Reliability performance of tin–bismuth–silver (Sn57.6Bi0.4Ag) solder joints with different content of carbon nano-tubes (CNTs) or nickel (Ni)-modified CNTs

In this study, the reliability performance of Sn57.6Bi0.4Ag solder joints containing different amounts (wt%) of CNTs and Ni-coated CNTs (Ni–CNTs) has been investigated. Thermal shock testing was used, as a means of harsh environmental stressing. The theory of the reinforcing effects for these two dopants was discussed. The difference of the dopant redistributing process during reflow soldering was also analysed. The formation mechanisms of cracks in the doped solder joints were elucidated through morphological analyses. Ball shear testing results were reported to substantiate the mechanical strength improvements of such joints. The characteristics of the fracture surfaces were analysed to clarify and correlate the changes in the related performance. Sn57.6Bi0.4Ag solder joints containing 0.03 wt% Ni–CNTs have been shown to offer the optimal reliability performance, as evidenced by its highest resistance to thermal shock. Ni coated ones showed a positive effect on the distribution of CNTs. However, solder joints containing excessive dopants of > 0.05 wt% yielded certain undesirable defects in the thermal shock testing, as evidenced by its deterioration in mechanical properties. The relative significance of such defects, e.g. aggregation of CNTs, formation of loose CNT-alloy structural phases and rapid propagation of cracks, varied considerably depending of the type and amount of dopants.