Hi Tom!

Have you done destructive analysis?  You simply take a diamond burr, a
Dremel tool or equivalent, and a few drops of water and carefully grind
through the failed device to get down to the interface without affecting
the failed solder joint.  Studying that joint usually shows what the
problem is pretty quickly.

Also, is there any pattern to the failures with respect to the BGA
geometry or the geometry of the pads it is being soldered too?

Wayne Thayer 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Monday, November 12, 2007 3:34 PM
To: [log in to unmask]
Subject: [TN] BGA failures after OMPAC underfill

We have a problem - we are using Loctite 3568 to underfill a 19mm
plastic BGA- 256 pin 1 mm pitch eutectic solder BGA. We test the board
before underfill and after (after underfill and conformal coat). We are
seeing about 10 percent of the BGAs failing after the second functional
test- the same part passes the first test but will fail the second.  The
boards fail for open solder joints- when heated with a heat gun they
work- intermittent open that seems to work when the part is heated
(expands) 
 
I have reviewed the process but can find no smoking guns that might
point to the root cause of the problem. THe BGAs are all baked out prior
to reflow, THe reflow profile peak temp is adequate- a little on the
cool side for me- but the time above reflow doesn't seem excessive. THe
boards are not subjected to any other mechancial fastening operations
that might subject the BGA joints to strain. The board is not routed out
after assembly. 
 
U10 is the BGA in question. THe peak temp is 212 C and is above reflow
for 71 seconds. Note the part is MSL Class 3 but the parts are baked out
and then stored in sealed Moisture Barrier Bags purged with nitrogen.
 
Anyone else run into something like this before? I have run into
situations where conformal coat under BGAs would expand and stress and
crack the solder joints but I would think that an underfill would be
designed to avoid this situation.
 
Thanks
Tom

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