Virtually any solder mask will bubble and flake when covering
     tin/lead. The solder melts under the mask and flows/puddles. During
     the melting, most adhesion to the liquid is lost. Since the mask
     should be fully cured by then, full strength adhesion is impossible,
     and non-adhesion can occur.

     I suspect that you also have solder balls/shorts on the sides of some
     traces, if you look hard enough.

     Go to HASL or OSP. Either option should eliminate this specific
     problem.

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Subject: [TECHNET] Solder mask material
Author:  JOHN OSULLIVAN <[log in to unmask]> at INTERNET