Influence of NiTi shape memory alloy as under bump metallization on thermal mechanical behavior of solder joints

Nowadays surface mount technology faces a critical problem that is the inelastic strain concentration accumulation in the solder joint during thermal cycling because of the mismatch of thermal expansion coefficient. Recent research has shown that the reinforcement of NiTi shape memory alloy particle in the solder matrix can suppress the inelastic strain concentration. The objective of this research is to investigate how NiTi shape memory alloy applied in the under bump metallurgy can affect solder joint reliability during thermal cycling. Multi layer thin film UBM of Cu/Ni/NiTi was prepared by physical vapor deposition. Photolithography technique and stencil printing was used to form well aligned and homogeneous solder balls on the deposited thin film UBM. The differential scanning calorimetry, transmission electron microscopy and X-ray diffraction were used to identify the crystal structure of NiTi and the crystalline temperature of amorphous NiTi thin film. Moreover, the cross section observation and ball shear test on the solder joint after different thermal cycles revealed the comparison of mechanical properties between samples with and without NiTi thin film. Furthermore, finite element simulation exhibited the influence of NiTi shape memory alloy on plastic strain distribution after thermal cycling.