In the Nov '95 issue of "SMT", pp.26-29, Ray P. Prasad wrote:

     "The critical surface finish requirement for boards is to ensure
sufficient solder thickness.  The general recommendation in the industry
about maintaining at least 0.0003" (300 microinches) is essential for
long-term solderability..."

He goes on to talk about the dewetting effects of <0.0003" thick coatings
and the benefits of 5um of tin and 7.5um of 60 Sn/40 Pb.

He also deduces from a round-robin test conducted by the IPC that coating
thickness alone is not all thats important, but also the quality of the
coating.

Ray P. Prasad is a "SMT" editorial advisory board member, engineer, author
of "Surface Mount Technology: Principles and Practice", and founder of the
Ray Prasad Consultancy Group.  Contact him c/o "SMT" or (503)297-5898;
fax:(503)297-0330.

Understand, I am not suggesting that you call him.  This is just the
blurb from "SMT". (So much for the disclaimer -- I'm probably taking a
big enough risk just quoting him...)

I DO suggest getting "SMT" magazine.

DonW...