Non-Visual Soldering Techniques

This series, which spanned several years in the Smith-Kettlewell Technical File (SKTF), describes how blind and low vision makers do electrical soldering. Bill Gerrey, Editor of the SKTF, originally intended this series to be the beginning of a book on soldering and the techniques used by blind technicians. As there are as many different soldering systems as there are fabrication processes and materials, the compilation of an exhaustive blind soldering tutorial would be a monumental undertaking. Since such a thorough tutorial has never been compiled, we offer two options: Bill Gerrey’s original teachings in the Technical File in Bill’s own prose, experience and recommendations about blind soldering, along with the suggestions and input from avid readers and contributors to the SKTF, and the following which is an edited version for folks looking for a more modern online resource. We have kept Bill’s wisdom and direct style as much as possible.

Choosing a soldering iron

The pros and cons of “instant heat” soldering irons.

Instant heat soldering irons heat up and cool down quickly, making them easy to handle. These irons have small tips that heat up fast but can overheat if not used properly. It’s important to use a soldering iron with enough power (at least 50 watts) because weak ones can damage parts by spreading too much heat. We also compare two types of soldering irons: transformer-based and battery-powered cordless. We suggest using both types for different tasks to have a more flexible soldering setup.

A variety of soldering irons and soldering guns are available whose features include “instant heat” capability; they warm up to soldering temperature within a few seconds of being turned on. However, only a particular type of these “instant heat” irons is of interest to us. We are interested in the irons whose tips have very low mass, and consequently have low heat storage capacity. These not only heat up quickly, they cool down quickly after being turned off. Unlike conventional soldering iron designs in which a large heating element heats the tip by thermal conduction, the irons to be described here use high current electricity to heat a small low-mass tip. The tip itself is the heating element. Within 60 seconds after the current through the tip has been turned off, the tip is cool enough to be touched.
These instant-heat, fast-cooling irons have two major disadvantages in comparison to conventional irons. The first is that the small surface area of the tip, which is the heating element, does not permit efficient heat dissipation in free air. If the current through the tip is left on while the tip is not in good thermal contact with the metals being soldered, the tip can reach an extremely high temperature which will quickly oxidize its surface and ruin the tinning job. The second disadvantage is that because very high current is necessary to heat the tip, a physically heavy high-current power source must be incorporated into the handle of the iron or gun. These disadvantages are something that can be lived with, and they are more than offset by the convenience offered to the user who wants to guide the iron into position with his fingers while the tip is cool to the touch. For the hobbyist who occasionally rolls up his sleeves and solders a project together, these irons eliminate the need for constant practice necessary to safely use a continuously hot iron.

Essential properties of a soldering iron:

For non-visual soldering, the best choice is an iron over 50 watts. The tip of the soldering iron must be made of a metal whose thermal conductivity is high. In addition, the tip of the iron must be able to accept and to retain a surface alloy of tin or solder (known as “tinning”) so as to afford the formation of a completely continuous metallic path between the iron and the work, which is necessary for efficient heat transfer. Finally, the iron must be powerful and efficient enough to heat up the localized area of interest faster than heat can be dissipated or transferred away from the iron.
A common fallacy is that the iron must be small enough not to damage the work. Used inappropriately, low-powered irons do more damage than soldering. Their intended application is in cases where a larger iron cannot be maneuvered into position for soldering. In general purpose soldering, low-powered irons do not heat up the localized area of the connection quickly, allowing considerable heat to be transferred to and absorbed by components of the work. In most cases, an iron of 50 watts or more can heat up the connection quickly and efficiently, allowing soldering to be accomplished without overheating the entire collection of circuit elements. The iron must be “big enough,” not “small enough.”

Two common types of instant heat soldering irons

Two kinds of these “instant heat” fast-cooling irons are commercially available.
The first is the transformer-type soldering gun. The tip of the transformer type soldering gun is a simple elongated loop of wire. The body of the gun contains a power transformer to match the high current load of the tip to the a.c. power line. These guns are available from a couple of manufacturers; however, in my opinion, the best is the Weller Model 8200 dual-heat soldering gun https://www.amazon.com/Weller-8200PKS-120-volt-Universal-Soldering/dp/B002YDMZF4. Its bare copper tip is cheap and simple; a piece of 12 gauge can be fashioned into a tip in a pinch. The binding posts for the tip are good solid construction so that the ends of the tip can be secured firmly (these binding posts need to be tightened occasionally.) The two position trigger switch provides two power ratings, 100 and 140 Watts; one for small jobs and one for larger work. The main limitation of these guns is that the tip is too large to be used on integrated circuits and other crowded assemblies.

The second type is a cordless-type soldering iron. With the advent of sealed rechargeable batteries, a junior member of this family of instant heat irons become available; instead of using a power transformer to supply current to the tip, a nickel-cadmium battery is incorporated into the handle. For practical reasons (not the least of which is that the high tip current must be controlled directly by a push-button switch), these cordless irons are very low power and are only good for very small work. They work fine for circuit board work (either printed circuits or point-to-point wiring on perforated boards), but they are not husky enough for soldering terminal lugs such as those found on plugs and jacks. In my opinion, the best of the cordless irons is Iso-Tip Quick Charge Cordless Soldering Iron Kit – Model #7700 or the newer model (same features with an upgraded battery) Model #8000 https://www.amazon.com/Soldering-Iron-ISO-TIP-inQuick-Chargein/dp/B004GHT6P8/
https://www.amazon.com/Model-8000-Cordless-Soldering-Iron/dp/B08L44SS7

Key Takeaways: Choosing a Soldering Iron (Non-Visual Soldering)

With a complement of two irons, a cordless iron and a transformer type gun, those who wish to use instant-heat soldering irons can cover the full range of electronic assembly work by using the appropriate tool.

