Jim, 

Most silicon strain gauges are based on diffused resistors. The shift in resistance, is the measurable
parameter. 

As these resistors vary in value from die to die let alone wafer lot to wafer lot, it is necessary 
to measure and store the strain gauge values of each die, prior to packaging, and again at whatever
point in the process interests you. This necessitates uniquely tracking each die/package, in order to
compare the pre Vs post values, which can be difficult. If possible, try to procure die with onboard 
traceability eg PROM which uniquely identifies that die. This would allow you to process your die/BGA's 
without having to worry about mixing, as you can read the PROM again whenever you require a strain gauge 
reading. 
These strain gauges usually are affected by temperature, so it will be necessary to accurately record
the temp at the time of measurement in order to compensate for temp. Ideally, this should be done
by means of an on-chip diode or similar structure.

If you are using a range of BGA and array-type packages, you may need a range of die sizes, each with 
its own strain gauge/s. The value of a strain gauge will vary depending on its location on the die -
corner Vs centre Vs side.... so if you are characterising a range of packages, you need to bear this in 
mind.
	This could rapidly mushroom into a major analysis project, where you end up devoting most of 
your time to procuring suitable die, packaging, reading the gauges, crunching the numbers etc. As 
pointed out by Jeremy Drake, you may be better off simply using reject die that match your intended
die sizes, and processing them through your reliability tests. This would allow you to devote more
resources to understanding other variables that might ultimately have a far greater effect on your 
final reliability - eg variations in ball size, ball placement, internal delamination, wire bonds,
registration of solder mask apertures Vs pads for ball attach, solderability, package construction, etc. 

	As far as I am aware, the Sandia Labs ATCXX series test chips, are the best open market source 
for your requirements. Contact David Palmer (Sandia) at (505) 844 2138.

	hope this helps.

	Olly

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