Effect of shear rate on lead free solder joint strength

Solder joint strength at high strain rates is a critical reliability requirement for portable electronic devices. Experimental observations showed low shear rate tests of solder joints cannot be used to accurately predict the mode of deformation and failure behaviors under high speed impact condition. Due to strain rate effect, brittle failure of solder joints under dynamic loadings may not take place at low strain rates. Thus, characterization of solder joints under impact becomes critical in package design and manufacturing for high reliability. This is particularly true for lead-free solder in handholds devices. Present study focused on the deformation and failure behavior of single solder ball joints under impact conditions. With a newly designed single-ball impact tester, impact strength of ball joints of two lead-free solders, i.e. Sn-1Ag-0.5Cu (SAC105) and Sn-1.2Ag-0.5Cu-Ni (LF35), were tested at a speed varying from 0.5 to 3m/s. Peak load and fracture energy obtained at different speeds were compared. The failure modes of the joints were studied by scanning electron microscope (SEM) and correlated to the peak load and total fracture energy which were strain rate dependent and changed with solder ball composition due to different strain hardening effects of the two lead-free solders.