Reliability Assessment of Cu-Al Wire Bonds under Thermal Aging: An Investigation of Interfacial Degradation and Mechanical Failure

Abstract

As the microelectronics industry increasingly adopts copper (Cu) wire for cost-effective interconnection, understanding its long-term reliability is paramount. The primary failure site in Cu wire bonding is the interface with the aluminum (Al) pad, where the formation of brittle intermetallic compounds (IMCs) under thermal stress poses a significant threat. This study provides a systematic investigation into the impact of isothermal aging at 175°C on the reliability of Cu-Al wire bonds over 1000 hours. A research framework was established to test the hypotheses linking thermal exposure, interfacial microstructure, and mechanical performance. Advanced sample preparation was performed using Focused Ion Beam (FIB) milling to obtain pristine cross-sections for analysis. Subsequent Scanning Electron Microscopy (SEM) revealed a progressive and detrimental growth of the Cu-Al IMC layer, leading to the formation of voids and micro-cracks. This microstructural degradation directly correlated with a severe decline in mechanical integrity, as measured by wire pull testing. The bond strength, after a slight initial increase, dropped by over 48% after 1000 hours of aging. This weakening was accompanied by a definitive transition in failure mechanism from robust ductile fracture to brittle interfacial failure. The findings conclusively demonstrate that uncontrolled IMC growth is the primary driver of Cu-Al bond degradation, highlighting the critical need for robust thermal management in modern electronic devices.