Smith-Kettlewell Soldering Series Part 7

Table of Contents

Soldering I
Soldering II
Soldering III: Tinning Stranded Wire
 Soldering IV: Popular RF Connectors
Soldering V: RCA and Motorola Plugs
Soldering VI: Resistance Soldering
Soldering VII
JA3TBW Solder Guide
Soldering Kinks
Vinther Fingertip Soldering Iron

SOLDERING--Part VII Resistance Soldering

The method of resistance soldering works by passing extremely high current through work pieces (the connection being soldered, not the components); the work pieces then become heated and act as their own soldering iron. The tool consists of either a pair of electrodes, or a single electrode associated with a ground-return clamp. These tools are sources of electric current, not sources of heat.

The resistance-soldering tool does not heat up and stay hot. The only heat energy it absorbs is transferred to it from the connection, and, unlike conventional irons, this transfer of heat to the tool is not very efficient, since the tool is not in a molecularly bonded solution of solder with the work. Therefore, the tool generally gets cool enough to touch very soon after a connection has been made. This has great appeal to some blind users; they position the tool and the solder with their fingers, and then they press a foot switch to initiate current flow.

You ask, "Why didn't you tell me that there was a perfectly safe way to solder, before you wrote hundreds of pages on lethal hot irons?" (The author has been criticized by three readers who are dyed-in-the-wool proponents of resistance soldering.) My answer is that things are not always as simple as they seem; there are major pitfalls which must be reckoned with. In fact, my respect for resistance soldering as a viable technique came not from my own experience, but from seeing some top-notch work being done in the laboratory of Rick Joy, a deaf-blind reader of SKTF.

Manufacturers' advertisements tout that, "This method allows connections to be made more quickly, so that heat won't be transferred to nearby components and to insulation which might otherwise melt!" "Quickly" is an undisputed truth--and at least one manufacturer, American Beauty, is honest enough to include the statement, "The eye is generally the best judge of when to remove the tool." I'll "second" that statement! The only limit to the temperature a connection can reach is its ability to dissipate power--the temperature can go sky-high very quickly.

My first casual experiments with this method were disastrous. I would, in my usual style, nonchalantly feel about the connection with the solder; when it melted, I withdrew the solder and then withdrew the soldering tool. More than once, I would find that as much as an inch or so of insulation had vaporized along adjoining wires--that's getting hot. (The literature suggests that you can also use this equipment to anneal and to temper metal parts--that's hot!)

American Beauty suggests that, before you use the equipment on actual work, you try several power settings on similar scrap work pieces. All the manufacturers recommend their equipment chiefly for assembly-line soldering, where the equipment can be preset for one size of joint.

Recognize also that there are electrical risks involved with resistance soldering. If, for some reason, good contact between wires at the joint is not established, you can inadvertently put components in series between the electrodes of the tool; this can test your smoke alarm systems and set the neighbors to wondering what you're cooking in there. It pays to know a little electrical theory and to use good judgment when you place the electrodes on the work. (These devices are powered by low-voltage, high-current transformers; the current supplied to the electrodes is therefore AC.) Problems can sometimes be eliminated by heating only one item of - the work pieces. If you heat one item, pick the one of largest heat capacity-the biggest piece of metal.

Finally, you can only solder stranded wire if it has been pretinned--otherwise, you might burn up a strand or two, and never have an effective "soldering iron" (the soldering iron being comprised of the work itself). Of course, this raises the question of the tool's effectiveness in tinning stranded wire, a necessary operation in all electronics work. Therefore, you might well consider keeping an "instant-heat, fast-cooling" gun or "cordless" iron on the premises for just this task. (A discussion of guns and cordless irons appeared in "Soldering--Part I," SKTF, Fall 1980.)

All in all, however, "resistance soldering" is a legitimate alternative for the blind technician. Though the above remarks may seem skeptical, most of the questions will find adequate answers in the discussion to follow.

General Description of Tools

Electrode Configurations

Three common styles of tools are readily available. They are described as follows:

The classic electrode configuration is a single rod, traditionally made of carbon, but nowadays often made of an alloy of metals. Along with this is a "ground-return clamp" which is attached to one of the work pieces a little ways away from the connection. This configuration is very good for large items of high heat capacity. Carbon has the advantage that it can tolerate higher temperatures, while metal has greater tensile strength.

