Soldermask Expansion

Posted on September 8, 2017
Tags: making, electronics

I’ve recently been designing some circuit boards with KiCad and then sending them off to OSH Park to be fabricated.

However, I ran into a little problem:

Missing bits of soldermask

I have a track running close to some pads, and there is a little bit of soldermask missing from the track as it passes by each pad. When I solder the connector in place, the solder bridges to that little bit of exposed track, creating a short circuit.

OSH Park responded:

Oh, the easy fix here is to reduce your mask expansion. We recommend 2 mil. When the mask expansion is larger than this, it can sometimes wind up exposing nearby traces. It’s sadly a problem that design tools don’t really warn you about, and a lot of folks simply have to discover this on their own.

In PCBNew, the mask expansion is under “Pads Mask Clearance” on the “Dimensions” menu. I’ve suggested that OSH Park add this to their page on how to set up KiCad for OSH Park. The default mask expansion in KiCad is 7.87 mil.

Pads Mask Clearance dialog