Kester 44 flux makes my throat hurt, and is not as effective as more modern fluxes. It actually has a non-zero halide content (0.44%, hence the name '44') but oddly enough, it's a no-clean flux that has been proven to be non corrosive despite its halide content. Still, you can get a range of alloys, and a range of fluxes, so for 63/37 alloy, I prefer Kester 245 flux. It's also a no-clean flux, it has lower odor and lower irritation than 44, and it also has a greater wetting capability than 44. So, it's better all around - you'd expect that from something designed in this century
The Kester part number for 0.031" wire with 245 flux is 24-6337-8800 (
Searching...). I've become a fan of 0.015" diameter (0.40 mm) wire, as it gives me better control over the fillet, but I work on smaller stuff anyway. Kester also sells that with a 245 flux (part number 24-6337-8806,
Searching...) and I use a bunch of that. In the US, Techni-Tool has a good fresh stock and good prices on Kester products.
For SMD, I use leaded paste since it reflows at a lower temperature and has greater wetting capability. I've been pretty happy using Kester EP256 with a 6 mil stencil, an infrared pre-heater, and a hot air wand. It's a 63/37 alloy with a nice flux and a useful particle size for stencil use. I've been able to print footprints for 0.5 mm lead pitch components (0.3 x 0.75 mm pads) reliably, as long as I get the screening right and do the proper temperature cycle with the preheater.
Someone mentioned earlier that lead solder leaches tin out of SMD components causing failures, but I don't think that's correct. SMD components frequently have nickel barriers on their solderable surfaces to prevent copper from being leached into the joint, and any surface coating on top of the nickel barrier is purely to assure solderability. The typical tin finish on many components is not a problem with any lead free alloy or 63/37.
Gold however will be a problem as it will form an intermetallic with tin or tin/lead solder alloys and makes the solder joint brittle, unless it is washed off first, so maybe that's what you're thinking of? I have been using leaded paste for SMD for several years now and have had zero problems with joint reliability that I can attribute to any alloy. The standard problems like bad footprints, or an improper thermal cycle are the usual culprits, but once a joint has been made successfully, it's stable with all manner of components that use tin finishes.