Bob,

Be afraid, be very afraid.  The BGA should collapse slightly if it's
properly reflowed.  Now you will have solder balls touching the traces
with just solder mask separating it.  I know that I've read solder mask
cannot be used as a insulator, but I do not remember where.

Jerry Dengler
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Croslin, Robert
Sent: Tuesday, May 11, 2010 3:07 PM
To: [log in to unmask]
Subject: [TN] Removing BGA Pads for Routing Clearance

Dear 'Netters,

I have an engineer laying out a very small board using a .050 pitch 113
pin BGA with two rows.  He's trying to rout out from the inside row and
in order to avoid using a via, wants to delete the two adjacent pads for
routing space.  If he does, the balls that would have met with those
pads will have about a 2 mil overlay on the routed trace.  We'll be
using an LPI mask and I've told him he could risk bridging from the
solder bump should the mask crack during reflow.  He then told me our
former designer had used that trick before, but wasn't sure on which
product.  We've never had a field failure related to that condition, but
it sure made me cringe.  

My question is how concerned should I be with this arrangement?  


Bob Croslin
The Nielsen Company


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