Hand soldering is process of joining two metals together with soldering iron by the use of a solder alloy to form a reliable electrical path. Soldering is not the simple task since it requires experience and knowledge of fundamentals. It is very important to make a proper soldering joint since faulty solder joints are one of the major causes of circuit boards failure.
For beginner soldering process looks like the solder simply sticks the metal like some kind of conductive glue. However, what happens during soldering process is far different. When hot solder comes into contact with surface of copper on circuit board, a metal solvent action takes place – the solder dissolves and penetrates the surface of copper. Copper and solder blend together and form new metal alloy that one part is copper and the other part is solder (and solder is already metal alloy of tin and lead). This can occur only if the surface of the copper and solder are hot enough and if the surface of the copper and solder are clean and free of oxide film. This oxide film forms when the metal is exposed to air. Sometimes a copper surface may look clean before soldering but actually it can be a thin film of oxide on top of copper. In that case, when solder is applied it will not stick to copper and no solvent and penetration action takes place because oxide film will prevent interaction of solder and copper. That is not god soldering joint and that solder can be easily scraped off copper surface. Therefore, for proper soldering bond, oxide film should be removed, and this can be accomplished with the use of fluxes. Fluxes are mixture of natural and synthetic rosins. Some solvents and abrasives could also be used for cleaning of surfaces to be soldered but in most cases they are not sufficient since oxide film forms very quickly on the surface of heated metal. On the other hand, flux removes oxide film and keeps removing it during soldering process. Flux is in solid state and it must melt at a temperature lower than solder. Soldering wire usually has flux in the middle of wire but there are different types of cored solder with different solder to flux rate. After soldering all fluxes should be removed with solvent.
The main requirement for soldering is the use of heat what is accomplished with soldering iron. The iron tip should be cleaned by wiping it on wet sponge before every use. During soldering operation, solder should never be applied against the iron tip - it should be applied to a clean and properly heated copper surface. That way solder will melt and flow without direct contact with soldering iron and provide a smooth surface. Otherwise, solder will form irregular appearance, built-up, and poor filling. Soldering is not fished at moment when soldering iron is removed from soldering joint because at that point solder is still fluid – components being soldered should be held tightly in place until the temperature decreases to solidify solder.
A hot soldering iron can significantly damage, deform and even lift the pads and conductors on circuit board if it is applied for too long or with to much pressure. Copper foil on circuit board is very thin –average thickness is .0014 – 0028 inches. The iron tip should never be pushed down on the copper surface – apply on iron same force as it would results from weight of soldering pencil resting on its point
After soldering is completed, soldering iron should be kept in a holder with iron tip cleaned and coated with thin layer of solder.