If you have a discrete sample of tin/lead of a given true ratio, an XRF analysis
should return that value with some amount of reasonable error. In the case of a
solder deposit on a copper substrate it is typical to detect an elevated level
of tin as the deposit become thinner due to the natural alloying of tin and
copper forming the intermetallic increasingly deficient in lead as you get
thinner. The Cu/Sn alloy become more prevalent, the Sn/Pb less so, thus the
higher tin values.

Hope that helps.

Joseph Webb
TET Halco

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