Why are electrical solders with antimony not more common?

Question

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?

Answer 1

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

Answer 2

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.

Answer 3

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.