GB2313961A

GB2313961A - Preventing dislocation of surface mounted components during soldering

Info

Publication number: GB2313961A
Application number: GB9612024A
Authority: GB
Inventors: Kevin Richard Keane; Paul Stephen Collis
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1996-06-08
Filing date: 1996-06-08
Publication date: 1997-12-10
Also published as: GB9612024D0

Abstract

A circuit board (9) comprising a solder pad arrangement for attaching a surface mount component to the circuit board (9). The solder pad arrangement is fixed to the circuit board (9) and includes at least one first portion (32) attached to the circuit board (9) for providing a mechanical anchorage of the surface mount component to the circuit board (9) and at least second and third portions (34), fixed to the circuit board and to the at least one first portion (32), projecting substantially away from the at least one first portion (32) and contiguous therewith, and defining at least one interstice therebetween. During reflow soldering of a component placed on the pad, opposing forces exerted by the holder on the first, second and third portions, so that the component does not become displaced from the pad.

Description

CIRCUIT BOARD Field of the Invention This invention relates to a circuit board, particularly but not exclusively to a solder pad of the circuit board for attaching mechanical parts to the circuit board.

Background of the Invention In an attempt to improve the efficiency and speed of manufacturing electrical goods, components are now often soldered to circuit boards during an automatic reflow soldering process. This reflow soldering process enables all of the components to be soldered onto the printed circuit board at the same time. The soldering process includes mechanical and electrical contacts.

The trend of reducing the size of electrical goods has led to the increased use of surface mount technology in the manufacturing process.

Surface mount components are positioned on top of the printed circuit board. They do not have feed through legs, as is the case with conventional devices, to assist in fixing the device to the printed circuit board during this reflow soldering process.

It has been recognised in the reflow soldering of surface mount components, that the surface mount components tend to move during the reflow soldering process. When a large solder pad arrangement is used to connect a large mass, or surface area, to the printed circuit board, a large amount of solder is required to fix the mass. When the solder is molten the solder tends to move the surface mount component, pulling the smaller mass (or surface area) part of the component, e.g. the legs, away from their electrical contact. A further problem associated with the reflow soldering of such surface mount components is the maintenance of a centred position of the component during the reflow soldering process. The viscous nature of the soldering process can cause the component to rotate or move in both the 'x' and/or 'y directions of the printed circuit board plane.

One solution to this problem is to use a glue on the device to fix it in place during reflow soldering. However, this method of glueing components to the printed circuit board typically requires manual intervention by applying the glue by hand to the desired position on the printed circuit board or surface mount component.

An automatic dispensing process could also be employed however it is an extra process step. This approach is very undesirable with regard to the speed and efficiency requirements of current automatic printed circuit board manufacturing.

A further solution has been to place a fixing clip to the printed circuit board to prevent any movement of the component. This is also undesirable as it adds an extra part count and hence, size and cost, to the production of the complete printed circuit board. In addition, to ensure the fixing is adequate, the fixing clip would need to be of a feedthrough arrangement, which may be contrary to the rest of the printed circuit board design and production methodology, thereby increasing cost and complexity to the process.

FIG. 1 shows a prior art approach to this problem. A rectangular solder pad 12 is used to solder, say, the casing of a crystal oscillator 10 to a printed circuit board to make a mechanical contact. Such a mechanical fixing arrangement is necessary to provide a better solder joint, thereby ensuring that the solder process does not cause the crystal body to be pulled away from the printed circuit board and hence, detaching the electrical leg contacts 14 from their solder pads 16.

Such a problem can occur with portable radios when the portable radio suffers a jarring effect, for example, when it is dropped from a height, thereby exerting increased "dislocation" forces on the joint, and in particular, the larger massed components. In addition, the operation of the crystal is sensitive and may well be affected by slight movements of the crystal position, with respect to the circuit board, e.g. if the body joint is broken then the component will vibrate against the printed circuit board and damage the crystal.

Thus, there is a need to provide a circuit board having an improved solder pad arrangement for fixing surface mount components whilst minimising undesirable pull-forces, and hence movement of the components, during reflow soldering processes.

