Hmmmm,

  Familiar Story:

  Board Works, Board Ships, Board Fails and is returned. Board works back at
factory.  You simply added a reflow cycle to "Fix" the problem.

  Next time you need to find out WHY a bad bard is failing, making it "Work"
with magic or reflow does not allow you to resolve the real problem.

Bob Kondner



  

-----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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