Soldering Fundamentals

Soldering is a process of joining two metals together with iron by the use of a solder alloy to form a reliable electrical path. It is not a simple task since it requires experience and knowledge. It is very important to make a proper soldering joint since faulty joints are one of the major causes of circuit boards failure.

Soldering is one of the oldest methods of joining two metals. There are several techniques of connecting two metals together like connecting with bolts and nuts, rivets, etc. Soldering process is still the most popular way of connecting two metals because of two reasons:

– joint is solid and there is not a movement in the joint

– there are no interfacing surfaces to oxidize

For beginner a soldering process looks simple. It looks that the solder simply sticks to the metal like some kind of conductive glue. However, what happens during this process is far different. When hot sold 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. Solder is already metal alloy of tin and lead. This process can occur only if the solder and surface of the copper are hot enough. In addition, surface should be clean and free of oxide film. This oxide film forms when the metal is exposed to air. Sometimes a copper surface may look clean but, actually, it can be a thin layer of oxide film on the top of the surface.

In that case, when solder is applied to copper surface it will not stick to copper. Oxide film forms very quickly on the surface of heated metal. No solvent and penetration action takes place because oxide film prevents interaction of solder and copper. That is not god joint and that solder can easily scraped off copper surface. Therefore, oxide film should be removed from copper surface. This can be accomplished with the use of fluxes. Fluxes are mixtures of natural and synthetic resins. Some solvents and abrasives could also be used for cleaning of copper surfaces but they are not as good and efficient as flux. Flux removes oxide film and keeps removing it during soldering process. Flux is in the solid state at room temperature. Melting temperature of flux must be lower than melting temperature of solder. Soldering wire usually has flux in the middle of wire. There are many different types of solder with different solder to flux rate. When soldering is completed, all flux should be removed with solvent.

The main requirement for soldering operation is a source of heat. Soldering iron is a tool that provides heat during soldering operation. The iron tip should be clean. Cleaning of iron tip should be done before every use by wiping it on wet sponge. During soldering operation, solder should be applied to a clean and properly heated copper surface and not to the iron tip. That way solder will melt and flow without direct contact with soldering iron and provide a smooth surface. Otherwise it will form irregular appearance, built-up and poor filling. Soldering is not finished at moment when iron is removed from joint because at point point solder is still fluid. Component being soldered should be held tightly in place until the temperature decreases to solidify solder.

If a hot soldering iron is applied for too long or with too much pressure to joint it can significantly damage, deform and even lift the pads and conductors on circuit board. Copper foil on circuit board is very thin. Average thickness of copper foil is 0.0007-0.0028 inches (0.0175-0.070 mm). Do not apply any additional force on soldering iron. The iron tip should never be pushed down on the copper surface.

After soldering project is completed, iron should be kept in a holder. Iron tip should be cleaned and coated with thin layer of solder.