Critical properties of Cu6Sn5 in electronic devices: Recent progress and a review. Issue 2 (April 2016) - Mu, D.K.
Highlights: A comprehensive review on the properties of Cu6 Sn5 is provided. Crystal structure, mechanical properties and solidification behaviour are covered. The review provides a rational basis for future scientific progress in Pb-free solders. Abstract: As the most common of the intermetallic compounds (IMCs) formed between Sn-based solders and Cu substrates during the packaging of integrated circuits (ICs), Cu6 Sn5 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 Cu6 Sn5 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 Cu6 Sn5 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 Cu6 Sn5 with a focus on: (1) identification of crystal structure and possible phase transformations of Cu6 Sn5 in real solder joints; (2) formation of Cu6 Sn5 during solidification of commonly used Pb-free alloys and its influence on the final microstructure; (3) the formation and growth texture of interfacial Cu6 Sn5 ; (4) thermal expansion and mechanical properties of Cu6 Sn5 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.