Effects of Grain Size on the Ag Dissolution and Ion Migration of Ag-4Pd Alloy Wires

Ion migration and corrosion behavior in Ag-4Pd alloy wires with fine and coarse grains in the outer regions were evaluated with water drop tests. The results from the in situ observations and measurements of Ag ion migration in both Ag-4Pd bonding wires indicated that the fine-grained wire had much shorter times of dendrite contact and short circuit than those of coarse-grained wire for various wire spaces. It was obvious that grain refinement in Ag-4Pd alloy wires accelerated the ion migration of Ag-4Pd alloy wires. Electrochemical polarization tests confirmed that the passive film on fine-grained Ag-4Pd wire was easily broken down, leading to a higher Ag dissolution tendency than that of coarse-grained wire. Although an annealing twin-rich Ag-4Pd alloy wire was shown to enhance the thermal stability of the grain structure and mechanical properties, the large number of twins had no influence on the formation of Ag dendrites on the cathode and Ag₂O oxide on the anode of the coarse-grained Ag-4Pd wire couple during ion migration.