Incorporating Tensile Stress Into Electromigration Life Prediction for Cu/SAC305/Cu Solder Joints

Abstract

Accelerated testing has been executed to examine the combined influence of electromigration (EM) stressors (elevated current density and elevated ambient temperature) and tensile stress on the lifetime of SAC305 solder joints (300 μm diameter) at two current densities (8500 and 9100 A/cm2), two ambient temperatures (100 and 150 °C), and five tensile stresses (0, 0.5, 1, 2.5, and 5 MPa). 60 total samples were tested, four of which survived the 500-h test duration limit. As tensile stress was increased, a significant reduction in lifetime was observed for each of the four EM conditions (current density–temperature pairs). Voltage drop across the solder samples was measured in situ, capturing the time to failure (TTF) for all samples and allowing for the development of life prediction models based on the multistress experimental scenario. Post failure analysis of the samples tested under combined electromigration and tensile stress showed necking or breakage at the Cu/SAC305 interface on the upstream side of electron flux. The cross-sectional analysis of tested samples is consistent with the findings from other studies regarding electromigration failure in Cu/SAC305/Cu solder joint assemblies, where the intermetallic regions at Cu/SAC305 interfaces grow asymmetrically. Inherent process voids in the experimental samples are discussed as a source of error and a brief computational examination of the impact of process-related voiding on stress as well as current density and self-heating within solder samples is provided.