GB2210818A

GB2210818A - A process for soldering a component to a printed circuit board

Info

Publication number: GB2210818A
Application number: GB8723863A
Authority: GB
Inventors: Vincent Curtis
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-10-10
Filing date: 1987-10-10
Publication date: 1989-06-21
Also published as: GB8723863D0

Abstract

The circuit board (4) comprises a number of plated through holes (6) and at least one surface-mounted component (2) having a number of leads (8) such that for each lead (8) there is at least one plated through hole (6). The board is subjected to a wave soldering process (10) whereby solder extends to the leads (8) of the surface mounted components (2) via the plated through holes (6), solder passing by capillary action through the holes (6). <IMAGE>

Description

A SOLDERING PROCESS AND CIRCUIT BOARD ADAPTED FOR USE THEREIN The present invention relates to a soldering process and to a circuit board adapted for use in the said soldering process.

Surface mounted components are designed to be as small as possible in order to offer the highest components density on a printed circuit board. Consequently the leads of surface mounted components are relatively small and offer very little surface area for soldering. Furthermore, solder applied to the leads of the surface mounted components must provide a dual function, namely a mechanical connection and an electrical connection.

A known solder process for the connection of surface mounted components comprises a reflow process. This is a process of fusion of preplaced solder applied to the leads in the form of a cream or paste.

The cream or paste may be hot dipped onto the surface of the circuit board, or applied as an electroplated coating. One disadvantage of this process is that heat is applied in the process of soldering which can be potentially damaging to the surface mounted components.

Another known solder process comprises a wave soldering process, whereby molten solder is permitted to flow across the surface mounted components. The printed circuit board with the surface mounted components positioned underneath may be passed over a wave of solder. The wave soldering process suffers from two disadvantages. Firstly, certain designs of circuit boards result in some of the leads being obscured from the direction of the solder and therefore being inadequately soldered. Secondly, the surface mounted components are exposed to thermal shock due to the molten solder flowing in such close proximity.

The object of the present invention is, therefore, to provide a soldering process for substantially alleviating the aforementiorled problems.

According to the present invention, there is provided a soldering process comprising mounting at least one surface mounted component on a printed circuit board (pcb) having at least one plated through hole, whereby for each lead of the surface mounted component there is at least one plated through hole, and subjecting the pcb to solder on the side remote from the surface mounted component, thereby enabling solder to flow via the plated through hole to each lead of the surface mounted component.

Also according to the present invention, there is provided a printed circuit board adapted to receive at least one surface mounted component, comprising a number of plated through holes, such that for each lead of the surface mounted component there is at least one plated through hole, thus enabling wave soldering of the leads via the plated through hole.

The present invention will now further be described with reference to the accompanying drawing; of which Figure 1 is a schematic diagram of a printed circuit board according to the present invention; and, Figure 2 is a schematic diagram of part of a printed circuit board according to the present invention incorporating modifications to the said pcb.

In Figure 1 there is shown, a surface mounted component 2 adhered to a printed circuit board (p.c.b.) 4. The pcb 4 is provided with a plated through hole 6. For each plated through hole 6 tl is a respective lead 8 of the surface mounted component 2. Th.

dimensions of the plated through hole 6 and the position of the 8 in relation thereto, is such that solder from the underside of the pcb 4 is capable of passing up through the plated through hole 6 by capillary action, thus affording solder to the lead 8.

During the soldering process, the solder is introduced to the underside of the pcb 4 by passing the pcb 4 across a wave of solder 10. The solder wave 10 is formed by pumping molten solder up through a slot (not shown) across the path of the feed and above the level of the solder bath (not shown). The molten solder passses underneath the pcb 4 at such a rate to enable solder to pass up through the plated through the hole 6 whilst preventing any "icicle" or "stalactites" from forming on the underside of the pcb 4.

Since the molten solder is not in direct contact with the surface mounted components 2, the possibility of damaging the surface mounted components 2 due to thermal shock is obviated.

Furthermore, this soldering process enables solder to extend to all of the leads 8 of the surface mounted components 2 regardless of the topography of the pcb 4.

The plated through hole 6 is positioned with respect to the lead 8 so as to permit solder to pass via the plated through hole 6 in order to connect with the lead 8. Thus the lead 8 may terminate at the perimeter of the plated through hole 6, as shown in Figure 2a.

However, the lead 8 may terminate over the plated through hole 6, thus providing an overhang of the said lead 8 as shown in Figure 1.

Alternatively the lead 8 may pass completely over the plated through hole 6 as shown in Figure 2b provided it does not creat gas-tight seal for the hole 6.

The foregoing description of the present invention has be.

given by way of example only and modifications may be made without departing from the scope of the present invention. For instance, the size and shape of the plated through hole may vary but it must be able to permit solder to be drawn through by capillary action. Furthermore, for each lead 8, there may be more than one respective plated through hole 8, thereby providing increased mechanical and electrical connection.

Claims

1. A soldering process comprising mounting at least one surf.~ mounted component on a printed circuit board (pcb) having at rt one plated through hole, whereby for each lead of the surface mounted component there is at least one plated through hole, and subjecting the pcb to solder on the side remote from the surface mounted component, thereby enabling solder to flow via the plated through hole to each lead of the surface mounted component.

2. A soldering process as claimed in Claim 1, whereby the p.c.b is subjected to a wave of solder.

3. A printed circuit board adapted to receive at least one surface mounted component, comprising a number of plated through holes, such that for each lead of the surface mounted component there is at least one plated through hole, thus enabling wave soldering of the leads via the plated through hole.

4. A printed circuit board as claimed in claim 3, wherein the plated through hole is dimensioned to enable solder to flow through the plated through hole under capillary forces.

5. A soldering process substantially as hereinbefore described.

6. A printed circuit board substantially as hereinbefore described.

7. A printed circuit board substantially as hereinbefore described with reference to the accompanying drawing.