[s-cars] More POS Failure Analysis

[Paul Gailus]

Mike,

Thanks very much for doing all this failure analysis work!

It's interesting that the failure was not the more typical
bond wire lift-off at the die bond, but it was a fracture of
the bond wire itself near the package pin wedge bond.

I'm guessing that the failure was due to metal fatigue
from differential expansion between the package, substrate
and bond wire. This would happen over a large number of
temperature cycles. The fact that the break occurred right
at the heel of the bond, where the maximum stress would
be located, reinforces this hypothesis. A misadjusted wedge
bonding machine could also accentuate the stress
concentration at this point.

I'm assuming that the package pin near the failure is
still firmly engaged in the plastic package so that it
can't vibrate or move.

I definitely agree that keeping the maximum temperature
down as much as possible using new thermal compound
on the POS's should help reduce fatigue and other failures.
But just  make sure that it's only a very thin layer, just to fill
any voids.

Thanks again,
Paul


----- Original Message -----
From: Mike Sylvester <msylvester at verizon.net>
To: <s-car-list at audifans.com>
Sent: Friday, March 12, 2004 8:20 PM
Subject: [s-cars] More POS Failure Analysis


I've looked at this a little more and am backing off the overheating, at
least in the case of my failure.
I've updated my site with another photo.
http://mysite.verizon.net/vze6ngsr/S6POS.htm

This photo was taken using the microscope.
You can see the break.
I'm told buy our component engineer that this is a classis bond wire failure
due to a process problem when the bond wire was attached.

I'm very interested to know if this is how they are all failing.

Mike Sylvester
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