Mechanical Characterization and Microstructural Evolution of SAC and SAC+X Lead Free Solders Subjected to High Temperature Aging

Abstract

Solder joints provide mechanical support, electrical and thermal interconnection between packaging levels in microelectronics assembly systems. Proper functioning of these interconnections and the reliability of the electronic packages depend largely on the mechanical properties of the solder joints. Lead free solders provide excellent thermomechanical properties and commonly used as interconnections in electronic packages. However, environmental conditions, such as, operating temperature, aging temperature, and aging time significantly affect these properties due to the microstructural evolution of the solder that occurs during aging. The most well-known and widely observed changes are coarsening of the $\text{Ag}_{3}\text{Sn}$ and $\text{Cu}_{6}\text{Sn}_{5}$ intermetallic compounds (IMCs)present in the eutectic regions between beta-Sn dendrites. In this work, we have investigated the variations in mechanical behavior of several SAC and SAC+X lead free solder alloys including SAC305 (96.5Sn-3.0Ag-0.5Cu)and SAC_Q subjected to high temperature aging. Specimens have been preconditioned at several extreme temperature aging conditions including T = 125 and 200°C. At each extreme temperature condition, several durations of aging were considered including 0, 1, 5 and 20 days. With each set of aging conditions, stress-strain tests were performed on the aged specimens. The evolution of the stress-strain behavior with aging temperature and time was determined. In addition, microstructural evolution of solder alloys during extreme high temperature aging has been explored, and aging induced coarsening of IMCs has been explored using Scanning Electron Microscopy (SEM)to validate our experimental results. Our experimental results show substantial degradations of the mechanical properties including initial modulus, yield stress, and ultimate tensile strength of lead-free solders with extreme temperature aging. The results suggested that there are large challenges to obtaining solder joint reliability when the joints are exposed to long-term high temperature aging. The addition of dopants (e.g. Bi)in the traditional SAC alloys significantly mitigated the high temperature aging induced degradations in both the microstructure and mechanical properties.