"Lloyd E. Sponenburgh" wrote in message
.. .

| Soldering, correctly done, is probably the best method for electrical
| conductance, but solder isn't an acceptable mechanical joint. In many
code
| jurisdictions, soldering is prohibited -- mostly because many people won't
| or can't do it well, but also because of the mechanical weakness of the
| joint.
|
| They always taught us in electronics schools, "Make a secure
mechanical-wrap
| before soldering; solder isn't glue."
|
| LLoyd

I stayed out of this one to see what would come up, but what I've seen
and worked has yet to be mentioned, so I'll offer my experience:

1. Cold joints started out as bad from the very beginning. It's only by
luck that they passed through life without causing a problem, so it's not a
matter of _if_ they fail, it's when. Only when corrosion and moisture can
work its way down inside the joint defect will the mechanical contact that
got the joint by for so long will send the joint into final electrical
failure. You can go a long way towards not having cold joint problems by
using eutectic solder (63/37) which doesn't have that temperature zone the
joint passes through where movement will cause crystallization and
separation of one component of the alloy. I can't recall for sure which
component has a lower melting point (tin?) that will crystallize.
2. What we often call a cold joint is really an incomplete flow of the
joint, in other words, a bad fillet with insufficient wetting. The flux
can't be removed from inside the joint, so eventually the above failure mode
pops up. Often there's a mechanical failure involved, too. Flux has this
lovely habit of attracting moisture, which is why the flux must be removed.
Acid fluxes are very bad for electrical and electronic connections for
similar reasons.
3. Stranded wires and soldering are rarely a good idea unless the wire
connection can be supported. If soldering stranded wires, a heat sink
should be used to prevent wicking of solder up inside the strands of the
wire. Where the rigid solder-soaked strands stop and flexible wire begins
there will be a spot highly prone to fatigue, and a guaranteed place to
break. When soldering stranded wires to terminations for whatever reason,
you MUST provide mechanical support, either in the form of a shrink
sleeving, mechanical support such as ties or clamps, or whatever suits the
application. It's more expensive tooling-wise to crimp wires, but the
strands have an evenly tapered flex section that distributes mechanical
movement over a larger part of the wire, thus reducing fatigue. Soldering
solid wire strands to a terminal is no big deal, usually, since the wire
itself is the mechanical support, but only in purely static equipment will
solid wiring ever be used. It's also a lot harder to work with, as many
know.
4. For the sake of welding cable, flexibility and high current capacity is
the reason for the high strand count. The less strands you have for a given
outside diameter the less circular area (thus ampacity) and less flexibility
you have. Since the individual strands are that much smaller, crimping is
even more critical due to the lower individual strength of the strands,
which will flake away at a soldered joint where the wicking ends. You can
support the wire all you want, but for the hassle of keeping the wire from
wicking and providing mechanical support either way, crimping is the more
reliable way to go hands down.
5. When the solder joint itself is the sole point of mechanical support, the
solder alloy will fatigue, work harden, and fail just like any other metal
will. Since the solder is so much softer than the metal it's joining, the
conductor needs the mechanical support (either inside or out of the joint)
to prevent the solder from being a stressed part of the connection.
Electronic technicians are familiar with large components on circuit boards
that must have mechanical support to keep things from ripping apart. I
recall capacitors on an A-4 upper beacon which was always coming in with
broken solder joints. I started securing it with a tie and RTV and never
had one of my own repairs ever come back to me. How the A-4 managed to have
a service life of over thirty years and not have that corrected baffles me
still.

I really like what's called copalum splices and terminals, even for copper
wire, because the intense pressure actually causes the aluminum to flow into
and around the wire completely, making a 100% sealed and perfect connection,
but they aren't cheap and the tooling even worse. The high quality of the
connection is the reason they're used for aluminum terminals, since the lack
of any resistance or corrosion entry point prevents any thermal movement
which causes fatigue and fires and gave aluminum wiring a very bad name. The
terminals allow aluminum wire to work to its best adantages.

If I made a boo-boo or need correction/clarification somewhere, by all
means have at it!