Hi All,

We just ran across a soldermask defect that I'm having trouble trying to
figure out how it may have been caused. We build a .093" thick 8-layer
assembly for a customer that is pretty picky as far as barrel fill is
concerned. So we started noticing a problem on one lead of a component that
we kept seeing no top solder fillet on that usually didn't have a problem.
Upon further investigation we saw that there was a solder mask ring down
inside the barrel:

http://stevezeva.homestead.com/Soldermask_plug.jpg

So then I pulled the bare boards that we had in stock and found this:

http://stevezeva.homestead.com/Soldermask_plug_2.jpg

I would say that at least 2/3rds of the boards I looked at had this
problem. What's kind of weird is that this mask ring would appear at the
top of the barrel and sometimes at the bottom, but always the same hole.
The mask is an LPI, and what I'm having a hard time understanding is that
the mask seems so well imaged around circumference of the pad, how does it
get cured inside the barrel like that? And why is the ring sometimes at the
top of the barrel and others down at the bottom?

I was able to clear the plug by carefully using a .033" drill bit under a
microscope so that I didn't damage anything else in the barrel. I double
checked the gerbers and there are no inner layer connections, the only
connection is the trace that you see on the outer layer in the photo.
Because of time constraints we could not send the boards back to the fab
vendor (even though I wanted to), but we put the next shipment on hold
until they inspected for the problem. Why doesn't it surprise me that they
said that they didn't see the problem? Sheesh...

So I'm just trying to understand what in the process could have caused
this, a scratch in the artwork maybe?

Steve

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