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The most common lead free electrical solder alloys are SAC305 (96.5%Sn, 3.0%Ag, 0.5%Cu) and Sn99 (99.3%Sn, 0.7%Cu), especially for hand soldering wire.

Small amounts of antimony (Sb) have been shown to improve the properties of tin-based solder. Most of these are mechanical (strength) benefits, similar to the benefits of silver while being cheaper. The most common electrical solder alloy with antimony is Ag03A (96.2%Sn, 2.5%Ag, 0.8%Cu, 0.5%Sb). It's still not as common as the above mentioned alloys in mass production, and is almost nowhere to be found for hand solder wire. So I question why this is.

Here are some advantages I see of the Ag03A alloy vs SAC305 (due to the antimony):

  • Increased wetting (probably, although 0.3% is more optimal for this)
  • Increased thermal fatigue resistance. [1]
  • Inhibits tin pest and other undesirable compounds.

And here are some more advantages that come specifically from replacing some of the silver with antimony:

  • Lower cost
  • Less silver oxide (which ruins the solder when heated for too long)
  • Reduced cracking

So why is this alloy not more common? Some sources say that AIM has a patent on it, but according to this it only applies in USA and Japan. But isn't SAC305 also patented?

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    \$\begingroup\$ Isn't it toxic? \$\endgroup\$
    – pjc50
    Nov 29, 2017 at 21:38
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    \$\begingroup\$ Lead also improves solder quality, but isn't used for the same reason. \$\endgroup\$
    – evildemonic
    Nov 29, 2017 at 21:41
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    \$\begingroup\$ Although China has the most Antimony in the Earth's crust, they also consume the most solder and this element can contaminate ground water. So it has been withdrawn from catalogs. \$\endgroup\$
    – Tony Stewart EE75
    Nov 29, 2017 at 21:45
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    \$\begingroup\$ Antimony isnt THAT toxic (compared to Lead) - however some of its compounds are less than savory - especially in the water table \$\endgroup\$
    – Rendeverance
    Nov 29, 2017 at 21:46
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    \$\begingroup\$ Bad for water? Then why is the plumbing solder 5% antimony? \$\endgroup\$
    – Electric-Gecko
    Dec 1, 2017 at 1:36

3 Answers 3

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Since the time you asked this question there are now more products containing antimony on the market. That said, concerns over antimony toxicity are still very much a problem that prevents higher quality lead-free solder from flourishing.

The possibility of antimony + zinc chloride toxicity are relevant, but this alone cannot explain the public perception of antimony toxicity [see comments to question for an example]. Chlorine and ammonia based cleaners have this same issue, and are definitely used in the same settings. Perhaps mandatory labeling is worth considering, but one harmful interaction with one specific soldering fluid does not fully explain the slow uptake.

The core answer to your question is that companies were and still are adverse to creating & marketing a product containing an element which is often assumed to be more toxic than it is. A compound known as antimony trioxide, formerly a commonly used fire retardant that is now outlawed or heavily restricted, and several related compounds used in plastic production add to this confusion. The deep mental relation between antimony and lead, and toxic compounds containing antimony make this a difficult perception to overcome.

None of this should matter, antimony itself is less toxic than silver based on the sources I could find that separated antimony as it is found in solder & electronics, (antimony and toxic compounds containing antimony are often lumped together in many reports). There seems to be an increase in research related to antimony and its manufacturing and use in solder and electronics, but it has a long way to go to outweigh the older body of research related to other antimony compounds and even more so, the perception of the public.

I know this question is old, but I felt there are still misconceptions about antimony. I hope we start seeing more effective lead-free solders that use this element, and I hope the relevant public finds there isn't as much reason to be afraid of it.

Sources

An industry source, so bias may or may not be included:

http://www.bomir.com/downloads/aim/Antimony_Study.pdf

Lots of interesting references to environmental impact of production and disposal:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.491.9436&rep=rep1&type=pdf

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    \$\begingroup\$ Interesting. Your answer is still useful (as I never got a better answer to this day). It's interesting that you're talking about public perception. I personally didn't know about Antimony Trioxide. I don't see anything on Wikipedia regarding it's ban. Also, I suspect that the type of people who buy solder are more likely to do their research, and less susceptible to public perception. \$\endgroup\$
    – Electric-Gecko
    Jun 26, 2020 at 7:01
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Because hobbyists and plumbers may be tempted to use zinc chloride as a soldering fluid with antimony solders. That produces stibine, a very poisonous gas. At least it smells like hell so you have a chance to flee.

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  • \$\begingroup\$ Oh no. I hope I didn't do this. I have a 0.15% antimony solder from Sparkfun. It contains a water-soluble flux core, but occasionally I have added extra rosin flux. Do either of these fluxes possibly contain ZnCl2? Given the small amount of antimony and the other fumes present, I may not have noticed the stibine. I wonder if the people at Sparkfun are aware of this problem. Thank you though. Of all the reading I have done on solder alloys, I have never found this. \$\endgroup\$
    – Electric-Gecko
    Dec 1, 2017 at 1:43
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    \$\begingroup\$ Stibine smells like rotten eggs as hell, you cannot miss it. Flux used in electronics almost never has zinc chloride because you needed to clean it off after soldering to avoid corrosion. Flux used for plumbing often has zinc chloride. \$\endgroup\$
    – Janka
    Dec 1, 2017 at 2:45
  • \$\begingroup\$ For plumbing? But isn't there a common plumbing solder alloy that's 5% antimony? Do plumbers need protection from this? \$\endgroup\$
    – Electric-Gecko
    Dec 2, 2017 at 2:17
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    \$\begingroup\$ You simply should never use zinc chloride with this. If I was the boss of a plumber gang, I had to seize all the old flux cans and cross fingers no-one of the other plumbers on a big construction site was still on zinc chloride so anyone could borrow that. It wouldn't be my favourite solder because of this, regardless of the price difference to SnAg. In a boiler factory, it is far more practical. \$\endgroup\$
    – Janka
    Dec 2, 2017 at 10:21
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Sb (Antimony) itself is not a toxicity problem. However, if Sb-containing solder mixes with a Pb-containing solder it can form a very low melting eutectic (think Wood's metal). If Sb-containing Pb-free solders always are used with Pb-free joints, it is OK. But the prevalence of Pb-containing solders (such as eutectic Sn-Pb) it's not reliable.

Luckily this will become less of a problem as all electronics switches to the Pb-free compositions.

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