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. 
- 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?