Hi, Rudolph
Can you provide a little more information on the prototype PWB?
Surface finish of the PWB?
How many layers and what type of PWB is it?
What is the uBGA?
If you suspect the uBGA solder joint is the culprit, your technician is
correct; 5Dx cannot detect a microcrack along the pad of either the uBGA
or between the solder ball and the PWB pad. Other types of X-rays may be
able to detect the crack however. Three that may are the Dage 7600,
Phoenix, or the YXLON.
I assume after you reflowed the assembly you fixed the issue, at least
for now.
Four other possible options next time this failure occurs are:
1. You may wish to perform Dye and Pry testing (see IPC 7095 for
information on this).
2. Werner's suggestion of hot testing is a good one, as either a cracked
solder connection or an internal open via should open up and begin
failing during the thermal expansion. Sometimes cold cycling will also
cause the failure to re-occur.
3. You may need to perform microsectioning through the suspect SJ. In
order to do this you will need to know exactly which node is failing.
4. Another option is to get a known good assembly that has passed all
testing and have the uBGA removed and replaced with a BGA socket. Get a
socket with crown pin contacts, not pin or coil spring contacts, because
then you can test uBGAs using your known good (golden) assembly with or
without the solder balls attached. After the socket is soldered to the
golden assembly, put the original uBGA in the socket and verify the
golden assembly is in fact still golden. If it is, use the golden
assembly as a troubleshooting tool to test suspect BGAs removed from
production units in the future, and to screen the incoming uBGAs prior
to assembly.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rudolph yu
Sent: Monday, December 22, 2008 12:35 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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