JPH07326853A - Method for forming ball bumps on printed wiring board - Google Patents

Abstract

(57) [Summary] [Purpose] To form ball bumps of a printed wiring board with relatively simple equipment without using expensive solder balls. [Structure] A solder resist layer having a dam hole overlapping a pad of a printed wiring board is formed, and a dry film resist layer having a solder plating hole having a diameter larger than the dam hole is formed thereon, and the dam hole and the solder plating are formed. Thick solder plating is applied to the pad through the hole, and only the dry film resist is peeled off and then reflowed to form a solder ball bump.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention connects a package such as an IC (also referred to as a subcarrier) to a printed wiring board (daughter board) or a relatively small wiring board (daughter board) to a printed circuit board (motherboard). Etc.) for forming ball bumps on a package or a relatively small printed wiring board.

[0002]

2. Description of the Related Art In ICs and the like, a flat package (QFP) in which pins (leads) are provided on four sides of the package has become widespread as the packaging density has increased. At present, a component having a lead pitch of 0.3 mm has been put into practical use as a component having this QFP structure, but it is conceivable to further reduce the lead pitch to further promote high density.

However, as the lead pitch becomes narrower, the problem of soldering failure due to the positional deviation between the package side lead and the substrate side pad and the solder bridge increases. Therefore, a method (Ball Grid Array: abbreviated as BGA) of using ball bumps without using leads as component connecting terminals has been proposed.

FIG. 3 is a plan view showing an example of dimensions of this BGA structure, FIG. 4 is a sectional view showing a typical structure example, and FIG. 5 is an explanatory view of a conventional ball bump forming method. In FIG. 4, reference numeral 10 is a printed wiring board that serves as a subcarrier. 1
2 is a via hole for conduction (VIA), 14 is a pad 16
It is a circuit pattern including. Reference numeral 18 is a resist applied to the chip mounting surface (upper surface in FIG. 4) of the wiring board 10, and 20 is a solder resist formed on the bump formation surface (lower surface). Dam holes 24 corresponding to the positions where the ball bumps 22 are to be formed are formed in the solder resist 20 (see FIG. 5).

An IC chip 26 is fixed to the chip mounting surface of the substrate 10 as shown in FIG. 4, and an electrode 28 of the chip 26 is bonded to a predetermined pad 1 by a bonding wire 30.
Connected to 6. Reference numeral 32 is a cap that hermetically covers the chip 26.

Heretofore, the ball bump 22 has heretofore been formed by using a solder ball 34 as shown in FIG. That is, a dam hole 24 is formed in the solder resist 18 that covers the bump formation surface of the wiring board 20, one solder ball 34 is put in the dam hole 24, and the solder ball 34 is heated by a heater and reflowed, as shown in FIG. Such a ball bump 22 is formed.

The solder resist 18 is applied by, for example, a printing method so as to cover all portions except the dam hole 24. Then, it is cured by a method suitable for the type of solder resist, such as heat curing or ultraviolet curing. In this way dam hole 2
Only 4 is covered with the solder resist 18 in a state where it has a predetermined size (see FIG. 5B).

Wiring board 10 covered with solder resist 18
A mask having holes formed only at the positions of the dam holes 24 is aligned and fixed on the surface of the above, and one solder ball 34 is supplied to each hole. The solder balls 34 are made to a predetermined size,
A predetermined surface treatment is applied to prevent surface oxidation.

When the solder balls 34 are reflowed by heating, the pad 1 is raised in a hemispherical shape from the dam hole 24 by the surface tension of the molten solder as shown in FIG. 5B.
6 is soldered. The size of the dam hole 24 and the thickness of the solder resist 18 are accurately controlled, and the size of the ball 34 is also accurately controlled.
The height of the solidified solder so that it swells up becomes uniform.

As shown in FIG. 3, the ball bumps 22 are formed so as to spread two-dimensionally at a pitch of 0.8 mm and a pitch of 1.5 mm, for example. Therefore, 27 mm x 27 mm
15 × 15 = 225 ball bumps 22 can be formed on the wiring board 10. That is, it is possible to easily increase the number of connection terminals while widening the distance between the ball bumps 22 as compared with the conventional QFP structure.

[0011]

2. Description of the Related Art As described above, the solder balls 34 used in the conventional method need to be subjected to various surface treatments in order to prevent their surfaces from being oxidized, and may have high dimensional accuracy. is necessary. Therefore, solder balls 3
4 becomes expensive.

The solder balls 34 are connected to the dam holes 24.
In order to reliably supply one by one, special equipment is required separately. For example, a metal mask having a hole corresponding to the dam hole 24 is prepared, the metal mask is aligned on the wiring board 10 with high accuracy, and one solder ball 34 is supplied to each hole, and the solder is used. A device for confirming the supply of the balls 34 is also required.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and does not require the use of expensive solder balls.
Another object of the present invention is to provide a method for forming ball bumps on a printed wiring board, which can be relatively simple in equipment.

[0014]

According to the present invention, the object is to provide a method for forming a ball bump for connecting a printed wiring board to another printed wiring board, including the following steps. Ball bump forming method of: a. A step of forming a via hole for conduction in a copper-clad laminate and performing copper plating; b. Forming a circuit pattern including a pad corresponding to the position where the ball bump is formed by etching; c. Forming a solder resist layer having a dam hole corresponding to the position where the ball bump is formed on the surface where the ball bump is formed; d. A step of crimping a dry film resist on both sides to form a solder-plated hole having a diameter larger than that of the dam hole so as to overlap the dam hole; e. Supplying solder to the dam hole and the solder plating hole by a thick solder plating process; f. Peeling the dry film resist; g. The step of heating to reflow the solder plating to form a ball bump is achieved.

The printed wiring board on which the ball bumps are formed is not limited to a printed wiring board (subcarrier or the like) which becomes a package of IC or the like, and may be another relatively small printed wiring board (daughter board or the like). . The printed wiring board is not limited to one having a circuit pattern formed on a copper clad laminate, but may be one having a circuit pattern formed on a ceramic substrate.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view of a process for forming a ball bump on a printed wiring board (subcarrier) for an IC package which is an embodiment of the present invention, and FIG. 2 is a flow chart of a processing process. In FIG. 1A, reference numeral 50 is a double-sided copper-clad laminate, and copper foils 54, 5 are provided on both surfaces of an insulating substrate 52 made of resin.
4 are bonded (step 100 in FIG. 2).

The copper clad laminate 50 has a diameter of about 0.2 m.
A large number of m conductive via holes 56 are machined by a drill (step 102), and a copper plating layer 58 is further formed.
Are formed (step 104). This copper plating layer 58
Is for imparting conductivity to the inner surface of the via hole 56, and is formed by electroless metal plating.
After imparting conductivity to the inner surface of the hole, it is formed by further performing electrolytic copper plating treatment.

A circuit pattern 60 is formed on both surfaces of the copper-clad laminate 50 plated with copper by a known etching method.
60 is formed (FIG. 1B, step 106).
For example, a resist is applied by a printing method and unnecessary copper plating layer 58 and copper foil 54 are removed by etching. The photoresist may be applied to both surfaces of the copper clad laminate 50, the circuit pattern may be exposed to remove unnecessary resist, and then the unnecessary copper plating layer 58 and the copper foil 54 may be removed by etching. By forming this circuit pattern 60, the copper clad laminate 50 becomes a printed wiring board 50A.

On one surface of the printed wiring board 50A, that is, a surface (hereinafter referred to as a bump forming surface) 64 on which the ball bumps 62 (see FIG. 1F) are to be formed, a part of the circuit pattern 60 is The pad 60A is formed with the ball bump 62. Next, a solder resist 66 is applied to the bump forming surface 64 (step 108 of FIG. 1C). As the solder resist 66, a heat-curable screen-printing resin containing epoxy as a main component is suitable.

The ball bump 6 is formed on the solder resist 66.
The dam hole 68 corresponding to the planned formation position 2 is formed.
That is, the solder resist 66 is applied by a screen printing method to a constant thickness so as to cover the surface other than the dam hole 68, and then is cured by heating. As a result, the bump forming surface 64 is covered with the solder resist 66 so as to overlap each pad 60A. The solder resist 66 used here is a dry film resist 70 described later,
It is not limited to the epoxy type as long as it does not fall off in the step of removing 72.

Next, dry film resists 70 and 72 are pressure-bonded to both surfaces of the printed wiring board 50A to form a dam hole 68.
A hole larger than the dam hole 68, that is, a solder plating hole 74 is formed at a position overlapping with (step 110). The dry film resists 70 and 72 are made of, for example, a photosensitive acrylic resin as a main component, and are exposed and baked by stacking a mask film and a mask glass plate, and then developed to remove unnecessary dry films. To do. After that, necessary characteristics such as solder heat resistance and solvent resistance are imparted by post-exposure and heat treatment.

