How hot are you setting your solder iron for hand soldering using solder wire or preforms? If you are cranking up the solder iron temperature because you have transitioned to lead-free solders, think again.
For both Sn63 and SAC solders, the suggested hand soldering temperature is 315°C. Using this temperature, the solder iron tip temperature is significantly hotter than the liquidus temperature of either alloy, although there is a 34° C differential or more between the liquidus temperature of these two alloys.
Hand soldering relies on the thermal conduction of heat from the solder iron tip to a solder alloy, relying on Fourier's law for thermal transfer. The hot temperature of the solder iron tip will quickly rise the solder temperature to its melting temperature at this setting. If the solder iron tip temperature is set higher you can quickly lose control of the solder quality.
One of the biggest issues associated with high temperature hand soldering has more to do with the flux than the solder alloy. Most solder fluxes, regardless of form (liquid, tacky, or flux core) are designed to withstand temperatures up to 350°C. Above these temperatures, they begin to decompose and blacken.
Solder alloy oxidation is also an issue associated with high temperature hand soldering. As a solder is exposed to high temperatures, the rate it oxidizes increases. Fluxes aid to remove surface oxides, but at high soldering temperatures, fluxes are quickly exhausted.
For optimal solder results, keep the solder iron tip temperature low.
Source: Indium Corporation