What is Resistance Soldering?

Kirsten C. Tynan

Resistance soldering is a process for joining metal pieces that uses an easily melted material called solder. In this form of soldering, the heat that melts the solder is generated through the application of an electrical current to the solder and soldering tool. Typically, a standard voltage source is used to provide this current. It is generally passed through a step-down transformer to provide a low voltage, high current output. This makes it possible to generate a high amount of heat in a very small area, thereby allowing for delicate soldering of small or closely spaced components.

Soldering irons are commonly used to fuse wires and electronic components together on circuit boards.
Soldering irons are commonly used to fuse wires and electronic components together on circuit boards.

Heat can be generated by applying a current to any resistive material, but materials of higher resistance generate more heat. Incandescent light bulbs, for example, generate light and heat due to the current running through filament wire inside the bulb. Just as a light bulb can get quite hot, so can other materials. Resistance soldering takes advantage of this to melt solder that is then used to join metal pieces.

There are a number of advantages to using this method of soldering over traditional methods. Heat is generated only in a confined area and dissipates quickly so the risk of damaging small or closely spaced components is lower than with some other methods. Rates of production may be increased with resistance soldering because heating is nearly instantaneous and the work piece cools much faster than with traditional soldering. Operational costs may be lower due to more efficient use of electricity and less wear and tear of soldering equipment.

Safety is another area in which resistance soldering offers advantages over other joining techniques. Heat is only generated during the actual soldering operation so there is no chance to bump into an iron that is hot but sitting idle near the operator. This method also avoids use of an open flame as is necessitated in some other joining processes. The rapid cooling of work pieces after soldering also helps to avoid mishaps.

While resistance soldering offers numerous advantages, it does have some drawbacks. The initial cost of the equipment is typically higher than that of traditional soldering. A typical resistance soldering operation takes place at a somewhat complex station as opposed to the simpler and more portable traditional method of soldering with an iron. This station generally includes a rather bulky power supply to generate the necessary current as well as soldering probes and a foot pedal for conducting the actual assembly procedures.

Welding and brazing are similar processes for joining metals but differ in some key ways. Welding involves melting not only the filler material, but also the two metals being joined, whereas resistance soldering melts only the solder applied to join the metals. Both soldering and brazing involve melting only the filler material, but in brazing this filler material has a much higher melting point than the filler used in resistance soldering.

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