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November 2007

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Subject:
From:
John Burke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Burke <[log in to unmask]>
Date:
Mon, 12 Nov 2007 14:06:01 -0800
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text/plain (81 lines)
 Couple of things - did you talk to Loctite?

You do not mention your cure schedule - the data sheet says 

"TYPICAL CURING PERFORMANCE
Suggested minimum heat cure conditions for either IR or
Convection oven is 15 minutes at 150ºC or 7 minutes at 165ºC.
It is essential that the bond lines attain these temperatures for
the stated times. Cure rate is very dependent on the oven type
used, the size and geometry of the parts being bonded as well
as their composition"


Also I too believe that that peak temperature is low - is that temp at the
BGA ball?

 
 
John Burke
 
(408) 515 4992

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Monday, November 12, 2007 12:34 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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