SUMMARY

Apparent FET failure due to mechanical stress from dimple on mounting clamp

Has anyone else any experience of this ???

+++++++++++++++++++++++++++++++++

DETAIL:


I have experienced an apparent FET failure mode caused by mechanical stress
that I have not encountered before.
Has anyone else experienced this or similar.

I have a design with two TO220 package MOSFETS which are mounted on a common
heatsink. The customer specified the mounting hardware and supplied samples
for prototype assembly. Silicon rubber thermally conductive but electrically
isolating pads are placed between each FET and the heatsink and the two FETS
are held against the  heatsink by a clamping bar which has a single tension
screw midway between the FETs.
                       SS
      CCCCCCSCCCCCCCC
          \/            S                   \/
       FFF         S              FFF                        Side View
  HHHHHHHHSHHHHHHHHHHHHH
  HHHHHHHHSHHHHHHHHHHHHH
                     NSN

HHHH = heatsink
CCCC= clamping bar
F = FETs
S = screw
N = nut

\/ = dimple in mounting clamp bar



HHHHHHHHHHHHHHHHHH
HHHFFFHHHHHFFFHHHHH   Top View
HHCCCCCCCCCCCHHHH
HHCCCCCSCCCCHHHHH
HHHFFFHHHHHFFFHHHHH
         |  |  |               |  |  |

There is a "dimple" in the clamp bar where it rests on each FET so that it
makes point contact to the FET body.
The dimple is punched in the clamp bar and while not sharp it concentrates
the force in a small area.

During testing, one PCB exhibited a Drain-Source short in one FET as
measured by meter in circuit and demonstrated by circuit (non) performance.
On removing the FET it was found to be non-short. Curious as to cause I
re-inserted the same FET and after assembling the circuit the short circuit
reappeared. On dismantly the short again vanished. Using a new FET cured the
problem.

It APPEARS that the localised clamping pressure is causing internal
shorting, possibly due to mechanical flexure.

This was originally  not a problem so some degree of "creep" with time and
or temperature may have occurred.

A friend simultaneously reported similar problems with a power IGBT actually
splitting across the mechanical contact point when under heavy electrical
stress. The clamp bars were supplied by the same customer (different
project). He also reports similar occurrences of failure in a USA produced
product some years ago where  a vast number of failures occurred in the
field some time after production. .

I would be interested to hear from anyone with similar experiences.




      Russell McMahon
_____________________________

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.