Bismuth-Based Transient Liquid Phase (TLP) Bonding as High-Temperature Lead-Free Solder Alternatives

Predominant high melting point solders for high temperature electronics contain lead (Pb), which will soon be banned by environmental regulations as in most of consumer electronics. In an effort to replace the Pb-based solders with a new high-temperature capable material, we developed a transient liquid phase (TLP) bonding of bismuth (Bi) and nickel (Ni). A molten Bi (m.p. of 271°C) strongly reacts with Ni to form a Bi
₃
Ni or BiNi intermetallic layer, both of which can withstand over 400°C. To study microstructural developments and their influences on reliability performance, the die attached coupons were assembled using Bi-Ni TLP bonds. It was shown that Bi
₃
Ni is the first phase to form, after which the diffusion of Bi controls the growth kinetics of this intermetallic phase with the activation energy of 65.5 kJ/mol for the temperature range from 160 to 240°C. The solid-state transformation of Bi
₃
Ni to BiNi follows, which is a slower process and only occurs at a long-term aging (activation energy of 17.8 kJ/mol, for the temperature range from 260 to 300°C). When tested at high temperatures (up to 350°C) or exposed under long-term storage at 200°C over 1000 hours, such bonds showed no degradation in die shear strength. It indicates the potential of the Bi-Ni TLB bonds as a Pb-free alternative to replace high-Pb solders for high temperature electronics that operate at 200°C or higher.