JPH10126047A - Formation of solder bump on ball grid array printed wiring board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form solder bumps having projections at the front ends of solder balls by supplying a prescribed amount of cream solder to each recess of a ball holding plate so that the solder may heap up above the surface of the plate and making the solder to cohere in ball-like shapes by utilizing the surface tension caused by reflow, and then, reflow-soldering the solder balls to a printed wiring board side. SOLUTION: Recesses 42 are formed on the surface of a ball holding plate 40 and the recesses 42 are filled up with cream solder 44. When the plate 40 is heated in a reflow furnace, the parts of the solder 44 exposed from surface of the plate 40 form spherical surfaces due to the surface tension of the solder and, when the solder coheres, solder balls 48 are formed. When a semiconductor package 50 is put in the reflow furnace while the package 50 is put on the plate 40, the molten solder balls 48 are stuck to the junction pads of a substrate 56. When the plate 40 is removed from the package 50 after the solder solidifies, the cream solder 44 in the recesses 42 reflows and remains as projections 44A at the front ends of the balls 48. Therefore, solder bumps 58 having projections 44A can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming solder bumps on a ball grid array printed wiring board.

[0002]

2. Description of the Related Art In recent years, high-density mounting of printed wiring boards has been further advanced, and the distance between electrode bonding pads (hereinafter referred to as "bonding pads") for mounting printed wiring boards on other printed wiring boards has become increasingly smaller. ing. Also, the number of bonding pads that need to be connected has been significantly increased.
Therefore, the area occupied by the bonding pads on the printed wiring board cannot be ignored, and the size of the bonding pads must be reduced.

Therefore, it has been considered to arrange the bonding pads in a two-dimensional grid. In this method, a protruding electrode called a solder bump is provided on each bonding pad, and this electrode is aligned with a bonding pad of another printed wiring board to be bonded, and reflow soldering is performed.

FIGS. 2A to 2F are process diagrams showing an example of this method. In FIG. 2, reference numeral 10 denotes one ball grid array printed wiring board (hereinafter, referred to as a BGA printed wiring board), and reference numeral 30 denotes another printed wiring board on the other side. The BGA printed wiring board 10 is
Has a large number of bonding pads 14 arranged in a lattice on the upper surface of the substrate. The bonding pad 14 is formed by etching a copper foil adhered to the substrate 12 by a known photo etching method.

A solder resist 16 is applied to the substrate 12 by printing using a metal mask or the like. The solder resist 16 covers portions other than the bonding pad 14 so as to slightly overlap the periphery of the bonding pad 14. Then, a flux 18 is applied to the bonding pad 14 of the substrate 10. The flux 18 is applied by printing using a metal mask or the like, or by using a dispenser or the like. FIG. 2A shows this state.

Next, spherical solder balls 20 prepared in advance and made of eutectic solder are supplied one by one onto each bonding pad 14 and temporarily fixed by a flux 18 (FIG. 2B). Then, the substrate 10 is placed in a reflow furnace and heated, so that the solder balls 20 are heated and melted (reflow). At this time, the molten solder rises in a hemispherical shape from the edge of the solder resist 16 due to its surface tension, and solidifies into a protruding solder bump 22 (FIG. 2C).

The other printed wiring board 30 includes a substrate 32
And a circuit pattern including the bonding pad 34 are formed. The bonding pads 34 are arranged in a grid corresponding to the bonding pads 14 of the printed wiring board 10, that is, the solder bumps 22. Eutectic cream solder 36 is printed on the bonding pad 34 using a metal mask to a thickness of about 200 μm (FIG. 2D). This cream solder 36
Is printed in order to improve the solderability while securing the amount of solder at the joints. The thickness (about 200 μm) of the
It is desirable that the thickness be the same as the thickness currently widely used for reflow soldering of a type LSI or the like.

