Failure Mechanism and Kinetics Studies of Electroless Ni-P Dissolution in Pb-free Solder Joints under Electromigration

Electromigration (EM) of Pb-free SnCu solder joints with NiPdAu surface finishes are studied. The failure mechanism and kinetics of electroless Ni-P dissolution under EM are discussed in detail. Under EM, depends on the Sn grain orientations, Ni atoms in Ni-P plating can be quickly consumed and the entire Ni-P plating can be transformed to P rich Ni3P. The volume shrinkage caused by this reaction can induce the Ni3P/Cu separation or accelerate the voids nucleation between IMC/solder. Two interesting phenomena were observed in kinetics studies of Ni dissolution: (1) the existence of long incubation time before the onset of Ni dissolution; (2) Depending on Sn grain orientations, Ni dissolution rate varies dramatically. The current density exponent n (shown in Black's law) for fast Ni dissolution is much larger than that for slow Ni dissolution.