Topic

Pad design

Through-hole components

For through-hole components, the pad configuration is a compromise between several competing requirements. Board density and insulation resistance demand the use of the smallest outside diameter possible, whilst joint strength and reliability are best served if the pad is as large as possible to increase the copper peel strength. Easy soldering, however, requires a pad that is wide enough for repetitive wetting and not too wide for solder to drain away from the lead wire.

The following guidelines should be followed:

Other areas of good practice include:

Figure 1: Over-large through hole

Over-large through hole

Figure 2: Cracks in weakened area caused by an over-large hole

Cracks in weakened area caused by an over-large hole

Figure 3: C-cut pad to eliminate solder fill

C-cut pad to eliminate solder fill

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Surface-mount components

For SM components, as in through-hole designs, layout density favours the smallest area, while joint stability indicates the largest pad possible. Manufacturing tolerances, including adhesive application and component placement, also dictate the need for large pads. Pad dimensions for wave-soldering of surface-mounted assemblies should also ensure as large a contact area as possible, to reduce any problems of lack of consistent wetting. Because of the geometry of the components glued to the surface, there are areas where gaseous material can be trapped, causing skipping or misses on small pads: the larger the pad, the less likely this problem is to occur.

Printed wiring board layout rules for reflow soldering are controlled by three considerations:

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Other design considerations

Not only does the copper area of the pad need to be considered, but so must the design and quality of solder resist. Resist is used to define the solderable areas, and needs to be both well-defined and resistant to the soldering process conditions.

As well as being used to connect to components, contact areas may also be dedicated as test pads. The majority of automatic testing is carried out by placing the board on an array of spring-loaded-probes (bed of nails). These probes make contact with the test points which should preferably be separate pads, away from the component joint in order to prevent component damage.

Recommended land patterns and other design information can be found in the IPC-SM-782A Surface Mount Design & Land Pattern Standard. This includes pad layouts for all types of passive and active components, with guidelines for both wave and reflow soldering. Whilst this a relatively old document, it is supplemented by a Land Pattern Calculator at http://landpatterns.ipc.org/default.asp where registered (free) users can look up surface mount land patterns, or calculate their own.

This IPC standard and its predecessors have formed the basis of design practice throughout the world, being designed to ensure acceptable solder joints under a wide range of conditions. However, it must be said that they sometimes represent a relatively conservative view: as with all standards, you should be prepared to discover differences between in-house practice and the published information.

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