I use 62 (Sn)/36 (Pb)/2 (Ag) low melting point (LMP) solder at 320°C. For 60/40 or 63/37, I'd recommend 360-370. 400 is too high IMO; flux may evaporate away too quickly. Thermal linkage is important when it comes to speed: https://www.circuitrework.com/guides/7-1-1.html