Fine wire for breakout board soldering that won't break

Question

I've been using silicone sheathed stainless steel wire (30 awg) with a 1.27mm breakout board. The wires cannot be secured (it's tight in the thing I'm building), and they easily break off at the point where the sheathing is stripped back. I notice this happens from the wires swaying back and forth. Eventually they wiggle so much that they break.

I can't really use any larger wire. I've noticed the same thing with 28 awg wire, so I don't think that will help either. Is this just because it is stainless? Would I have better luck with copper? Any pointers on how to do this correctly?

Answer 1

As a general rule, any direct wire-soldered-into-PCB connection is prone to mechanical failure at the edge of the solder joint, if it is allowed to move at all. This should be avoided, unless mechanical strain can be relieved off the joint, and even then I would avoid it, depending on the product context.

For reliability and repairability, you would want some sort of PCB screw terminal, or a connector system with crimped wires and a header soldered onto the board. Some sort of termination that isolates the solder from the wire.

A 50-thou connector on the PCB would be compatible with a small 1.27mm JST or Molex PicoBlade etc.

Stainless steel wire? That's a strange thing to be using in any electronic application. It isn't solderable under normal conditions, and to terminate it reliably you would probably want to use a screw terminal. At high currents, differential thermal expansion will make the joint become loose and potentially catch fire (this is a well known problem with old aluminium wire in buildings, terminated into copper connectors) and therefore terminal blocks made out of stainless steel would need to be used.

In a corrosive environment such as a marine environment, just use ordinary copper wires, maybe tin-plated copper, and use appropriate enclosures, sealants, gaskets, conformal coatings etc. Stainless steel will cause you no end of trouble.

Wire with very fine strands tends to be more flexible, particularly if silicone rubber insulation is used. This can make it less likely to put stress on the soldered joint and break.

Adafruit sell some nice little spools of thin, flexible 30AWG stranded wire, tin-plated Cu, in a range of colors.

Answer 2

Mostly comment (need 50 to post comment): I might know what you have - How do you know it is stainless? Stainless is not used for electrical interconnects AFAIK. You can't solder stainless (at least not without special materials that are corrosive IIRC)

I am 're-working' an $80K machine that was designed by a novice builder (an engineer no less). He mistakenly ordered silicon insulated wire from China, that turned out to be (copper flashed) ALUMINUM!. Plain aluminum is also not solderable. I think the Chinese found an outlet for their reclaimed aluminum scrap! It also does not seem to be pure annealed, but is very brittle.

Check if you wire is actually aluminum, or perhaps a poor grade of tinned copper. Insulated stainless would be pretty rare.

I would recommend abandoning that wire and rewiring it, if you aren't too committed into this. I expect it will only continue to cause failures, even if you manage to get them all successfully connected at the same time.

PS I am also curious about your wire specs; post a pic or share the label text? Copper with tin plate or tinned would be silver in color too. If it is a poor quality of copper, it can also be very brittle. Proper copper wire is very ductile, and should stand up to multiple 'bendings'.

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@Jasen Good point. I didn't know if network cabling went down to 30ga. (personal note about myself, I didn't know I would have to start out needing 50 reputation again on another SE board! Anxious to qualify, this is my area of expertise!)

Answer 3

If the wires break right at the point where the insulation is stripped, then there's a very good chance that your stripping tool is nicking the wire. All it takes is a tiny scratch on the wire, and it will break at the scratched spot from vibration.

Either adjust your tool so that it doesn't nick the wire, or get a better tool.

Alternatively, use a thermal stripping tool. These use heat to melt the insulation off of the wire. They are available as handtools.

You might also use a tinning pot to remove insulation from wires.

Another thing you need to keep in mind is that stainless steel wire is stiff and springy.

If you have to twist and hold the wire in place against its natural springiness, then it will over time pull itself out of the solder.

You need to arrange your wires so that they are not under mechanical tension or so that the mechanical tension forces the wire into contact with the solder pad .

I've had steel wires pull out. The solution in that case was a tie wrap to hold the whole cable assembly in place such that the one steel wire lay exactly in place when the whole assembly (housing, PCB, and cable) was put together.

Gluing the wires in place will help both with wires breaking and wires pulling out.

To be honest, it sounds very strange that you are using stainless steel wire. I don't know where I could order stainless steel wire.

For connecting breakout boards in a cramped space, I'd probably use 30 AWG Kynar wire wrapping wire.

It is readily available and easy to solder. It isn't as stiff as steel wire, so it won't pull itself out of the soldered joint.

The use of "breakout boards" makes it sound like you may need to reconsider what you are doing. It might be more productive (and more reliable) to have a printed circuit board (PCB) made that can be properly installed rather than wiring in some standard breakout board with jumper wires.

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At one place I used to work, we modified electronic equipment by installing small modules inside the equipment.

The modules had connectors with 24AWG wires, and we used them that way if there was room in the equipment.

If there wasn't room, we'd either have a flat flex connector made, or else use 30 AWG Kynar wire wrapping wire to connect them. Many times, the choice fell to wrapping wire because flexes were expensive to have made and difficult to get right.

There were two of use who did the installation work.

The other fellow tried every mechanical wire stripper known to mankind.

I used a melted blob of solder on the tip of my soldering iron to melt the insulation off of the last millimeter of the wire.

His were beautiful, a joy to look at.

Mine were melted, burned, and ugly.

I worked there for over 10 years. We did a lot of that kind of stuff.

The only devices that came back because of broken wires were the beautiful, mechanically stripped wires from my coworker.

The ugly, melted and burned wires that I produced never, ever came back for a broken off wire.

I used the melting technique because way back in the beginning I had tried using the (admittedly crappy) wire strippers we had and found that nicked wires would sometimes break while I was installing the modules. I switched to melting the insulation and never looked back.

The other fellow thought the melted insulation was ugly and unprofessional and kept looking for the perfect setting on the perfect mechanical wire stripper - and kept producing joints that failed in use.

Answer 4

Glue

If you can't get a mechanical strain relief to hold the wires, glue them down after soldering them. epoxy, hot glue, or neutral-cure silicone are all good options.