Shear Strength and Fracture Surface Studies of Ball Grid Array (BGA) Flexible Surface-Mount Electronics Packaging Under Isothermal Ageing

Abstract

Electronic systems are known to be affected by the environmental and mechanical conditions, such as humidity, temperature, thermal shocks and vibration. These adverse environmental operating conditions, with time, could degrade the mechanical efficiency of the system and might lead to catastrophic failures. The aim of this study is to investigate the mechanical integrity of lead-free ball grid array (BGA) solder joints subjected to isothermal ageing at 150 and 175 °C, for up to 1,000 h. Upon ageing at 150 °C the Sn-3.5Ag solder alloy initially age-softened for up to 200 h. This behaviour was linked to the coarsening of grains. When aged beyond 200 h the shear strength was found to increase up to 400 h. This age-hardening was correlated with precipitation of hard Ag₃Sn particles in Sn matrix. Further ageing resulted in gradual decrease in shear strength. This can be explained as the combined effect of precipitation coarsening and growth of intermetallic layer. Samples aged at 175 °C showed a similar behaviour with a reduced initial age-softening period and higher shear force values. Investigation of the fracture surfaces under a Scanning Electron Microscope (SEM) revealed that higher ageing temperature would expose the solder joints to brittle failures.