Kinetics of interfacial reaction between eutectic Sn-Pb solder and Cu/Ni/Pd metallizations

Abstract

The interfacial microstructure and the kinetics of interfacial reaction between the eutectic Sn-Pb solder and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) were studied both in the liquid- and solid-state of the solder. The liquidstate reaction was carried out at 200°C, 225°C, and 250°C for 30 s, 60 s, 150 s, and 300 s at each temperature. The solid-state aging was carried out at 125°C for up to 43 days. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope. Depending on the thickness of the Pd-layer, both PdSn₄ and PdSn₃ phases were observed near the solder-substrate interface. These results were correlated with the initial thickness of the Pd-layer and the diffusion path in the calculated Pd-Pb-Sn isothermal sections. For the aforementioned isothermal reactions, only one Ni-bearing intermetallic (Ni₃Sn₄) was observed at the solder-substrate interface. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Pb-Sn isothermal sections. Selective etching of solder revealed that PdSn₄ and PdSn₃ had a faceted rod morphology, and Ni₃Sn₄ had a faceted scallop morphology which gave rise to rugged Ni₃Sn₄-solder interface. Segregation of Pb on the facets of PdSn₄ and PdSn₃ was also observed. The growth kinetics of the Ni₃Sn₄ intermetallic layer at the solder-substrate interface was analyzed using an Arrhenius-type of equation. In the thickness regime of 0.16 to 1.2 µm, the growth kinetics yielded a time exponent n=3.1, an apparent activation energy of 9260 J/mol both in the liquid- and solid-state of the solder, and a temperature dependent pre-exponential factor. The latter was attributed to the presence of one or more phases ahead of the growing layer.