A Brief Review on High-Temperature, Pb-Free Die-Attach Materials

Abstract

High-Pb solders have been used as die-attach and interconnect materials for decades in discrete power components. Because of the known harmful effects of Pb to human health and the environment, as well as the demand of the wide-band-gap SiC/GaN power devices serving under high power density and high junction temperature, alternative Pb-free solders/materials and solutions have been studied intensively. Pb-free alternatives to replace high-Pb solders are still in their infancy. The exemption of using high-Pb solders has been extended to 2021, although it may be terminated anytime if a new technology or material were to be accepted industry-wide. The potential high-temperature Pb-free materials, including solders (AuSn/AuSi/AuGe, ZnAl and BiAgX®), Ag-sintering material, and transient liquid-phase bonding (TLPB) material, are reviewed in this paper with an emphasis on BiAgX® solders. The extremely high cost of Au-based solders limits their applications, although they have been used in high-temperature electronics. Zn-Al has a high melting temperature, good mechanical properties, and good thermal performance. However, the high reactivity makes Zn-Al solders only available in wire and preform. BiAgX® solder (available in paste, wire and preform) has been developed to replace the high-Pb solders for use in low-power discrete components, with relatively low cost and better reliability than the high-lead counterpart. Ag-sintering material and TLPB material form joints by atomic diffusion (either solid diffusion for sintering or solid–liquid interdiffusion for TLPB). The challenges in mass-production and the cost have restricted their success to be widely used in industry. Although it is likely that none of these materials/technologies are ideal to satisfy all the requirements of the variety of high temperature Pb-free die attach applications in terms of processing/reliability/cost, each solution has the potential to satisfy a niche within this broader categorization.