Lead-Free Solder Reinforced with Multiwalled Carbon Nanotubes S.M.L. NAI, 1 J. WEI, 2 and M. GUPTA 1,3 1.—Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576. 2.—Singapore Institute of Manufacturing Technology 71 Nanyang Drive, Singapore 638075. 3.—E-mail: mpegm@nus.edu.sg In this study, varying weight percentages of multiwalled carbon nanotubes were successfully incorporated into 95.8Sn-3.5Ag-0.7Cu solder to synthesize novel lead-free composite solders. The composite solders were synthesized using a powder metallurgy route consisting of blending, compaction, sintering, and extrusion. The extruded materials were then characterized for their phys- ical, thermal, and mechanical properties. With the addition of increasing weight percentage of carbon nanotubes, the composite solders experienced a corresponding decrease in density values and an improvement in wetting properties. The melting temperatures of the composite solders were found to be unchanged with additions of carbon nanotubes. However, improvements in the mechanical properties, in terms of microhardness and tensile properties, were observed with increasing weight percentages of carbon nanotubes. Key words: Lead-free solder, carbon nanotubes, composites INTRODUCTION For several decades, tin-lead (Sn-Pb) solders have been used extensively as interconnect materials. However, environmental concerns over the use of Pb, which is toxic, have led to the banning of lead usage in electronics manufacturing by Japan and the countries in the European Union. Thus, this created a need to move beyond such solders. Lead- free solders are the proposed candidates to address the environmental concerns. 1,2 In addition, through the years, as micro- and nano- systems technologies advanced, the size of electrical components shrank and the number of input/output terminations increased. To cater to these ever- increasing changes, the numbers of solder joints per package have increased while the dimensions of the solder joints have decreased. Furthermore, the trend in the micro-/nano-systems industry today is to make products more personal by making them lighter and smaller. It is also essential to make them more user-friendly, functional, powerful, and reliable. Thus, this necessitates the development of a new generation of interconnection materials. These new interconnection materials must be equipped with a combination of good mechanical, electrical, and thermal properties, in order to fulfill ever- stricter service requirements. 1,2 One feasible way to improve solder performance is to introduce second phases to a conventional solder alloy, forming a composite solder. The presence of these second phases has been proposed as a poten- tial mechanism controlling reliability of the solder joints. 3,4 In this study, the influence of such second phases on the material’s physical and mechanical properties is studied. Enhanced mechanical proper- ties can be achieved based on the mechanisms to impede dislocation motion, such that higher applied stress is needed to cause deformation of metal. The addition of second phases can result in two strengthening methods, namely, precipitation hardening and dispersion hardening. Precipitation hardening results from shearing of dislocations through deformable precipitates, while the disper- sion hardening results from the bypass of disloca- tions around particulates which do not deform. The second phase used here results in dispersion hard- ening, as it does not deform and has minimum solubility in the solder matrix. Thus, such a second phase is more resistant to growth or overaging. 5 To date and to our knowledge, there has not yet been any literature report of multiwalled carbon nano- tubes being used in the development of composite solders. In this study, an attempt was made to synthesize SnAgCu/CNT solder composites using blend-press- sinter powder metallurgy techniques. The compo- sites obtained were then extruded and characterized (Received November 29, 2005; accepted January 26, 2006) Journal of ELECTRONIC MATERIALS, Vol. 35, No. 7, 2006 Regular Issue Paper 1518