JPH0794852A - Solder film forming method - Google Patents

Abstract

PURPOSE:To form a solder film by nonelectrolytic solder plating which can easily form uniform thickness without deterioration of solder wetability and short-circuit due to fused solder bridge, and is applicable to the mounting of present fine SMT parts. CONSTITUTION:The title method consists of the following; a process for forming a solder-plated resist film 2 on the copper circuit 1 of a printed wiring board, a process for forming a first solder film 3 having dense structure on the copper circuit 1 of the part where the solder-plated resist film 2 is not formed, a process for eliminating the solder-plated resist film 2, a process for forming a second solder film 4 having porous structure on the copper circuit 1 of the part where the first solder film 3 is not formed, and a process for fusing the first solder film 3 and the second solder film 4 and unifying them in an integral body by heating and for forming a solder film 5 on the copper circuit 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solder film forming method,
In particular, it relates to a method for forming a solder film on a printed wiring board.

[0002]

2. Description of the Related Art The surface of a copper circuit of a printed wiring board is coated with solder for the purpose of soldering and corrosion protection. As a conventional method for coating a copper circuit of a printed wiring board with solder, a molten solder plating method is used. This method
Generally, this is a method in which a printed wiring board having a copper circuit pattern formed thereon is dipped in a molten solder bath, and then solder excessively attached to the copper circuit is removed by blowing air heated to about 240 ° C. .

In recent years, with the progress of surface mounting technology, a solder film formed on a printed wiring board is required to have a uniform thickness of about 20 μm, and a plating thickness of molten solder is 0.5 to 30 μm. A big problem is that there is a large variation within the range. When the solder film formed by the hot-dip solder plating method is thin, the copper-tin alloy layer diffuses on the surface of the solder film, which significantly impairs solder wetting. Conversely, when the solder film is thick, the printed wiring board However, there is a problem that short circuit problems frequently occur due to the bridge of the molten solder in the portion where the circuit interval is narrow.

On the other hand, a method of forming a solder by remelting an electric solder plating film used as an etching resist in a pattern plating method of a printed wiring board is practiced. In this case, the thickness of the solder plating film is reduced. 1
It is difficult to set the thickness to 0 μm or more because of the limitation of the thickness of the plating resist. As a method for improving it, Japanese Patent Laid-Open No. 63-1
Japanese Patent No. 428993 discloses a method of forming a solder film having a sufficient film thickness by forming an auxiliary solder film on an electric solder plating with a solder paste or the like and then remelting it. 0.5m due to restrictions
m pitch SMT (Surface Mount Te)
It is difficult to apply this technology to the chology) component pad.

[0005]

As described above, in the conventional hot-dip solder plating method, it is difficult to form a solder film having a uniform film thickness, the solder wettability is deteriorated, and a short circuit occurs due to a bridge of the hot-melt solder. There was a problem such as.

In addition, the electric solder plating method has a problem that a sufficient solder film thickness cannot be obtained, and even with the technique of replenishing a sufficient amount of solder with a solder paste or the like, the present minute SMT is used.
There is a problem that it cannot be applied to component pads.

An object of the present invention is to provide a solder film forming method which can easily form a uniform solder film without deterioration of solder wettability and short circuit due to a bridge of molten solder and can be applied to the present minute SMT component pads. To do.

[0008]

A method for forming a solder film according to the present invention comprises a step of coating a copper circuit of a printed wiring board with a solder plating resist film made of a photosensitive resist containing a dry film resist, and the solder plating resist. A step of forming a first solder film by applying thin electroless solder plating having a dense structure on the copper circuit in a portion where the film is not covered; a step of removing the solder plating resist film; A step of forming a second solder coating by applying electroless solder plating for thickening having a porous structure on a portion of the copper circuit where the first solder coating is not formed; and heating the first solder coating. A step of melting and integrating a solder film and the second solder film to form a solder film on the copper circuit.

[0009]

Embodiments of the present invention will now be described with reference to the drawings.

1 (a) to 1 (e) are sectional views showing the first embodiment of the present invention in the order of steps. In the first embodiment of the present invention, first, as shown in FIG. 1A, a copper circuit 1 having a width of 0.3 mm is 0.3 m thick by a subtractive method.
Formed at a pitch of m, a solder protective film 6 is printed between the copper circuits 1 by screen printing or the like to form a printed wiring board.
Next, as shown in FIG. 1B, a photosensitive dry film resist having a film thickness of 50 μm is laminated, and a solder plating resist film 2 having a width of 0.2 mm is formed in the central portion of the copper circuit by exposure and phenomenon.

