Critical properties of Cu₆Sn₅ in electronic devices: Recent progress and a review

As the most common of the intermetallic compounds (IMCs) formed between Sn-based solders and Cu substrates during the packaging of integrated circuits (ICs), Cu₆Sn₅ is frequently involved in the fabrication of solder joints and plays an important role in the integrity of electronic devices. This is especially true for recently developed micro-bumps in 3-dimensional (3D) high-density integrated circuits (ICs), in which the volume fraction of Cu₆Sn₅ is significantly higher than in conventional ball grid array (BGA) or through hole pin (THP) arrangements. Recently, with the use of advanced characterization techniques, significant progress has been made in the understanding of Cu₆Sn₅ intermetallics in terms of their crystal structure, solidification behaviour, role in interface reactions, thermal expansion and mechanical properties. This improved understanding is of fundamental importance for the production of next generation electronic devices, however there is no existing comprehensive summary of this research available. Here, we provide a review on the properties of Cu₆Sn₅ with a focus on: (1) identification of crystal structure and possible phase transformations of Cu₆Sn₅ in real solder joints; (2) formation of Cu₆Sn₅ during solidification of commonly used Pb-free alloys and its influence on the final microstructure; (3) the formation and growth texture of interfacial Cu₆Sn₅; (4) thermal expansion and mechanical properties of Cu₆Sn₅ and the relationship between crystal structure and temperature. The effects of selected alloying elements that have remarkable influences on the above properties are also discussed. The aim of this paper is to identify the key factors that affect the properties of this important IMC and the relationship between these properties and the integrity of solder joints under various conditions.