Fracture Behavior of Sn-3.5Ag-0.7Cu and Pure Sn Solders as a Function of Applied Strain Rate K.E. YAZZIE, 1 J.J. WILLIAMS, 1 and N. CHAWLA 1,2 1.—Materials Science and Engineering, School for the Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287-6106, USA. 2.—e-mail: nchawla@asu.edu The demand for environmentally benign Pb-free solders is increasing, and the push toward smaller portable electronics will make it more likely for solder interconnects to encounter mechanical shock through dropping or mishan- dling. Thus, fundamental understanding of the relationship between solder microstructure and mechanical shock resistance is essential for developing reliable numerical models of mechanical shock behavior. In this paper we report on the strain rate-dependent mechanical behavior of pure Sn and Sn-3.5Ag-0.7Cu solders, measured from tensile tests conducted in the strain rate range from 10 3 s 1 to 30 s 1 . Local strain and strain rate distributions were measured by digital image correlation. Finally, the strain rate depen- dence of fracture mechanisms is discussed. For a given strain rate, water- quenched tin–silver–copper (SAC) had the greatest ultimate tensile strength (UTS), followed by furnace-cooled SAC, then pure Sn. Furnace-cooled SAC had lower ductility than water-quenched SAC, due to large Ag 3 Sn needles that nucleated elongated voids which easily coalesced. Key words: Pb-free solder, mechanical shock, fracture INTRODUCTION The need to develop environmentally benign electronic packages has generated great interest in Pb-free alloys. 1–6 As electronic packages are made smaller for portable devices, there is an increased probability that solder joints may fail by accidental dropping during manufacture, shipping, or use. The strain rates experienced by solders during drop, i.e., mechanical shock, are in an intermediate range between the quasistatic and dynamic regimes, i.e., between 10 1 s 1 and 10 2 s 1 . 7–13 The microstruc- ture, creep, and thermal fatigue behavior of Pb-free solders is well understood, 14–17 and a good under- standing of the mechanical shock behavior of Pb-free solders is being established. 18–22 However, funda- mental understanding of the relationship between solder microstructure and mechanical shock resis- tance is needed. Understanding this relation- ship requires quantitative analysis of strain rate-dependent fracture mechanisms. In this study the mechanical behavior of Sn-3.5Ag- 0.7Cu solder was systematically studied, and com- pared with that of pure Sn, at strain rates ranging from 10 3 s 1 to 30 s 1 . Local values of strain and strain rate were measured at the onset of necking, in the necking region of the tensile specimen, using digital image correlation (DIC). Fracture surfaces of failed tensile specimens were quantitatively ana- lyzed using scanning electron microscopy (SEM). The first part of the paper describes the micro- structural characterization and analysis of experi- mental tensile test data. The second part discusses strain rate-dependent fracture mechanisms. EXPERIMENTAL PROCEDURES Sn (99.999% pure; Alfa Aesar, Ward Hill, MA, USA) and Sn-3.5Ag-0.7Cu (SAC; Indium Corpora- tion, Ithaca, NY, USA) ingots were used in this study. The ingots were reflowed in a graphite-coated aluminum mold with the following dimensions: 10.5 cm long, 1 cm wide, and 0.8 cm high. A ther- mocouple placed at the bottom of the solder was (Received March 3, 2012; accepted June 14, 2012; published online July 6, 2012) Journal of ELECTRONIC MATERIALS, Vol. 41, No. 9, 2012 DOI: 10.1007/s11664-012-2180-9 Ó 2012 TMS 2519