Solder mask vs non solder mask defined pads

The impact is low on larger pitches and pad sizes. as things get to the .4mm pitch the pad differences start affect your ball size and yields. The trace size going into the pads and the solder mask relief can cause some odd shaped pads with significant differences in surface area. It is best to make the pads larger and make them SMD for uniformity. The corner where the mask and pad meet can cause some concerns as crack starters so you will need to evaluate your application and do the testing to be sure that is not a problem. Good process control on the mask removal at your fabricator can also impact that.

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The size of the pad changes among the two. A solder mask defined pad will always be larger than a copper defined by your mask expansion setting. This will affect paste volume, if your standard paste aperture is designed for copper defined pads you may not have enough solder volume if wetting action on the pad surface acts first and spreads before reaching the ball. Also, thermal uniformity is important under the component to ensure even and sufficient heat profiles for each ball. Embedding the pad in copper will certainly affect reflow results. Consider thermally relieving the pad, if you connect two spokes that are 50% the pad width you have already equaled the maximum current capacity to the terminal.

This message was posted the Electronics Forum @ SMTA

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