I was at Potters' electrical supply a few days ago and got to wondering about our plan to use Wood's metal for printing electrical circuits. Wood's metal has always bothered me a bit because of it's use of lead. While I know that it will be some time before individuals are prohibited from using SNPb solder it strikes me that it is only a matter of time before we see that happen.
What I looked at at Potters' that got me to thinking was a roll of SnBi solder that melts at 138 degrees Celsius. That is about the same temperature that we extrude CAPA. What went "click" in my head was that the solder had a diameter of ... about 3 mm.
Wouldn't it be cool if we could just use a Mk II to print that stuff?
On the minus side, it might have enough thermal capacity to melt too far into the CAPA. If we built up a conductor trace out of a series of very thin threads like we do with CAPA, however, we might be able to control that.
On the plus side, a lot of components that we regularly deal with like the 754410 and the L298N get quite warm in operation. That would make using Wood's metal with them a bit dicey. Having a higher temperature conductor trace material could be an advantage there.
One thing that I saw when checking the characteristics of Bismuth is that it has an electrical resistance ranging from half to a full magnitude higher than materials ordinarily used like copper, lead and tin. Mind, half of that SnBi solder is tin, which has quite a low electrical resistance, so maybe tin is going to be the active conductor while bismuth will simply be the enabling material for the eutectic mix.
I think I am going to buy a roll of that next time I'm in Potters' and play with it a bit.
I'm always off on my own tangent... but I have to wonder how well that stuff would mix with CAPA... If they were 'friendly', they would probably mix easily when melted, shifting the thermal capacity more towards pure CAPA, and probably lower the viscosity of the compound, making it easier to 'extrude' feedstock...
On that last point... I have to wonder if the MkII would be capable of handling the raw solder; it's my understanding that the CAPA is a lot more viscous than solder... but then the solder has surface tension and loves to cling to hot metal... I wonder if you could attach a needle or a paperclip or something at the end of the extruder to 'guide' and 'lay' the solder onto the board... if you did it right, you could have the 'needle' either melt a trough into the underlying capa to accept/hold the solder, or melt the underlying solder enough to properly bond to the layer you're depositing... You could even use it to 'spot solder' components that are manually inserted into the piece while it's being made... (so long as could compensate for colisions/etc, maybe a preprogrammed path done by hand or something...)
What if you layed a piece of the solder into a 'valley' in the circuit board and extruded hot CAPA over it, would the CAPA be hot enough to melt the solder enough to 'impregnate' it into the board? It's a little 'labor intensive', but if it works... it'd be a good repstrap technique...
I think the only way to find out is to try that sort of thing. :-)