Materials Science and Engineering A 445–446 (2007) 73–85 Thermomechanical fatigue damage evolution in SAC solder joints M.A.Matin, W.P. Vellinga ∗ , M.G.D. Geers Materials Technology, Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands Received 2 May 2006; received in revised form 3 September 2006; accepted 6 September 2006 Abstract Thermomechanical fatigue in lab-type Sn–Ag–Cu solder interconnections between two copper plates has been investigated under cyclic thermal loading within a number of temperature ranges. Fatigue mechanisms have been studied using optical and scanning electron microscopy. Among the various fatigue mechanisms observed the occurrence of persistent slip bands is notable. From the correlation of the microscopic observations and stress fields calculated using elastic finite element modelling, it appears that three crucial factors govern the evolution of fatigue damage: thermal mismatch between Cu and solder, intrinsic thermal mismatches caused by Sn anisotropy and the mechanical constraints posed by the Cu on the soldered joint. The stress fields resulting from these combined sources determine the location and severeness of fatigue damage in solder joints. The relative predominance of these factors is discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Pb-free solder; Thermomechanical fatigue; Persistent slip band; Orientation imaging microscopy; Finite element analysis 1. Introduction Soldered joints have an essential contribution to the proper functioning of micro-electronics and therefore their potential failure is an important reliability issue. Two major challenges faced by the microelectronics industry are: (1) continuing minia- turization and (2) the replacement of traditional near-eutectic Sn–Pb solder by Pb-free, Sn-rich alternatives [1–3]. Some potential Sn–Ag, Sn–Bi, Sn–Zn, Sn–Cu binary eutectic and Sn– Ag–Cu, Sn–Ag–Bi, Sn–Zn–Bi ternary eutectic alloys have been developed as substitutes for Sn–Pb alloys [4–6]. Recently, indus- try has focused its interest on eutectic Sn–Ag–Cu (SAC) because of its comparatively low melting temperature, the competitive price, and apparently good mechanical properties [7]. Numerical tools for lifetime prediction that could guide engi- neering efforts are in great demand, and considerable progress is being made in some areas (see, e.g. [8] and references therein). To be successful, quantitative experimental input in compu- tations is crucial in a number of areas: (i) the evolution of microstructure; (ii) the initiation and propagation of damage dur- ing thermomechanical fatigue and (iii) the mechanical behavior ∗ Corresponding author. Present address: Materials Science, Applied Physics, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. Tel.: +31 50 363 4821; fax: +31 50 363 4881. E-mail address: w.p.vellinga@rug.nl (W.P. Vellinga). of various types of interfaces present. Since it is not clear a priori at which scale the relevant structural evolution will take place, a combination of techniques operating at different lengthscales is called for. In this paper such an approach is used. Several types of in situ microscopic techniques are combined to provide insight in the relevant microscopic mechanisms occurring dur- ing thermomechanical loading of soldered joints that eventually govern the lifetime of the joint. A lot of effort is currently undertaken to provide baseline data that describe the behavior of Pb-free solders under ther- momechanical loading conditions [9–26]. A solder material is generally exposed to thermomechanical fatigue during ser- vice. The thermomechanical coupling in solder joints originates globally from the mismatch in coefficients of thermal expan- sion (CTE) between the chip and printed circuit board (and other layers) and locally from differences in CTE between the various phases or grains in the solder. Another important issue is the intrinsic anisotropy of Sn. The Sn crystals have a body-centered-tetragonal structure with lattice parameters of a [1 0 0] = b [0 1 0] = 0.5632 nm and c [0 0 1] = 0.3182 nm at 25 ◦ C, in which the c/a ratio equals 0.546 [27]. At 30 ◦ C, the CTE’s in the principal directions are α [1 0 0] = α [0 1 0] = 16.5 × 10 -6 K -1 and α [0 0 1] = 32.4 × 10 -6 K -1 [27]. Note the large thermal anisotropy in comparing the c-axis with the other two axes. Sn is also anisotropic in its elastic properties. The following values for the elastic moduli [28]: c 11 = 73.5; c 12 = 23.4; c 13 = 28.0; c 33 = 87.0; c 44 = 22.0; c 66 = 22.65 (all values in GPa) show 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.037