Next, the BGA printed wiring board 10 on which the solder bumps 22 are formed as described above is turned upside down, and the solder bumps 22 are aligned with the bonding pads 34 of another printed wiring board 30, ie, the cream solder 36, and are overlaid ( (E of FIG. 2). Then, if the solder bumps 22 and the cream solder 36 are melted (reflowed) by heating in a reflow furnace, the solder bumps 22 and the cream solder 36 are mixed and integrated. If the solder solidifies, both printed wiring boards 10,
The 30 bonding pads 14 and 34 are bonded by solder 38 (FIG. 2 (F)).

[0009]

In the method described above, it is necessary to supply the solder balls 20 to each bonding pad 14 of the BGA printed wiring board 10 one by one accurately. In particular, when the number of connection pads is remarkably increased and, for example, 225 or more connection pads are provided, it is practically impossible to supply manually.

For this reason, the solder balls 20 are sucked individually or collectively into a jig by vacuum suction and placed directly on the electrode of the wiring board or on the cream solder printed thereon. However, a dedicated mounting machine controlled by a computer is required. However, not only the development of this mounting machine is required, but also a problem that capital investment becomes expensive arises.

In the BGA printed wiring board, it is necessary to improve the flatness (coplanarity) of the mounting terminal surface in order to improve the reliability of soldering. That is, it is necessary that the tips of the solder bumps have the same height. However, as the area of the mounting terminal surface increases and the number of electrodes, that is, the number of solder bumps, increases, the flatness tends to deteriorate. For this reason, it has become difficult to increase the size of the BGA printed wiring board, and it has been difficult to increase the size of a semiconductor package such as an LSI or HIC (hybrid IC) using the printed wiring board.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and it is possible to form solder bumps on a BGA printed wiring board without using solder balls, and to maintain flatness even when the area of a mounting terminal surface becomes large. It is an object of the present invention to provide a solder bump forming method which can be maintained well and does not require a special dedicated mounting machine.

[0013]

According to the present invention, there is provided a method of forming solder bumps on a large number of electrode bonding pads of a ball grid printed wiring board, the method comprising: (a) forming a depression at a position corresponding to the electrode bonding pad; A step of preparing a ball holding plate having poor solder wettability on the surface; (b) a step of supplying cream solder to the depression of the ball holding plate so as to rise from the depression; (c) a reflow of the cream solder raised in the depression (D) positioning the electrode bonding pads on the solder balls and stacking the printed wiring board on a ball holding plate; (e) Melting the solder balls by reflow and soldering them to the corresponding electrode bonding pads; (f) soldering after solidification of the solder; Step of removing the retaining plate from the printed wiring board; solder bump forming method of a ball grid array printed circuit board and having a, is accomplished by.

In the step (b) of supplying cream solder to the depression of the ball holding plate, the depression is filled with cream solder,
It is desirable to supply cream solder to a predetermined thickness thereon by printing using a stencil mask. This is because the supply amount of the cream solder to the depression can be easily and accurately controlled.

In this case, the filling of the depression with the cream solder is as follows.
This can be performed by directly rubbing the surface of the ball holding plate with a spatula. After filling the depression with cream solder, 1
It is preferred to heat to 00 to 130 ° C. for drying. This heating and drying process evaporates the solvent in the filled cream solder, and improves the integration with the cream solder printed thereon in a later step.

It is natural that the surface of the ball holding plate used here has a sufficiently good flatness. In the step (e), a large number of solder balls are held in a state of being held by the ball holding plate having a good flatness. Since reflow soldering is performed on the printed wiring board side, a sufficiently high flatness of the solder bumps can be ensured. In step (f), after the printed wiring board is peeled off from the ball holding plate and removed, and the protrusion at the tip of the solder bump is pressed against a flat plate having good flatness, the flatness of the solder bump is further improved.

[0017]

1 (A) to 1 (G) are process diagrams showing the method of the present invention. First, a ball holding plate 40 is prepared (FIG. 1).
(A)). The ball holding plate 40 is made of a plate having good flatness such as a stainless steel plate or a ceramic plate, and has a printed wiring board 50 (FIG. 1F) on its surface.
A depression 42 is formed at a position corresponding to the bonding pad of (G)). These depressions 42 may be, for example, drilled small holes having a constant diameter and depth.