Summary of the Invention According to the preferred embodiment of the invention there is provided a circuit board comprising a solder pad arrangement for attaching a surface mount component to the circuit board. The solder pad arrangement being fixed to the circuit board and including at least one first portion attached to the circuit board for providing a mechanical anchorage of the surface mount component to the circuit board and at least second and third portions fixed to the circuit board and to the at least one first portion, projecting substantially away from the at least one first portion and contiguous therewith, and defining at least one interstice therebetween.

By providing a solder pad arrangement with second and third portions having an interstice therebetween, the surface mount component is maintained substantially at the same position at which it is placed during a reflow soldering process. Thus, electrical and/or mechanical contacts are maintained between the surface mount component and the printed circuit board via the solder pad arrangement.

A further advantage of the circuit board having the solder pad arrangement according to the preferred embodiment of the invention is that when the surface mount component has an uneven weight distribution any undesired pull-effect of the surface mount component due to the uneven weight distribution during the re-flow soldering process is reduced.

In addition, when the surface mount component has at least one mechanical contact connected via a first solder pad arrangement to the printed circuit board, and at least one electrical contact connected via a second solder pad to the printed circuit board, the first solder pad arrangement is so designed as to reduce the pull-effect of the at least one mechanical contact affecting the location of the at least one electrical contact during the reflow soldering process.

Specific embodiments of the invention will now be described, by way of example only, with reference to the drawings.

Brief Description of the Drawings FIG. 1 shows a prior art solder pad arrangement of a circuit board for fixing a surface mount component; FIG's 2a-2c shows a series of solder pad arrangements of a circuit board that are improvements over the prior art solder pad arrangement in accordance with embodiments of the invention; and FIG. 3 shows a solder pad arrangement of a circuit board in accordance with a preferred embodiment of the invention.

Detailed Description of the Drawings Referring first to FIG. 2, a series of solder pad arrangements are shown that provide an improvement over the prior art solder pad arrangement of FIG. 1. FIG. 2a shows a duplicative two-part solder pad with a first part 20 of smaller surface area being connected to a second part 22 of larger surface area.

In operation, the arrangement of the solder pad and hence the distribution of the solder, in comparison to the prior art design of FIG. 1, restricts the surface area for the molten solder to meniscus on. The restricted area of molten solder prevents the surface mount component moving to an undesired position during a reflow soldering process.

Similar improvements over the prior art using the distributed solder pad methodology are shown in FIG's 2b and 2c. FIG. 2b uses a number of solder-fingers to distribute the solder contact and FIG. 2c combines these fingers with the two part approach of FIG. 2a. The fingers 26 being the first part and the base 28 being the second part of the solder pad.

FIG. 3 shows a circuit board 9 having a solder pad arrangement in accordance with a preferred embodiment of the invention. The solder pad arrangement comprises a two-part form, in a comb arrangement - a first portion, the spine 32, for providing a mechanical anchorage of, the surface mount component to the printed circuit board, connected to a second portion, the fingers 34, preferably projecting away from the first portion, for reducing a pull-effect of the surface mount component during a re-flow soldering process. The solder pad is positioned in such a manner that the surface mount component, e.g. the crystal oscillator 10, is soldered primarily to the spine 32 part of the pad with the fingers 34 being so designed as to provide a meniscus of solder up the pad during the reflow solder process. To ensure the solder joint is adequate the fingers 34 are positioned in substantially the same plane as, and substantially perpendicular to, the spine 32, to allow the solder to meniscus up the fingers from the spine during the reflow soldering process.

In operation, solder paste is applied to the circuit board 9 to cover all of the tracks and solder pads. The solder paste is preferably applied to areas extending beyond the solder pads which is drawn onto the pads during the reflow soldering process to form a fillet.

Typically the reflow soldering process comprises of a bare board with a complete circuit board layout pad arrangement. Paste is applied to the pad of the board, usually with a simple screening (stencil and squeegee) process. The component is then placed in the paste which is a complex mixture of flux and solder alloy. The circuit board, paste and component are then fed through a reflow oven. The temperature I time graph for the heat input to the circuit board is controlled in the oven. The temperature is increased in a controlled manner so that the complete mass is increased above the liquidus temperature of the solder paste.

