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February 1997

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Sun, 16 Feb 1997 22:09:27 -0500 (EST)
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Misri Mohd replied to me personally that they are using the Methane Sulfonic
Acid based electrolyte.

The Methane Sulfonic Acid based system has certain properties that must be
attended to when plating Tin or its alloys that may be causing some of the
problems alluded to in the previous message.   Tin has two oxidation states
besides the metal, Stannous ( 2+) and Stannic (4+).   The Stannous Tin, which
is the normal, and preferred, state in this bath,  readily oxidizes to
Stannic Tin in the presence of either atmospheric Oxygen, or if the anode
area is too small for the current being pushed through the bath.

When Stannic Tin forms, it is insoluble, and forms a milky looking
precipitate in the bath.   The Methane Sulfonic Acid bath is clear, (not to
say uncolored) when there are is no Stannic Tin in the bath.   This milky
precipitate normally stays suspended in the bath, and can plate out,
(co-deposit) and become part of the plating.  The amount of the Stannic Tin
plated out will change with the current density, with greater current density
tending to plate more Stannic Tin.   This could be the cause of some of the
color change of the plating.

The Stannic Tin is difficult to filter out, but there are ways to get it out
of the bath, and my guess is that there are people reading this that can give
this gentleman some directions at finding the appropriate technology.
  Alternately the plating bath supplier may be able to assist in this matter.

Hope this helps

Rudy Sedlak
RD Chemical Company

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