Effect of Isothermal Aging and Electromigration on the Microstructural Evolution of Solder Interconnections During Thermomechanical Loading T. LAURILA, 1,3 J. KARPPINEN, 1 V. VUORINEN, 1 J. LI, 1 A. PAUL, 2 and M. PAULASTO-KRO ¨ CKEL 1 1.—Department of Electronics, School of Electrical Engineering, Aalto University, PO Box 13340, 00076 Aalto, Finland. 2.—Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. 3.—e-mail: tomi.laurila@aalto.fi The effect of different pre-aging treatments on the microstructural evolution of lead-free solder and growth of interfacial intermetallic compound layers under thermal cycling has been investigated in this work. The results show that there are distinct differences in the microstructural changes between samples with no pretreatment, samples that have experienced thermal annealing at 125°C for 750 h before thermal cycling, and those that have had direct current (DC) stressing for 750 h as pretreatment. The microstructural evolution of the solder matrix is rationalized by utilizing the science of microstructures and analysis of the influence of electron flow on the precipi- tation phenomena. The finite-element method is utilized to understand the loading conditions imposed on the solder interconnections during cyclic stressing. The growth of intermetallic reaction layers is further analyzed by utilizing quantitative thermodynamic calculations coupled with kinetic anal- ysis. The latter is based on the changes in the intrinsic diffusion fluxes of elements induced by current flow and alloying elements present in the system. With this concurrent approach the differences seen in thermal cycling behavior between the different pre-aging treatments can be explained. Key words: Electromigration, solid-state diffusion, intermetallic reactions, lead-free, recrystallization, reliability INTRODUCTION The adoption of high-density packaging technol- ogies has enabled the creation of novel electronic devices with advanced features and functionality. The higher level of integration has, however, introduced new challenges in reliability engineer- ing. These novel devices are continuously exposed to temperature fluctuations caused by internal heat dissipation of the active and passive components. Temperature fluctuations induce thermomechanical strains in electronic assemblies due to differences in the thermal expansion of materials used. Additional thermomechanical loads may be induced by changes in the ambient temperature of the service environ- ment. On the other hand, many products are equally likely to be constantly exposed to high ambient temperatures or kept inactive in such conditions for long times between service cycles. Further, the increasing internal power levels have led to higher current densities in electrical conduc- tors, which promote material degradation through electromigration. To date, the reliability implications of multiple loads on board-level interconnections have not been assessed by employing test methods with multiple interacting loading conditions. Instead, reliability has generally been verified using several individual standardized test methods. The ability of intercon- nections to withstand thermomechanical fatigue has been assessed with standardized accelerated (Received December 12, 2011; accepted August 2, 2012; published online August 30, 2012) Journal of ELECTRONIC MATERIALS, Vol. 41, No. 11, 2012 DOI: 10.1007/s11664-012-2223-2 Ó 2012 TMS 3179