Pb-free solder—microstructural, material reliability, and failure relationships

The establishment of near-eutectic Sn–Ag–Cu (SAC) alloys as replacements for eutectic Sn–Pb marked as the beginning for the development of lead-free solder alloy in electronic packaging industry. Second-generation lead-free alloys with lower Ag content were introduced to address the shortcomings such as poor mechanical shock performance and higher cost. Yet, the evolution has not stopped. In response to higher reliability requirements, third generation lead-free alloys are being developed to serve the applications operated in increasingly aggressive environments. In this chapter, case studies on thermal fatigue performance of various SAC-based lead-free solders and Sn–Zn-based low temperature solders have been made. The effect of solder size, solder composition, Sn grain morphology, printed circuit board surface finish, and thermal cycling profile on solder joint microstructure and reliability is evaluated. The relationship between microstructural evolution and thermal fatigue failure mechanism is discussed.