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Perspectives of High Temperature Lead-Free Solders
Keywords: Lead-free, Die attachment, Solder
High-Pb solders have been used as die- attach and interconnect materials in discrete power packages. Due to the demand of SiC device serving the high power regime and the harmful effects of Pb to human health/the environment, the alternative Pb-free solders, and the novel bonding materials, as well as the solutions have been studied intensively in the recent decades. The exemption of using high-Pb solders has been extended again to 2021, although it may be terminated anytime if any new technology/material were to be accepted industry-wide. This review is presenting the potential materials/technologies for high temperature lead-free die-attachment, focusing on the alternative solders. The sintering materials and the TLPB (transient-liquid-phase-bonding) materials have been briefly covered as well. AuSn/AuSi/AuGe solders have shown the exceptional high cost, which limited their application. BiAg and BiCu based solders, especially BiAgX paste/wire/preform with the improved wetting, exhibit the remelting temperature of 262oC and 270oC, respectively. The acceptable reliability performance on temperature cycling/thermal aging as well as the low material cost has made them the most competitive candidate for the low power discrete die-attach devices. SnSbAgCu, with the well-designed compositions in the recent studies, offers the remelting temperature above 320oC. Together with the decent thermal conductivity (>35W/(mK)) rooting from the composition, SnSbAgCu is accountable for the devices with mid- to-high power regime. SnSbAgCu pastes have been designed and the reliability from the various die attach vehicles is being studied. Zn-Al has the remelting temperature above 380oC and the extremely low material cost (comparable to or even lower than the high lead solders). Although the bonding process is stringent, the excellent thermomechanical behavior and the superior thermal/electrical conductivity have allowed Zn-Al a potential candidate for high temperature/high power die attachment competitive to AuSi/AuGe solders. Sintering materials form the bonds through solid state interdiffusion, while TLPB materials create the joint through solid-liquid interdiffusion, in which the remelting temperature is enhanced by forming the massive IMCs. The desired high thermal/electrical/mechanical/melting performance as well as the relatively low processing temperature (<350oC) are shining the sintering materials especially Ag-sintering ones. The intrinsic high porosity (>20%) and the evolution of pores from the pressureless sintering, may shadow the reliability. In addition, the immaturity of the processing (Time/Temperature/Pressure/Atmosphere/E quipmental Availability etc) may struggle the industrial adoption of sintering materials. So far, none of these materials/technologies are ideal to satisfy all the requirements of the variety of high temperature lead-free die attach applications in terms of processing/reliability/cost. However, each material/solution has the potential to satisfy a niche within this broader categorization.
Hongwen Zhang (Ph.D.),
Indium Corporation
Clinton, NY

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