Wetting and Spreading of molten solder on the surface of solid metal is essential to soldering technology. There is several expressions of wetting ability, wetting angle is one of them. Smaller wetting angle, better wetting ability.
Conclusion: All of the Lead-free solders have worse wetting ability than Sn-Pb eutectic. Selection of flux is essential for lead-free soldering. Assessment of a number of different fluxes from different suppliers is recommended to find the most suitable one.