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September 1998

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Date:
Tue, 15 Sep 1998 15:54:53 EDT
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 Hi Ken:
- Although Werner could be correct about the possibility of poor solder
interconnect, another thing to consider is that the mass of the component is
being restrained during shock test solely by the compliance of the leads and
the movement of the component body is exceeding the shear capability of the
connection.  If you are satisfied that connection integrity is adequate, you
might try simply restraining the component body with a dot of adhesive under
the component (watch the z-axis CTE of the adhesive).
- An obvious, but frequently overlooked, source of trouble could be your shock
test fixture and system calibration.  Go over the fixture carefully to assure
that it is holding the specimen satisfactorily and check the calibration of
test instrumentation.
- Another question is the type/direction of the applied shock pulse (per
TM-650 or some other method?).  Is the shock applied to the individual pwa, or
to the end item.  Seems to me that a half sinewave, 11 ms pulse at 20g peak
would be sufficient for qualification testing of most products.  I presume
that repetitive shock is applied in all 3 primary axis.  You might be able to
determine which axis is doing the most damage by simply using an optical
microscope to examine the specimen after exposure in each axis.  Additionally,
you might benefit from putting a small tri-axial accelerometer on the specimen
and measuring the cross-axis transmissibility ratios during shock to detect
any ringing (reflex pulsing or oil-canning) that is taking place.
- Guess you have probably already done most of the above, but thought I'd
throw in my tupence.
Regards, Jim Moffitt

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