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 Celsius. 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 most common are 18 swg, used for general work, and the thinner 22 swg, used for fine work on printed circuit boards.
How to do it
Good soldering is a skill that is learned 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. Using the correct size bit is critical for this step. If the parts are not at the solder's melting temperature in a couple seconds consider using a bigger size tip. 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.
Even solder that states to be lead-free it still contains some lead. It is a good practice to wash your hands after you are finished soldering. 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.