Growth and Ripening of (Au,Ni)Sn₄ Phase in Pb-Free and Pb-Containing Solders on Ni/Au Metallization

The evolution of an interfacial microstructure between Cu/Ni/Au bond-pad metallization and Pb-free or Pb-containing solder bumps during solid-state aging was investigated. A distinctive difference between the Pb-containing and Pb-free solder bumps was observed. A continuous (Au,Ni)Sn₄ intermetallic compound layer was readily detected in the Pb-containing solder bump on the bond-pad. No such layer exists in the Pb-free case; instead a large number of (Au,Ni)Sn₄ particles scatter in the bulk of the solder. The different morphologies of the (Au,Ni)Sn₄ have been explained on the basis of the contact angle in Young's equation and of the growth of a flat layer by ripening. A simple kinetic analysis of ripening between a set of monosize spheres and a flat surface is given to describe the layered growth having a time dependence around 0.5.