, and CSP to be mounted, and solder bumps 16 are formed on each pad 14. The solder bumps 16 can be formed by a known method using cream solder or a method using solder balls.

For example, in the method using cream solder,
A predetermined amount of cream solder is supplied to each pad 14 by using a squeeze method in which a metal mask having a predetermined thickness is stacked and the cream solder is spathered from above, and the cream solder is melted by heating (reflow). A substantially hemispherical solder bump 16 can be formed by utilizing the surface tension. In the method using a solder ball, a solder ball having a certain size is supplied to each pad 14 and heated (reflowed) to form a solder bump 16 in the same manner.

Reference numeral 18 denotes a holding block, which is made of a metal which is not easily wetted by solder, for example, a chromium-copper (Cr-Cu) alloy, titanium, stainless steel or the like. It is to be noted that a metal having a coefficient of linear expansion as close as possible to the substrate of the printed wiring board 10 is suitable for the holding block 18, and therefore a Cr-Cu alloy is particularly preferable. A bank-shaped edge 20 is formed on the periphery of the holding block 18. The height of the edge portion 20 is made equal to the height of the flat surface 16A of the solder bump 16 after flattening.

The holding block 18 is connected to the printed wiring board 1.
0, the holding block 18 is connected to the solder bump 1
6 and stops (the virtual line position 18A in FIG. 1).
reference). In this state, the holding block 18 is pressed vertically against the printed wiring board 10. And about 150 ° C
Heat to The pressing force F at this time is
6 must be large enough to be simultaneously plastically deformed while being heated to about 150 ° C.
When a total of about 1500 solder bumps are to be obtained, for example, when P is simultaneously processed, the pressing force F is preferably about 30 kg. In this case, the pressing force per bump is about 200 g.

When the solder bumps 16 are held for about 20 seconds under the conditions of heating and pressing, the solder bumps 16 are plastically deformed, the tops are crushed, and the edges 20 of the holding blocks 18 are
(FIG. 2). Therefore, the top of each solder bump 16 becomes a flat surface 16A having a constant height by the flat lower surface of the holding block 18 (FIG. 3). This flat surface 16A
Depends on the height of the edge 20 described above. For example, when the thickness of the pad 14 is 50 μm and the solder bump 1
When the thickness of the edge 6 is set to 50 μm, the height of the edge 20 may be set to 100 μm.

At the time of pressing by the presser block 18,
The effect of heating the solder bump 16 to about 150 ° C. will be described. Generally, the solidified solder is more likely to be deformed as the temperature increases. The shear strength is used as a criterion for the ease of deformation. The table below shows the shear strength (unit: MP) for changes in solder temperature (20 ° C → 100 ° C).
a) is shown for each type of solder alloy. As can be seen from this table, when the temperature changes from 20 ° C. to 100 ° C., the shear strength of the solder decreases significantly. Therefore, the top of the solder bump 16 is easily deformed and flattened by the pressing by the pressing block 18.

[0020]

[Table 1]

When the pressing block 18 is pressed,
The pressing force F is applied to the printed wiring board 10 and the holding block 12.
Need to be added perpendicular to the However, in practice, all the solder bumps 1
The plastic deformation of No. 6 does not proceed simultaneously and changes slightly. For this reason, the holding block 18 may not be perpendicular to the printed wiring board 10 but may be inclined.

FIG. 4 shows a state in which the holding block 18 is inclined, and FIG. 5 shows a shape of the solder bump 16B in this case. The holding block 18 is inclined from the state in which it is in contact with the top of the solder bump 16 (the position indicated by the solid line in FIG. 4) as indicated by the imaginary line 18B due to the start of the deformation of the solder bump 16. In this state, the holding block 18 is lifted and the solder bump 16 is lifted. 5A, the top surface 16B of the solder block 16
Tilt as shown.

FIG. 6 is a diagram for explaining a method for solving such a problem. In this figure, the holding block 18 and the holding block 12 in FIGS. 1 to 5 are turned upside down, and the upper surface of the upper block 32 holding the printed wiring board 30 is pressed via the ball joint 34. First, a lower block 36 located on the lower side is provided with a heating table 38 heated by an electric heater, a block 40 made of a Cr-Cu alloy adhered and held thereon, and a table made of Invar which covers the block 40. The plate 42 is formed. The reason why the invar front plate 42 is used is that the coefficient of linear expansion is extremely close to that of the substrate of the printed wiring board 30. A spacer 44 is provided on the periphery of the upper surface of the lower block 36.
Is installed.

