I have obtained a sample quantity of something closely resembling unobtainium.
It is lead-free solder with a melting point of 217C, cored with activated rosin flux. CASTIN alloy from Aim, with composition Sn96.5Ag2.5Cu0.5Sb0.5
Aim claims that the wetting action of SAC type alloys decreases with increasing Ag content, and that the 2.5% Ag gives a more optimum wetting than 3.0% Ag typical of SAC305 which is likely to be the industry standard lead-free.
They implicate Motorola in having a lot to do with the choice of SAC305, though the fact that CASTIN is patented probably doesn't help.
Also, the 0.5% antimony helps to keep grain size consistent over a range of cooling rates which is a large plus for an alloy to be used in hand soldering/rework. Slow cooling tends to cause large grain sizes, resulting in the very frosty appearance of lead-free solder joints.
If you have experimented with any SAC305 hand soldering, you will discover that it is possible for the alloy to harden to a shiny joint, but the cooling must occur at a very rapid rate, and having a lot of flux helps as well.
The CASTIN's antimony content seems to help make the frosty surface remain as finely grained as possible, and also if a cross section of the joint is examined under a microscope, the CASTIN is more homogeneous with respect to grain size, whereas non-Sb bearing SAC alloys are much more heterogeneous. Thus CASTIN could be expected to deliver more consistent joint strength in non-automated soldering methods.
In my initial experiments with the RA-cored CASTIN, I am greatly impressed by how well it wets and flows, as well as its tendency to give a more mildly frosty surface appearance than what often results with SAC305.
I think it is the RA flux though, not just the alloy composition that leads to the very good wetting action of RA-cored CASTIN. But perhaps the two factors together are as close to a optimum lead-free as is possible to achieve. It really is not much different from working with Sn60Pb40.
However, due to market forces, it appears likely that we will not see rosin cored lead-free wire solders available in the future. At least not from Aim, but they seem to think that this will be the case throughout the solder industry. They don't want such a large matrix of flux/alloy combinations, so they want to stock only no-clean and water-clean flux cored wire solders.
I think this sucks, partly because for all the non-automated soldering that goes on out there, rosin is something that people are familiar with. Even just the smell. No-clean smells like a burning chemical stink, whereas rosin is quite pleasant. (At least for those of us who don't suffer allergic reaction to rosin.)
But I am convinced that rosin is a better flux, compared at least to the no-cleans that I have had the opportunity to try. I like the RA CASTIN wire *much* better than my Kester no-clean SAC305 wire. When I add some rosin to a joint prior to soldering with SAC305, then it performs much better. But I don't want to have to dispense bonus flux to ever joint I perform. That's the point of flux core in the first place. The fact that no-clean wires tend not to have more than 2% flux, where RA may be up to 3%, may also be a factor.
It is actually possible to custom order CASTIN with RA flux, but one would have to order 24 lbs. Hmm, that makes me wonder: Is anyone interested in a group purchase of 0.031" RA cored CASTIN wire? I suppose if we could get it down to 1-2 rolls per person, it might not be a bad deal. Or maybe I should swing for 24 rolls and sell the surplus on Ebay.
I hope that there will ultimately wind up to be RA cored SAC305 wires available in the future, when the RoHS misfortune has fully taken hold. Perhaps the manufacturers will get enough complaints from folks who are just used to rosin and its pleasant smell to stock it despite their present judgements of the market.