Role of diffusion anisotropy in β-Sn in microstructural evolution of Sn-3.0Ag-0.5Cu flip chip bumps undergoing electromigration

The anisotropy of beta-Sn grain is becoming the most crucial factor to dominate the electromigration (EM) behavior with the downsizing of solder bumps. When the c-axis of beta-Sn grain is parallel to the electron flow direction, excessive dissolution of cathode Cu occurs due to the large diffusivity of Cu along the c-axis; when the c-axis of beta-Sn grain is perpendicular to the electron flow direction, limited dissolution of cathode Cu occurs even in the current crowding regions. However, there is no evident dissolution of cathode Ni regardless of the orientation of beta-Sn grains, due to the protection of a stable interfacial (Cu,Ni)(6)Sn-5 intermetallic compound (IMC) layer and the extremely low solubility of Ni in beta-Sn. Cu6Sn6-type protrusions selectively precipitated in specific Sn grains with small angle 8 (between the c-axis of Sn grain and electron flow direction) but not in the neighbor grains with large angle 8 or along the direction of c-axis of beta-Sn. Sn hillocks are squeezed out to relieve the compressive stress generated by the formation of Cu6Sn5-type IMCs. The high diffusion anisotropy in beta-Sn grains, which is calculated by a proposed model, accounts for the novel diffusion behavior of solute atoms, dissolution of cathode and consequent precipitation of IMCs in Ni/Sn-3.0Ag-0.5Cu/Cu flip chip solder joints. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.