Another configuration is to have two electrodes, mounted in a "fork" with a fixed spacing, which can be adjusted (by bending or grinding) to fit a particular job. This style is often used in assembly-line settings. On the other hand, PC-board connections are just standard enough to make this the instrument of choice for doing this kind of work.

The style of most interest to us is the tweezer-type tool. This consists of two pointed electrodes (made of some metal alloy which does not take solder) mounted in a spring-loaded insulating handle. This device, though limited in the size of work it can accommodate, can handle most any of the commonly found electronic connections. (These can be gotten in large enough sizes to solder 14-gauge wire, for example, or small enough for wire of less than 50-gauge.)

All of the above configurations are available in extreme variations, if you jump from one manufacturer to another. For example, the Hot Tip company makes very small single-electrode probes. Other companies make a "plier-type" tool which has carbon blocks for jaws--you can temper steel with those things. Yet another style is a panel containing electrodes--this panel being mounted directly onto the power transformer.

Power Transformer Types

The power transformers are either multi-tapped or variable types; they are good for 100 watts or greater. Their on-off switch is usually something separate. Most often, you buy a foot switch which has a piggyback AC plug and socket at the end of its cable; the transformer plugs into this receptacle. On the other hand, American Beauty sells a series of units which are controlled from a separate connector on the "power unit;" this facilitates the option of having a pushbutton on the handle of the soldering tool.

An embellishment is to add a timer which shuts the power off after a preset period. The American Beauty literature cautions that this is not a recommended feature where variations in the size of work pieces are expected.

Solder Preforms

Resistance soldering equipment is often used with pre-formed units of solder; they may be in the shape of balls, beads, or tiny washers--with or without flux. These "solder preforms" are marvelous for industrial applications on an assembly line; you just slip them over a lead, place the tool on the work, and press the foot control.

Use of solder preforms takes away an important source of feedback for a blind technician. Now that you are not holding onto the solder, how will you know when it melts? It may be possible to depend on alternative cues. (The following tips refer to "Tactile Feedback" as discussed in "Soldering--Part II," SKTF, Winter 1981.)

Looking for a sharp rise in temperature of an adjoining component lead may still work, but only if the lead you are feeling is not one being electrically heated by the tool. If you are sure that the lead you are monitoring is not being directly heated by the tool, a sharp temperature rise will probably mean that wetting of this lead has occurred.

The "squeakiness" of solder-wet metals (when subjected to slight motion) will always tell the story. However, this squeakiness will not be as apparent through the resistance-soldering tool as it is through the conventional iron. This is true because the electrodes of the resistance-soldering tool are never "wet" themselves.

Finally, solder preforms are usually miniscule in size; fitting them in place is not easy. I prefer to hook the solder around a lead (3/4 of a turn being enough for small socket pins in a PC board), and this puts me back in the position of being able to monitor the solder flow directly.

Selection of Equipment

Everyone agrees that "trial and error" (or perhaps having a "sales consultant" drop by) is the way to decide which setup best suits your application. It's not quite as bad as all that--you can make a good guess from the tools listed here.

If you're going to err, do it on the large side. For example, if you are going to solder house wiring; get the largest tweezers you can find, a single electrode of perhaps 1/4 inch diameter, and a power transformer of 250 watts. If you are interested in soldering point-to-point wiring on perforated board, whether IC's are involved or not, tweezers whose electrodes are pointed and whose shank diameters range from 0.04 to 0.08 inches will do fine, and the power transformer need only be good for 100 watts.

A continuously variable transformer may be a convenience; then again, it means that tried-and-true settings will be harder to repeat exactly. The variable transformers are a nicety where fine control over a particular industrial operation is being set up. Your work may involve a variety of work sizes; close monitoring of melting of the solder will be necessary anyhow, so why not make do with a less expensive multi-tapped unit.

