JPH04343293A - Soldering method for parts onto printed wiring board - Google Patents

Abstract

PURPOSE:To provide a method of soldering parts on a printed wiring board, where solder cream is prevented from creeping up the lead of the parts or protected against a wicking action and the printed board can be kept high in connection reliability. CONSTITUTION:In a soldering method where the lead 5 of a part 4 is soldered with solder cream 6 to the foot print 2 covered with a solder coat 3, the melting point of the solder coat 3 is set lower than that of the solder cream 6 or the lead 5 is previously coated with high-melting solder 7 whose melting point is higher than that of the solder cream 6 before a soldering operation takes place.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to printed wiring boards, and more particularly to a method for soldering components to printed wiring boards.

0002

[Prior Art] Solderability during soldering of components on a printed wiring board on which a conductor pattern such as a footprint is formed on the surface through a patterning process, or
A solder leveling process is performed to cover the footprint with solder in order to improve storage stability and corrosion resistance during use. In this solder leveling process, for example, a printed wiring board that has been degreased, derusted, and cleaned is immersed in flux, pulled up, and then dipped in solder.
Furthermore, when pulling up the printed wiring board from the solder,
In order to remove excess solder adhering to the printed wiring board and to make the layer thickness of the solder coat uniform, the surface of the printed wiring board is heated at a temperature of, for example, 180 to 200°C.
The footprint is covered with a solder coat having a thickness of about 1 μm by blowing high-temperature, high-pressure air at a pressure of about 3 to 5 kg/cm 2 from a nozzle.

Furthermore, as shown in FIG. 3, when soldering the leads 5 of the component 4 to the printed wiring board 1 whose footprint 2 has been completely coated with the solder coat 3, the upper layer of the solder coat 3 is After applying a solder cream 6 containing a mixture of flux and solder, for example, by a method such as screen printing, the tips of the tin-plated leads 5 are placed on the solder cream 6 using means such as an automatic placement machine. (Figure 3(a)). Further, the solder cream 6 is applied with the solder coat 3 by a method such as a reflow soldering method or a vapor phase soldering method (VPS method).
The component 4 is soldered to the printed wiring board 1 by heating at a processing temperature of 15° C. to volatilize the flux component and melt the solder.

Generally, the solder forming the solder coat 3 and the solder cream 6 is a eutectic solder having a composition (weight ratio) of tin (Sn):lead (Pb)=63:37 and a melting point of 183°C. Based on this melting point, the blowing air temperature during solder leveling and the processing temperature during reflow are set.

[0005]

However, for example, a trace amount of tin contained in the solder coat 3 forms an alloy with the copper of the footprint 2. For this reason, even though the same eutectic solder is used for both the solder coat 3 and the solder cream 6, the melting point of the solder constituting the solder coat 3 is lower than that of the solder cream 6 during the soldering process.
The melting point of the solder cream 6 may be slightly higher than the melting point of the solder coat 3, and immediately after the start of reflow, the solder cream 6 melts before the solder coat 3, as shown in FIG. 3(b).

Then, as shown in FIG. 3(c), only the melted solder cream 6 flows up the leads 5 of the component 4 and concentrates at the bent portions of the leads 5, causing a phenomenon called wicking. As a result, the amount of solder connecting the lead 5 and the footprint 2 becomes insufficient, resulting in a situation where the conductivity between the component 4 and the footprint 2 is reduced.

[0007] This kind of wicking is used in soldering 1
It has been confirmed that this occurs with a probability of approximately 1/104 at each location.For example, when a component with 20 leads is
In a printed wiring board where 100 pieces are mounted per board, this occurs at a rate of approximately one in five boards, and is a bottleneck in ensuring connection reliability of a given printed wiring board. The present invention has been proposed in view of the above-mentioned conventional circumstances, and is a printed wiring that prevents the occurrence of wicking in which solder cream runs down the leads of components and ensures connection reliability of printed wiring boards. The purpose is to provide a method for soldering components to a board.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the present invention uses solder cream 6 to solder leads 5 of components 4 to footprints 2 of printed wiring board 1 coated with solder coat 3. Assuming the method of soldering components to the printed wiring board, the following methods are adopted.

That is, the melting point of the solder coat 3 is lower than that of the solder cream 6. moreover,
Prior to the soldering, the leads 5 are coated with a high melting point solder 7 having a higher melting point than the solder cream 6.

0010

[Function] According to the above structure, as shown in FIG. 1, when reflow soldering the component 4, the melting time of the solder coat 3 is earlier than that of the solder cream 6, and due to the surface tension of the melted solder coat 3, , the solder cream 6 is adsorbed to the footprint 2 side.

Furthermore, as shown in FIG. 2, by performing pretreatment to coat the surface of the leads 5 of the component 4 with the high melting point solder 7, it is possible to prevent the solder cream 6 from forming before the solder coat 3 immediately after the start of reflow. Even when melting is started, the high melting point solder 7 and solder paste 6 maintain low wettability.

0012

EXAMPLES The present invention will be explained in detail below based on examples. FIG. 1 is a procedure explanatory diagram of an embodiment according to the present invention. As shown in FIG. 1(a), a printed wiring board 1
Prior to soldering the component 4, solder leveling is performed on the footprint 2 formed on the surface of the component 4 by conventionally known means and methods, and a uniform solder coat 3 with a layer thickness of about 1.0 μm is formed. A solder cream 6 containing a mixture of flux and solder is applied to the upper layer of the solder coat 3 by a screen printing method. after that,
The tip of the lead 5 of the tin-plated component 4 is placed on the footprint 2 to which the solder cream 6 has been applied using an automatic placement machine or the like.

