The impact of 45 to 28nm node-scaling on the electromigration of flip-chip bumps

Abstract

The impact of 45nm to 28nm node-scaling on the electromigration of lead-free flip-chip bumps has been studied. Specifically, different UBM and PI open sizes as well as plated Ni and sputtered Cu UBM thicknesses were experimentally investigated. UBM sizes between 84 and 65 μm directly impact lifetime, while PI open diameters between 30 to 20 μm did not. A current-density exponent (n) value of 1 was measured when Joule heating is neglected, thereby suggesting a void-growth-limited mechanism. This value increases to 2 or greater when Joule heating is included, showing that Joule heating can strongly impact Imax extrapolations. Also, a thin plated Ni layer (1.5μm) and a thin sputtered Cu layer (2kÅ) independently led to significant early fails due to improper barrier coverage (rather than the thickness of the layer), which enhanced intermetallic transformation and therefore electromigration failure.