Formation of Ag3Sn plates in SnAgCu solder bumps

Special experiments are designed to obtain the solid reactants directly from a liquid solder during phase transformation. Series of such tests performed throughout reflow, which enables to investigate the entire formation process of intermetallic Ag3Sn plates out of liquid SnAgCu solder bumps. The results show that Ag3Sn plates are formed first in the middle of the cooling stage. In the plane, they have two preferable growth directions. By varying thermal conditions during reflow, the formation mechanism of these plates is discussed.