The upper block 32 for holding the printed wiring board 30 is made of Cr-Cu, and has a hemispherical concave portion 46 formed in the center of the upper surface thereof. Is engaged from above. The recess 46 and the ball 50 form the ball joint 34 described above.

Accordingly, the printed wiring board 30 is placed on the lower block 36 with the solder bumps 52 facing down, and the upper block 32 is further placed thereon. And the lower block 36
The heating rod 38 is heated to keep the front plate 42 at about 150 ° C., and the holding rod 48 is pressed down vertically. Then, the solder bump 52 is plastically deformed.

At this time, if all the solder bumps 52 are not uniformly plastically deformed and the deformation is varied, the printed wiring board 30 and the upper block 32 are slightly inclined. However, since a part of the printed wiring board 30 hits the spacer 44, if the holding rod 48 keeps pushing down vertically, the upper block 32
The inclination of the spacer 4 is uniform over the entire circumference of the printed
It is naturally corrected to hit 4. Therefore, a flat surface 52A having a uniform height is provided on the tops of all the solder bumps 52.
((D) of FIG. 6) is formed.

[0027]

According to the first aspect of the present invention, as described above, the pressing block is pressed against the solder bump, and the solder bump is plastically deformed while being heated to a temperature lower than the solder melting temperature to flatten the top of the solder bump. As in the case where the bump is melted to flatten the top, the melted solder is not sucked by the surface tension of the pad of the printed wiring board due to the surface tension, and the height of the solder bump is not uneven. For this reason, the tops of all the solder bumps can be flattened at a uniform height, and the solder bonding can be performed while performing high-precision positioning without causing displacement of the area array terminals of the semiconductor package.

In this case, the heating temperature is preferably about 150 ° C. in the case of eutectic solder. At this time, the wiring board having about 1500 solder bumps is pressed at about 30 kg, and this state is maintained for about 20 seconds. (Claim 2). Also, if one of the holding block and holding block is fixed and the other is held so as to allow the inclination, both blocks may be temporarily non-parallel if each solder bump undergoes plastic deformation not at the same time but with a time lag. It is suitable for flattening the top while making the height of all solder bumps uniform (claim 3).

[Brief description of the drawings]

FIG. 1 is a diagram showing a state before plastic deformation of a solder bump according to an embodiment of the present invention.

FIG. 2 is a view showing a state after plastic deformation.

FIG. 3 is a view showing a printed wiring board processed by this method;

FIG. 4 is a view for explaining the cause of uneven plastic deformation.

FIG. 5 is a view showing a printed wiring board having poor smoothing.

FIG. 6 shows another embodiment according to the present invention.

FIG. 7 is a view for explaining a conventional solder ball forming method.

FIG. 8 is a view for explaining a method of joining the solder balls to a wiring board;

[Explanation of symbols]

 10, 30 Printed wiring board 12 Holding block 14 Pad 16, 52 Solder bump 16A, 52A Smooth surface 18 Holding block 32 Upper block as holding block 34 Ball joint 36 Lower block as holding block

Claims (3)

[Claims] 1. A method of flattening a solder bump for flattening a top portion of a solder bump formed on a pad of a printed wiring board, comprising: pressing a flat metal holding block, which is hardly wetted by solder, to the solder bump; A method for flattening a solder bump, wherein the solder bump is plastically deformed by heating the solder bump to a temperature lower than a solder melting temperature to make a distance between the solder bump and a printed wiring board constant. 2. The solder has a melting point of about 183 ° C., and the holding block is pressed at about 30 kg against a printed wiring board having about 1500 solder bumps, and is pressed at about 150 ° C.
2. The method according to claim 1, wherein the solder bump is plastically deformed by heating for about 20 seconds. 3. One of the holding block and the holding block for holding the printed wiring board is held so that the pressing direction is vertical, and the other is held so that the inclination with respect to the pressing direction can be changed. Solder bump flattening method.