Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

----- Original Message -----
From: "Stefan Trethan" <stefan_trethan@...>
> I have the choice of:
>
> ROSOL 1S S-Sn96,Ag4 250g 15eur
>
> ROSOL 3 S-Sn97,Cu3 250g 10eur
>
> Now what is the difference between paste with copper and paste with
> silver? I don't really mind the price since it will go a long way.
> I remember some leadfree solder (wire) has silver in it too?

It's too late in the day for hard science, but here's the reader's digest
version. Eutectic tin (liquidus, or molten, that is) alloys freely with many
metals, including silver. Adding silver to the solder reduces the silver
taken from the solderee in the intermetallic layer formed during soldering.
Sn96Ag4 also melts at a higher temperature than Sn63Pb37. This is useful for
reflowing boards with components on both sides. The first side is reflowed
at a high temp profile using tin/silver. The second side is then populated
and reflowed with a suitable paste on a lower temp profile. If lead-free is
a concern (one presumes it is), tin/copper melts about 20^C lower than
tin/silver, 220^C versus 240^C; tin/bismuth/silver is lower still. The more
familiar tin/lead melts at about 180^C, much lower than any of the lead-free
alloys.

Since the mechanical or electrical properties aren't the major concern for
choosing between the two, tin/copper would work as well or better than
tin/silver for our use. For that matter, I'm still using Kester 44. Just
ordered another 1/2 lb., in fact. Nothing wets as well as that old crap.