What lead-free solder type is convenient to use for hand-soldering?
Lead is a magic element that makes solder melt at a lower temperature and protects the tip of your iron against oxidization. Too bad it is a potent neurotoxin.
Most lead-free solders have high melting points. (Bismuth solder paste is lead free and has a lower melting point than leaded solder, but because it is a paste, using it with an iron is not very convenient.)
Without lead, and at higher temperatures, tip oxidization is your #1 problem. That means shutting off your iron immediately when you aren't using it. Don't let it bake. Some fancy lead-free irons have an auto-setback that reduces the tip temperature when it is placed in the cradle.
And stay away from "CLEAN" solder! Water-soluble flux is very aggressive but corrosive! It will eat vias and traces if it is not immediately cleaned off (should be called "MUST CLEAN"). It is only used by industrial processes that include an ultrasonic bath after soldering.
"NO CLEAN" solder is what you want and means the flux is not corrosive and does not need to be cleaned. Yes, the terminology is confusing.
FWIW I like Kester Sn96.5/Ag3/Cu0.5. The fumes are no worse than lead solder.
"SAC305" and Sn96.5/Ag3/Cu0.5 are the same thing, SAC is just an acronym for Sn/Ag/Cu or Tin/Silver/Copper. The "305" refers to percentages of Ag and Cu (ie 3% and 0.5%). This is really on the only readily available lead free solder in widespread use. SAC105 is also available but offers no real advantage.
All the lead free solders are higher soldering temperature, SAC305 melts at 217C vs SnPb (37% Pb) lead solder at 183.
https://en.wikipedia.org/wiki/Solder
For hand soldering I prefer wire solder, but paste is possible too. The paste has a shelf life though, after a year it may be too thick to extrude from the syringe.
For fine pitch SMT parts you will want the finest gauge solder.
The key to a good lead free joint is plenty of flux. The fine gauge solder has so little flux in it I always add flux, either liquid from a dropper or "flux pen", or tacky paste which helps hold the parts in place while soldering.
Flux comes in 3 basic types:
No Clean: Requires no cleaning or solvents as the residue is inert. Note that this is invariably the least effective kind of flux in my experience. It is also very difficult to clean should you want to.
Water Soluble: Very strong and produces very irritating fumes when soldering. This must be washed with water to remove to avoid corroding the assembly. And then the assembly must be dried with something like a hair dryer. I avoid this type for hand soldering.
RMA or "Rosin, Mildly Activated" flux. This is the kind I prefer and gives good solder joints w/o noxious fumes. It should be clean off with Isopropyl Alchohol and a brush after.
I would recommend a fan to create a gentle air flow to push the fumes away in any case. Too much air will make it hard to solder due to cooling.
And lastly you need a soldering iron designed to reach the higher lead free temperatures. I always recommend a Metcal with interchangeable tips. They are expensive new, but are built like tanks and older quite functional units can usually be had for less than USD$200 on ebay.
Bit of an old question but:
It's worth looking into SN100C. It's a eutectic alloy of 99% Sn, 0.7% Cu, 0.05% Ni, and less than 0.01% Ge, with a slightly-higher-than-SAC305 melting point of 227°C (compared to 63/37 at 183°C and SAC305 at 217~220°C (non-eutectic)).
It has some of the same problems as other lead-free solders, but it has no silver content (making it cheaper than silver solders), and I've found it easier to work with than SAC305 or pure tin. It flows better and forms good, shiny joints where silver solder doesn't. It seems reasonably good at preventing tip oxidation too, but it's still good to keep some tip tinner on hand.
The main downside is that it's a little harder to find as of 2020, since the patent on it only expired last year and manufacturers other than Nihon Superior (the patent holder) are only just starting to make it. I was able to find it from Chip Quik, who call it CQ100GE, but you may find other sources.