Mechanisms of deformation in high-ductility Ce-containing Sn–Ag–Cu solder alloys H.X. Xie a , N. Chawla a,⇑ , Y.-L. Shen b a Materials Science and Engineering, Arizona State University, Tempe, AZ 85287-6106, United States b Department of Mechanical Engineering, The University of New Mexico, Albuquerque, NM 87131, United States article info Article history: Received 22 December 2010 Received in revised form 7 February 2011 Accepted 10 February 2011 Available online 5 March 2011 abstract Rare-earth-containing Pb-free solders have gained widespread attention due to their superior ductility relative to conventional Pb-free alloys. Our previous work has shown that new Ce-based alloys are also extremely oxidation resistant compared to La or Y-containing alloys. In this paper, we report on a mech- anism-based model for the large increases in ductility with small addition of rare-earth element to Sn–3.9Ag–0.7Cu. The mechanisms of ductility enhancement by Ce were observed in a scanning electron microscope, in interrupted shear-tests, where CeSn 3 particles served as microscopic fracture and void nucleation sites. Micro-mechanical modeling using the finite-element method was used to examine the plastic strain field in solder affected by the particles. The concentrated deformation band was seen to be disturbed by the particles, resulting in a more uniform deformation pattern with reduced strains and thus enhanced ductility of the lap-sheared joint. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The toxic nature of lead has prompted the electronic packaging industry to seek environmentally-friendly Pb-free alloys as replacements to Pb–Sn solder alloy. Although a series of near-eu- tectic Sn–Ag–Cu (SAC) alloys are being used [1], there are some drawbacks for SAC alloys. These include higher melting tempera- ture and poor mechanical shock resistance, relative to Pb–Sn alloys [2–4]. Rare-earth (RE) elements have been used as fourth alloying element to SAC as a means of enhancing both physical and mechanical properties [5–8]. Past investigations have shown that by adding RE elements the microstructure of SAC solder can be refined [5–11]. Wetting behavior [11–14] and mechanical perfor- mance [9–11,15] can be improved when the RE content is less than 0.5 wt.%. In particular, enhanced mechanical properties, such as tensile strength [9,11], ductility [15,16], and creep resistance [15,17], of RE-containing solders have been reported. In our previous work [18,19], we have shown that small additions of La and Ce (0.1 and 0.5 wt.%) to SAC alloy refine the Sn dendrite microstructure, reduce the Cu 6 Sn 5 intermetallic compound layer thickness, and significantly increase the ductility of solder joints compared to SAC alloy. The oxidation resistance of La-containing solders is relatively low and the resulting LaSn 3 particles are prone to whiskering during oxidation [20–22]. Ce-based alloys are less prone to oxidation but still exhibit the desirable attributes of micro- structural refinement and increased strain-to-failure, relative to SAC, as in the La-containing solders [8,23]. For La-containing Sn–Ag–Cu solder, we hypothesized that the main mechanism for the higher ductility in these solders is based on microscopic voids nucleating at LaSn 3 throughout the solder volume [18]. This cavitation based mechanism, however, needs to be validated experimentally. In this study, we have conducted systematic interrupted shear experiments, coupled with micro- structural characterization using scanning electron microscopy (SEM), on Ce-containing SAC. The behavior of the RE-containing materials was compared to that of SAC solder alloys. It will be shown that plasticity around CeSn 3 intermetallic particles, as well as debonding and fracture of the particles takes place. The shear strain is distributed more homogeneously in the RE-containing sol- ders, compared with SAC. Finite-element method (FEM) was used to model the effects of CeSn 3 intermetallic compound on strain dis- tribution. The FEM results qualitatively corroborate the experi- mentally-observed behavior. 2. Materials and experimental procedure Vacuum-melted ingots of Sn–3.9Ag–0.7Cu with trace amounts of Ce (0.5 wt.%) were prepared. High purity Sn–3.9Ag–0.7Cu ingots (Indium) were cut into small rectangular pieces (6.5 mm  6.5 mm  13 mm) and mixed with Ce shot (ESPI, Ashland, OR). Due to the reactive nature of pure Ce with oxygen, the materials were mixed in a quartz ampoule (12 mm in diameter) under a sealed glove box with helium atmosphere. The quartz ampoule was then evacuated to 10 À5 torr and sealed. The sealed ampoules were heat treated at 1000 °C for 4 h, and periodically mixed by rotation of the 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.02.005 ⇑ Corresponding author. E-mail address: nchawla@asu.edu (N. Chawla). Microelectronics Reliability 51 (2011) 1142–1147 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel