I posted this in May last year, but to save you searching for it,
here it is again…
To solder (braze) successfully it helps to understand how it works.
Soldering refers to the process whereby pieces of metal are joined
by melting another metal (the solder) so that it wets the joint and
holds it securely when it freezes. That’s basically it, but there are
a few “gotcha’s”.
The important word is “wets”. It’s just like water. Put a nice clean
pipe in water and freeze it; it’s then difficult to get the pipe
out, but use an oily pipe and it’s much easier. The difference is
that the water wetted the clean pipe, but couldn’t wet the oily one.
If the pipe wasn’t oily but dirty, the effect would be the same.
Solder works in exactly the same way. So all you have to do is make
sure the joint is nice and clean and you’ll get a good joint, right?
Well, no, it’s a little more complicated than that, but not a lot.
To melt the solder you have to heat it up. The trouble is that the
very act of heating it also makes it dirty. The oxygen in the air is
only too eager to oxidise everything it touches, and, as far as
solder is concerned, metal oxide is dirt. Most metals oxidise rather
slowly at room temperature, but heat them up and the effect is very
rapid - so rapid as to make soldering impossible, unless you prevent
the oxygen from reaching the hot joint.
There are only three ways to do this: solder in a vacuum, solder in
an atmosphere devoid of oxygen, or coat the joint with an oxygen
barrier that can withstand the heat. The first two options are rather
impractical, but the third is fine; the barrier is called a flux.
The job of the flux is to cover the joint with a barrier to stop the
oxygen from oxidising it. Resin is a good flux for soft solders, and
borax for hard solders or brazing. If the joint and the solder are
nice and clean then these fluxes work rather well, but that’s all
they do; they are known as inactive fluxes. If the joint is a little
dirty these simple fluxes do nothing except act as a barrier, but
there are others that can do a small amount of cleaning too. Baker’s
Fluid is one such active flux for soft solder and EasyFlo is one for
hard solder. (I use a product called Auflux, which is a sort-of
luminous green liquid. It is used straight from the bottle and
doesn’t froth up as much as the powdered fluxes.) The active fluxes
are certainly better than the inactive ones, but they are not magic,
you shouldn’t rely on them to do the cleaning for you. Active fluxes
usually contain some rather toxic and dangerous additives (potassium
biflouride?) which do the cleaning bit.
So, now you have a nice clean joint and a flux, so it’s plain
sailing, yes? Well almost. Most problems are caused by heating the
solder rather than the joint. All that happens is that the solder
melts, goes into a ball, and refuses to flow into the joint because
it freezes before it can wet it. The secret is to heat the joint, not
the solder. When the joint gets hot enough it will melt the solder
which will then flow nicely into the clean, fluxed joint. The final
thing that can go wrong is to burn the flux. If you heat the flux for
too long, longer than a minute or so, it will lose its properties and
allow the oxygen to pass. This is normally the result of insufficient
heat, so if it happens, remove the heat, clean the joint, and start
over, perhaps with a better source of heat, or better insulation to
prevent the heat from leaking away.
So, the four main points for a good soldered joint are:
- Make sure the joint and solder are both clean.
- Use a good appropriate flux.
- Heat the joint not the solder.
- Complete the joint quickly.
I hope this helps.
Regards, Gary Wooding