SOLDERING: THE TRILOGY OF SOLDERING
In soldering there are three key elements that are important and inseparable:
A. Heat;
B. Solder; and,
C. Soldering Flux
If any of the three is missing, good quality soldering cannot be achieved. The soldered joint and the entire process becomes an exercise in futility.
We, at NUMERIQUE LIMITED, describe these three component parts as the “Trilogy of Soldering”.
What is Soldering?
Soldering is a process in which two or more metal items are joined together by melting a filler metal. The melted filler metal is then allowed into the joint. The filler metal (usually called solder) has a relatively lower melting point than the metals being joined together.
The soldering process accomplishes a strong bond between the metal items by joining them together. In this procedure, the solder, an alloy of tin and lead flows over the two pre-heated metal items and binds them together. Only the solder melts, not the metal items that are being soldered.
To successfully achieve this process all the three key elements of the trilogy must be present.
A. HEAT
Heat (the first element of the “Trilogy of Soldering”) is applied to the items to be joined and the solder. The heat melts the solder and raises the temperature of the items to be joined. It is inseparable from the process. The heat comes from a soldering iron or soldering station. Soldering irons come in different forms, sizes, and power ratings. Usually, they are powered from the mains but can come as battery-powered or butane gas-powered.
There are different methods for applying heat. Heat can be applied using Hand Soldering, Dip Soldering, Wave Soldering, Reflow Soldering, etc.
B. SOLDER
Solder is the alloy material used to bond two metals together in the soldering process. A soft alloy that melts when heated and allows the formation of an intermetallic bond between itself and the metals to be bonded together. It has a lower melting point and is an alloy of Tin (60%) and Lead (40%). For electronic soldering, the preferred solder alloy is Tin (63%) and Lead (37%). Solder is the second part of the “Trilogy of Soldering”. It is inseparable from the soldering process.
Solder for electronic hand soldering comes as solder wire. They come in rolls of different wire lengths, wire diameters, and Tin/Lead alloy compositions. Solder also comes as Solder Paste (for SMT Hand and Reflow Soldering), Solder Bars (for Dip and Wave Soldering).
C. SOLDERING FLUX
To understand the role of soldering flux in the soldering process, let us look at the soldering process in detail.
When hot solder comes in contact with a metallic surface, a metal-solvent action takes place. The solder dissolves and penetrates the metal surface. The molecules of the solder and the metal blend to form a new alloy, one that's part metal and part solder. This solvent action is called WETTING and forms the intermetallic bond between the parts. Wetting can ONLY OCCUR if the surface of the metal is free of contamination and from the oxide film that forms when the metal is exposed to air.
When metals are exposed to the air they oxidize, i.e. they react with oxygen to form an oxide of that metal (e.g. rust is formed when the iron is exposed to air). Copper, tin, lead, etc. also similarly form oxides. When heat is applied in the soldering process, the rate of oxide formation increases with the temperature increase. Oxides do not allow the wetting action of solder to take place properly to form the intermetallic bond necessary for a good solder joint.
Soldering Flux, the third part of our “Trilogy of Soldering” is the material that removes the oxides from the metal surfaces. Without the soldering flux, the soldering process becomes a nightmare as oxides form uncontrollably and continuously.
Rosin flux is used for the soldering of electronic equipment. It comes in paste form to be applied directly onto the surfaces to be soldered. It also comes as an integral part (the core) of solder wires. Good quality solder wires always come with a rosin flux core.

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