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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