Hi Rudolph,
Sounds like you have a failure that only shows up under certain conditions---this could include fractured vias, pad cratering with trace fracture, pillow-on-head, brittle interfacial failures, etc.
You might try to test the assembly at elevated T [say 50C], or while slightly bending or vibrating it.

Werner


 


 

-----Original Message-----
From: Rudolph yu <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, 22 Dec 2008 1:34 pm
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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