In this example, the solder cream 6
As with the conventional solder, the composition (weight ratio) is tin (Sn):lead (Pb) = 63:37, and the melting point is 183.
Solder coat 3 as opposed to using eutectic solder at ℃
The composition (weight ratio) of the solder constituting the solder coat 3 is set to tin (Sn): lead (Pb): indium (In) = 62:36:2, and the melting point of the solder of the solder coat 3 is 178 to 173°C.
Use low melting point solder.

In this way, the printed wiring board 1 on which all the components 4 are placed has an atmosphere (temperature) in the furnace of 215°C.
By heating the solder cream 6 together with the solder coat 3 using a vapor phase soldering method (VPS method) set as follows, the flux component is volatilized and the solder is melted. At this time, as shown in FIG. 1(b), since the melting point of the solder constituting the solder coat 3 is lower than that of the solder in the solder cream 6, it melts first, and its surface tension causes the solder cream 6 to spread over the footprint 2. Adsorbs to the side.

Next, the solder cream 6 melts, and together with the solder coat 3, the tips of the leads 5 are attracted to the footprint 2 by surface tension, completing the reflow as shown in FIG. 1(c). In this embodiment, the solder in the solder coat 3 melts before the solder cream 6 during reflow, so the solder cream 6 is attracted to the footprint 2 side, and in this state, the solder cream is in direct contact with the lead 5. It is possible to prevent the occurrence of wicking in which the wire 6 travels up the lead 5.

In the present invention, the solder coat 3
It is sufficient that the melting point of the solder paste 6 is lower than that of the solder paste 6, and in addition to the combination of solders adopted in the above embodiments, it is also possible to use a combination of two types of solder having different melting temperatures as shown in Table 1. Can be done. However, even if, for example, eutectic solder (melting point 183°C) is used for solder coat 3 and high melting point solder with a higher melting point than the eutectic solder is used as solder paste 6, the above wicking can be prevented, but during reflow The temperature inside the furnace must be made higher than before, which increases thermal stress on the parts 4, which may cause other problems. Therefore, it is desirable to use a combination of solders as in the above embodiment.
In this case, the air blown during solder leveling and the furnace temperature for vapor phase soldering (VPS) can be set in the same way as in the past.

FIG. 2 shows another embodiment of the present invention.
The method of soldering components to a printed wiring board was performed as shown in the figure below. That is, as shown in FIG. 2(a), prior to soldering, the surface of the lead 5 of the component 4 is coated with a material having a composition and melting point higher than the melting point of the solder cream 6 described later, for example, as shown in Table 1. Cover with high melting point solder 7. However, in order to improve connection reliability, it is desirable that the melting point of this high melting point solder 7 be lower than the temperature of the subsequent reflow process. Thereafter, the footprint 2 is placed on a printed wiring board 1 coated with a solder coat 3 and a solder cream 6, as shown in FIG. 2(b).

In this embodiment, the solder coat 3
Both solder creams 6 and 6 have the same composition (weight ratio) of tin (Sn):lead (Pb) = 63:37 as the conventional solder.
A eutectic solder with a melting point of 183° C. was used. After this,
By using the vapor phase soldering method (VPS method), the furnace atmosphere (
Temperature): Reflow the solder coat 3 and solder cream 6 at 215°C. Immediately after the reflow, as shown in Fig. 2(c)
As shown in , if solder cream 6 is solder coat 3
Even if the solder cream 6 melts earlier than the solder cream 6, the wettability between the melted solder cream 6 and the high melting point solder 7 covering the leads 5 is poor, so the solder cream 6 is repelled by the surface of the leads 5, causing the above-mentioned wicking. There's nothing to do.

Next, the solder coat 3 and the high melting point solder 7 are melted in this order, and the lower end of the lead 5 is connected to the footprint 2 as shown in FIG. 2(d), completing the mounting of the component 4. In this embodiment, wicking of the solder cream 6 running up the leads 5 can be prevented by simply coating the surfaces of the leads 5 of the component 4 with high melting point solder 7 prior to soldering.

[0020] The compositions of the solder coat, solder cream, and high melting point solder used in the present invention are not limited to those shown in Table 1. Of course, it is sufficient to set .

[0021]

[Table 1]

[0022]

[Effects of the Invention] As described above, according to the present invention, it is possible to prevent the occurrence of wicking in which only the solder cream melts immediately after reflow and runs up the leads of the component, thereby improving the connection reliability of printed wiring boards. There are effects that can be enhanced.

[Brief explanation of drawings]

FIG. 1 is a procedure explanatory diagram of an embodiment of the present invention.

FIG. 2 is a procedure explanatory diagram of another embodiment of the present invention.

FIG. 3 is an explanatory diagram of the procedure and problems of the conventional example.

[Explanation of symbols]

1 Printed wiring board 2 Footprint 3 Solder coat 4 Parts 5 Lead 6 Solder cream 7 High melting point solder

Claims (2)

[Claims] [Claim 1] A printed wiring board in which a lead (5) of a component (4) is soldered with solder cream (6) to a footprint (2) of a printed wiring board (1) coated with a solder coat (3). A method for soldering components to a printed wiring board, characterized in that the melting point of the solder coat (3) is lower than the melting point of the solder cream (6). [Claim 2] A printed wiring board in which a lead (5) of a component (4) is soldered with solder cream (6) to a footprint (2) of a printed wiring board (1) coated with a solder coat (3). In the method of soldering components to
A method for soldering components to a printed wiring board, characterized in that the solder cream (6) is coated with a high melting point solder (7) having a higher melting point than the solder cream (6).