Hi All,

Seeing that there is a recent solder mask thread, I have a question about
double masking. We have an assembly that we're building that has a 15.7-mil
pitch QFP and 0201's on the assembly, so when I ordered my stencil I spec'd
.004 thick foil and 8-mil wide apertures for the QFP (since there's no
solder mask web between the pads) and 13-mil squares for the 0201 pads.

The 0201's did great but I had a ton of bridging on the QFP! That really
surprised me. I started looking at the PCB a little closer and noticed that
there was a double layer of solder mask on the board. Here's a couple of
photos:

https://photos.google.com/share/AF1QipNeiheoq_Go0K7zsQY_iF6vfJk_YNws-6UlCC6i3z7p5LHn-HSQ31rVj-Cl-EJ-xg?key=bUJEdkRsQzlaOUkzcWdLaXI3dEI2TV91V2lhbHlR

I inquired the board shop why there was two layers of mask on the board and
was told that because of the 2-oz. copper weight being called out on the
outer layers, it required two layers to get complete coverage and not have
it thin out on feature edges. Lines are at 8-mils and space is 6-mils. That
doesn't seem too extreme to me. Is that really true? I've built boards
before that had way more than 2-oz. copper that didn't require double
layers of the mask to ensure good coverage, but those weren't boards that
had 15-7-mil pitch devices and 0201's on them either.

-- 
Steve Gregory
Kimco Design and Manufacturing
Process Engineer
(208) 322-0500 Ext. -3133

-- 



This email and any attachments are only 
for use by the intended 
recipient(s) and may contain legally privileged,
 confidential, proprietary 
or otherwise private information. Any 
unauthorized use, reproduction, 
dissemination, distribution or other 
disclosure of the contents of this 
e-mail or its attachments is strictly
 prohibited. If you have received 
this email in error, please notify the
 sender immediately and delete the 
original.