Non-SMOBC-soldermask? If soldermask is not SMOBC (Solder Mask Over Bare Copper), then the solder will easily creep under the solder mask and cause "swelling". Difference is that SMOBC is put on the PCB before pads are plated, (thus no plating between solder mask and copper), but non-SMOBC boards will get plating for whole PCB and solder mask is put there only after plating.
Regards,
Janne