Ingemar:

Consider what could reasonably be in the via holes:

1. Residual solder mask.
2. Residual nomenclature ink.
3. Misc. cleaning solutions.

1. SOLDERMASK
Presuming the solder mask is an LPI of the common type, you're looking a
polymers. Vias are the toughest part of the PWB to remove unwanted solder
mask due to their relatively small size. The "bubbling", however, bothers me
-- even if it were uncured solder mask (these vias aren't plugged on one side
by any chance?). Time for a micro-section view.

2. NOMENCLATURE
Presuming your PWB has nomenclature, there is a solvent-based ink that may be
in the holes as a residual. The ink could be in the vias due to original
design placement, or the PWB vendor may have had a "wash-down", whereby the
first attempt to screen the ink failed and the panels were washed in a
solvent to remove the uncured ink, but perhaps the panels were not thoroughly
cleaned and dried (again, the vias are most difficult to cleanse).

3. CLEANING SOLUTIONS
Alcohol is a common cleaner, but again I wouldn't expect it to "bubble".
There's a whole world of specialty cleaners on the market so that I won't
even begin to examine those.

=The Big Guess=
Because you mentioned the PWB finish is gold we can discount HASL, OSP and
other sources of mischief. Because you described it as polymeric (I'm
thinking solder mask) -- and because you said there was too little of it to
analyze (I'm thinking residual), my first suspect is Residual Solder Mask (if
the plated hole integrity is sound).

Please tell us what you ultimately discover!

Bob Lazzara
Circuit Connect, Inc.

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