Effect of Ni thickness on the IMC and reliability of ultrathin ENEPIG

Electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) is a common surface finish featured by good solderability, wide application and reliable mechanical properties. With the miniaturization and high integration of the integrated circuits, high thickness of Ni-P will affect the signal integrity because of the gigantic resistance. This study focused on the effect of the Ni layer thickness on the morphology and evolution of IMC and the reliability of ENEPIG. Commercial SAC305 solders were reflowed on different thicknesses of Ni (0.3um and 3um) to study the Cu/Ni IMC evolution while OSP sample was used as a control sample. Aged experiments were conducted at 150°C to evaluate the long term thermal stabilization and After every aging stage, we conducted high speed shear test to estimate the mechanical strength of the IMCs and analysis of fracture morphology. After aging, scallop-shaped IMCs of (Cu,Ni)₆Sn₅ was formed and its thickness increased with aging as well. Brittle IMC (Cu,Ni)₃Sn first began to appear in ultrathin ENEPIG and the shear strength dropped slightly. After 256 h aging, ultrathin Ni layer had been completely consumed and a large number of (Cu,Ni)₃Sn formed while 5um ENEPIG still had a good barrier effect. Even if Ni layer had been consumed, ultrathin ENEPIG still had a certain inhibitory effect on IMC growth compared with OSP, probably because of the formation of NiSnP layer. Further drop tests and temperature cycle tests will be conducted and the failure models together with cause will be also presented combined with previous results of IMC evolution.