It would not be at all unexpected to observe a higher relative
concentration of tin in an area determined to have a  "thin" HAL deposit.
The XRF is responding to the ratio of tin to lead, and given you are
approaching the copper/solder interface, and more specifically the
tin/copper interface, it would be naturally preferential to tin
concentration.

Sounds like you are within expected conditions, other than the thin solder
deposits. May I assume they are on larger ground areas? There is a direct
correlation between deposit thickness and area size under given conditions

Hope this helps!

Joseph Webb
TET/Halco

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