Grant,

Without actually seeing the boards I cannot be 100 percent.... but

Passing this around the table it is thought it could be incomplete mask cure
from incomplete volatile excursion or incomplete cross polymerization. These
can be cleared up with a double pass through a UV tunnel at correct intensity
and belt speed.  Relative to conformal coating, I have assembled and coated
parts of this description without any problems; adhesion, outgas, or
otherwise.  This would be the nice scenario.
A worse case condition is the boards were not completely dried after cleaning
prior to soldermask.  The result being entrapments being forced out the vias.
 The final bake (or boiling in this case) is especially hard on tented,
plugged or flooded vias will exhibit the "hues" or halo's, worsening to
actual blistering.  In this condition it is the call of the board shop QA
prior to shipping.
One "cure" test is cotton swab with iso-al; green denotes cure issue.
The adhesion test: 3/4" wide tape pull at 90 deg with force.

One last thing, with the decreasing thickness of solder mask there can be an
absence or decrease of actual pigment on top of lands (traces, vias, etc).
If your discoloration includes flat areas this is probably not the case.

Boston Brad
781 858 0783

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