On Sat, 14 Jun 1997 19:51:38 -0700 Terry Zhu <[log in to unmask]> writes:
>Phil:
>	You are right. Solder and tin boards are not compatible with
CuCl2 etchant. The reason is that tin is more active metal than copper
and hydrogen and tin will replace copper and release hydrogen out in
CuCl2+HCl system. Tin itself is oxdized as Sn(2+) and dissolved into
solution. Resists used for Cupric chloride etchants are screened inks,
dry film, gold and tin-nickle. I do have difficult to explain why Sn-Ni
can survive from CuCl2+HCl. Someone else may help me straight it out.
******************************

Terry,

Electrodeposited tin/nickel is not actually an alloy, it is an
intermetalic compound with properties quite different from either tin or
nickel.  It is much harder and more chemical resistant than either of the
parent metals.  

The tin/nickel deposit passivates quickly and remains virtually untouched
by normal circuit board etchants, it isn't even attacked by hot 50%
nitric acid.  The same characteristics that protect it from attack also
cause it to require an activated flux for soldering.

Its main uses are as a wear resistant layer for bearing surfaces, as a
replacement for chrome (mostly Europe), and as a surface finish for
circuit boards (particularly where wear resistance is required).

I hope this helps.

Regards,
Paul Galatis
Electrochemicals

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