During a theory class in my Guitar Repair and Construction class, we watched a vintage demonstration by Dan Earliwine. Dan is a woodworking wizard, but like most guitar repairmen he should leave the soldering to someone else. I think the video lasted about a half hour and I quit counting after Dan had executed at least twenty cold solder joints. The man simply can not use a soldering iron and feed solder at the same time. He’s not alone. The video reminded me of how little guitar manufacturers and repair techs know about solder, electrical circuits and components, metallurgy, chemistry, and mechanical engineering. Or maybe they know about this stuff but don’t care about it. From wall-to-wall cold solder joints to misused components, the electrical portion of what are otherwise works of craftsmanship and artistry are an embarrassment.
The primary problem is that many people do not understand how solder works. While solder “is essentially metallic glue used to join metal parts together,” as Earliwine says. it might be helpful to think of the soldering task more in terms of brazing or welding. Thinking of solder as “glue” negates needing to understand how the allow process works and makes it seem like you just have to pour on the “glue” to solder, which isn’t unlike how Earliwine solders.
The basic concept is that we are trying to use a metal alloy with a low melting temperature (tin-lead or tin-silver-copper) to produce a bond between two similar materials (usually copper, brass, or nickel). The lower right corner of the alloy type illustration (right) is, ideally, the result of a solder joint’s contact point. For this bond to ideally occur, the two materials to be soldered will be in direct contact as much as possible. A copper-to-solder-to-copper connection is not only weak (structurally) but is dubious electrically. What we’re going for is as much copper-to-copper as we can get. Lead is mediocre conductor, one of many reasons that industry’s leaving tin-lead solder is a good thing (outside of the obvious environmental reason). Lead oxide (and tin oxide, for that matter) is a mediocre semiconductor and, as such, introduces distortion and noise when it reacts differently to current flow in one direction vs. the opposite direction (aka AC, alternating current). So, the traditional guitar tech’s “solder bead” tactic is a terrible idea because the components are, often, not in contact with anything but solder. In manufacturing terms, we call this tactic “reflowing” and it is generally an undesirable practice. Reheating solder produces crystalizing effects that are mechanically undesirable, introduce nonlinear electrical properties, and make the solder brittle. Reflowing without additionally applied flux is a terrible idea, but much of the available tin-lead flux is extremely corrosive (RA stands for “rosin activated,” meaning the corrosive materials, salts, in the rosin are activated by heat). Reflowing is a lose-lose proposition.
The next thing guitar techs get really wrong is poor preparation for the work. First, all components need to be as clean as possible. Wires to be soldered should be new, bright and shiny copper or tinned. Metal parts, like the lids of potentiometers in a guitar’s controls, should be scrubbed clean of old flux and contaminates with a metal brush followed by sandpaper or Scotch-Brite™. After the parts are clean, they should be carefully tinned as quickly as possible to apply a fresh base of tin-lead or tin-silver to protect the newly cleaned or exposed surfaces. When you apply heat, apply as much as possible of the soldering iron tip surface to the largest component. In the case of an old fashioned metal potentiomenter cover, heat the cover and the wire will follow if it is in contact with the cover. If everything goes well, your solder will flow like water to both components and if you are very careful and steady the connection will cool and cure without any vibration or motion upsetting the connection. If it doesn’t go well, you either have to clean off the old solder and start over or add flux when you try to re-heat the connection. Reflowing solder always requires additional flux.
