In situ electromigration and reliability of Pb-free solders at extremely small length scales

Over the past several years, the density of integrated circuits has been increasing at a very fast rate, following Moore’s law. The advent of three dimensional (3D) packaging technologies enable the increase in density of integrated circuits without necessarily shrinking the dimensions of the device. Under such constraints, the solder volume necessary to join the various layers of the package is also extremely small. At smaller length scales, the local cooling rates are higher, so the microstructures are much finer than that obtained in larger joints (BGA, C4). The fraction of intermetallic compounds (IMCs) present in solder joints in these volumes will be larger. The Cu6Sn5 precipitate size and spacing, and Sn grain structure and crystallography will be different at very small volumes. These factors will most certainly affect the performance of the solder. Examining the mechanical behavior and reliability of Pb-free solders is difficult, primarily because a methodology to characterize the microstructure and the mechanics of deformation at these extremely small length scales has yet to be developed.

In this study, Sn grain orientation and Cu6Sn5 IMC fraction, size, and morphology are characterized in 3D, in pure Sn based solder joints. The obtained results show differences in morphology of Sn grains and IMC precipitates as a function of location within the solder joint indicating influence of local cooling rate differences. Ex situ and in situ electromigration tests done on 250 um and 500 um pure Sn solder joints elucidate the evolution of microstructure, specifically Sn grain growth, IMC segregation and surface degradation. This research implements 3D quantification of microstructural features over micro and nano-scales, thereby enabling a multi-scale / multi-characterization approach.