On Wed, 08 Dec 2010 19:02:38 -0500, Tony Miklos wrote:

On 12/8/2010 5:47 PM, wrote:
On Wed, 08 Dec 2010 11:44:23 -0500, Tony Miklos
wrote:



A few weeks ago I installed a couple of 35 amp bridge rectifiers. The
leads were, as they often are, blackish colored and do not tin very
well. Seems they always come that way, even from different
distributors. They always get hand cleaned with the same brass wool I
use for the soldering iron before being installed.

It sounds like these are contaminated. Any abrasive cleaning can damage the
plating and could *easily* cause problems down the road.

No problem with them 30 years later. I'm not too worried.

Black coloured component leads??
Sure sounds like silver plating. Sulphur in the air tarnishes silver,
making a very black coating. Removing the tarnish by abrading should
make a reliable solder joint.

Actually it does look like silver... oxide? I've been told that it is
a great electrical conductor, but not great to solder to. Do you know
if the black stuff does really make a better conductor for something
like a switch?

The black tarnish is Silver Sulfide, which is almost as conductive as metallic
silver. Silver oxide, while still a conductor isn't nearly as good. Scraping
it isn't a good idea because it can reveal the metals below. I doubt that
it's silver sulfide, though, because it still should solder well. It's likely
a tin-lead coating that's oxidized. You probably didn't scrape through it, if
that's what it is.