Effect of Intermetallic Content on Shear Deformation of Thin Sn-3.0Ag-0.5Cu Solder Micro-joints Between Copper Substrates

Abstract

In 3D electronic packages, stacked dies are connected vertically using through-silicon vias and solder micro-bumps, which are typically between 1 μm and 50 μm thick. Solder micro-joints undergo significant shear deformation due to various loading conditions, which can occur during usage of microelectronic devices, such as thermal cycling, mechanical bending, and drop impact. A limited amount of work has been done in shear deformation and failure mechanism of these joints. To explore this, 25-μm-thick joints of SAC305 solder between two Cu substrates were tested, containing three different Cu₆Sn₅-to-Cu₃Sn ratios, in shear at strain rates from 1 s⁻¹ to 100 s⁻¹. The joint shear strength is correlated with observed failure mechanisms such as Sn, Cu₆Sn₅, Cu₃Sn and Cu₆Sn₅/Cu₃Sn interface failure. The growth kinetics of intermetallic compounds (IMCs) in thin Sn-3Ag-0.5Cu joints attached to Cu substrates have been analyzed, and empirical kinetic laws for the growth of Cu₆Sn₅ and Cu₃Sn in thin joints are reported. By combining the shear deformation results, we infer that increased IMC content due to heat treatment deteriorates the mechanical properties of the joint due to the presence of disconnected incipient micro-cracks. Deformation and damage are controlled by the intermetallics, and not the strain-rate sensitive solder for the aged samples. Both Cu₆Sn₅ and the Cu₆Sn₅/Cu₃Sn interface fracture are the dominant mechanisms with increasing aging under the shear deformation.