Are you sure there is anything there? It may be an optical effect due to 
the refraction of the coating. In investigating, note the orientation 
effect: more pronounced N-S and less so E-W. If there is something 
there, this may give you a clue as to its origin. If it is optical, 
reorient the lighting (or the sample) and see whether the max/min moves 
with it.

Brian

On 03.10.2013 03:18, Lum Wee Mei wrote:
> Steve has kindly post this picture for me.
>
> Anyone saw such phenomena before on delivered bare boards? Are those grey matter found on recess between solder pads and soldermask acceptable? My PCB supplier said that since the PCBs passed isolation test, those residues should not pose any problem. In addition, as IPC has no such reference in 6012 and 600, my PCB supplier suggest that I use them as UAI. I am concern about long term effect on reliability after assembly, especially such PCBs will be used for space application. Your expertise and opinion will be greatly appreciated.
>
> Thanks and Regards,
> ~wee mei~
>
> "Successful people aren't born that way. They become successful by establishing the habit of doing things unsuccessful people don't like to do. The successful people don't always like these things themselves; they just get on and do them." - William M Thackeray
>
> From: Steve Gregory [mailto:[log in to unmask]]
> Sent: Thursday, 3 October 2013 2:46 AM
> To: Lum Wee Mei
> Subject: RE: Grey Matter
>
> Hi Wee Mei!
>
> Hmmm, not so sure, depends on what it is? Is this on bare boards that have not been assembled yet? Have you tried cleaning it?
>
> Anyways, I have your picture posted, it's here: http://stevezeva.homestead.com/LumWeeMei.JPG
>
> Maybe you can get some good input from the group from someone who might have seen this before...
>
> Kind regards,
>
> Steve
>
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