Tin Bismuth Solder + PCB hotplate

My first SnBi soldering project

With my pcb hotplate built, I needed a test for my new tool. My goal is to be able to easily do prototypes of pretty much anything in-house, to avoid the delay, cost, and general PITA of outsourcing board fab. I also want to switch to a lead-free process.

Motivation to go Lead-Free

There are a couple of advantages I see with moving to lead-free SnBiAg solders, especially at home. The first is that the liquidus temperature is much, much lower- around 140 C. Eutectic tin-lead solders come in around 185C, and SAC lead free solders melt around 220. This lower temperature has two pleasant benefits. First, you won’t end up scorching your parts/silkscreen/soldermask with hot air trying to get some chip off the board. Second, everything is easier/faster to heat up since less energy is required.

This lower temperature also seems like it should reduce the energy in the flux fumes, which I can only assume is better than the alternative (use a fume extractor, kids).

The other advantage is that there is no lead in lead free solder. It’s 2023- we know lead is bad. I have done a lot of soldering with lab/food separation, and while leaded solder is probably fine if you have a lab setting, its not something you want to get in your carpet/personal desk/place where you might sleep or eat. Realistically there’s always a little loose solder created when soldering (think of what is the bucket of a brass sponge).

That said, I’m keeping my tin-lead solder (for good reasons).

How Does It Compare?

A quick test- most of the paste printed well (look at C12 and C19)

One limitation is that SnBi solder can’t be used with leaded solder, since this can form a Sn-Pb-Bi alloy with a very low melting point (below 100C). And, for some reason, it does not seem to be available in small wire diameters. Hopefully this will change, since having a huge 1mm solder wire sort of prevents any kind of small SMD rework using wire.

Another limitation (as a result of not playing nice with lead) is that leaded pad finishes are inappropriate for SnBi alloys. This, combined with fine-pitch parts, lead me to use OSP. Both leaded and unleaded solder seemed to have less-good wetting on this surface than on ENIG/HASL type finishes. I think that SAC HASL is probably OK for a surface finish but I have not researched it. ENIG would have been preferable for the QFN.

It also seems to take a long time for the solder to get wicked up into solder wick, which can be frustrating. I think a larger tip would help here, but my best “normal size” tip is currently reserved for leaded soldering.

And that’s about it. Overall a good replacement, especially for prototype boards that don’t get hot!

A First Project

I wanted to really see what I could get away with, so I designed a board with a QFN, some fine pitch USB-C connectors, and a handful of of fine pitch leaded SMD components. There were also some larger parts like buttons, crystals, and a whole module, that represent “bulky” parts. To raise the stakes, I decided I would go crazy and use all kinds of $10 ICs.

To solder this, I used the chip-quick NC191LT50, which is a paste that does not need to be refrigerated. It printed nicely using a stainless stencil.

Test run showing bridged pins, some issues wetting

I then put it on the PCB hotplate and took it up to about 160C. Since this is a four layer board, it took a few seconds for the top to heat up. The soldering went surprisingly well, but the QFN had some bridged pins.

Since the solder is low temp, removing the IC was a breeze and the board stayed totally un-scorched. I quickly wiped up some of the solder, and hot-aired it back on. It is now working just fine!

Larger parts were soldered with the same alloy, but with solder wire. I only had a very small soldering iron tip to use (a non-leaded one), so it was hard to get good contact, but the solder did eventually wet everything nicely.

Overall this went shockingly smoothly, given the diversity of parts and using a while new process!

Conclusion: Tin Bismuth Silver solder is here to stay

I am pretty happy with this stuff, especially for projects where a stencil makes sense/is affordable. I’d hate to solder fine pitch components with the huge wire, but I think a paste (even from a syringe) and a hotplate will go a long way.

Posted in: ENG

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