Do you have an image of the other side of the TH, the secondary side? I have never seen solder resist perfectly inside just the hole without it being on the outer ring. 

Ted T.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, November 10, 2015 12:32 PM
To: [log in to unmask]
Subject: [TN] Unusual solder mask defect...

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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