Introduction to Soldering

Soldering is the process of a making a sound electrical and mechanical joint between
certain metals by joining them with a soft solder. This is a low temperature melting
point alloy of lead and tin. The joint is heated to the correct temperature by soldering
iron. For most electronic work miniature mains powered soldering irons are used.
These consist of a handle onto which is mounted the heating element. On the end of
the heating element is what is known as the "bit", so called because it is the bit that
heats the joint up. Solder melts at around 190 degrees Centigrade, and the bit reaches
a temperature of over 250 degrees Centigrade. This temperature is plenty hot enough
to inflict a nasty burn, consequently care should be taken.

It is also easy to burn through the PVC insulation on the soldering iron lead if you
were to lay the hot bit on it. It is prudent, therefore, to use a specially designed
soldering iron stand. These usually incorporate a sponge for keeping the bit clean.

Soldering irons come with various ratings from 15W to over 100W. The advantage of
a high wattage iron is that heat can flow quickly into a joint, so it can be rapidly made.
This is important when soldering connectors as often there is a quite a large volume of
metal to be heated. A smaller iron would take a longer time to heat the joint up to the
correct temperature, during which time there is a danger of the insulation becoming
damaged. A small iron is used to make joints with small electronic components which
are easily damaged by excess heat.

Always use a good quality multicore solder. A standard 60% tin, 40% lead alloy
solder with cores of non-corrosive flux will be found easiest to use. The flux
contained in the longitudinal cores of multicore solder is a chemical designed to clean
the surfaces to be joined of deposited oxides, and to exclude air during the soldering
process, which would otherwise prevent these metals coming together. Consequently,
don't expect to be able to complete a joint by using the application of the tip of the
iron loaded with molten solder alone, as this usually will not work. Having said that,
there is a process called tinning where conductors are first coated in fresh, new solder
prior to joining by a hot iron. Solder comes in gauges like wire. The two commonest
are 18 swg, used for general work, and the thinner 22 swg, used for fine work on
printed circuit boards.

How to do it (soldering, that is)

Good soldering is a skill that is learnt by practice. The most important point in
soldering is that both parts of the joint to be made must be at the same temperature.
The solder will flow evenly and make a good electrical and mechanical joint only if
both parts of the joint are at an equal high temperature. Even though it appears that
there is a metal to metal contact in a joint to be made, very often there exists a film of
oxide on the surface that insulates the two parts. For this reason it is no good applying
the soldering iron tip to one half of the joint only and expecting this to heat the other
half of the joint as well.

When the iron is hot, apply some solder to the flattened working end at the end of the
bit, and wipe it on a piece of damp cloth or sponge so that the solder forms a thin film
on the bit. This is tinning the bit.

Melt a little more solder on to the tip of the soldering iron, and put the tip so it
contacts both parts of the joint. It is the molten solder on the tip of the iron that allows
the heat to flow quickly from the iron into both parts of the joint. If the iron has the
right amount of solder on it and is positioned correctly, then the two parts to be joined
will reach the solder's melting temperature in a couple of seconds. Now apply the end
of the solder to the point where both parts of the joint and the soldering iron are all
touching one another. The solder will melt immediately and flow around all the parts
that are at, or over, the melting part temperature. After a few seconds remove the iron
from the joint. Make sure that no parts of the joint move after the soldering iron is
removed until the solder is completely hard. This can take quite a few seconds with
large joints. If the joint is disturbed during this cooling period it may become
seriously weakened.

The hard cold solder on a properly made joint should have a smooth shiny appearance
and if the wire is pulled it should not pull out of the joint. In a properly made joint the
solder will bond the components very strongly indeed, since the process of soldering
is similarly to brazing, and to a lesser degree welding, in that the solder actually forms
a molecular bond with the surfaces of the joint.

It is important to use the right amount of solder, both on the iron and on the joint. Too
little solder on the iron will result in poor heat transfer to the joint, too much and you
will suffer from the solder forming strings as the iron is removed, causing splashes
and bridges to other contacts. Too little solder applied to the joint will give the joint a
half finished appearance: a good bond where the soldering iron has been, and no
solder at all on the other part of the joint.

Remember it is much more difficult to correct a poorly made joint than it is to make
the joint properly in the first place. Anyone can learn to solder, it just takes practice.
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