We have a PCB that two traces have shorted and blown a set of traces on the PCBs.  The trace width and spacing is correct for 150 V and is not a new design.  We recently changed PCB shops.  We found some residual moisture about 1/8" square in close proximity of two traces after assembly and after test.  The traces were entirely covered with LPI soldermask.  

I understand that the moisture should be removed prior to test.  My question is shouldn't the soldermask have insulated the two traces from the moisture?

Scott

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