P. Ajay Kumar

In 3D electronic packages, stacked dies are connected vertically using through-silicon vias and solder micro-bumps, which are typically between 1 μ m and 50 μ m 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- μ m-thick joints of SAC305 solder between two Cu substrates were tested, containing three different Cu ₆Sn ₅-to-Cu ₃Sn 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, Cu ₆Sn ₅, Cu ₃Sn and Cu ₆Sn ₅/Cu ₃Sn 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 Cu ₆Sn ₅ and Cu ₃Sn 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 microcracks. Deformation and damage are controlled by the intermetallics, and not the strain-rate sensitive solder for the aged samples. Both Cu ₆Sn ₅ and the Cu ₆Sn ₅/Cu ₃Sn interface fracture are the dominant mechanisms with increasing aging under the shear deformation. A few results will be shown on the evolution of microstructure of Cu/Sn-58Bi/Sn-3.0Ag-0.5Cu composite micro solder joint as well.