Au–Sn solders applied in transient liquid phase bonding: Microstructure and mechanical behavior

Transient liquid phase bonding offers one option to generate a robust lead-free die attach with sound thermal and electrical conductivity in microelectronic packages. However, it is a challenge to characterize the microstructure and mechanical behavior of the bonding layer because of its ultra-thin thickness and its nano-sized constituents. We show that microstructure s and local mechanical properties of such a modern solder systems are accessible with state-of-the-art methods including transmission electron microscope, synchrotron X-ray nano-beam diffraction as well as micro-scale mechanical testing. Three sub-regions with different morphologies have been identified within the bonding area, and their contained phases have been mapped. Two of the sub-regions contain nano-sized intermetallic compounds (IMC) while the third one is mostly composed of an FCC Au–Cu solid solution with a Cu concentration gradient. On top of that, micro-cantilever bending testing has been conducted to investigate the mechanical behavior of the bonding region. The two IMC sub-regions show brittle behavior while the Au–Cu sub-region is ductile.