Effects of isothermal aging on interfacial microstructure evolution and shear behavior of Au-12Ge/Ni(P)/Kovar Solder Joints

Abstract

Au-12Ge(wt.%) eutectic solder is widely used in electronic packaging thanks to its good electrical conductivity, solderability, and chemical stability. To investigate the stability of Au-12Ge solder during high-temperature service, the effects of different isothermal aging conditions on the microstructure and performance of Au/Ni(P)/Kovar substrate after soldering were investigated. Solder joints produced at 400 °C for 1 min showed excellent wetting behavior with NiGe/Ni5Ge3/Ni3(P,Ge) intermetallic layers formed at the interface. The thickness of generated interfacial intermetallic compounds was estimated to only 1.56 μm due to the amorphous nature of Ni-P coating that hindered the diffusion of Ni atoms. As aging increased with time and temperature, the interfacial intermetallic compounds grow further, leading to deterioration and failure of solder joints. The growth mechanism of interfacial intermetallic compounds was then examined, and activation energy was estimated to 95.4 kJ/mol. In turn, shear strength of solder joints declined and fracture mode became brittle as aging time and temperature increased. Meanwhile, fracture position evolved from IMC layer to Ni-P layer. At aging temperatures above 290 °C, Ni-P layer showed crystallization transition to porous Ni3P, leading to formation of holes and cracks, and resulting in exfoliation of coating with Kovar substrate.