Ok, one last thing before the springs are being made, as the BeCu 1mm wire has been ordered and shipped.
We initially, or rather I initially, chose zinc as plating as that would be the best compromise between cost, mechanical properties, and electrical conductivity.
However, I have thought about using nickel tin for plating instead. Because tin has a lower melting point, it would be easier to solder a bypass to than zinc, if you actually need to haha, and since it is a bit softer than zinc, it would provide about better contact area with the battery, and provide about the same electrical conductivity. And nickel substrate below the tin.
Zinc is very difficult to solder, and the joint will be weak.
I suggest you get some information about the solderability of the nickel-tin plating before committing.
Gold plating would be superior. Hence the ubiquity of it for torch springs.
I think ENIG (electroless nickel immersion gold) plating would be the best, and is a standard PCB manufacturing process, though maybe unfamiliar for a spring manufacturer.
One of the reasons high quality PCBs are made with ENIG on the pads, to maintain consistent solderability in storage. Or cheap ones are “HASLed” (hot air solder levelled) during manufacture, whilst the copper is still clean. HASL has a shelf life, depending on environmental conditions, it can become pretty much unusable after a short while.
Clean copper has good solderability with standard fluxes. Beryllium copper however is much less easy to solder even with activated fluxes. Specialist organic fluxes are needed for best results. So some sort of surface plating is necessary. Again, I’d suggest gold, if data is not forthcoming for the nickel-tin proposed.
These springs have to be soldered to tail PCB or driver. An easy, reliable, not brittle, solder joint using standard solder and standard flux is paramount if they are going to be good.
The problem with gold plating is that it costs significantly more in small quantities.
It would actually cost double for a gold coating since it is a small order.
It would go from to 350$ to 700$+
Also, I did know about ENIG.
Finally, nickel-tin is actually a great coating material for soldering.
Nickel is first off deposited onto the substrate, then a layer of tin is deposited on it. This allows for good contact on non-even surfaces, and good solder ability, even better than gold in most cases, while also having a shiny silver finish.
TLDR: Gold is too expensive for small batches, so nickel-tin is the way to go for our battery springs.
But nickel-tin is not necessarily as you describe, it’s can also be done in a co-deposition electrolytic process resulting in a uniform nickel-tin alloy.
I’ve never used it, so have no experience to offer.
Do check that the nickel-tin that you are offered has the good solderability, that is important.