My two cents -

The tin depletion is a product of the oxidation of it verses the lead
component of the solder. The rate at which this occurs can be coupled to
the presence of certain contaminants, namely Al, and As, but is not
indicated in the described analyses. A much more likely candidate is/are
the operating conditions.

Elevated temperatures promote drossing (tin oxidation). Elevated
temperatures in conjunction with bath turnover (exposure to the atmosphere)
furthers the process. Both occurring without product running through, which
as least partially protects the molten alloy with flux drag-in (that factor
has changed dramatically wit the use of no-clean fluxes), further
aggravates the situation.

Bottom line - tin depletion is common, only the extent varies. I would
suspect the line that shows a markedly lower tin was likely kept at
operating temps and conditions with sporadic workflow. There is a dynamic
equilibrium that may be reached if 63/37 from the HAL or reflow process is
introduced, but it will not be sufficient to offset losses due to the
conditions I described.

JMW
TET/Halco

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