To solder 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
The important word is wets. Its just like water. Put a nice clean
pipe in water and freeze it; its then difficult to get the pipe out,
but use an oily pipe and its much easier. The difference is that the
water wetted the clean pipe, but couldnt wet the oily one. If the
pipe wasnt 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 youll get a good joint, right? Well, no, its 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 stand 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 (ie
lead solder) and borax for hard (ie. silver solder).
If the joint and the solder are nice and clean then these fluxes
work rather well, but thats 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. Bakers 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 shouldnt rely on them to do the cleaning for
So, now you have a nice clean joint and a flux, so its 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.
You can practise on copper or brass quite satisfactorily. Use small
pieces first 'cos that will teach you to remove the flame as soon as
the solder flows - you stand a good chance of melting the work piece
Progress onto larger and heavier pieces to get experience of heating
quickly so as not to burn the flux, then go back to small items
Regards, Gary Wooding