Next, as shown in FIG. 1 (c), this printed wiring board is pretreated with an aqueous solution of hydrogen peroxide at 30 ° C. for 1 hour.
After etching for 1 minute and washing with water, it was dipped in an electroless solder plating solution for thinning at a liquid temperature of 55 ° C. for about 30 minutes to obtain a first solder film 3 having a dense structure of about 3 μm. As an electroless solder plating solution for thinning, thiourea 100 g / l, tin borofluoride 0.1 mol / l, lead fluoride 0.03 mo
A composition of 1 / l was used. Next, as shown in FIG. 1 (d), after removing the solder plating resist film 2 with a dry film resist stripping solution, the solder plating resist film 2 is immersed in an electroless solder plating solution for thickening for about 1 hour to form a porous first layer of about 20 μm. A solder film 4 of 2 was obtained. As the electroless solder plating solution for thickening, a beam solder PC manufactured by Uemura Industry Co., Ltd. was used. Then, Figure 1
As shown in (e), the first solder film 3 and the second solder film 4 were melt-integrated by immersing in a molten oil at 200 ° C. for about 20 seconds to obtain a solder film 5 having a film thickness of 20 μm.

As a result, as shown in FIGS. 1 (d) and 1 (e), the surface of the copper circuit 1 is concave, the thickness of the solder film 5 in the concave portion is about 20 μm, and the thickness of the solder film 5 in the shoulder portion of the concave portion is About 3μ
It became m. This is because the first solder film 3 has a dense structure, and therefore the plating solution does not cause a substitution reaction with the copper of the copper circuit 1 when the second solder film 4 is formed. This is because the solder film 4 is deposited only on the copper circuit 1 which is not covered with the first solder film 3.

FIGS. 2A to 2E are sectional views showing the second embodiment of the present invention in the order of steps. In the second embodiment of the present invention, as shown in FIGS. 2A to 2E, a solder plating resist film 2 having a width of 0.3 mm is formed on a copper circuit 1 having a width of 0.3 mm. The same as the first embodiment shown in FIGS. 1 (a) to 1 (e). In the second embodiment, FIG.
As shown in (d) and (e), the copper circuit 1 had a rectangular cross section, and the solder film 5 having a thickness of about 18 μm was uniformly formed on the copper circuit 1. On the other hand, since the first solder film 3 and the second solder film 4 have good wettability, no defects such as short circuits and solder balls due to bridges between adjacent copper circuits 1 occurred.

[0014]

As described above, according to the present invention, the first solder film having a dense structure on the copper circuit and the porous structure on the copper circuit on which the first solder film is not formed are provided. By forming a second solder film and heating and melting the first solder film and the second solder film to form a solder film on the copper circuit, deterioration of solder wettability and bridge of molten solder may occur. There is an effect that it is possible to provide a solder film forming method which can easily form a uniform solder film without short circuit and can be applied to the mounting of the present minute SMT components.

[Brief description of drawings]

1A to 1E are cross-sectional views showing a process sequence for explaining a first embodiment of the present invention.

2 (a) to 2 (e) are sectional views showing a second embodiment of the present invention in the order of steps.

[Explanation of symbols]

 1 Copper Circuit 2 Solder Plating Resist Film 3 First Solder Film 4 Second Solder Film 5 Solder Film 6 Solder Protective Film

Claims (4)

[Claims] 1. A step of coating a solder plating resist film on a copper circuit of a printed wiring board, and a thin electroless half-plating for thinning is applied to a portion of the copper circuit not covered by the solder plating resist film. No. 1 step of forming a solder film, a step of removing the solder plating resist film, and a step of applying thick electroless solder plating on the copper circuit in a portion where the first solder film is not formed, And a step of forming a solder film on the copper circuit by melting and integrating the first solder film and the second solder film by heating. Forming method. 2. The method for forming a solder film according to claim 1, wherein the solder plating resist film is a photosensitive resist including a dry film resist. 3. The solder film forming method according to claim 1, wherein the first solder film has a dense structure. 4. The method for forming a solder film according to claim 1, wherein the second solder film has a porous structure.