DIY Chip Quik style desoldering alloy (#P37)

And finally closure on the fake China solder wire topic back from WHL #68 / WHL #68F1. I bought a bunch of Bismuth metal in early June (1KG Bismuth Metal Ingot 99.99% Pure Crystal Fr Making Crystals/Fishing Lures – 20.03€ including shipping, before the “no more VAT free imports” EU legislation went into effect) and with the high lead content solder from the other review, I should be able to make some low-melting point solder.

First of all, the Bismuth was delivered in a big chunk as advertised, which I did not expect. They are somehow able to split that crystallized cake precisely into pieces of slightly below 1kg and then add in small bits to make exactly 1000g (at 2 cents a gram, they clearly do not give away free stuff). I did measure something like 1003g and the big one covered pretty much 99% of that.

A consequence of that big blob was that the “parcel”, the usual grey plastic bag without any cardboard, was, let’s say, dense. At 9,78g/cm³, one kg is just 102ml plus inner ziplock bag, plus plastic bag, plus label, no big gaps in between. Just check the photo, it’s barely 7x5x3cm³ in size. Since it was delivered with Hermes which is a new thing, probably in preparation of the EU legislation requirements, I’m very pleased the delivery guy left it on the stairs and didn’t throw it through the window. Some say, Hermes doesn’t have the best reputation in Germany. Or, like anywhere.

Since this block is very hard to split (and not make a mess), I melted it down with the Solder Pot from WHL #48. Melting point is similar to lead-free solder so that’s fine, I did try to clean it out first but impurities added to the stuff really aren’t a problem here. And, because I also happened to have something like a table hot plate with damaged coating, I wouldn’t really call it a grill although it was advertised that way when my grandma apparently bought that in the late 90s, I had the perfect casting mould for it. Long, thin stripes and heat-resistant plus excellent heat-sinking across the plate. So this went well, except for the fact it took ages and my entire apartment was filled with blood-hungry pests when I finally closed the door at like midnight.

Anyway, these sticks are now a good way to dose the bismuth, plus I had lots of fun handling the molten metal. The coloured ones are when the stuff gets too hot and oxidizes, that’s how people make the very shiny bismuth crystals. As shown above, even the mass produced stuff crystallizes nicely, check out the rightmost stick that formed small crystal islands in the reservoir upside down when disturbed during cooling.

Bismuth, by the way, has a negative coefficient of expansion like water does, so it expands during the (apparently double) phase transition from liquid to solid. That’s pretty rare and also present in bismuth alloys like Wood’s metal, which has a 50% bismuth content by weight.

I haven’t yet melted down the rest of the two solder spools in preparation for the mix, but I did make a small test batch with all the crumbs from the bismuth block and the density tests of the various known solders. That made this blob of bismuth alloy:

It has such terrible solder properties that it a) flows in every direction, making it a very flat puddle with very low surface tension, and b) is of such low mechanical strength that it is easily broken in two pieces by hand. Contrary to the bismuth structure shown it has very small grains with no long-range order, gaps in between and also a dull shine, unlike the leaded solder and the bismuth it is made of. So while that is absolutely terrible solder, it is perfect for desoldering.

For tests, I recovered this heavy-duty connector from an IBM power supply distribution board, not too dissimilar to the board from #P35, but bearing the load of two PSUs. Probably a four-layer board with at least 2oz copper on the important layers, this connector would allow dozens and dozens of amps and the board is required to handle all that.

While it is basically impossible to desolder this without damage to board, connector or both due to aggressive use of hot air, desoldering the thing with the leaded bismuth allow is great. I’m not claiming I just went near it and the thing dropped out on its own, but since the solder stays liquid until well under the threshold the soldering iron can regulate (“100°C”), getting and keeping everything into liquid state is pretty easy. From then on one just needs to pull out the connector carefully. After all, it’s a lot of pins, the load bearing ones even connected together, and those still present a lot of mechanical friction even with the solder out of the way.

After removal from the board it however is imperative to clean the thing you intend to re-use (not done here, just sucked off the solder). With traces of bismuth on the pads or legs, the next attempt to solder will produce a weak version of that alloy which also has bad mechanical properties. So re-tin, desolder with a pump or braid, maybe do it again, and then re-use the component. And save the desoldered stuff, because it will be useful for the next desoldering action on heavy-duty parts!

Bismuth solder alloys might damage the tip of the soldering iron if used long-term, so in addition to not contaminate every solder joint afterwards, it would be a good idea to have a separate (old) tip for desoldering with bismuth.

Now, for the Chip Quik comparison: Chip Quik might be the most well-known brand out there for desoldering aids. Their identically named desoldering alloy contains, according to their own datasheet, tin, silver, bismuth, indium and copper “in varying percentages” (I bet you fuckers have a fixed mixture, you just don’t wanna share it – there are however different products, some leaded, some lead-free and some indium-free at different price points). It also seems to contain rosin and other de-oxidizing agents which my DIY solder will not have, so I will need to externally add some flux if needed.
Their best stuff has melting temperatures below 80°C which requires indium to be practical. Wood’s metal does contain cadmium (which is highly toxic, so a commercial no-go) and has 60°C melting point. Field’s metal which is 51% indium, 33% bismuth and tin follows closely at 62°C, but they likely make something different that is not so heavy on indium content. Current price of indium is like 350€ per kg and Chip Quik still needs to make a profit on their stuff – at over 1000€ per kg however (24€ per 22g), that should be possible.

Of course every Chip Quik product will beat this rough DIY alloy hands down, but given the low price of less than 0.03€ per gram compared to more than 1€ per gram of the high-end alloy, the increase in cost doesn’t really translate to handling difference due to lower melting points. For my occasional heavy-duty desoldering that’ll do, and if I ever have to desolder something critical at work I might just deposit a little chunk of that stuff in my drawer. It’s difficult to get something so specialized ordered in time unfortunately, since it is not widely available here. Mouser for example will ship, but it takes a couple of days and we only order once or twice a year from them. For people doing this on a regular basis however: Feel free to require the original product and enjoy. But throwing in a chunk of bismuth will also do…

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