Next, the printed wiring board 50A is subjected to thick solder plating 76 ((D) of FIG. 1, step 112). The solder plating 76 does not deposit on the dry film resists 70 and 72, but deposits on the pad 60A facing the dam hole 68 and the solder plating hole 74. By strictly controlling the processing conditions such as the composition and temperature of the solder plating bath, the deposition amount of the thick solder plating 76 can be controlled with high accuracy. Further, since the thickness of the dry film resist 70 is also controlled with high precision, the amount of the solder plating 76 deposited can be controlled with higher precision.

After a predetermined amount of solder plating 76 is deposited,
The dry film resists 70 and 72 are removed ((E) of FIG. 1, step 114). When the dry film resists 70 and 72 containing acrylic as the main component are used as described above, this peeling treatment can be performed with a 2% caustic soda solution. As a result, as shown in FIG. 1E, the solder plating 76 is projected from the dam hole 68 of the solder resist 66 to a predetermined thickness by the size of the solder plating hole 74.

The printed wiring board 50A is then placed horizontally in a heating device (not shown) and heated to about 230 ° C. (FIG. 1 (F), step 116). This heating causes the solder plating 76 to reflow, and becomes a spherical shape due to the surface tension of the molten solder. Since the diameter of the dam hole 68 is constant and the amount of molten solder is also constant, the size and height of the molten solder are also constant. As a result, after solidification, the solder ball bumps 62 having a predetermined size are formed (step 118).

In the above embodiment, the epoxy solder resist 66 and the acrylic dry film resist 70 are used.
However, the combination of both is not limited to this. That is, the dry film resist 70, 7
In the second stripping process (step 114 in FIG. 2), only the dry film resists 70 and 72 can be selectively stripped, and the lower layer solder resist 66 is not stripped at this time.

The printed wiring board on which the ball bumps 62 are formed may be a printed wiring board (subcarrier) which is a substrate of an IC package as in the above-mentioned embodiment.
When connecting another relatively small printed wiring board (daughter board) to another large printed wiring board (motherboard), a small printed wiring board (daughter board) may be used. The printed wiring board on which the ball bumps are formed is not limited to the one using a resin insulating substrate, and may be one using a ceramic substrate.

[0027]

As described above, the invention of claim 1 forms a solder resist layer having a dam hole overlapping a pad of a printed wiring board, and a dry layer having a solder plating hole having a diameter larger than the dam hole is formed on the solder resist layer. Forming a film resist layer,
Thick solder plating is applied to the pad through the dam hole and the solder plating hole, and only the dry film resist is peeled off and then reflowed to form a solder ball bump.

The thickness of the dry film resist can be precisely controlled, the dimensions of the solder plating hole and the dam hole can also be precisely controlled, and the thickness of the thick solder plating can be controlled extremely accurately by controlling the plating conditions. it can. Therefore, the capacity of the solder plating supplied to each solder plating hole and dam hole is also managed with high accuracy, and the dimensions of many ball bumps can be made uniform. In addition, since each process used in this method can be the one used in the normal manufacturing process of a printed wiring board, there is no need to prepare a special device, and the equipment is relatively simple.

[Brief description of drawings]

FIG. 1 is a process explanatory view of an embodiment of the present invention.

[Fig. 2] Flow chart of processing steps

FIG. 3 is a diagram showing an example of dimensions of a BGA structure.

FIG. 4 is a diagram showing an example of a typical structure of BGA.

FIG. 5 is an explanatory view of a conventional ball bump forming method.

[Explanation of symbols]

 50 Copper-clad laminate 50A Printed wiring board 56 Bayer hole 60 Circuit pattern 60A Pad 62 Ball bump 66 Solder resist 68 Dam hole 70, 72 Dry film resist 74 Solder plating hole 76 Thick solder plating

Claims (1)

[Claims] 1. A method for forming a ball bump for connecting a printed wiring board to another printed wiring board, comprising:
A method for forming ball bumps on a printed wiring board, which comprises the following steps: a. A step of forming a via hole for conduction in a copper-clad laminate and performing copper plating; b. Forming a circuit pattern including a pad corresponding to the position where the ball bump is formed by etching; c. Forming a solder resist layer having a dam hole corresponding to the position where the ball bump is formed on the surface where the ball bump is formed; d. A step of crimping a dry film resist on both sides to form a solder-plated hole having a diameter larger than that of the dam hole so as to overlap the dam hole; e. Supplying solder to the dam hole and the solder plating hole by a thick solder plating process; f. Peeling the dry film resist; g. The process of heating and reflowing the solder plating to form ball bumps.