1868 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 3, NO. 11, NOVEMBER 2013 Recrystallization and Ag 3 Sn Particle Redistribution During Thermomechanical Treatment of Bulk Sn–Ag–Cu Solder Alloys Uttara Sahaym, Babak Talebanpour, Sean Seekins, Indranath Dutta, Praveen Kumar, and Peter Borgesen Abstract—Sn–Ag–Cu (SAC) solders are susceptible to appre- ciable microstructural coarsening during storage or service. This results in evolution of joint properties over time and thereby influences the long-term reliability of microelectronic packages. Accurate reliability prediction of SAC solders requires predic- tion of microstructural evolution during service. Microstructure evolution in two SAC solder alloys, such as, Sn-3.0Ag-0.5Cu (SAC 305) and Sn-1.0Ag-0.5 Cu (SAC 105), under different ther- momechanical excursions, including isothermal aging at 150 °C and thermomechanical cycling (TMC) was studied. In general, between 200 and 600 cycles during TMC, recrystallization of the Sn matrix was observed, along with redistribution of Ag3Sn particles because of dissolution and reprecipitation. These latter effects have not been reported before. It was also observed that the Sn grains recrystallized near precipitate clusters in eutectic channels during extended isothermal aging. The relative orientation of Sn grains in proeutectic colonies did not change during isothermal aging. Index Terms— Electron backscatter diffraction (EBSD), lead- free solders, recrystallization. I. I NTRODUCTION S n–Ag–Cu (SAC) solders have largely replaced Pb–Sn sol- ders in microelectronic components because of legislation restricting the use of Pb. During service and/or storage, SAC solders are subjected to temperatures ranging from 0.4 to 0.8 T m (where T m is the melting temperature), making them highly prone to in situ microstructural coarsening [1]–[3]. This affects the mechanical properties over time, and thereby influences the long-term reliability of microelectronic packages [4]–[8]. Manuscript received September 24, 2012; revised April 9, 2013; accepted June 17, 2013. Date of publication July 18, 2013; date of current version Octo- ber 28, 2013. This work was supported in part by the Strategic Environmental Research and Development Program SERDP under Contract W912HQ-10-C- 0041 and the National Science Foundation sponsored REU program under Grant DMR-1062898. Recommended for publication by Associate Editor K. Ramakrishna upon evaluation of reviewers’ comments. U. Sahaym, B. Talebanpour, and I. Dutta are with the School of Mechan- ical and Materials Engineering, Washington State University, Pullman, WA 99164 USA (e-mail: usahaym@wsu.edu; babak.talebanpour@gmail.com; idutta@wsu.edu). S. Seekins is with the School of Chemical and Biological Engineering, University of Maine, Orono, ME 04469 USA (e-mail: sean.seekins@umit.maine.edu). P. Kumar is with the Department of Materials Engineering, Indian Institute of Sciences, Bangalore, Karnataka 560012, India (e-mail: praveenk@materials.iisc.ernet.in). P. Borgesen is with the Department of Systems Science & Industrial Engineering, Binghamton University, Binghamton, NY 13902 USA (e-mail: pborgese@binghamton.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2013.2272543 Accurate quantification of the aging behavior of solder joints is therefore critical for package reliability predictions. Several studies have addressed the issue of microstruc- tural coarsening in lead-free solders during both isothermal aging and thermomechanical cycling (TMC) [4], [9]–[17]. The microstructure of SAC alloys consists of two components: 1) proeutectic β -Sn grains and 2) eutectic microconstituent consisting of Ag 3 Sn and Cu 6 Sn 5 precipitates dispersed in β -Sn matrix. Both Ag 3 Sn and Cu 6 Sn 5 particles coarsen by Ostwald ripening, when subjected to thermal or thermome- chanical treatments [13]–[17]. This coarsening is facilitated by diffusion of Ag and Cu atoms from the shrinking particle to the growing particle through the β -Sn matrix. The rate of diffusion and hence the rate of coarsening increases during TMC because of increase in the excess vacancy concentration during straining at high rates [1]. Such coarsening behavior is very typical of Sn–Ag-based solder alloys and is a function of the size of the solder joint and the bond pad material [13], [14], [18], and [19]. It is also noteworthy that Cu 6 Sn 5 coarsens much faster than Ag 3 Sn, and its volume fraction is much smaller than that of Ag 3 Sn in most commercial SAC solders. Assuming all Ag and Cu are in compound form, the Ag 3 Sn and Cu 6 Sn 5 volume fractions for SAC 387 have been calcu- lated to be ∼ 5.5% and 1.9%, respectively [13], [15]. Hence, the contribution of Cu 6 Sn 5 to the evolution of mechanical properties is negligible relative to that of Ag 3 Sn [13]–[15]. A model that captures the effect of aging and TMC on the coarsening of Ag 3 Sn particles in bulk SAC alloys was proposed in our previous paper. Sn grain size and grain orientation have also been reported to influence the mechanical behavior of SAC solder alloys because of anisotropic nature of Sn. Many researches have addressed this issue by characterizing proeutectic grain struc- ture in SAC solder joints using electron backscatter diffraction (EBSD) (see [20]–[28]). Generally, it is believed that the proeutectic phase in a solder joint consists of β -Sn dendrites with only a few unique orientations. In one of their recent papers, Arfaei et al. [28] studied the effect of joint size and solidification temperature on Sn grain morphology. Larger SAC solder joints (750 and 500 μm) showed beach-ball Sn grain morphology but the smaller joints (100 μm) had inter- laced morphology because of interlacing of Sn dendrites. No research has been done to date to study Sn grain morphology in bulk SAC solders. This paper elucidates the effects of thermal and thermome- chanical treatments on both precipitate and Sn grain structure 2156-3950 © 2013 IEEE