[s-cars] POS failure analysis

[Mike Sylvester]

A colleague and I performed a failure analysis on my failed POS.

1st we did some non-destructive testing.
x-ray didn't reveal anything except mildly interesting photo.
The curve tracer confirmed that ports 1 and 3 were OK and port 2 was open circuit.

The part was then opened up.

Inside are 3 FETs or IGBTs potted in some very soft clear material, almost liquid.

The dies look OK, so the failure is not from an overheating condition.

Under magnification we found that a bond wire failed where it connects to the output terminal.

This failure is most likely from thermal shock  ( Rapid temperature change) or fatigue.

As someone else had mentioned, hitting this part with water when washing your engine could rapidly cool the part.


This finding does not totally discount the theory of poor heat dissipation.

It is possible that the bond wire joint was marginal from the factory, but if the heat dissipation was poor due to lack 
of thermal paste the internal temperature extremes may have fatigued the bond wire joint over time.


I am curious if this is the common failure.  If anyone has a bad POS that they would be willing to give up, I could take a look at it.

Mike Sylvester

P.S.

This weekend I will post the photos of the FA to my website.