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Exploring Bismuth as a New Pb-Free Alternative for High Temperature Electronics

Predominant high melting point solders for high temperature electronics (operating temperatures over 200°C) are high Pb containing alloys. The new regulation will have banned the use of Pb in all electronics by 2019. We designed a "new" class of solder alloys with high bismuth (Bi) content (≥70 wt.%) to evaluate their potential as a high temperature Pb-free solder. Although having high melting point (271°C), pure Bi exhibits poor thermal conductivity, limited plasticity and wetting issues on common metallized surface such as Ni and Cu. In this work, alloying elements such as Sb and Cu were added to Bi in an attempt to negate such shortcomings by creating innovative alloy microstructures and optimum alloy compositions. As an extension to this work, a transient liquid phase (TLP) bonding between Bi and Ni was also investigated as Bi in a molten state reacts strongly with Ni to be converted into a Bi3Ni or BiNi intermetallic layer. Once formed, these phases can be stable until reaching - 470°C (with Bi3Ni), or - 617°C (with BiNi). We tested the reliability of the die attached coupons using Bi-Sb-Cu solders and Bi-Ni TLP bonds under high temperatures (up to 200°C for Bi solders and up to 350°C for TLP bonds), thermal shock (from -55 to 200°C), and high temperature storage (at 200°C). The resultant data indicate the potential of these two Bi systems as Pb-free alternatives for high temperature electronics.