The obnoxious dude that said the LED lead had separated at 720 is most
likely mistaken.
Regardless of what makes him think so (digital display on his soldering
station or whatever), it's very unlikely.
Component connections on mass produced equipment can fail without any
appreciable heat.

I've got a soldering station analyzer and a thermocouple calibrator, and
these instruments are barely adequate for an accurate analysis of the
proposed scientific test. Many types of thermocouples degrade/derate quickly
when exposed to high temperatures.

A component's leads soldered to copper traces on a resin/glass board will
prove to be individually different. Another factor is if it's a factory
solder, or if it's a generic consumer solder.

I would be certain that a fairly accurate comparison (not an accurate
definitive test) could be accomplished with a thermocouple securely attached
to a soldering iron tip, and having the tip positioned where you could hang
a loop of wire solder on the tip. As you monitor the increasing tip
temperature rise to the point where the solder loop lets go, this temp can
be used to compare it to a different alloy or brand.
The final result will only be a fairly decent comparison.

Soldering iron temperatures are normally much higher than the melting point,
otherwise the iron tip would have to sit on the connection until the
connection equaled the minimal tip temp. This could take a while, depending
upon the thermal mass of the iron, the heat transfer rate (a dry tip doesnt
transfer heat well), the thermal conduction rate/dissipation of the
connection, and finally the solder melting point.

If all of our electronic stuff was soldered at a minimal temp, none of the
connections would be any good, because of the previously mentioned cold or
dry joint connection. Thermal cycling would probably cause the components to
fall of the circuit boards.
Mass production solders for surface mount components are generally pastes
and can be "set" by oven heat.

WB
................

"Tom Quackenbush" wrote in message
...
Specifically, how do you know when a lead alloy has reached it's
liquidus temperature?

On another newsgroup, a poster mentioned that the solder attaching
his LED to the PCB melted, therefore he knew that the temperature must
have reached 720 degrees F. I responded, asking what kind of solder he
was using, since 60/40 melts somewhere around 370 deg. F.


R,
Tom Q.

p.s. I'll be asking on r.c.m for advice.