electric coil for a DIY solder bath, wrong idea?

[mcinque]

I need a medium size (about 20x20cm) solder bath for a small production and I can't afford a proper one at the moment.

I'm trying to build it one just by using a proper sized metal tank and a washing machine's electric coil (I will use an MCU and a K thermocouple to drive a solid state relay to control the temperature, keeping in consideration the thermal inertia of the system).

If the coil can stay inside the tank, touching directly the solder, it would be much simple to build for me (just two holes...), but I've seen that the commercial solder baths have the coils under the pot or inside some pipes to protect them, probably from corrosion.

I don't need long term reliability, and if the coil's coating gets damaged by the solder exposing the live on the melted solder, the earthing would trip the circuit breaker, so no safety risks.

What do you suggest about this project? Do you think someting is wrong my details?

[NiHaoMike]

I don't think the insulation will last very long at such a high temperature. Maybe use stove elements? They are designed to run red hot.

[Ian.M]

Apart from the element sheath possibly slowly dissolving in the solder, I see two potential problems:

1: Element overheating during first melt due to poor thermal contact between solder pellets and the element.  This one's probably resolvable by melting the initial solder charge to cover the element in a ladle over a gas burner.

2: Differential contraction damaging the element on cooldown.  Resolvable by fully emptying the pot on switchoff before the solder solidifies.

An externally heated pot would be a lot easier to use.   What about a stainless pan on an induction hob, with extra fan cooling of its induction coils?  Add a glassfibre mat between the pan and the hob top for less thermal loss.

[Gyro]

I've seen it done in a flow soldering machine, It had more than one element though - and I remember that was important once, when one of the elements failed! There were crossed fingers all round that the remaining one would survive long enough to fully melt the solder in the tank.

I think they were probably stainless steel sheathed.

[SeanB]

The problem with standard immersion elements is that they are made from nickel plated brass, so any fracture in the nickel plate leads to a very fast erosion of the underlying brass. If the base is flat and stainless steel, simply take a flat plate stove element, clean it down to bare steel and use a thin layer of silver solder paste to bond it to the base of the pan. You use the element to heat the silver solder up to melting, but it will only have to do this once, as normally a lead load will draw enough heat to prevent it melting again. The element will probably survive the silver solder at least once, as they run at red heat in cooking with some settings. You get silver solder paste used for preform brazing of stainless steels, which have a rather agressive flux carrier, and which only melt at over 500C, so are perfect for this. The standard use id for brazing stainless steel mesh or for bonding sheet edges in industry. will probably melt at red heat, but as long as you clamp the pot firmly to the element and only rely on the braze for heat transfer it should work for a few dozen cycles. If it fails at least you will be able to undo it with a gas torch.

[mcinque]

hmm... thank you for all your advices.

@SeanB, nice advice for the silver solder for bonding the stove element.

@Gyro, nice, so if I find a stainless stove element I could use it without issues...

@Ian.M, yes, an induction heater would probably do the job, and it's a nice idea to use fiberglass to insulate the pot. Unfortunately I don't have an induction coil/heater to use

#NiHaoMike, what about a ceramic insulated element?

[mcinque]

About the induction heater: won't the inductive field affect and possibly damage any component being soldered on the dipped pcb?