The solder is softer than copper and under the pressure of the screw deforms quicker than bare copper would. Deforming is often enough to make the terminal loose after a short while.
The solder can wick up the strands and make the ore stiff (the strands stuck together behave somewhat like a single solid strand. If the transition is outside of the terminal, then you have a point where all the strands flex where they transition from individual strands to a single core, and they tend to break off there.
Soldering the wires into the terminals with no screws? Ok.
Crimp terminal then solder? Ok.
Clamp in a screw terminal then solder? Ok, but unconventional.
Solder then stuff into a screw terminal? No.
Solder then crimp? No.
Why? As BG has said, solder flows/creeps over time. Fine if you crimp/crew down hard then fill the gaps with solder to keep the muck out, but bad bad news if you solder first then crimp (or use a screw terminal) as the solder will flow and the joint will relax over time/become high resistance.
Historically a big problem in aerospace and automotive. No clue on what the wiring regs say, but from a practical perspective the above is true.
That TLC crimper works very well by the way. Buy the ferrules with a plastic shroud and for the odd occasion that you need the clearance of bare ferrule you can just snip off the plastic.
Interested to see markcosic's summary as I've just been helping a friend fit a new wiring loom to a 1946 Riley. It looked as if originally the end of the multistrand had been dipped in solder and then crimped. It seemed obvious that sometime in the intervening 60+ years the switch had given problems, but perhaps it wasn't the switch (which appeared sound) but the crimping/solder.
It WAS indeed common practice some years back to "tin" wires with solder before fitting them into screw terminals. It's one of many things where it's taken a few years for us to work out that it's actually not a good idea. Go back several decades and you'd find doctors advising people to smoke for instance...
I just re-terminated a lampholder for a pendant and that was pretty finely stranded, and the good old strip and double over method worked perfectly well
There is a difference between tinned copper wire and soldered copper wire.
Tinned copper wire has thin layer of tin with a specified thickness that is strictly controlled. Soldered wire has a layer of solder and with multistranded wire the spaces between strands is filled with solder.
One reason that ends were soldered was to keep the strands together during assembly of wiring looms. Often this was achieved by induction heating the wires to weld the tin coating together over a short length inside the insulation before the wire was cut and stripped.
No stray strands meant less problems with assembly.
In theory a tinned strand is less likely than copper to corrode ( oxidise ). Hence the tinning of copper wire that may be exposed to damp or atmospheres with corrosive gas ( sulpher dioxide from burning coal in an open fire for example ). Improvements in housing improved the quailty of the air so domestic cables were no longer made from tinned copper.
If the strands are twisted tightly before being bulk tinned ( soldered ) then there may be an improvement to the life of the connection if in in an adverse enviroment.
Tinning of whole of the conductor end is not permitted, neither is using a fine and very fine stranded conductor tinned conductor end if there is going to be any relative movement of the wire and the terminal.
There should never be any relative movement at or close to the terminal. I recall twice the minimum bend radius being the minimum distance from the terminal at which flexing could be allowed.
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