JP3468876B2 - Printed wiring board and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printed wiring board having a soldering pad, which is used for reflow soldering a surface mount component, by plating a metal conductor as a base of the pad with solder. It relates to a manufacturing method thereof.

[0002]

2. Description of the Related Art A reflow soldering method is widely known as a method for soldering a surface mount component to a pad of a printed wiring board. Figure 3 shows the surface mounting I
It is a figure which shows the state which soldered C10.

In FIG. 3, reference numeral 12 is a printed wiring board, and 14 is a printed wiring board.
Is a metal conductor such as copper formed on the surface of the pad by a method such as etching and serves as a base of the pad. 1
Reference numeral 6 is a lead of the IC 10 as an electrode of the component. The lead 16 is aligned with the conductor 14 and reflow-soldered by the solder 18.

As a method for this reflow soldering, there are a method using cream solder (solder paste) and a method using solder thickening plating. FIG. 4 is a diagram showing a method using cream solder. FIG. 4A shows a state in which the cream solder 20 is supplied to the pad 22 by, for example, a dispenser. In this case, it is necessary to immerse the substrate 12 in a molten solder bath and attach the solder coat layer 24 to the conductor 14 to prevent the conductor 14 from oxidizing and improve the solderability.

The lead 16 of the IC 10 is placed on the cream solder 20 as an electrode of the component, and the cream solder 20
IC10 is temporarily fixed by its own viscosity. Then, when placed in a reflow oven and heated, the cream solder 20 and the solder coat layer 24 are melted and soldered as shown in FIG. 4 (B).

However, in the method using the cream solder 20, when the interval between the pads 22 becomes narrow (for example, 0.3 mm pitch), a solder bridge is apt to occur between the pads, and in order to prevent this solder bridge, the cream solder is supplied. If the amount is reduced, soldering failure is likely to occur due to insufficient supply. Therefore, this method is considered unsuitable for mounting narrow-pitch components.

Therefore, solder thickening plating is used as an alternative method. FIG. 5 shows a method using this solder thickening plating. In FIG. 5A, 26 is a tin / lead solder plating layer formed on the conductor 14. The plating layer 26 is formed by immersing the whole of the conductor 14 in the electrolytic solder plating bath with the periphery of the conductor 14 covered with the plating resist 28. Since the plating resist 28 is peeled off after this plating, the edge of the electrolytic solder plating layer 26 slightly projects to the outside of the conductor 14 and overhangs.

However, since the electrolytic solder plating layer 26 is a porous solder, there is a problem that it is poor in oxidation resistance and corrosion resistance and cannot be stored for a long time. Therefore, it is necessary to perform fusing. In this fusing, the electrolytic solder plating layer 26 is melted by heating and is bonded to the copper conductor 14 by a strong intermetallic bonding force, and at the same time, it is converted into a uniform solder alloy.

When this fusing is performed, the surface tension of the molten solder causes the solder to become hemispherical and solidify. Figure 5
(B) shows this state, and 30 is a solidified solder layer.

[0010]

2. Description of the Related Art In the method using the solder thick plating as described above, since the solder layer 30 after the fusing has a hemispherical shape, there arises a problem that the surface mount component cannot be stably temporarily fixed. That is, in this method, IC1
Surface mounted components such as 0 are fixed to the printed wiring board 12 with an adhesive and reflowed, but the electrodes of the components, for example, the leads 16 are unstable even if they hit the apex of the spherical solder layer 30. Tends to slip on the spherical surface of the solder layer 30 and deviate from the correct fixing position with respect to the conductor 14.

Further, in a chip type capacitor or resistor, the electrodes between both ends are fixed to the printed wiring board 12 with an adhesive. There is a possibility that the component may be attracted to the printed wiring board 12 and the electrode may slip down the solder layer 30 and be displaced in position.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and eliminates the need to perform fusing of a solder thickening plating layer when reflow soldering a surface mount component to a solder thickening plating pad. A first object of the present invention is to provide a printed wiring board in which the pad surface does not become hemispherical and components can be stably soldered. Also to provide a method of manufacturing the printed wiring board and the second object.

[0013]

According to the present invention, a first object of the present invention is a printed wiring board having a soldering pad for reflow soldering a surface mount component, which is a metal conductor which is formed on an insulating substrate and serves as a base of the pad. And a thick solder plating layer laminated on the surface of this conductor, and a thin low temperature solder plating layer laminated on this thick solder plating layer are formed by fusing at a temperature between the melting points of both plating layers. A printed wiring board comprising a soldering pad formed with a low temperature solder layer that flatly covers the upper surface of a thick solder plating layer.

