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Soldering iron tips
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KL27x:
The "soiling" of the iron tip is part and parcel of the high temps you need in the small iron tip in order to heat up relatively large pieces of steel. Just wipe off the burnt flux residue on a sponge or piece of brass wool and carry on.

You could try flux designed for lead free. In some cases it will be formulated for higher temps, the way that brazing flux melts at much higher temps.

I think the white powder flux for brazing is anhydrous boric acid. The salt form of folic acid is folate. I dunno why not "borate?" But, yeah, that would sound kinda dumb.
Ian.M:
The O.P's trying to solder to HO gage model rail.  Due to steel's lower thermal conductivity than copper, it shouldn't be a problem with eccessive flux burning while getting the joint up to soldering temperature, as any decent temperature controlled iron should be able to solder to solid copper wire of similar cross sectional area, reasonably quickly if a suitable large hoof bit is used.   OTOH if he's using an underpowered iron with a poncy little long conical bit, you could well be on to something.
KL27x:

--- Quote ---copper wire of similar cross sectional area
--- End quote ---
I am assuming the tracks are relatively thick for w/e tip he is using. And there is thermal mass to consider in addition to heatsinking. I use regular rosin flux to solder steel a few times. Works fine. And my soldering iron is constantly "soiled" with black residue even when soldering tiny SMD parts.
Domagoj T:
I don't think ultrasonic equipment would be justified, but thanks, the technology looks interesting.
The crud on the soldering iron from that specific flux was not your everyday crud. I spent about half an hour getting it off my tip using brass wool and it was never the same again.
Unfortunately I don't have any lead free stuff on hand. I might look into getting some.
Yeah, brazing stuff is based on boric acid, but it's got something else in it too. In any case, activation temperature for this particular flux is way too high for this application.
The crossection of track is comparable to some thicker wires, thermal mass should not be the issue. I've soldered thicker copper wires without much problem.
I have the Weller PU 81 station and WSP 80 iron. It's 80W, so should be good enough. The tip is a 2,5mm chisel.

All in all, yesterday, I tried with Copper Sulphate, but had no luck. Decide to give it another go and leave the crystals to dissolve overnight. Today, I left a drop of it on the rail for some 20 minutes (I got distracted by other stuff) and after that the solder made a good bond (stronger than a 0,14mm^2 copper wire, which is good enough for me).

Thank you, Ian (and the rest of you guys)!
zomehi:

--- Quote from: Domagoj T on May 28, 2019, 09:03:05 am ---I don't think ultrasonic equipment would be justified, but thanks, the technology looks interesting.
The crud on the soldering iron from that specific flux was not your everyday crud. I spent about half an hour getting it off my tip using brass wool and it was never the same again.
Unfortunately I don't have any lead free stuff on hand. I might look into getting some.
Yeah, brazing stuff is based on boric acid, but it's got something else in it too. In any case, activation temperature for this particular flux is way too high for this application.
The crossection of track is comparable to some thicker wires, thermal mass should not be the issue. I've soldered thicker copper wires without much problem.
I have the Weller PU 81 station and WSP 80 iron. It's 80W, so should be good enough. The tip is a 2,5mm chisel.

All in all, yesterday, I tried with Copper Sulphate, but had no luck. Decide to give it another go and leave the crystals to dissolve overnight. Today, I left a drop of it on the rail for some 20 minutes (I got distracted by other stuff) and after that the solder made a good bond (stronger than a 0,14mm^2 copper wire, which is good enough for me).

Thank you, Ian (and the rest of you guys)!

--- End quote ---
Great. Might try copper sulphate someday.
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