• Use a soldering iron with at least 50 watts of power for better heat control.
• The tip should conduct heat well and be able to hold a layer of solder (“tinning”) for good heat transfer.
• A stronger iron heats the joint quickly and efficiently, avoiding damage to nearby parts.
• Small irons aren’t safer—they often overheat components by taking too long to do the job.
• The goal is a tool that’s powerful enough, not just small.

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Two Common Instant-Heat Soldering Irons

1. Transformer-Type Soldering Iron
   • Best pick: Weller 8200
   • High power (100–140W), heats fast
   • Simple, replaceable copper tip
   • Great for general work, too big for tight spaces
2. Cordless Soldering Iron
   • Runs on rechargeable batteries
   • Best for small, delicate jobs like circuit boards
   • Not strong enough for bigger parts
   • Recommended: Iso-Tip Model 8000 or 7700

✔️ Using both types covers most soldering needs.

Putting Tools in Hands: Designing Curriculum for a Nonvisual Soldering Workshop, ACM ASSETS 2020 Poster by Lauren Race, Joshua A Miele, Chancey Fleet, Tom Igoe, Amy Hurst

A note about preparing your work space

Since splattering and dripping of the solder is inevitable, choose a work surface on which marring is of no concern. Because the instant heat guns and irons can be heavy, a collection of blocks, books, or heavy transformers against which your hands can be braced may help slipping of the iron off of the connection.

Your paraphernalia should include good solid holding devices which can rigidly support the work. A small table vise with a swivel is good for most applications. (More on this in Part II.) In designing the layout for your projects, try to arrange for the soldering to be done in accessible places. When stringing connecting wires around your project, make them long enough so that they can be gotten out of the way during soldering.

Tinning your solder iron

“Tinning” the iron specifically refers to applying a coat of fresh solder to the tip. This can be done in two ways. If the iron is cold, wrap about three inches of solder around the tip and turn the iron on. If the iron is to be “tinned” while hot, solder must be “brushed” along the tip. To make sure that the entire tip is being bathed in fresh solder, turn the iron slowly while “brushing” the solder over the tip.

Tinning a cold iron and preparing them for use is a simple matter if you carefully monitor what is happening by holding on to the solder. Spool off a couple of feet of solder and wrap about three inches of it around the tip of the iron or gun. Hold on to the solder about an inch away from the tip. Turn on the iron and wait for the solder to melt, which will disconnect the iron from the solder in your hand. Immediately and simultaneously release the button or trigger and give the iron a quick little shake in a direction away from you. After a minute, feel the tip and inspect your tinning job. The tip should feel smooth and perhaps a little gummy from the flux. If the tip feels rough in spots or if it has a glob or an “icicle” of solder that might get in your way during the first connection, repeat the process. (To some people, these three inches may seem like an excessive amount of solder, but the point is to assure that the entire surface of the tip is bathed in fresh solder. Overdoing the amount harms nothing.)

Tinning a hot soldering iron. If the iron is to be “tinned” while hot, solder must be “brushed” along the tip. To make sure that the entire tip is being bathed in fresh solder, turn the iron slowly while “brushing” the solder over the tip. Finding the tip of a hot iron with a piece of solder is no easy task.
To aid in doing so, rest both hands against some familiar object, such as your vise or the rest stand, so that you have some idea as to where the iron and the piece of solder will intersect. I often extend the piece of solder an inch or so beyond my projected point of intersection, thus allowing the iron to “cut” the solder to the exact length. Do the “tinning” procedure over an unimportant surface which is not flammable. It is a good idea to “tin” the iron over a wet cleaning sponge, but remember to retrieve the solder droplets from the sponge; they will be larger than those left in the sponge after normal wiping, and you do not want the iron to pick them up later. The iron can be cleaned by wiping it on a wet cloth or a wet cellulose sponge. Whichever you use, the item must contain nothing which will contaminate the iron. For example, no cloth having components of polyester are acceptable. Also, many general purpose cleaning sponges contain chemicals which can soil the tip. Above all, the item used must be wet (dripping wet is acceptable). This is essential, since while the iron is being wiped free of its protective excess solder, it is very vulnerable to charring foreign matter.

🔧 Essential Tools & Equipment

• Soldering iron or soldering gun
  • Recommended: 50+ watts
  • Includes tip suitable for tinning
• Solder
  • Comes on a spool
  • Should be enough to wrap around the iron tip and for soldering joints
• Soldering iron stand or rest
  • Used to safely hold the hot iron when not in use
• Work surface
  • Heat-resistant and not damaged by splatter
  • Should be in a well-ventilated area and not flammable
• Hand support aids
  • Heavy objects (e.g. blocks, books, or transformers) to brace hands and prevent slipping
• Vise or holding device
  • Example: Small swivel table vise
  • Used to firmly hold the workpiece in place
• Wet cleaning sponge or cloth
  • Must be non-synthetic (no polyester or chemical additives)
  • Used to clean the tip of the soldering iron
  • Must be wet to prevent charring and tip damage