Hello Brian,

I have been following your contributions to this forum with interest.

In regard to the two points you raise:

1.  Dross

There are many factors that affect dross production but, given that in the
case quoted all other factors were more or less the same, I think that the
lower rate observed with the stabilised Sn-0.7Cu solder is mainly a
reflection of a lower tendency of that alloy to oxidise compared with the
Sn-37Pb.   There is a way of expressing that property in terms of the
physical chemistry but I cannot immediately recall it at the moment.

2.  Low solids fluxes

You are right that the conditions of leadfree soldering do strain the well
recognised limits of low solids fluxes.   There have been advances in the
development of activation systems that can survive reasonably long heat
exposure but perhaps a move back to higher solids content fluxes will be one
of the tradeoffs involved in the adoption of leadfree solders.   Just how
high will depend on the situation.   For single sided boards the heat
exposure need not be as great as when you need to get a leadfree solder to
penetrate a through hole in a multilayer board.   Of course the Japanese
electronics industry never moved as far in the direction of low solids fluxes
as did the industry elsewhere in the world so for them the adoption of
leadfree soldering is not requiring a major change in their liquid flux
technology.

Regards
Keith Sweatman
Nihon Superior Co Ltd

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