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In my previous question I focused on lead-based solder alloys (see Best solder wire - Sn63Pb37 vs Sn60Pb40 vs ...?).

Some of you brought new exciting solder alloys to my attention. The two most promising types contain either Bismuth or Indium.

Some questions:

  • How do they compare to each other on properties like:

    • wetting of pads and pins
    • interaction with fluxes
    • joint strength - brittleness (is this directly related to the "tensile strength (PSI)" column in the table below? Please enlighten me.)
    • price
    • ...

  • How do they compare to standard solder alloys, like Sn60Pb40?

  • Any drawbacks?

  • Most of them are sold without flux core. Wouldn't that make the solder process very difficult?

  • What kind of flux should one use with Bismuth and/or Indium based solders?

Note:

Perhaps the following table might be of help:

enter image description here

This table comes from "Indium Corporation".

K.Mulier
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2 Answers2

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They are very expensive. Some of them are outright forbidden (e.g. all Cadmium based alloys). These special alloys are made for special needs: Low temperature soldering (e.g. if you have a 100°C thermal fuse, you cannot solder it with normal solder), low EMF (e.g. if you want to measure voltages with <100µV of accuracy), etc pp. Unless you are doing very special stuff you will not need them.

Attila Kinali
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  • Low EMF is a concern for me, as I sometimes make sensitive analog circuits. Please tell me more about how these solder alloys can improve the EMF... – K.Mulier Sep 20 '17 at 18:12
  • Even if your analog circuit is sensitive, it does not mean you will get any benefit from low EMF solder. The usual electric contacts we use have something like 1-10µV/°C. Your average opamp will have have an order of magnitude higher temperature coefficient or even just plain drift, not to mention the sensor you are using. Also keep in mind this is a DC offset. Unless you are measuring very low frequencies, it does not matter. To learn more, just google for EMF or Seebeck effect. – Attila Kinali Sep 20 '17 at 18:37
  • Very interesting. Did you ever use Bismuth or Indium based solders? – K.Mulier Sep 20 '17 at 18:39
  • I didn't. Although there were cases where I considered it, the benefits never justified the additional cost. – Attila Kinali Sep 20 '17 at 18:44
  • Is it brittle? Will the joints crack under vibration, or will the ductility of Indium prevent that from happening? – K.Mulier Sep 20 '17 at 18:46
  • Sorry, I have no idea about the mechanical properties of those solders under stress. – Attila Kinali Sep 20 '17 at 18:51
  • @K.Mulier, if you are concerned with vibration and other mechanical stresses, I believe that 63/37 or 60/40 lead-based alloys are still preferred (and mandated for use) for test equipment and military applications, and this kind of equipment is officially excepted from the idiocy of ROHS regulations. – Ale..chenski Sep 20 '17 at 19:22
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    @AliChen though I obviously agree that normal lead-free tin is really annoying to work with – please don't deride those regulations as idiotic. In prototyping and specialty applications, the amount of lead may not pose a health or environmental risk, but in mass-produced consumer applications it does sum up to a significant one. And high-tech industrial production has proven to be quite capable of making good, reliable electronics without any lead, so what's the point... – leftaroundabout Sep 20 '17 at 21:07
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    @leftaroundabout, I maintain that ROHS is idiocy. The logic behind is inherently faulty. The regulation replaces well-studied lead with a bunch of different kind of heavy metals, for which medicine has no foggiest clue about their health impact, simply because there were no sufficient studies, yet. Lead is harmful in a form of salts. Yet there are miles and miles of car salvages with broken lead-acid batteries leaking into soils and underground aquifers. No problem here, right? – Ale..chenski Sep 20 '17 at 21:23
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    @AliChen Sure there's a problem there! And you're probably right, car batteries are a _bigger_ problem. However these batteries are rather well-controlled and a big part recycled; the obvious and much more direct danger of the corrosive acid makes this somewhat inevitable. Electronics are far more commonly dumped, smashed, perhaps even chewed on... that has little _direct_ harm, making long-term issues easier to sneak along. — The main components of the common SAC solder _are_ studied just as well as lead. Sure, other heavy metals are used too, but in far lower proportion if only due to cost. – leftaroundabout Sep 23 '17 at 11:22
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Soldering alloys have many properties that target different applications: malleability, melting point, solidification point, expansion, etc. See this selection from major supplier, Indium Corporation. There used to be some open tables with bunch of parameters, but I can't find it easily.

Ale..chenski
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  • Thank you very much. Do you have personal experience with either Bismuth- or Indium-based solders? – K.Mulier Sep 20 '17 at 18:16
  • @K.Mulier, yes, I do have some experience. At one point (~10 years ago) I made an alloy with T=100-120C per Indium recipe, out of individual metals, and soldered a heat pipe to exposed pad of a video adapter chip, with a set of flat fins at the other end, making a silent heat sink. – Ale..chenski Sep 20 '17 at 19:17