Microstructure Characterization and Creep Behavior of Pb-Free Sn-Rich Solder Alloys: Part I. Microstructure Characteriza
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SOLDERS are used as interconnect materials in microelectronic packaging.[1] Traditionally, eutectic and near-eutectic Pb-Sn solder alloys have been used. Due to the toxic nature of lead, environmentally-benign Pb-free solders are being developed. Among various Pb-free alternatives, Sn-rich solders have emerged as viable candidates, primarily because of their high strength and superior resistance to creep and thermal fatigue.[1–4] In particular, near-eutectic alloys of Sn-Ag, Sn-Cu, and Sn-Ag-Cu are leading candidates for replacement of Pb-Sn solders.[1–7] The mechanical properties of solder alloys are controlled by their microstructure, which depends strongly on solidification characteristics. Thus, it is important to understand the relationship between cooling rate, microstructure, and mechanical behavior of Sn-rich solders. Above the eutectic melting point, molten Sn will react with Ag and Cu, to form Ag3Sn and Cu6Sn5 intermetallics, respectively. Thus, the solder microstructure of Sn-rich alloys containing Ag or Cu consists of a combination of Sn-rich dendrites and a eutectic mixture of Ag3Sn or Cu6Sn5 intermetallics in a Sn-rich matrix.[1,8,9] When solder alloys are reflowed on Cu substrates to produce solder joints, an additional component of the microstructure is a Cu6Sn5 intermetallic R.S. SIDHU, formerly Graduate Research Assistant, School of Materials, Fulton School of Engineering, Arizona State University, is with the Assembly Technology Development Department, Intel Corporation, Chandler, AZ and N. CHAWLA, Professor, is with the School of Materials, Fulton School of Engineering, Arizona State University, Tempe, AZ 85287-8706. Contact e-mail: [email protected] Manuscript submitted January 12, 2006. Article published online January 9, 2008 340—VOLUME 39A, FEBRUARY 2008
layer that forms at the solder/Cu interface.[10,11] Several investigations have been conducted on the effect of cooling rate on the microstructure and mechanical behavior of Sn-rich solders.[12–19] In general, at faster cooling rates (i.e., water quenching), the microstructure consists of fine spherical intermetallics particles dispersed in a Sn-rich matrix. At slower cooling rates (i.e., furnace cooling), the microstructure is coarser, and the Cu6Sn5 particles exhibit more irregular morphologies. It has been hypothesized that faster cooling rates suppress the growth of intermetallic particles due to decreased time for diffusion and growth of the second-phase particles. Cooling rate induced changes in microstructure have a profound influence on the mechanical behavior of the solder; faster cooled specimens exhibited increased strengths due to the finer and more uniform dispersion of intermetallic particles. The slower cooled samples exhibit higher ductility, but decreased strength due to the coarser microstructure. The cooling rate also has an effect on the Cu6Sn5 intermetallic layer that forms between the solder/Cu interface.[20–24] Deng et al.[20] showed that at faster cooling rates, a thinner and more planar intermetallic morphology was ob
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