Growth and selection of intermetallic species in Sn-Ag-Cu No-Pb solder systems based on pad metallurgies and thermal histories

The adoption of Pb-free solders will affect manufacturing processes and joint reliability for electronics packages. Since SAC solder has a higher melting temperature than eutectic Pb-Sn solder, higher processing temperatures will be required. The higher processing temperatures allow for increased solubility of some elements, such as Ni, within the solder melt. The specific selection of pad metallurgies can influence both the type of intermetallics that form at the pad interfaces, and the age related evolution of those intermetallics. In joints where both Ni and Cu pad metallurgies are present, the fast diffusing Cu species can allow the intermetallic compound, (Cu,Ni)₆Sn₅ to form at the opposite, Ni, interface. The absence of a Cu pad can starve the system of Cu and effectively stop the growth of this (Cu,Ni)₆Sn₅ intermetallic compound. The Ag component of the SAC solder can, under proper conditions, result in the growth of Ag₃Sn plates in the solder melt before the Sn phase nucleates. This is controlled by the specific concentration of Ag in the solder, by the cooling rate of the solder, and by the tendency of the solder joints to undercool by a typical 30°C before the Sn phase nucleates. The critical cooling rate for the onset of this phenomenon is between 3°C/sec. and 0.3°C/sec., within the range of normal reflow practices of the electronics industry.