Good morning. I suppose the best way to address this is to differentiate
the IMC formation and structure from the basics of XRF analysis.

I simplest terms, the IMC, by definition, will be predominately tin rich
RELATIVE to the ratio of tin and lead you would see in an adjacent area
that has more than 80 microinches of solder on it. To the XRF, this would
be akin to measuring a thin plated tin deposit. Little lead would be
detected, and the readings would reflect that. Given the IMC is a
tin/copper construct, little lead will be present.

As to your comment on the fact that the thin area moves on subsequent HAL
treatments, and it not being associated with copper condition, I would
disagree. If a marginal wetting condition exists across the annular
surface, it is rather common for that surface to be able to retain some
amount of solder yielding the thicker areas. That deposit can be tenuous at
best, prone to moving with each pass based on the dynamics of air flow.

I have been witness to, and have created intentionally, healthy looking
solder deposits on pads and annular rings, only to have them exposed or
dewetted on subsequent reflow/wave soldering.

Looks can be deceiving!

Regards,

Joseph Webb
TET/Halco

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