Basic Procedure

Stated more than once in the American Beauty book, "The Principles of Resistance Soldering," initial setup of the equipment will necessitate a trial-and-error process. It is for this reason that this paper cannot say specific things like, "Set the PDQ-5 power supply to position 3, and so forth. Students of this series of articles know enough about the metallurgy of soldering to do educated experimentation on their own--say yes. The basic steps are:

  1. Set the transformer for a setting suitable for the job (based on your experience and by making an educated guess.)
  2. Place the solder on the work in such a way that you can judiciously monitor its status; you will want to know, with certainty, when it melts. Wrapping the connection with a measured amount is one method; you can then monitor the solder by pulling on it slightly. Then again, some people like to feed solder when the time comes (this takes less setup time); in this case, keep it against the connection and press forward gently.
  3. Place the electrodes on the connection with the power off (doing so with the power on will cause impressive arcing).
  4. Close the power switch (be it a foot control or a pushbutton on the soldering tool).
  1. Watch that solder like a hawk; put enough tension on it so that you will know immediately when it melts. If a pre- determined amount has not been wrapped around the connection, apply an appropriate amount-and be quick about it!
  2. Simultaneously release the switch, remove the tool and the solder.
  3. Modify the transformer setting as necessary, based on the previous similar connection.

If you can pass current through all the connecting work pieces (and if they are all of reasonable size), everything will get hot enough to take solder. However, this does increase the chance that you might pass current through circuit elements and not through the connection--if the items are not in firm contact, etc. You can heat the largest item at the joint and make this your "soldering iron" (instead of the whole collection of items). If you do the latter, wetting of all metals will not occur simultaneously; firm contact between the pieces is again necessary in order to promote rapid heat transfer to them all.

In choosing the "correct" heat setting, all of the literature specifically says to "choose the highest setting which the operator can control effectively." I prefer to interpret this statement loosely; if taming the system down a bit means that I have enough time to make a decision or two during the process, I'll do it that way. Remember, these tools are intended to be sold to assembly-line shops; they want to keep the times minimal.

I would think that turning up the heat would have diminishing returns. Nowadays, the solder we use often contains so-called "activated flux." According to the Kester soldering manual, "activated fluxes," when compared to the most basic rosin flux of yesteryear, have inferior breakdown characteristics in the presence of excessive heat.

On the other hand, setting the heat too low carries its own set of risks. As with any soldering, too small an iron or too little heat will allow a damaging amount of heat to be imparted to connected components before the joint reaches soldering temperature. Given the wrong conditions--poor lead and electrode placement--these ill effects could be magnified with resistance soldering. The energy to one item could be terrific, enough to damage this component, but not enough to heat its neighboring leads at the joint.

Troubleshooting

I love this section in books. "Make sure the power setting is correct, and make sure all connections are secure." Of course, all that stuff works, or you wouldn't be reading the book.

American Beauty suggests that the electrodes could have gotten dirty--coated with flux or with oxides. With their units (which have solid alloy electrodes; i.e., no plating), they recommend that you clean them with a strip of emery paper or a small wire brush. Not knowing all the variations you may run into, as far as electrode materials are concerned, I would have to advocate reading the instructions of your particular tool.

I have seen one interesting problem with a set of Hot-Tip tweezers which were not working. The electrodes had bent so that the chucks that held them were shorting ahead of the connection; installing heat-shrinkable tubing around one of the chucks would prevent this from ever happening.

Tools

Sorry, my direct experience with these devices is so old that I would not feel comfortable in recommending anyone product specifically. I can outline (basically restate) principles discussed earlier, so that you know what to watch for. Meanwhile, price is not much of a factor; everybody, except for Teledyne Kinetics, wants from between $100 and $120 for a setup (by the time you get a foot control and such).

Of the three users I have interviewed on the matter, two use "tweezer-type" instruments. Tim Cranmer, on the other hand; has used a single-electrode device to heat the posts of wire-wrap IC sockets, around which he would wrap the wires being attached (just one turn around the post), then wrapping solder around the post further up. There are a couple of single-electrode units small enough to permit this, and they are listed here. Remember, you can buy one power unit (and an accompanying foot control if it is not included)--then pick an array of hand pieces, including both tweezers and single-electrode holders. (Of course, all single-electrode assemblies come with the necessary "ground clamp" which serves as the return path.)

All of the advertisements are misleading when they speak of "precise temperature control" and "no heat being transferred to adjacent components." What they really mean is that, given a particular repeatable situation, you can set up the equipment for--and get the operator used to--a given solder connection, and make the connection efficiently enough to protect nearby components.

They all refer to their adjustable power supplies as having something to do with "temperature control," which is true, only indirectly. One manufacturer called their power supply "a solid-state temperature controller;" right away I jumped for joy. Could this mean that they sense the temperature of the connection and thus control the temperature? Nah, it's just a transformerless variable power unit which is small enough to plug into the wall directly.

