Lead-Free Alternatives for Interconnects in High-Temperature Electronics

Abstract

Predominant high melting point solders for high-temperature and harsh environment electronics (operating temperatures from 200 to 250 °C) are Pb-based systems, which are being subjected to RoHS regulations because of their toxic nature. In this study, high bismuth (Bi) alloy compositions with Bi-XSb-10Cu (X from 10 wt % to 20 wt %) were designed and developed to evaluate their potential as high-temperature, Pb-free replacements. Reflow processes were developed to make die-attach samples made from the cast Bi alloys. Die-attach joints made from Bi-15Sb-10Cu alloy exhibited an average shear strength of 24 MPa, which is comparable to that of commercially available high Pb solders. These alloy compositions also retained original shear strength even after thermal shock (TS) between −55 °C and +200 °C and high-temperature storage (HTS) at 200 °C. Brittle interfacial fracture sometimes occurred along the interfacial NiSb layer formed between Bi(Sb) matrix and Ni metallized surface. In addition, heat dissipation capabilities, using flash diffusivity, were measured on the die-attach assembly and were compared to the corresponding bulk alloys. The thermal conductivity of all the Bi–Sb alloys was higher than that of pure Bi. By creating high volume fraction of precipitates in a die-attach joint microstructure, it was feasible to further increase thermal conductivity of this joint to 24 W/m·K, which is three times higher than that of pure Bi (8 W/m·K). Bi–15Sb–10Cu alloy has so far shown the most promising performance as a die-attach material for high-temperature applications (operated over 200 °C). Hence, this alloy was further studied to evaluate its potential for plastic deformation. Bi–15Sb–10Cu alloy has shown limited plastic deformation in room temperature tensile testing in which premature fracture occurred via the cracks propagated on the (111) cleavage planes of rhombohedral crystal structure of the Bi(Sb) matrix. The same alloy has, however, shown up to 7% plastic strain under tension when tested at 175 °C. The cleavage planes, which became oriented at smaller angles to the tensile stress, contributed to improved plasticity in the high-temperature test.

Silicon Carbide Converters and MEMS Devices for High-temperature Power Electronics: A Critical Review

“…Gold-based solders such as AuSn, AuGe, and AuSi, have a good reputation because of their melting points of 280 °C, 365 °C, 363 °C or so, respectively, but the gold-based solders are costly and not suitable for mass production. Other alloys such as zinc and bismuth alloys have been studied for their potential as well [37,38]. Silver sintering has better long-term reliability compared to leaded-free solders, but the challenge of microstructure evolving or coarsening at elevated temperature is a hindrance.…”

High Electrical and Thermal Conductivity of Nano-Ag Paste for Power Electronic Applications

“…However, traditional die-attach materials, such as Pbbased and Sn-based solders, exhibit the poor bonding reliability at high temperature and low thermal and electrical conductivity [5][6][7]. Many researches have recently been directed toward developing high-temperature materials for power electronics, which can be used to attach chips and have high thermal and electrical conductivity [8,9].…”