I agree with Ted. Clearly something happened to the solder mask which left
exposed metal. The defect occurred prior to application of the photo-defined
legend. They decided the best course of action was to cover the exposed
metal with "something handy". For class 3, they should have told you about
it and gotten a waiver for the rework. At least they didn't try to hide it!

By the way, for aerospace where you often have concern about particulates,
using photo-defined legend might not be the best path: If you cross section
it, you will note that there are very fine overhangs at the edges of the
letters due to the physics of the light-based polymerization process and the
extremely high loading of pigment in the coating. These overhangs can and do
break off occasionally. (But these overhangs certainly do contribute to a
very sharp-looking image!)

Not sure why the board designer used mask encroachment on the vias (I'm not
seeing any high density features in the limited area you shared), but those
can be a source of particulates as well (due to slight mis-matches causing
some amount of unsupported mask). Looks like there might be an area for that
kind of concern to the right of "R574".

Also, what is the surface finish? I know how hard it is to take pix of shiny
metal, but I'm not seeing yellow (indicating gold), and it looks too smooth
to be HASL and too shiny for ImAg.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Theodore J Tontis
Sent: Tuesday, June 02, 2015 7:09 PM
To: [log in to unmask]
Subject: Re: [TN] PCB blob defect

It looks like a solder resist repair and they used blue instead of green. It
would be interesting to find out when the repair happened, at the board
house or assembly house. It's difficult to tell because the silkscreen looks
to be repaired as well. 

Ted T.

Sent from my iPhone

> On Jun 2, 2015, at 5:56 PM, Watson, Howard August <[log in to unmask]>
wrote:
> 
> Hi all,
> 
> My PCB fab knowledge is limited, and a very unusual defect was discovered
(too late) in assembly. I'm wondering if anyone has seen something like this
before, or has an idea what this could be. The blue color seems noteworthy
to me. This is for IPC class III, space. I will return this to the
fabricator, but I wanted to get educated in case there is a debate regarding
the cause. Thanks for your help!
> 
> Steve put the pictures here:
> 
> http://stevezeva.homestead.com/Howards_Blob.jpg
> 
> http://stevezeva.homestead.com/Howards_Blob_close.jpg
> Howard A. Watson
> 
> ______________________________________________________________________
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