PCB design rule: solder mask expansion

Since I've been writing about PCB via size rules, I thought that I'd discuss another rule:  solder mask expansion.  Solder mask is a thin layer of polymer used to prevent solder bridges and is usually green [wikipedia].  OSH Park boards use purple solder mask.

Solder masks are applied to the entire surface of a PCB except where pads are located.  In a perfect world the solder mask opening would be the exact size of the pad.  In reality the solder mask application will be offset a bit from the pad.  With a pad-sized opening, solder masks offsets would cause some of the pad to be covered by the mask.  To prevent this PCB layout tools define a 'Solder Mask Expansion' Rule which defines how much larger the opening should be than a pad.

Since I couldn't find a solder mask expansion rule on OSH Park's website, I had to take a guess.  I used a 4 mil expansion rule on bityExpress-B and a 2 mil expansion rule on a other design.  Check out the pictures below for some examples of how the offset can vary.  Based on this limited sample set it looks like a 2 mil expansion rule might be right for OSH Park.  I'll keep tracking the accuracy of the mask on my next few orders and see if this holds.

0.8mm BGA pads, 2 mil solder mask expansion, board #1

0.8mm BGA pads, 2 mil solder mask expansion, board #2

0.8mm BGA pads, 2 mil solder mask expansion, board #3

1.0mm BGA pads, 4 mil solder mask expansion, board #1

1.0mm BGA pads, 4 mil solder mask expansion, board #2