This is standard FR4 with immersion silver finish.
  The standard soldermask colour we spec is clear, not green.  Allows us to better see the copper, and often some inner layer features as well, but if you aren't used to working with FR4 with clear soldermask, it can definitely look a little strange, at first.
  The dark colour seen is due to an inner layer that is primarily ground plane.
  I'm pretty sure that it isn't soldermask bubbling
  We have not yet done any microsectioning.  So far we have only seen this on fully populated boards that have experienced some hand-soldering, and we are reluctant to sacrifice one, if it can be saved for production.
  I have a blank two-up panel available from each manufacturer we've used.  We are planning to run some hand-soldering tests.  Vary temperatures, one unbaked and one baked from each panel, etc.  Any other ideas?


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