Component Footprint or Pad - Jargon Buster
18 February 2008
Surface mount and area array pads need to be a particular shape that corresponds to the size and shape of the component termination which is to be mounted on it. The proper shape and size of the pads will allow satisfactory, printing, placement and soldering joint formation to meet relevant IPC inspection and reliability criteria.
IPC 610D specification provides guidelines for all the most common joint terminations on both reflow and wave soldering. As a general guide reflow pads will be smaller than those designed for wave soldering, generally the length of the pads will be increased for wave to improve soldering yield.
All device pads should be the same size and be thermally isolated from other connection points for the same device. Example guidelines provided on the SMART Group CD-ROM and are based on the IEC/JEDEC component specifications which may not be suitable for all devices of a similar pitch and package size or from different suppliers. There should ideally be no via holes located directly in the pad surface unless the size will not allow the solder paste to starve the joint during reflow. Large vias should be separated from a pad by a minimum of 0.010" to prevent solder reflowing down the via. Separation distances of less that 0.010” does not allow the effective use of solder resist preventing solder loss. Solder mask clearance from the edge of a solder or test pad is normally 0.003”.
Generally placing vias in pads should be avoided as they lead to loss of solder, voiding, open joints and contamination. Using micro vias of 0.2mm 0.008” or less is becoming more common practice on some boards and should be reviewed separately. Working with your PCB supplier there are often ways of filling the via making use of via in pad practical.
A selection of design rules and pad sizes are often supplied by component manufactures. It is important to check where these design rules have come from, if any actual production trials were conducted and that the rules are proven in manufacture. Consider actually measuring the component and the terminations, it may seem like a lot of work but there is no better way of confirming the accuracy of footprints. One of the most common process defects “Solder Beading” during reflow could be eliminated if pads were designed the correct size.
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