It could be the functional test itself. Perhaps the board is fine but the
test is flakey. 
Do the board fail your functional test on return? Does reflow result in a
pass? 
Picture this: boards pass your test. Boards fail their test. Presume they
are the same test. Return boards . . . Reflow, pass test. Send back, pass
tests on customer's site, pure coincidence. 

Possible issues, program problems at functional test, clock problems on
functional testers. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rudolph yu
Sent: Monday, December 22, 2008 1:35 PM
To: [log in to unmask]
Subject: [TN] What could be the root cause for a failed board that passed
functiona test after going through reflow for the 2nd time?

Hi:
 
We have a proto assembly that passed the functional test initially.  But
after we shipped it to the customer, it failed at the their site for the
same kind of test.  After we received the board back, we reflowed it using
the same reflow profile, and the board would funcation once again... 
We checked the boards under 5DX  after we received it back from the customer
and found no defects on it.
 
What could be causing this? and why the assembly would pass the test in the
first place? 
 
Here iare couple areas that I could think of...but not sure if they are
valid.
 
1.  Certain parts (microBGA) may be better off to have underfill underneath
for thermal mismatch? ( Need to check spec) 2.  During handling and
shipping, the board may be stressed to a degree that a microfacture is
formed.
 
 What other areas could cause this problem.
 
Thanks
Rudolph 
 
 
 
 
 
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