A cross section of the "pimples" should help isolate the cause.

If there is copper in the pimple - the problem goes all the way back to board
fab - most likely strip and etch.  These would have been tight geometries, so
incomplete stripping of image film, and resulting marginal etching would
leave these copper bumps.  They are now covered with nickel.

If there is no copper in the bumps, the nickel plating is implicated.  It
would help to know if it is electroplate or electroless nickel.  I suspect
electroless - as the pads seem to be for a peripheral lead device.  Thus they
would have been nickel plated after etching of circuits. Over activation, or
dried on activator from an interrupted process sequence might give this
pattern.  But, I would expect even more extraneous plate - even on top of the
mask dams, perhaps.  On the chance they were electroplated at the start of
the fab cycle, "burning" is the creation of electroplated nodules when the
electroplate current density is too high.

Do you know the fabrication sequence? Can you get a cross section of a
"pimple"?

Denny Fritz
MacDermid

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