The surface of the ball holding plate 40 has a depression 42
It is necessary that the solder including the inner surface has poor wettability, in other words, it must repel molten solder. When a stainless steel plate is used, in order to sufficiently deteriorate the solder wettability, the surface and the inner surface of the depression 42 are preferably coated with Teflon.

The depression 42 of the ball holding plate 40 is filled with cream solder 44 (FIG. 1B). This step can be performed by directly rubbing a spatula on the surface of the ball holding plate 40 on which the cream solder is mounted, without using a stencil mask. In this case, unnecessary cream solder is removed with a spatula. Note that the amount of the cream solder 44 filled in the recess 44 becomes constant.

Next, the ball holding plate 40 is
Heat to 0 ° C. to dry the cream solder 42. Then, the cream solder 46 is placed on the dried cream solder 42 to a predetermined thickness by printing using a stencil mask (FIG. 1C). Each recess 4 in the stencil mask
An opening having a diameter larger than that of the depression 42 is formed at a position corresponding to 2, and the opening is aligned with the depression 42, and the cream solder placed on the stencil mask is rubbed with a spatula so that the cream solder having a predetermined thickness is formed. 46 can be supplied to each depression 42.

The ball holding plate 40 is placed in a reflow furnace and heated, so that the cream solders 44 and 46 are simultaneously melted (reflowed) (FIG. 1D). At this time, the cream solder 44 has been dried at 100 to 130 ° C. in advance and the solvent therein has evaporated.
6 is smoothly integrated. The melted cream solders 44 and 46 are united, and the portion projecting from the depression 42 becomes spherical due to surface tension.

Then, when cooled and solidified as it is, a solder ball 48 is obtained. Since the supply amounts of the cream solders 44 and 46 are constant, the diameter of the solder balls 48 is also constant, and the vertices of all the solder balls 48 are arranged on a common plane with high precision.

A semiconductor package 50 as a BGA printed wiring board is mounted on the solder ball 48. Here, the semiconductor package 50 has a semiconductor component 52 such as an LSI or HIC mounted on an upper surface thereof and hermetically sealed with a resin 54. A bonding pad is formed on a lower surface of a substrate 56 made of a printed wiring board. I have.

With these bonding pads aligned with the solder balls 48, the semiconductor package 50 is overlaid on the ball holding plate 40 from above. Then, if these are put in a reflow furnace and the solder balls 48 are reflowed, the melted solder balls 48 are soldered to the bonding pads of the substrate 56 of the semiconductor package 50 (FIG. 1E).

After the solder is sufficiently solidified, the ball holding plate 4
0 from the semiconductor package 50, the solder balls 4
8 is fixed to the semiconductor package 50 side and the ball holding plate 4
0 (FIG. 1 (F)). At this time, the cream solder 44 in the recess 42 is reflowed and remains as a protrusion 44A at the tip of the solder ball 48. As a result, the solder bump 58 with the protrusion 44A is completed.

Since the surface of the ball holding plate 40 is finished with a sufficiently good flatness, a large number of solder balls 48 held here are also arranged with a good flatness. Therefore, the protrusions 44A of the solder bumps 58 transferred to the semiconductor package 50 are also arranged with good flatness. However, when the ball holding plate 40 is peeled off from the semiconductor package 50, some of the solder bumps 58 may be slightly inclined.

Then, it is desirable that the protrusions 44A be pressed against the flat plate 60 having good flatness to improve the flatness of the arrangement of the tips of the protrusions 44A (FIG. 1 (G)). Preferably, the surface of the flat plate 60 is also made of a material having poor solder wettability.

