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Acid-Flux Solder
April 1951 Radio & Television News Article

April 1951 Radio & Television News
April 1951 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Acid-Flux Solder, April 1951 Radio & Television News - RF Cafe

Liquid RMA Rosin Flux (left) and Acid Flux (right). I prefer liquid flux for PCB work and paste for chassis work.

If you do repair or restoration work on electronics assemblies with leaded components (i.e., not surface mount), large metal mounting tabs, or other relatively high mass surfaces to be soldered and the surfaces are contaminated and/or not easily tinned, then the advice offered in this 1951 Radio & Television News magazine article is well worth considering. Acid flux is normally taboo around electronics, but, as author Herbert Brier points out, there are instances where, with careful work, it can be used to create surfaces which can then be soldered using standard rosin flux. The key is to use the acid flux sparingly and being sure to thoroughly clean off the acid flux residue before moving on to rosin flux. I have done this with both electronics and mechanical parts (such as music wire for model airplane landing gear). I do NOT recommend ever using acid flux on insulated wire if the flux can wick up under the insulation, and NOT on stranded wire since the flux will get between the individual strands and can eat away at it over time. An exception might be if the aforementioned wires are just being tinned and can be placed in an ultrasonic cleaner with isopropyl alcohol or even a strong soapy water solution to assure all traces of acid are removed.

Acid-Flux Solder

By Herbert S. Brier

Electronic technicians, who are sometimes tempted to circumvent Ohm's Law, would rather eat oysters in July than to violate the command "Thou shalt not use acid-flux solder." As with most laws, however, there are times when it can be safely ignored. Consider the following:

You are servicing a receiver with, say, a defective filter condenser. Electrically, the repair is simple, but mechanically it is a headache. The old condenser, apparently the first part mounted, is riveted to chassis and completely surrounded by other parts. There is no room to drill out or file the rivet without removing the obstructing parts; therefore you bend the mounting strap back and forth until it breaks. Although the old condenser is now out, the rivet is still there. After searching unsuccessfully for another spot to put the replacement, you decide to solder the new mounting strap to the chassis. Usually, the chassis is dirty and, with no room to work, it refuses to tin. Of course, with patience, the job can be done at the cost of a few charred parts and burned knuckles.

This is what happens with rosin-flux solder. But what a difference a little acid-flux solder makes. The chassis tins easily and, after wiping with a bit of cloth to remove any excess flux, you solder the mounting strap to it with regular rosin-flux solder.

Occasionally, a receiver comes in with a pulley, swedged to a control shaft, loosened. Repair without damaging the control or other components generally requires removing the assembly from the chassis. Probably every technician facing the problem has tried soldering at least once. He usually discovers that either the shaft or the pulley is impervious to rosin-flux solder. Acid flux makes child's play of the job. Tin both parts with it and finish with rosin-core solder.

Rubber-covered "a.c. cord" sometimes solders with the greatest difficulty only after meticulous scraping of each individual strand of wire. Tinning first with acid-flux makes the task easier and usually results in a better job. Use it exceedingly sparingly - just enough to tin the wire, wipe off immediately, and finish with rosin-flux solder, applying heat long enough to cook out any residual acid. The last step is especially important because, of the three samples given, it is the only one where a little corrosion would do any important damage.

Other spots where acid-flux solder could be used to advantage have undoubtedly come to your mind. The important thing to remember is that it is a special-purpose tool. Used as above, it is invaluable, while, if used to  solder something like an i.f. or r.f. coil, it is almost certain to result in an open coil or a very noisy receiver.

In my work, a single dime spool of acid-flux solder outlasts pounds of rosin-flux solder, but it is always ready when needed.

 

 

Posted January 27, 2022

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