Hi Steve-

Because of this issue, we try to eliminate mask beneath 0402 and smaller parts.  (If the designer is forced into routing between some of the pads because of route density, he owes the assembly team Mountain Dews or equivalent!)

Doesn't that double mask also cause printing issues?  I would think you'd get way too much solder deposited.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of stephen gregory
Sent: Sunday, July 14, 2013 11:03 PM
To: [log in to unmask]
Subject: [TN] Soldermask thickness causing 0402 tombstones?

Hi all,
 
Have any of you ever had thick solder mask cause you tombstones? I'm thinking that maybe that has happened to me. We built a board last week that we built before without any problems. It does have quite a few 0402's. We had a few random tombstones last build, but not anything like we had this time! Tombstones all over the board! Nothing changed from the last build except the datecode on the PCB's. They were supplied to us from our customer.  
 
I'm pretty sure that the board was double masked. The fab drawing calls out for SMOBC, and then when you look farther down in the notes section it calls out for a matte black mask. It looked to me that there may have been a green LPI first put down, then a matte black mask put over top of it. 
 
I don't have pictures now, but I'll get some tomorrow. I've found some links that talk about mask causing tombstones:http://blog.screamingcircuits.com/2007/11/soldermask-and.html
http://communities.mentor.com/mgcx/message/2726
 
So I know it can happen. I just want to check with you guys to find out if it happens often.
 
Thanks!
 
Steve

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