During the reflow process the flux acts as a cleaning agent to clean the component terminations and the circuit board pad. When the temperature is increased above liquidus the paste turns into a molten liquid. The terminations of the component and the pad on the circuit board are then "wetted" by the liquid. The liquid forms a fillet up the side of the component terminations if the pad on the circuit board is not large enough to drain away the solder from the joint. (N.B. the solder will only flow and attach to a surface that is solderable, e.g. gold, silver, copper. or another solder coated surface.) The pad and paste deposition is designed to put excess paste on the pad so that a fillet is created during reflow soldering, solder resist being used around the pad.

In the preferred embodiment of the invention, the spine of the solder pad is of rectangular shape with one side of the longer parallel sides being attached to each of the fingers. Each finger is substantially rectangular in shape. The width of the fingers 34 'x' is arranged to be half of the length of the fingers 34 and of the depth of the spine 32 each dimension being approximately '2x' in size.

It is within the contemplation of the invention that each finger does not need to be of equal dimensions, nor the relative dimensions given above.

In addition, it is within the contemplation of the invention, that the fingers 34 are arranged to project away from the spine 32 at any angle, with the perpendicular arrangement being the preferred embodiment only.

Further, it is within the contemplation of the invention that the improved solder pad is used for electrical as well as mechanical fixing arrangements.

When the surface mount component has an uneven weight distribution, a larger mass part would typically cause an undesired pulleffect of the surface mount component during a re-flow soldering operation. An uneven pull effect can also occur if the pad arrangement is not symmetrical. However, with the preferred solder pad arrangement, the surface mount device is now pulled to the desired final position for fixing when the solder solidifies on cooling. e.g. the component centres itself. In particular, when the surface mount component has a mechanical contact and an electrical contact with the mechanical contact area being of larger mass than the electrical contact area, the pull-effect of the mechanical contact does not adversely affect the location of the electrical contact of the surface mount component during the reflow soldering process.

Thus, an improved solder pad arrangement for fixing surface mount devices is provided. The improved solder pad provides self centring movement of the surface mount components during reflow soldering processes.

Claims

1. A circuit board comprising a solder pad arrangement for attaching a surface mount component to the circuit board, the solder pad arrangement being fixed to the circuit board and comprising: at least one first portion attached to the circuit board for providing a mechanical anchorage of the surface mount component to the circuit board; and at least second and third portions fixed to the circuit board and to the at least one first portion, projecting substantially away from the at least one first portion and contiguous therewith and defining at least one interstice therebetween.

2. A circuit board comprising a solder pad arrangement for reflow soldering of a surface mount component to a circuit board, the solder pad comprising at least one first portion and at least second and third portions defining at least one interstice therebetween and arranged such that during a reflow soldering process component dislocation forces exerted by solder at the at least one first portion are opposed by forces exerted by solder at the at least second and third portions.

3. The circuit board in accordance with any one of the preceding claims, wherein the at least second and third portions of the solder pad arrangement are substantially in a same plane as the at least one first portion.

4. The circuit board in accordance with any one of the preceding claims, wherein the surface mount component is so arranged such that it extends at least partly along the at least second and third portions thereby bridging across the at least one interstice.

5. A circuit board in accordance with any one of the preceding claims, wherein the at least second and third portions of the solder pad arrangement are a plurality of fingers projecting away from the at least one first portion such that the forces exerted on the surface mount component during the reflow soldering process are distributed between each of the fingers.

6. A circuit board in accordance with any one of the preceding claims, wherein the solder pad arrangement is comb-like having a spine and a plurality of fingers projecting away from the spine in a substantially perpendicular manner.

7. A circuit board in accordance with claim 6, wherein, the solder pad arrangement comprises a first quadrilateral portion and a second quadrilateral portion, a minor side of the second quadrilateral portion adjoining a major side of the first quadrilateral portion.

8. A circuit board in accordance with claim 7, wherein the first and second quadrilateral portions are rectangular in shape.

9. A circuit board in accordance with any one of the preceding claims, wherein a portion of the solder pad arrangement is at least one of the following: 'L' shaped or T shaped or 'U' shaped or w shaped.

10. A circuit board substantially as hereinbefore described with reference to, and as illustrated by, the solder pad arrangement of FIG. 2a or FIG. 2b or FIG. 2c or FIG. 3 of the drawings.