Lead-Free Solders: Focus on Fundamentals, Reliability, and Applications

Although lead-free solders have been adopted by much of the commercial electronics industry, the implementation of lead-free processes still faces some challenges. The performance of lead-free solder joints in high-reliability aerospace, military, and medical applications is still a concern. There is a clear shortcoming in the ability to predict the lifetime of lead-free assemblies in service. Thus, a common objective of current researchers is to develop a fundamental understanding of the initial solder microstructure, its evolution under various reliability test conditions, and the failure mechanism of lead-free solder joints. Their research focus often includes control of the solidification microstructure by understanding the nucleation of Sn, investigating the role of precipitates and intermetallic compounds at the interface during the deformation of solders, and examining the effect of Sn grain morphology and orientation on the thermomechanical response of solder joints. Researchers are trying to better understand strain-enhanced coarsening, recrystallization, and crack initiation and propagation to develop models that can explain the failure mechanism in different reliability experiments. One goal of this research is to provide a microstructurally adaptive model that can accurately predict the life of soldered electronic devices. Concurrent with the transition to lead-free manufacturing, the electronics industry is experiencing a major revolution associated with the introduction of 2.5D and 3D packaging technologies. There, the lead-free challenges faced by materials scientists become even more complex as the size of electronic packages and solder interconnects shrink to enable deployment of thinner, faster devices. New challenges will be introduced as the entire solder joint may be transformed into intermetallic compounds during assembly or operation. Reducing the size of the solder bumps additionally results in higher current density and increased power dissipation, making electromigration reliability more of a concern. More efficient thermal interface materials are needed to increase the heat flux from die to thermal solution. Moreover, the electronic industry is seeking new solder alloys that are low cost but have acceptable reliability performance in both high-strain applications such as drop/shock and vibration and low-strain applications such as thermal and power cycling. The addition of various fourth and fifth alloying elements to SnAgCu solder alloys is being investigated. The effects of these additional elements on mechanical properties, failure mechanisms, and other reliability phenomena such as Sn whisker formation are not fully understood. In addition, there is a desire among the industrial electronics community to develop new solder alloys for high-temperature applications such as aviation engine controls and downhole drilling electronics as these industries anticipate a future transition to lead-free electronic assembly. Several of these topics were discussed during the Pb-Free Solders and Emerging Interconnect and Packaging Materials Symposium at the TMS 2014 Annual Meeting in San Diego, California. Researchers from industry and academia from different parts of the globe shared their latest research findings in this leading symposium on electronic packaging. More than 60 works were presented in 8 different sessions. A poster session was also organized to include the numerous additional contributions. Three of those technical presentations are presented in this issue of JOM, while others will be published in a special issue of the Journal of Electronic Materials as well as other journals published by TMS. The first article, by E. J. Cotts et al., discusses the effect of impurities on the solidification behavior Babak Arfaei is the guest editor for the Electronic Packaging & Interconnection Materials Committee of the TMS Functional Materials Division, and coordinator of the topic Progress with Lead-Free Solders in this issue. JOM, Vol. 66, No. 11, 2014