Resistance soldering is an alternative to conventional “pencil” soldering iron which offers some unique capabilities.
The image of the electrical engineer or hacker bent over and using a soldering iron on a circuit board is a classic visual cliché of TV and movies (Figure 1).
In most of these soldering images, the engineer is using a basic pencil-style soldering iron. That makes sense because the pencil soldering iron is the one most users start out with. More-advanced units feature adjustable temperature control and maintain the temperature over a narrow band.
Until a few decades ago, the ability to hand-solder electronic components and wires was a typical requirement for electrical engineers — and a rite of passage. Soldering was a standard connection technique for breadboarding, prototyping, and building production units.
In fact, at one point in the 1960s, one color-TV manufacturer claimed their TVs had true “craftsmanship” because these televisions were hand-soldered rather than assembled using the unproven and radical new technology (at least for the time) of printed circuit boards. We know how that competition turned out.
Undoubtedly, times have changed. Hand soldering no longer has the same place or demand. Nonetheless, there are many applications when soldering is a viable option or the only option for making a mechanically sound electrical connection or a non-electrical one.
What is soldering? Soldering is a process that joins similar or different types of metals, such as copper or tin-coated metals, by melting solder. Original solder was made of tin and lead and is melted with a heat source such as a hot iron. The connection is then allowed to cool to create a low-resistance, mechanically strong electrical bond.
Solder comes in different alloy combinations. “Soft” solder has a typical melting-point range of 90 to 450⁰ C (190 to 840⁰ F) and is widely used in electronics, sheet-metal work, and plumbing. The alloys that melt between 180⁰ and 190⁰ C (360⁰ and 370⁰ F) are the most common ones. Soldering performed using alloys with a melting point above 450⁰ C (840⁰F) is called “hard soldering” and is rarely used in electronics.
Historically, almost all solders contained lead, but environmental and health concerns have increasingly dictated the use of lead-free alloys for electronics and plumbing purposes. In contrast to welding, the workpieces are not melted when soldering. This is the same as with brazing. It does not melt the metal of the workpiece, but the filler metal melts at a higher temperature than it does than with soldering.
A well-made solder joint is sturdy and reliable. For many engineers, the act of creating a well-made solder joint, along with the smoke and smell of the melting flux (needed to prevent oxidation) is a mix of artistry and personal satisfaction, admittedly on a small scale.
Basic soldering is nothing new
Why is the tool called a soldering “iron?” The answer is historical: The metal-joining technology of soldering pre-dates electrical circuits and electronics. Centuries before our convenient, electrically heated units were available, a soldering iron was just that: an iron placed in a fire to heat up, then pulled out with a working time of about a minute until it cooled down (Figure 2).
These irons were used by plumbers, tinsmiths, and other craftsmen, who typically had several irons in the fire at a time. This meant there was always a hot iron ready, eliminating the need to wait for one to reheat before use. Managing this set of irons took time and attention, leading to the cliché about “having too many irons in the fire.”
When portable gas-fueled torches were developed, the “in the fire” soldering iron was upgraded to one that could be heated directly by the flame (Figure 3). However, the availability of electricity soon led to our pencil-style soldering irons with integral heating elements, making externally heated irons obsolete.
Some insist that today’s electrical engineers spend more time at their keyboards, rather than soldering. The reality is that for many hands-on EEs, soldering by hand is still an important and useful skill when there’s a cable to be made, a broken connection to fix, or a large component to be soldered to a PC board.
There are several video tutorials and online resources available that show how to solder, though some are better than others. For example, a short column in Popular Mechanics from several years ago made it sound easy (it was in print only; see Figure 4). Certainly, it can be but the points #1 and #4 from this article stand to be corrected and will result in an unreliable, high-resistance “cold solder” joint. Point #3 does not apply when using a standard rosin-core solder (which almost everyone does).
Although a conventional pencil-style iron can do the job — whether an unregulated or advanced temperature-controlled unit — it has limitations related to the placement, precision, and the risk of heat damage to the adjacent components or materials (such as plastic housings).
Resistance soldering is an alternative to soldering with a hot iron, and it will be discussed in Part II.
Related EE World Content
A Reliable Alternative to Manual Soldering
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References
About Mechanics, “What is Resistance Soldering?”
American Beauty Tools, “Resistance Soldering”
American Beauty Tools, “American Beauty Resistance Soldering Systems”
Model Train Forum, “Resistance Soldering?”
Engineering Choice, “What Is Soldering?- Types And How To Solder”
Model Railroader, “Resistance Soldering”
NMIA, “Build your own resistance solderer”\
The World of Rail, “Resistance Soldering Unit”
National Model Railway Association, “Making a Low-cost Resistance Solderer”