If the marketing jesters were forced to study engineering (thus spending a short while in a "Control Systems" class), they would learn that the only systems that you can really call" "controlled" are ones which have corrective feedback. "Open-loop" systems have adjustments; one can adjust them so that, all things remaining constant, the resultant "control" over their operation will be predictable. Open-loop systems, however, are not controlled, in the true sense of the word.

What you need to do is, by experiment, set up the variable power supply so that a solder connection can be made in a reasonable time--in a range of perhaps 2 to 7 seconds. This "control" will change for every piece of work you're doing.

American Beauty (American Electric Heater Co.)*

For each hand piece, a "power rating" is listed. This indicates with which power unit the tool is to be fitted. It does not indicate the power delivered to the work; this cannot easily be determined, since every item has its own current density and resistance.

Note that the foot control comes separately from the power unit. Although this is not true of other manufacturers, the prices of the eventual setups are still quite competitive.

Single-Electrode Handpieces:

Tweezer-type Hand Pieces:

Power Units

(add suffix of 220/240V for these voltages):

Foot Switch:

Hot-Tip*

Single-Electrode Hand Piece:

Tweezer-type Hand Pieces:

Power Transformers

(120V 50/60 cycles shown in U.S. Catalogues):

(Note: The foot switch apparently comes with these power units.)

[The editor would start with the T-10S tweezers, and possibly include the P-40 probe. The H-191A transformer has enough power for anything I would be doing.]

Teledyne Kinetics*

These are basically self-contained; there is no power transformer on the bench. The RSC-1 ($85) has a solid-state variable supply that plugs into the wall. The RS-1 is simpler ($60); they don't say how this one is powered. Neither have foot controls. Rather, they seem to have a pushbutton control. They don't say whether the pushbutton also controls closing of the tweezer noses, but it sort of looks like that from the pictures.

Triton*

This device is a unit out of which protrudes electrodes that look like square long-nosed pliers. It has a trigger that serves two functions: a slight pressure closes the points. Additional pressure turns on the power.

Square carbon tips are standard. However, small-diameter "special metal tips" can be gotten which are mounted into appropriate adapters.

The power transformer has two taps, high and low.

Suppliers and Manufacturers

Marshall Industries 788 Palomar Avenue Sunnyvale, CA 94086 (408) 739-8720

Macdonald 1736 Standard Street Glendale, CA 91201 (800)423-2453 (Outside CA)

American Electrical Heater Co. (American Beauty) 6110 Cass Street Detroit, MI 48202 (313) 875-2505

Triton Manufacturing Co., Inc. East Haddam, CT 06423

Hot Tip 6 Elm Avenue Hudson, NH 03051 (603) 883-7708

Teledyne Kinetics 410 S. Cedros Avenue Solana Beach, CA 92075 (619) 755-1187

THE G3JYT SOLDER GUIDE

by Frank Jeanmonod, G3JYT Pinner, Middx., England

I have used this guide successfully for many years; it works very well on the pins of IC chips, as well as for soldering larger connections. (I use a small Weller iron for the IC projects.)

The basis of this tool is a darning needle (which we call a "bodkin"). The handle is made from a bit of 3/4-inch doweling. I drill a tiny hole in one end and ram the bodkin into the hole so that the pointed end protrudes about 2-1/2 inches.

Though not sharp, the point is such that when the tool is braced against the connection, the darning needle tends to stay in position. These needles are stainless steel, so solder does not adhere to them.

I first wrap solder around the bodkin--2, 3, or 4 turns, depending on the connection--, after which I locate the joint with its pointed end. Then, I slide the solder down the bodkin so that it contacts the work.

The bodkin is easy to find with the hot soldering iron; I then slide the iron down to the work.

[Editor's Comments: Well now, we have another good inventor in our midst. This tool has one distinct advantage over the tubular guide (SKTF, Spring 1983); this guide will never clog up. It has the disadvantage that you can no longer monitor the melting of the solder, which is ordinarily the main cue that the connection is hot enough. However, you still get other indications: Small motions of the iron against the work will feel (and sound) "squeaky" when the flux has done its cleaning job. The smell of flux will be apparent. The adjoining leads and components will exhibit a sharp rise in temperature as the solder-wet pieces promote efficient heat transfer. Bravo, and thank you, Mr. Jeanmonod.]