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

Abstract

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.