Effects of the Interlayer Thickness and Alloying on the Reliability of Transient Liquid Phase (TLP) Bonding

In an effort to replace Pb-based solders commonly used in high-temperature electronics (operating at 200°C or higher) with a new high-temperature capable material, we have developed a transient liquid phase (TLP) bonding between bismuth (Bi) and nickel (Ni) in our previous study. To address the reliability concerns and also to warrant the manufacturing efficiency of the TLP bonds, the current study was focused on the optimization of the interlayer structure and alloying via: i) thin bond-line-thickness (BLT) (<; 10 um), ii) intermediate BLT (20-40 um), and iii) thick BLT (> 60 um). These TLP bonded coupons were then tested for bonding reaction, microstructure development, and mechanical reliability. A thin BLT was made via sputter deposition of Bi on Ni-metallized die, which enabled a Ni layer (≈ 1 um) remaining during reflow without being completely consumed. The intermediate BLT sample was made using a Bi preform as in our previous study. For the thick BLT case, a powder/paste of Bi-xNi (from x=0 to 21.9 wt.%) was used. In particular, as pure Bi will not react with the Cu metallized surface (or Cu substrate), an interlayer consisting of Bi-Ni mixed powder/paste will enable Cu surface to be bondable with this TLP system. In this paper, the reflow conditions and the interlayer microstructures for the Bi-Ni TLP bonding with the three different approaches are discussed.