Soldering a thin spring steel wire to a PCB

We solder stainless steel from time to time. It needs more heat than I'm really happy bringing to a PCB, and the flux (we use a HCl flux, as you say) will quickly eat the tracks off the pcb if you leave any residue. Our process is to tin the steel with the acid flux, then clean very thoroughly, then solder to the PCB in a separate operation. If you can buy the wire pre-tinned, all the better.

We can't use conductive epoxy for this for various reasons. If we could, we might consider it. But in my experience it comes a poor third place for mechanical strength, and good epoxies (i.e. Epotek H20E, not "silver paint for PCB repair") can be expensive and a little faffy to work with.

Another approach we've used is to crimp a ferrule to the end of the steel wire, then solder that to the PCB. That avoids the nasty flux, and gives a mechanically very satisfactory join. But it does take up a little more space.

Since you seem to be thinking about possible quantity production, consider sourcing plated wire which is solderable. This kind of material is used for such applications as battery holder springs.

You'll have to consider how the high temperatures in reflow soldering will affect the spring metallurgy. Soldering temperature is within the normal range of tempering for carbon steel (and much briefer), so I'm guessing it won't have a huge effect.

I don't have a lot of optimism about how sturdy a soldered carbon steel wire would be stuck through a PCB hole. I think the joint will tend to fracture near or at the wire-soft-solder interface if stressed. For small quantities consider silver soldering the wire to a ferrule (can be done easily with a propane or MAPP torch) and staking/soldering the ferrule into the board.

Rather than soldering, consider using a conductive epoxy.

Example.

Steel can be soldered using tin/lead solder and the correct flux. The amount of heat this needs to work is higher than most PCBs can withstand without de-laminating.