JPH05259631A - Surface mounting of printed wiring board - Google Patents

Abstract

PURPOSE:To make it unnecessary to bond parts to either side of a substrate using adhesive and thus reduce the number of processes by reflow-soldering a first part using first cream solder having a high melting point and subsequently reflow-soldering a second part using second cream solder having a low melting point. CONSTITUTION:First cream solder 16A is applied to pads 12 on side A of a substrate, and a first part 20A is mounted thereon by reflow soldering. Second cream solder 24A is applied to pads 14 on side B, with the melting point of the second cream solder 24A lower than that of the first 16A. Leads 28A of the second part 26A are temporarily bonded to the second cream solder 24A, and subsequently, the board is placed in a second high temperature atmosphere. The temperature is then controlled and kept at approx. 50 deg.C, for example, in order that the second cream solder 24A will be melted but the first 16A will not. This makes it unnecessary to fix parts on either side using adhesive, shortening the production process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for mounting surface mount components on both front and back surfaces of a printed wiring board.

[0002]

2. Description of the Related Art In recent years, in order to increase the mounting density of electronic parts and to reduce the size of electronic devices, it has become common to use a printed wiring board having parts mounted on both sides. The parts to be surface-mounted here (surface-mounted parts) are suitable for automatic mounting using an automatic mounting machine.

FIG. 2 is a process diagram of a conventional method in this case. Reference numeral 10 is a printed wiring board, and pads 12, 12 and 14, 14 for soldering are formed on both surfaces thereof ((A) of FIG. 2). First, with one surface A of the printed wiring board 10 facing upward, cream solder 16, 16 is supplied to the pads 12, 12 on this surface A by a dispenser or printing. Further, a thermosetting adhesive 18 for fixing components is supplied to this surface A (FIG. 2 (B)).

In this way, the cream solder 16 and the adhesive 18
The surface A supplied with the
0 is attached (FIG. 2 (C)). For example, when mounting a component 20 such as a flat pack IC, the lead 22 of this IC is aligned with the corresponding pad 12 and I
The bottom surface of the C package is adhered to the adhesive 18. Then, the whole is heated in a reflow oven such as an electric oven to cure the adhesive 18. The heating temperature at this time is a temperature at which the cream solder 16 is not melted and only the adhesive 18 is cured, for example, 150 ° C.
Is set in the rank.

Next, the printed wiring board 10 is turned upside down and the other surface B is turned up. Then, the cream solder 24, 24 is supplied to the pads 14, 14 (FIG. 2 (D)). Two
Reference numeral 6 is a second surface mount type component such as a flat pack IC, and its leads 28, 28 are cream solders 24, 24.
It is aligned and temporarily fixed. Then, while keeping the whole in this state, put it in the reflow furnace, and in a high temperature atmosphere (about 2
The cream solders 16 and 24 are melted and soldered at 30 ° C. (FIG. 2 (E)). At this time, the first component 20 on the lower surface A
Is fixed by the adhesive 18, it does not fall.

[0006]

However, according to this conventional method,
It is necessary to supply the adhesive 18 for fixing the first component 20 and also to put it in an electric furnace in order to cure the adhesive 18. Therefore, there is a problem that the number of manufacturing steps increases.

For parts such as ICs, the lead is inward.
SOJ (Small Outline J-bend P
ackage) or QFJ (Quad Flat J-bend Package)
The type such as is known. When these components are surface-mounted, the gap between them and the surface of the printed wiring board becomes large, so that they cannot be fixed with an adhesive or a large amount of adhesive must be used.

FIG. 3 is an explanatory view when a large amount of this adhesive is used. In this figure, 30 is a printed wiring board, 32,
Reference numeral 32 is a pad formed on one surface of the pad, and 34 and 34 are cream solders applied to the pads 32 and 32. Three
Reference numeral 6 is an SOJ type IC, and 38 and 38 are its J-bend type leads. Since the leads 38, 38 are thus bent inward, the distance L between the lower surface 40 of the package of the IC 36 and the printed wiring board 30 is increased.

For this reason, a small amount of the adhesive 42 is supplied in FIG.
In the case of (A), the IC 36 easily peels off and falls off. Therefore, it is also possible to supply a large amount of the adhesive 44 as shown in FIG. However, at this time, the adhesive 44 flows around and adheres to the pad 32 and the cream solder 34, which causes a problem that the reliability of soldering is lowered.

Therefore, it is necessary to manually solder such an SOJ IC by using a soldering iron. However, in this case, not only the productivity is poor, but there is also a problem that thermal stress is applied to the component by the soldering iron and the reliability of the component is reduced.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and eliminates the need for fixing components to be mounted on one surface with an adhesive when the components are surface-mounted on both surfaces of a printed wiring board. The number of processing steps can be reduced by eliminating the need for supplying an adhesive and curing the adhesive by heating, and especially when applied to the surface mounting of a component having a J. bend type lead. , No need for manual soldering,
It is an object of the present invention to provide a surface mounting method which has good productivity and eliminates the risk of reducing reliability of components due to thermal stress.

