Topping off solder pot

[bpiphany]

I have a solder pot that I use to dip solder headers on my small projects. I like it a lot over hand soldering all those joints..



I filled the bath up with lead free Sn96.5Ag3.0Cu0.5 solder, and I will soon need to top it off with more. There's plenty of dross formation, probably more than what ends up forming actual joints.. Still the time saving is a blessing.

My question is if there's a better alloy to top off with to have a better chance of keeping the composition of metals correct. I obviously have no way to analyze what's actually in my pot. I dip only "gold flash" boards, and do the occasional tinning of wires.

[Ian.M]

I have a solder pot that I use to dip solder headers on my small projects. I like it a lot over hand soldering all those joints..

<snip video>

I filled the bath up with lead free Sn96.5Ag3.0Cu0.5 solder, and I will soon need to top it off with more. There's plenty of dross formation, probably more than what ends up forming actual joints.. Still the time saving is a blessing.

My question is if there's a better alloy to top off with to have a better chance of keeping the composition of metals correct. I obviously have no way to analyze what's actually in my pot. I dip only "gold flash" boards, and do the occasional tinning of wires.

Is that video *your* pot?  It seems like you don't keep much solder in it.   Unless you regularly replace a significant part of the solder volume with fresh alloy of the desired composition, you are liable to run into problems with buildup of gold, nickel and copper, and loss of tin by oxidisation eventually resulting in poor wetting and brittle joints due to unwanted gold and nickel intermetallics.    Without quantitative analysis of the alloy, topping up with other metals or alloys will do more harm than good.  With analysis, there may be some benefit in adding pure tin as it will be lost at a higher rate, or using Sn96.5Ag3.5 for bulk replenishment to compensate for copper buildup.

The most cost effective option for a small pot would be to buy SAC305 bar solder, and be prepared to dump the existing solder when you notice an excessive melting point shift or it gets too 'claggy'.

[bpiphany]

Yes, that is my pot. I was more concerned with which metal was primarily skimmed off in oxide form. But probably tin then? The bars available seem to be either SAC305 or pure tin. So if anything else than SAC305 pure tin would be the candidate.

In total I've added the full 2.5lb bar by now (roughly 1kg).

Say that my pads are 1mm diameter (including the header pin filling the hole), 40 per board, and I've done 1000 boards, then that equates to about 125000mm².

How thick is the plating? I have no idea, say 1µm. If we assume all the plating dissolves we're at 125mm³. In pure gold that is 2.5g. Dissolved into 1kg that is 0.25%

Some of those contaminants would have left the pot in joints, which also the base alloy have done. I'm not going to try to make up a differential equation based on shaky assumptions over that =D

I have no idea how off that calculation was, what concentration of gold is bad, how much other junk the chinesium headers release..

I think your advise to simply add SAC305 until the solder stops behaving is sound. Nothing I build is particularly critical.