"mc" wrote in message
.. .
Given that it's desirable to do repairs with the same type of solder as
was used originally, in order to avoid alloy mixing or partial melting
problems...

(1) How do I recognize lead-free solder when I see it?

(2) What temperature should I set my iron to, when working with SnSb or
SnAg solder?

Thanks!


It has been recommended by the creators of the half-arsed RoHS directive,
that manufacturers mark their boards with the alloy that has been used. To
date, I think I have probably seen about 2 or 3. In general, boards made
with lead-free, look as though every joint is bad ( and often, this is
pretty much the case !! ). Instead of the joints having a shiny appearance,
and being domed or meniscus-shaped, they are dull and grey, decidedly
'crystalline' looking, and tend to be volcano-shaped, with straight sides.

Pretty much any commercial equipment from the big far east manufacturers -
Pan, Sony, Hitachi etc - built in the last two or so years, can be assumed
to have been built in lead-free.

When you get a lead-free board for repair, as well as finding your fault,
which may well be joint-related, check the solder joints on anything large
such as connectors, power transistors, heatsinks etc. It's also worth
checking the soldering on any LSIs fitted to the board - particularly the
rows of legs on the downstream side of the soldering process, which is often
marked on the board by an arrow. Give anything suspicious, a good rocking.
Bad joints just don't look the same as with leaded solder. I have had
components just come out in my hand, leaving behind a perfect-looking
volcano of solder. This is because the manufacturers run their solder
process plants at as low a temperature as they can, to avoid damage to LSIs
etc. With the known inferior wetting properties of lead-free, this tends to
result in insufficient heat to properly solder components with a high
thermal inertia. SM LSIs seem to suffer as a result of the inferior flowing
properties of lead-free, resulting in poorer capilliary 'draw-in' of the
solder, under the legs.

As Graham says in this thread, 50 deg C hotter is about right. The
difference in actual melting temperature, is around 30 - 40 deg, depending
on the exact mix. As a matter of interest, the recommended alloy for general
bench rework, is Tin-Copper-3% Silver. This has a melting point about 10 deg
lower, and apparently, rather better wetting properties, but I can't vouch
for this, not having tried any yet.

Arfa