Soldermask Expansion is the value applied to SMD pads
Soldermask is a protective layer for PCB boards. But the pads for components assembling on the circuit boards cannot be covered with solder mask ink. These pads part should be avoid applying soldermask.
Soldermask Expansion - the value applied to the initial pad/via shape to obtain the final shape on the solder mask layer. Enter a positive value to expand the initial pad/via shape, enter a negative value to contract it. It is the expanded distance between SMT pads and solder mask layer. In the production of printed circuit board there should be some space for pads on solder mask layer, which avoid the overlapping of solder mask layer and pads. With PCB designs, you need to make sure that the solder mask is never on top of SMD pads. The solder mask expansion standard is 4mil.
Soldermask Expansion
In practice, things never align perfectly. The actual holes punched in the solder mask may be slightly less than what you specified ("shrinkage"), and the holes are always be placed in a slightly different location than what you specified ("movement"). If the solder mask expansion is too small, then these misalignments cause the solder mask to partially or completely overlap SMT pads and through-hole pads, perhaps causing uneven placement of the components on the board. If the pads are covered with the solder mask, the SMT part will be disconnected from the pads. If the solder mask expansion is too big, it won't prevent neighboring pads from bridging during soldering.
