Effect of thermal aging on electromigration failure of a flip-chip SnAgCu solder joint

Abstract

Lead free solder materials have replaced lead based solder materials nowadays for increased environmental concern. Further miniaturization of electronic solder joints in packages has caused electromigration to dominate among all the reliability issues found in electronic packages. This current investigation deals with the review of mechanical property degradations of SAC (SnAgCu) solder materials of different Ag percentages due to thermal aging and their effects on electromigration oriented failure of small scale flip chip solder bumps. Thermal aging causes degradation of strength as well as elastic modulus of the SAC solder material. This degradation in stress-strain behavior plays an utterly important role in electromigration oriented mass diffusion and subsequent failure in the solder joints. This research highlights the linkage between thermal aging oriented strength degradation and its effect on electromigration oriented void propagation rate and time to failure. Structural-electric-diffusion analyses with commercial finite element analysis software have been performed for different aging conditions (1 to 20 days of aging at 100 oC) at different electromigration temperatures. Percentage of Ag generally varies between 1-4% in solder joints of commercially available packages. Presence of Ag in a higher amount causes formation of Ag3Sn in a large amount which increases mechanical strength of SAC materials. So the effect of Ag percentage on SAC solder materials has also been analyzed in the current study. It is also found in literature that with aging for significantly long time, different intermetallic compounds (IMC) form at the interface of Cu-SAC joint. So, the effect of IMC on the electromigration has also been observed in the current research. Finally, the effect of random vibration observed in many real world applications on electromigration oriented failure of flip-chip solder joint has also been investigated. Findings from the analysis indicate that mass diffusion due to electromigration and associated stress migration actually reduces with the increment of aging time and thus increases time to failure in case of a flip chip solder joint. Moreover, increment in Ag percentage increases diffusion flux as well as plastic work per volume (PLWK) in SAC material. Furthermore, it has been found that PLWK and diffusion flux significantly reduce with IMC formation. Finally, analysis revealed that inclusion of random vibration expedites electromigration failure significantly. Simulation results have been found to be in good agreement with the electromigration experiment results that are available in literature.