Thermal ageing monitoring in Cu Al intermetallic joints through electrical resistance drift: Comparative study of lifetime potential in pure and alloyed copper wires

Copper has pervasively replaced gold as preferred wire bonding material in Integrated Circuits (IC) plastic packaging. Different types of wires are available today in the market, with specific annealing treatments, coating and doping solutions aimed at optimizing bondability and reliability performances of the joints. In this study, the electrical resistance drift of packaged daisy chains has been analyzed to compare the lifetime potential of pure and alloyed copper wires under accelerated High Temperature Storage (HTS). Two aluminum-based bond-pads with different composition and thickness have been included in the experimental matrix. The results obtained through this statistically efficient and non-destructive methodology have been correlated with more “classical” readout data based on wire pull test, polished cross sections for Inter-Metallic Compounds (IMC) thickness measurement and TEM lamellas for phases stoichiometry characterization. Comparative analysis of the drift plots has pointed out a specific electrical signature for the consumption of the aluminum source under the IMC joint, confirmed by a Finite Element Method (FEM) simulation. A systematic delay in the IMC evolution has been demonstrated in all the samples with alloyed copper wires, correlating their lower ohmic drift with a lower thickness and a different composition of the IMC phases growing during HTS.