Intermetallic Brittle Phase in Solder

Intermetallic phases form when the solder joins with the base material. They are hard and brittle, that's normal and not always a problem. Only when the layer gets too thick does the joint become brittle and can crack. You have to keep soldering time and temperature as short and low as possible, then the layer stays thin. Too much heat or prolonged reheating makes the phase really grow. If you break the joint later and it looks gray and crystalline, there was too much energy in it.