[0012]

According to the present invention, an object of the present invention is to provide a surface mounting method of a printed wiring board for mounting surface mounting type components on both front and back surfaces of the printed wiring board, the printed wiring board having the following steps. Surface mounting method of board: supplying first cream solder to a surface mounting pad provided on one surface of the printed wiring board; temporarily mounting a first surface mounting type component on the pad on the one surface A step of stopping; a step of melting the first cream solder in a first high temperature atmosphere and reflow-soldering the first surface mount type component; a surface provided on the other surface of the printed wiring board A step of applying a second cream solder having a melting point lower than that of the first cream solder to the mounting pad; a step of temporarily fixing the second surface mount type component to the second cream solder; The second lower than the first high temperature atmosphere Wherein in a high temperature atmosphere first chestnut - the second without melting the solderless
Of melting the cream solder and reflow soldering the second surface mount type component. Achieved by.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a process drawing of an embodiment of the present invention. In this embodiment, SOJ type ICs are used as the surface mount components 20A and 26A.

First, a printed wiring board 10 similar to that described with reference to FIG. 2 is prepared, and one surface A of the printed wiring board 10 faces up (FIG. 1A). First cream solder 16A is supplied to the pads 12, 12 on this surface A (FIG. 1 (B)). The first cream solder 16A used here has a melting point higher than that of the second cream solder 24A, which will be described later, and is of a type that starts melting at about 180 ° C., for example.

Next, the component 20A is mounted on this surface A. That is, by aligning the leads 22A, 22A of the component 20A with the cream solder 16A, 16A, the cream solder 16A is aligned.
The component 20A is temporarily fixed by the viscosity of A and 16A itself.
Then, in this state, the whole is put into a reflow furnace such as an electric furnace and the first high temperature atmosphere (about 240 ° C.) is applied to melt the cream solders 16A and 16A. As a result, the first component 20A is mounted on the surface A by the reflow soldering method (FIG. 1C).

Next, the printed wiring board 10 is turned upside down and the other surface B is turned up. Then, the second cream solders 24A and 24A are supplied to the pads 14 and 14 on the surface B (FIG. 1D). This second cream solder 24
As described above, A has a melting point lower than that of the first cream solder 16A and is melted at, for example, about 150 ° C. or lower.

After the leads 28A, 28A of the second component 26A are temporarily fixed to the second cream solders 24A, 24A, the whole is put in the second high temperature atmosphere in the reflow furnace. The temperature of the second high temperature atmosphere is controlled to about 150 ° C., for example, so that the second cream solder 24A is melted but the first cream solder 16A is not melted. Therefore, at this time, the first component 20A soldered to the lower surface A by the reflow soldering method does not fall off (see FIG. 1).
(E)).

Although SOJ type ICs are used as the components 20A and 26A in the above embodiments, the present invention is not limited to this, and flat packs and other types of ICs, or chips such as resistors and capacitors. It may be a mold part or the like.

[0019]

As described above, the invention of claim 1 changes the type of the cream solder supplied to each surface of the printed wiring board, and utilizes the difference in the melting temperature of both types of the cream solder. First, the first component (20A) is reflow-soldered with the first melting solder (16A) having a high melting point, and then the second component (24A) is used with the second component (24A) having a low melting point. 26A) is reflow-soldered. Therefore, since it is not necessary to fix the components on one surface with an adhesive, the steps of supplying the adhesive and curing the adhesive become unnecessary, the production process becomes short, and the productivity is improved.

As a component used here, an SOJ type IC
Even in the case where the gap between the printed wiring board (10) and the lower surface of the component becomes large as in the above, it is not necessary to increase the amount of the adhesive or use the manual soldering method as in the conventional case.
Therefore, unlike the case where a large amount of adhesive is supplied to fix a component, the adhesive does not flow out onto the pad and the reliability of soldering is not deteriorated. Further, the productivity is improved as compared with the case where the manual soldering method is used, and the reliability of the parts is not lowered because the soldering iron does not apply excessive heat stress to the parts (claim 2).

[Brief description of drawings]

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

FIG. 2 is a process diagram of a conventional method

FIG. 3 is an explanatory view of soldering an SOJ type IC by a conventional method.

[Explanation of symbols]

 10 Printed Wiring Boards 12 and 14 Pads 16A First Cream Solder 20A SOJ Type IC as First Part 24A Second Cream Solder 26A SOJ Type IC as Second Part

Claims (2)

[Claims] 1. A surface mounting method for a printed wiring board for mounting surface mounting components on both front and back surfaces of the printed wiring board, comprising the following steps: Supplying the first cream solder to the surface mounting pad provided on one surface of the plate; Temporarily fixing the first surface mounting type component to the pad on the one surface; First high temperature atmosphere In which the first cream solder is melted to reflow-solder the first surface-mounting type component; the first cream is attached to a surface-mounting pad provided on the other surface of the printed wiring board. A step of applying a second cream solder having a lower melting point than that of the cold solder; a step of temporarily fixing the second surface mount type component to the second cream solder; a step lower than the first high temperature atmosphere 2 in the high temperature atmosphere The second without melting the ream solder
Of melting the cream solder and reflow soldering the second surface mount type component. 2. The surface mounting method for a printed wiring board according to claim 1, wherein the first surface mounting type component has a J-bend type lead.