There is no such thing as electroless Tin, if you mean a Tin plating system
that acts like "electroless" Copper, which can build, in theory, to any
thickness.   There is immersion Tin, which is a displacement plating bath,
which means that it dissolves one atom of Copper for each atom of Tin that is
plated.   The implications of this are that when the substrate Copper is
completely covered, plating ceases, thus severely limiting the thickness of
Tin that can be plated.

Immersion Tin is used a little, but it is not much in favor, as a
solderability protectant, likely due to the short life of the solderability.
 This is due to the fact that Tin and Copper spontaneously alloy, forming an
intermetallic compound.  When the entire deposit of Tin has alloyed, and
become an intermetallic, the surface is no longer solderable.   The length of
time required for this to happen is a function of the thickness of the Tin
deposit, and the storage temperature of the boards.  The thicker the deposit,
and the lower the temperature they are stored at, the longer the boards
remain solderable.

There are essentially two types of immersion Tin plating baths available.
  Baths that are supplied as a liquid, and designed to be used at, or close
to room temperature, and baths that are supplied as a dry salt, which must be
used at elevated temperatures, and preferably as high as 150 F (65 C).  The
baths which are meant only to be used at elevated temperatures will typically
plate at least 3 times the thickness of the liquid baths.   I could quote
absolute numbers, but it might result in chemistry vendors making some
interesting claims which are very difficult to substantiate...... At least in
the hands of this chemist.

Vendors of this type of chemistry include:

Shipley Corp.
Electrochemicals Inc.
RD Chemical Company

If you want more information, please contact me directly.

Rudy Sedlak
RD Chemical Company
[log in to unmask]
fax 415-962-0370
ph. 415-962-8004

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