Effects of thermal history on the intermetallic growth and mechanical strength of Pb-free and Sn-PbBGA solder balls

Purpose - To investigate effects of the thermal history on intermetallic thickness and morphology and on the resulting shear strength of the ball attachment for a variety of BGA components. Design/methodology/approach - In this study, a variety of BGA components with balls made of Pb-free Sn-Ag-Cu (SAC) 305, Sn-Pb eutectic and high-temperature 90Pb-10Sn alloys, were subjected to different thermal histories, including up to ten reflow cycles, and aged at 125 degrees C from 24 to 336 h. The intermetallic thickness and morphology after these thermal events were then examined under optical and scanning electronic microscopes. Ball shearing tests were conducted to investigate effects of the thermal history and intermetallic thickness and morphology on shearing strength of these solder balls. Findings - The results show that effects directly from intermetallic layers may or may not be detectable; and the shear strength of solder balls is largely dependent on the solder alloy and its microstructure. Shear strength increases are observed after multiple reflow cycles and ageing at elevated temperature for the two Pb-bearing alloys, while the SAC305 lead-free alloy shows slight reductions in both strength and ductility after thermal exposure. Practical implications - Presented results can be used for estimation of reliability for electronic assemblies subjected to multiple rework and repair operations, which expose sensitive components, such as BGAs, to elevated temperatures. Originality/value - It is believed that a sound understanding of the effects of intermetallic morphology and thickness on reliability of BGA solder balls can lead to more intelligent choice of soldering processes, as well as to rework/repair process optimisation and to establishing their operational limits.