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 electical solder alloy with antimony is Ag03A (96.5%Sn, 2.5%Ag, 0.8%Cu, 0.5%). 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 '17 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 '17 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 So it has been withdrawn from catalogs.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 Sunnyskyguy EE75 Nov 29 '17 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 '17 at 21:46
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    \$\begingroup\$ Bad for water? Then why is the plumbing solder 5% antimony? \$\endgroup\$ – Electric-Gecko Dec 1 '17 at 1:36

Because hobbyists and plumpers 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 '17 at 1:43
  • \$\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 '17 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 '17 at 2:17
  • \$\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 '17 at 10:21

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