Effect of thermal ageing and thermal cycling on the reliability of solder joints of TAB outer lead bonds

The reliability of outer lead bonding (OLB) joints of gold plated tape automated bonding (TAB) leads after hot bar soldering was investigated. Reliability in the solder joints was affected when it was subjected to thermal ageing and thermal cycling treatments. Solder joints degraded with increasing ageing hours and thermal cycles. Thermal ageing resulted in coarsening of ridges at crack initiation zone, whereas thermal cycling resulted in the decrease of the area of crack propagation zone as treatment time were increased in both cases. From the cross section of fracture surfaces, solder was found to be the crack initiation site. A proposed terminology, grain size equivalent (GSE), was introduced to explain the effect of solder grain size on the pull force. An empirical equation was derived and results implied that solder grain growth was the major factor that caused degradation of solder joint reliability within 750 hours and 1500 cycles.