BGA failure when chamber test

Hi It's me again. Our products need to pass a chamber test 45c/36hrs after the PCBA completed batch of functional tests and assemble in to case.

We found 1 unit was functioning for 24hours in the chamber, but hang after that. We tried to re-boot the unit but it fail. We had encounter this problem for few time (about 0.001% failure rate), we suspected is BGA failure, so we perform test as below:-

1) Remove the BGA from the board. 2) Re-ball the BGA and mount on a new board. 3) Take a new BGA and mount on the failure board. 4) Put these 2 units into chamber as test as normal. (These was done by gone through the same production re-flow oven)

Well, these 2 units was able to passed the chamber test which take 36hrs. Our curious point:-

1) If the BGA have solder crack, it should have detect at ICT and shouldn't able to sustain chamber test for 24hrs. 2) If the BGA defective, it should fail at the new board. 3) If other components on the board are defective, it should fail with new BGA. 4) We suspected the heat sink was not fit correctly, so we check the heat sink once we open the unit. But it was normal. 5) Re-flow oven and profile is same when we running for re-balled BGA.

I have no idea how to identify the problem, customer is desperately hope to see what is our founding. If anyone having the same problem please share your founding.

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