Have You Tried Resistance Soldering Yet? You Should.

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).

Figure 1: The electrical engineer working on a circuit board with a soldering iron is an often-used image, with considerable truth. (Image source: Radio Shack of Bozeman)

There’s good reason for that: soldering is the dominant technique for making electrical connections and the only practical way to electrically and mechanically connect most electronic components to a board. There are also techniques such as crimping and wire wrapping, but those are for, well, wires.

It’s not hard to learn to solder properly, but it takes practice and patience to master making a good solder joint with solid electrical and mechanical performance, rather than a “cold” one. The latter can mislead you into thinking you have a viable connection when you really don’t.

In most of these soldering images, the engineer is using a basic pencil soldering iron. That makes sense, as the pencil soldering iron is the one with which most users start out when they are getting into electronics. However, many do-it-yourself (DIY) enthusiasts and most professionals eventually graduate to a temperature-controlled unit which allows setting of the desired tip temperature, a useful feature when soldering components and leads with different thermal masses or sensitivities.

When done properly, a well-made solder joint is reliable, no doubt of it. 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.

So why is it called a soldering “iron”? The answer is historical: soldering as a metal-joining technology pre-dates electronics. 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).

Figure 2: The term “soldering iron” is a carryover from the historical tool which was heated in a fire, then used to solder two pieces of metal. (Image source: Pinterest)

These irons were used by experimenters and researchers (the “makers” of their time) as well as plumbers, tinsmiths, and other craftsmen. Serious users would have several of these irons in the fire at the same time, all heating up so they would always have a hot one ready. Managing this set of irons took attention and time; this challenge gave rise to the phrase, “Having too many irons in the fire.”

When portable gas-fueled torches were developed, the “in the fire” soldering iron was upgraded to one which could be heated directly by the flame (Figure 3). If you think you have soldering challenges, just remember that the crew laying the first trans-Atlantic telegraph cable in the 1850s used tools such as these to successfully splice and solder the thick undersea cables, all while on a ship on the open sea.

Figure 3: The development of gas-powered heaters lead to “self-heating” soldering irons. (Image source: Steve's Antique Technology/StevenJohnson.com)

The availability of electricity soon led to pencil-style soldering irons with integral heating elements and the obsolescence of externally heated irons. Even though circuit boards in a production setting are now generally soldered using wave or reflow-soldering technology, there’s still a need for hand soldering for rework, special components, or attaching wires to connectors.

There are some who insist that as today’s electrical engineers spend more of their time at the keyboard, they don’t need to know how to solder. Despite that supposition, the reality is that for many hands-on EEs, soldering by hand is still an important and useful skill. While the traditional pencil-style iron can do the job—whether a basic unregulated unit or a more advanced temperature-controlled one—it has limitations and shortcomings with respect to placement precision, and risk of causing heat damage to adjacent components or materials such as plastic housings.

Turn dissipation to advantage

Fortunately, there’s a convenient, easy-to-use alternative soldering technique called resistance soldering which has been available for decades that offers many benefits. It leverages the well-known principle of electrical self-heating to create a very localized “hot spot” which is hot enough to melt solder and so create a high-quality, reliable connection. The solder is the same type as used for pencil-style soldering irons. A representative resistance-soldering system is the American Beauty Tools Model #10502 light-capacity tweezer-style unit shown in Figure 4.

Figure 4: Model #10502 is a low-power resistance soldering unit that offers power control and a handset with pin-point tweezers for precise placement. (Image source: American Beauty Tools)

This unit can deliver up to 250 watts at 2.8 volts AC to the connection to be soldered. Operation is simple: the current from the system is applied across the joint via a pair of tweezers in the handpiece; the tips of the tweezer are positioned to “straddle” the connection to be made. The operator steps on a foot pedal, the current flows between the tips, the joint heats, the operator applies the solder, lets it melt, and releases the foot pedal.

The whole sequence takes only a few seconds, while the heating which might otherwise damage or melt adjacent components or materials is nearly non-existent. As the applied voltage across the tips is low—on the order of a few volts—there is no danger of shock to the user or damage to nearby components.

The handpiece and its tweezer are a key part of the system’s flexibility, performance, and convenience. Here’s why: the tip of a pencil-style soldering iron needs to touch the joint to be soldered with sufficient thermal contact area to provide the necessary heat transfer to the joint to melt the solder. This can be a challenge in tight spaces or with small components. In contrast, the tweezers of the resistance soldering system direct the flow of current, which is more precisely locatable and efficient in tight situations.

Vendors offer these tweezers in multiple sizes to handle different soldering situations and spaces. The tweezer tips can be adjusted to facilitate access to really tight spots or around corners. For example, the American Beauty Tools Model 105133 handpiece is 6 inches (in.)/15.24 centimeters (cm) long and features 0.04 in./0.10 cm diameter nickel-chromium (“nichrome”) tapered-pin electrodes. The electrodes have a 0.375 in./0.95 cm opening between them and can easily be bent or notched, allowing the user to customize the handpiece to specific applications (Figure 5).

Figure 5: The Model #105133 handpiece features nichrome electrodes with spacing of 0.375 in./0.95 cm; they can be adjusted for better access to tight spots. (Image source: American Beauty Tools)

Conclusion

Resistance soldering is much more than just an alternative to the conventional pencil-tool soldering approach; instead, it offers distinct advantages. It concentrates the heat in a highly localized spot; eases access to tight spots; reduces soldering time; minimizes the possibility of heat-induced damage; uses a lighter handpiece for reduced operator fatigue; cuts energy consumption since it is not “on” unless actually soldering; and the electrodes do not wear out, nor do they require constant re-wetting and cleanup.

If you haven’t used resistance soldering, then, as the saying goes, “Try it, you’ll like it.”

Related Content

Recommended Soldering Techniques

https://www.digikey.com/en/articles/recommended-soldering-techniques

American Beauty Tools Video References

  1. Through-Hole components being soldered to a PCB using resistance soldering equipment.
  2. Resistance soldering multiple wires to the solder terminals of a multi-tap switch assembly.
  3. Resistance soldering of a multi-pin audio jack and cable assembly.
  4. Wire terminations to a micro switch with resistance soldering.
  5. Electrode Replacement: 105133 Micro Tweezers Hand Piece

About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

More posts by Bill Schweber