Small Wire Soldering
Background

Especially in products containing cyclically flexing wires, or wires which are bent or twisted as part of the function of the device, broken connections nearly always become a problem. Wires often aren’t designed to handle stresses introduced by excessive bending. Because of this, devices like headphone signal cables, phone chargers, computer mice and especially the voice coils in headphones and speakers usually fail after a relatively short time.

Some of the smallest repairs that I have yet performed have been made to the voice coil wires attached to headphone driver diaphragms. With the repeated bending of the thin copper wires, sometimes large in magnitude and usually a few million times per song, failure is inevitable due to the fatigue failure limits of all metals, including copper, which is most often used to form voice coils due to its high conductivity. I’ve had two pairs of headphones fail in this way and have repaired the fine wires leading to the voice coils on both pairs to bring their drivers back to life.

In my experience, as they experience cumulative wear, USB charging/data cable wires tend to fray and internally detach, thus breaking the connection between the ends of the cable. I can recount more than 10 different times that I have tried to repair these frayed connections by cutting out the damaged section of the cable and re-soldering the wires to the contacts at the head of the cable. In these situations, the wires end up being load bearing structures, though, which is undesirable, so I usually choose to fill the open space left behind in the head with hot glue to aid in supporting against any loads.

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Wires soldered to rigid structures are horrible at withstanding repeated bending loads, a lesson that I learned after improperly repairing a few charging cables. Quite similarly, headphone cables seem to disconnect from the audio drivers found within headphone earpieces, likely due to solder joints handling the tensile stress that arises when the headphones are tugged around by the cable. I’ve had to repair that kind of failure as well as reconnecting the wires in pairs of headphones that I purchased online whose wires had been sliced. At one point, I decided to entirely do away with the original cable from one such pair of headphones and replace it with my own cable, which ended up being a whole large modification project in itself. Fine soldering is a very useful skill which has allowed me to create and repair many circuits.

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Detailed Description

I have seen many headphone and device charging cables designed with exposure to repeated bending in mind, usually reinforced by surrounding the wires with threads meant to take up tensile stresses, however these still tend to fail in certain applications. I have had countless iPhone chargers fail at the interface between the cable and the charging head which tends to experience the worst of the stress from repeated plugging and unplugging of devices. I’m rather surprised that Apple still has yet to find a stable material to use in charging cables to give it the rigidity and cyclical lifetime necessary for their cables to outlast those made by other charging cable brands.

In another case but on a finer level, about halfway through college, I noticed a loud popping sound in one ear whenever I shifted my Bose QC15 headphones around. It was bearable, but the intensity progressively worsened, and it began occurring as a result of not only my movements but also the music itself. I dissected the headphones, de-soldered the driver and examined the frail connection between the voice coil wires and their rigid contacts on the driver, through which they received audio signals.

One wire looked sharply bent to the naked eye, but when I magnified it further, I could see that its ends were detached and would momentarily lose contact with each other if the diaphragm moved to pull them apart. My solution was to remove the entire diaphragm, voice coil and all, unwind the copper coil slightly to free up a small amount of undamaged wire and very lightly adhere it to the diaphragm leaving part of it free to absorb the motion without introducing large stresses, and then I re-soldered the ends of the voice coil to the driver contacts. That pair of headphones has functioned perfectly for another 3 years so far without any issues, which has made it worth the time that I invested in repairing them.

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