In the above-described embodiment, the cream solder 44 is filled in the recess 42 with a spatula, and then a certain amount of cream solder 46 is placed by printing using a stencil mask. As long as it can be supplied to the depression 42, it may be supplied by another method. For example, depression 4
2 may have a shallow shape with a large opening, and a fixed amount of cream solder may be supplied by one printing using a stencil mask.

If the recess 42 is formed as a small-diameter hole as in the above embodiment, a sufficiently long projection 44A is formed at the tip of the solder bump 58.
Can be formed. Since the projections 44A are easily crushed by being pressed against the flat plate 60, it is particularly convenient to increase the flatness.

The cream solders 44 and 46 used are preferably eutectic solders having the same properties. In this case, the cream solders 44 and 46 are instantaneously melted, the reflow is performed quickly, and the component composition of the solder does not become uneven.

[0031]

According to the first aspect of the present invention, as described above, a predetermined amount of cream solder is supplied to the dent of the ball holding plate so as to swell from the dent, and the solder is reflowed using the surface tension. The solder ball is solidified into a ball shape, and the solder ball is reflow-soldered to the printed wiring board to form a solder bump having a projection at the tip of the solder ball. Therefore, it is not necessary to prepare spherical solder balls in advance and supply them to the respective bonding pads one by one, and a special dedicated mounting machine becomes unnecessary. For this reason, facilities such as a normal printing apparatus and a reflow furnace are sufficient.

Further, since the flatness of the tip of the solder bump can be improved by sufficiently improving the flatness of the ball holding plate, a semiconductor package having a large mounting terminal surface area can be soldered with high reliability. Will be possible.

Here, in order to supply a fixed amount of cream solder to the ball holding plate, after filling the depression with cream solder, the cream solder is printed to a predetermined thickness thereon by printing using a stencil mask. (Claim 2).
In this way, a fixed amount of cream solder can be easily supplied with high precision. In this case, if the cream solder filled in the depression is heated and dried at 100 to 130 ° C., the subsequent printing using the stencil mask can be performed smoothly and is suitable for integrating both cream solders ( Claim 3).

When the ball holding plate is peeled off from the printed wiring board in step (f), the heights of the solder bumps may become uneven. In this case, the projections are pressed against a flat plate having high flatness. The flatness can be improved by squashing (claim 4).

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention.

FIG. 2 is a diagram showing a conventional method using a solder ball;

[Explanation of symbols]

 Reference Signs List 40 Ball holding plate 42 Depression 44, 46 Cream solder 44A Projection 48 Solder ball 50 Semiconductor package as printed wiring board 58 Solder bump 60 Flat plate

Claims (4)

[Claims] 1. A method of forming solder bumps on a large number of electrode bonding pads of a ball grid printed wiring board, comprising the steps of: (a) holding a ball having poor solder wettability on a surface having a depression formed at a position corresponding to the electrode bonding pad; A step of preparing a board; (b) a step of supplying cream solder to the depression of the ball holding plate so as to rise from the depression; (c) a step of melting the cream solder raised in the depression by reflow and forming into a ball by surface tension. Solidifying to form a solder ball; (d) stacking the printed wiring board on a ball holding plate while aligning the electrode bonding pad with the solder ball; (e) melting the solder ball by reflow to respond (F) removing the ball holding plate from the printed wiring board after the solidification of the solder. Removing the solder bumps from the ball grid array printed wiring board. 2. The step (b) comprises: (ba) a step of filling the depression of the ball holding plate with cream solder; (bb) a step of further applying another cream solder on the cream solder filled in the depression by using a stencil mask. And printing the solder bumps to a predetermined thickness. 3. The method according to claim 2, wherein in the step (ba), the depression is filled with cream solder, and then heated to 100 ° C. to 130 ° C. and dried. 4. The following step after the step (f): (g) a step of pressing a projection corresponding to the shape of the depression formed at the tip of the solder ball against a flat plate to secure the flatness of the tip of the projection. 2. The method for forming solder bumps on a ball grid array printed wiring board according to claim 1, wherein: