Integrating copper at the nanometer length scale with Sn–3?5Ag solder to develop high performance nanocomposites P. Babaghorbani 1 , S. M. L. Nai 2 and M. Gupta 1 In the present study, copper at the nanometer length scale is integrated with Sn–3?5Ag using the technique of powder metallurgy incorporating energy efficient microwave sintering. Superior mechanical characteristics were realised for the formulations containing nanometer length scale copper in excess of 1 vol.-%. Sn–3?5Ag reinforced with 2?5 vol.% nanosize copper particulates exhibited the best overall mechanical characteristics. Particular emphasis is placed on studying the effect of the increasing presence of nanosize copper particulates on the microstructure and property evolution of the Sn–3?5Ag matrix. Keywords: Lead-free solder, Metal matrix nanocomposites, Mechanical behaviour, Microwave sintering Introduction In the electronic packaging industry, solders play a crucial role as an interconnect material. 1 As a joining material, solder provides electrical, thermal and mechan- ical continuity in electronic assemblies. The performance and quality of the solder are crucial to the integrity of a solder joint, which in turn is vital to the overall functioning of the assembly. Owing to the inherent toxicity of lead, environmental regulations around the world aim to eliminate the usage of Pb bearing solders in electronic assemblies. This has prompted the development of Pb free solders, and has enhanced the research activities in this field. 1–8 These lead free solders are mostly based on Sn containing binary and ternary alloys. Among the new generation of lead free solders being developed, Sn–3?5Ag (wt%) alloy used in this study has immense potential because of its good wett- ability, higher strength and superior resistance to creep and thermal fatigue than the eutectic Pb–Sn solder. 1,9–12 Moreover, through the years, as micro-/nanosystems technologies are advancing, the size of electrical components are shrinking, leading to an increase in the number of input/output terminals. As a conse- quence, the number of solder joints per package have increased while the dimensions of the solder joints have decreased. For solder joints with reduced dimension to stay functional, powerful and reliable necessitates the development of a new generation of interconnection materials. One potentially viable and economically affordable approach to improve the microstructure stability and mechanical properties of a solder is the addition of secondary particles to a solder matrix so as to form a composite solder. The presence of the second phase has been shown to trigger the microstructural mechanisms that enhance the reliability of the solder joints. 13–15 It has been shown by investigators that composite technology is a promising approach to engineer and stabilise a fine grained microstructure and homogenise solder joint deformation to improve the mechanical properties of the solder joint, especially creep and thermomechanical fatigue resistance. 16–20 In other words, composite solders with high strength show improved service performance. In addition to the use of reinforcements, the selection and innovation of the processing techniques also play an important role on the end properties of the compo- sites. 21–23 Since the powder metallurgy (PM) technique offers advantages such as near net shaping, greater material utilisation and a more refined microstructure, it is used as one of the most common processing approaches to produce high performance metallic materials for various applications. Sintering is an important step in the powder metallurgy technique, where densification and bond formation take place, and is traditionally carried out using conventional resistance furnaces. 8,27,31 Recently, investigations have shown that microwaves can be utilised to sinter both monolithic and composite powder compacts much more rapidly than conventional sintering, producing materials with better microstructural and mechanical properties. 8,24–33 In addition, the use of microwaves can lead to energy saving of up to 90%, and even higher in some cases. More recently, for flip chip circuit assembly, bonding processes including thermal compression, thermosonic and ultrasonic bonding have been introduced. 34–37 In these approaches, solders do not go through a conven- tional melting and solidification and the properties of the original solder are mostly retained, particularly in ultrasonic (US) bonding. Kim et al. 37 have shown the 1 Materials Division, Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576 2 Minerals, Metals and Materials Technology Centre National University of Singapore, 9 Engineering Drive 1, Singapore 117576 *Corresponding author, email mpegm@nus.edu.sg 1258 ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 8 August 2008; accepted 13 August 2008 DOI 10.1179/174328408X378582 Materials Science and Technology 2009 VOL 25 NO 10