A second object of the present invention is, in a method of manufacturing a printed wiring board having a soldering pad for reflow soldering a surface mount component, a metal conductor which is formed on an insulating substrate and serves as a base of the pad is thickened with solder. Plating plating,
A low-temperature solder having a melting point lower than that of the thick-plated solder is thinly plated on this, and the upper surface forms a soldering pad by fusing at a temperature between both melting points of the low-temperature solder and the thick-plated solder. And a printed wiring board manufacturing method.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing one step in the manufacturing process of an embodiment of the present invention, and FIG. 2 is a diagram showing its completed state.

In FIG. 1, the same parts as those in FIG. 5A are designated by the same reference numerals. 1 is different from FIG. 5A in that the low temperature solder plating layer 32 is electrolytically plated on the thick solder plating layer 26. Here, the low-temperature solder plating layer 32 is a tin-bismuth-based solder, and its melting temperature is sufficiently lower than that of the tin-lead-based solder plating layer 26. For example, the solder plating layer 26 has a melting point of about 190 ° C. or higher, and the low temperature solder plating layer 32 has a melting point of about 130 ° C. or lower.

Of course, the tin-bismuth low-temperature solder plating layer 32 may contain other components such as lead and cadmium in addition to tin and bismuth. Similarly, the tin-lead solder plating layer 26 may also contain other components such as silver and indium. Further, it is desirable that the tin-lead solder plating layer 26 has a thickness of 5 to 100 μm, and the tin-bismuth low temperature solder plating layer 32 has a thickness of 1 to 20 μm.

The printed wiring board 12 plated with the low temperature solder plating layer 32 in this manner is fused. In this fusing, irradiation with infrared rays (IR) (infrared ray method), immersion in a fusing tank containing a hot liquid (hot liquid immersion method), or passage of heated organic solvent vapor (gas phase fusing method) By heating, only the low temperature solder plating layer 32 is melted. That is, heating is performed at a temperature between both melting points of the low-temperature solder plating layer 32 and the solder plating layer 26, for example, 160 ± 10 ° C.

When only the low temperature solder plating layer 32 is melted by this fusing, the low temperature solder plating layer 32 spreads to the surface of the solder plating layer 26, and the surface of the solder plating layer 26 is covered with the low temperature solder layer 34, resulting in the state of FIG. Therefore, the porous solder plating layer 26 is shielded from the outside air and protected from oxidation and corrosion.

The pad 34 thus produced has a flat upper surface, so that the electrodes of the component can be stably held. Therefore, for example, when the leads 16 of the IC 10 are placed on the pad 36 and reflowed at a temperature higher than the melting point of the solder plating layer 26, the solder plating layer 26 and the low temperature solder layer 34 are melted as shown in FIG. The solidified solder 18 is soldered.

[0021]

As described above, according to the invention of claim 1, a porous thick solder plating layer (26) is formed on a metal conductor which is a base of a pad on a substrate, and the thick solder plating layer is formed thereon. (26) The pad is covered with a thin low-temperature solder plating layer (32) having a lower melting point than that of (26), and only the low-temperature solder plating layer (32) is melted and solidified to form a pad (36) having a flat upper surface. The upper surface of (36) becomes flat, and the electrode of the surface mount component can be stably placed on this and reflow soldered. Therefore, the electrode does not move and the relative position of the pad (36) with respect to the underlying conductor (14) does not change, and accurate soldering is possible.

[0022] According to the method of the invention of claim 2, Ru obtained method for manufacturing a printed wiring board according to claim 1.

[Brief description of drawings]

FIG. 1 is a diagram showing one process during manufacturing of one embodiment of the present invention.

FIG. 2 shows the completed state

FIG. 3 is a diagram showing a mounting state of an IC.

FIG. 4 is a diagram showing a method using cream solder.

FIG. 5 is a diagram showing a method using thick solder plating.

[Explanation of symbols]

10 ICs as surface mount components 12 Printed wiring board 14 conductor 16 Leads as electrodes 18 Solidified solder 26 Thick solder plating layer 32 Thin low temperature solder plating layer 34 Low temperature solder layer 36 pads

Claims (2)

(57) [Claims] 1. In a printed wiring board having a soldering pad for reflow soldering a surface mount component, a metal conductor formed on an insulating substrate as a base of the pad and a thickness laminated on the surface of the conductor. And a thin low-temperature solder plating layer laminated on the thick solder plating layer are formed by fusing at a temperature between the melting points of both plating layers to flatly cover the upper surface of the thick solder plating layer. A printed wiring board comprising a soldering pad formed with a low temperature solder layer. 2. A method for manufacturing a printed wiring board having a soldering pad for reflow soldering a surface mount component, wherein a metal conductor, which is formed on an insulating substrate and serves as a base of the pad, is thickly plated with solder, A soldering pad having a flat upper surface by thinly plating a low-temperature solder having a melting point lower than that of the thick-plated solder on this, and further performing fusing at a temperature between both melting points of the low-temperature solder and the thick-plated solder A method for manufacturing a printed wiring board, comprising: