Whereas Werner mentioned testing at an elevated temperature, my instinct
would have been to test it at lower temperature, in order to see if you can
make it fail due to shrinkage, if in fact you have a microcrack.
Pressing lightly on a suspect component can sometimes give a failed unit
continuity to pass
I just went through a situation in which a failed unit would pass after
reheating the connection but after the unit had fully cooled to ambient, it
would fail again.
Initially the SMT process was blamed, but after closely evaluating the
solder joints of the LGA in question, we determined this was a test limit
issue relative to temperature, and nothing was wrong with the solder joints
Cross section analysis (I know, destructive) is what most would do if we
want to determine the root cause....otherwise we are just guessing.
Phil
At 01:50 PM 12/22/2008 -0500, Werner engelmaier wrote:
> 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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