Micro-additives and their impact on tensile and fracture performance of solder

Abstract

High-performance components and sustainable electronics require new and durable interconnection technologies. In addition to Solid-Liquid Interdiffusion (SLID) Bonding and silver sintering, established solders are also advancing. Especially in the field of power electronics, where high-temperature applications require high demands on assembly and interconnection technology, the lead-free SnAgCu solders has established a good track record. This basic solder is then improved and optimized for the requirements of its specific application, for example by adding micro-additives. This paper deals with the tensile strength of micro-alloyed SnAg0.4Cu0.5 (SAC) solders, building on our previous work considering shear tests and phase formation behaviour published in Steenmann et al. (2021). In power electronics, solders as joining partners between component and base plate have to accommodate high mechanical and thermal loads resulting, e.g., from a mismatch in coefficients of thermal expansion of the joined materials. The cracking behaviour of the solders in tensile tests provides information on its mechanical resistance and load-bearing capacity. In this work, we examine SAC based solders with small amounts of Sb (3 wt%), Bi (6 wt%) and Ni (0.2 wt%) to investigate the effects of the micro-additives on tensile strength. The micro-additives are studied individually in quaternary systems “SAC + X”, but also in the combination SnAg4Cu0.7Bi3Ni0.1Sb1.5 and SnAg0.5Bi3Sb5 and compared respectively to SAC solder. We achieve this by melting the solders using a standard soldering process at different times and temperatures to replicate the solders' phase formation behaviour during soldering. We vary the peak temperature and holding time of the process to investigate the respective effects on phase growth and the associated change of solder behaviour under mechanical load. The specimens prepared at different parameters are tested under identical conditions on a tensile testing machine based on DIN EN ISO 6892 1 “Metallic materials — Tensile testing”. We investigate the intermetallic phases on scanning electron microscope images by energy dispersive X-ray (EDX) spectroscopy and derive the relationships with the micro-additives. In this paper, we discuss in detail the differences in tensile behaviour depending on the micro-additives. By considering all results holistically, the influence of the individual and combined micro-additives in the solder can be deduced.