Thin copper wire under extreme HTSL stress duration: Crack failure mechanism characterization

Mission profiles for specific automotive applications are becoming more and more demanding from the reliability point of view. Translating this challenging requirements into reliability targets, it means performing trials for longer duration, or using more accelerated conditions (increasing temperature or voltage, etc…). This study is focused on the understanding of the failure mechanism and the characterization of a thin copper wire over Aluminum pad submitted to a very long stress duration at high temperature: more than 5000hrs @150°C. Going beyond AEC-Q100 Grade 1 (1000hrs @150°C) [1] and AEC-Q006 (2000hrs @150°C) [2] specified conditions for copper wires, it has been possible to observe the effects of the isothermal stress experienced by the Cu/Al bonding system, up to its wear out. Such critical conditions often activate failure mechanisms not significantly observable in standard qualifications. After thousand hours at high temperature a crack between Cu ball and CuxAlx IMC (Inter-Metallic Compounds) propagates from the ball edge to the ball center, affecting all the bonding area and causing an open contact. Crack propagation has been evaluated as second order effect of IMC growth and evolution, so its appearing is predictable by the Arrhenius law. The activation energy for the failure rate prediction has been estimated from the experimental data.