Fatigue model based on average cross-section strain of Cu trace cyclic bending

This study focuses on quasi-static mechanical cycling durability of copper traces on printed wiring assemblies (PWAs). PWA specimens populated with Land Grid Array (LGA) components on copper-defined pads were cycled to failure under zero-to-max, three-point bending. Failure is defined in terms of electrical opens due to fatigue damage propagation through the entire cross section of the trace. Failure statistics were collected and failure analysis was conducted to identify fatigue failures in the copper traces, near the connection to the solder pad. Cyclic bending of this assembly was modeled with 3D, elastic-plastic, finite deformation (geometrically nonlinear) finite element analysis. Due to the complexity of the geometry, a two-step global-local approach was used to identify the cyclic strain history and the mean stress at the copper trace failure site. A generalized strain-based fatigue model is proposed, to characterize the fatigue durability in terms of the amplitude of cyclic strain and the cyclic mean of the hydrostatic stress. The strain and stress values are averaged over the entire cross-section, to be consistent with the failure criterion defined above. Average cross sectional model constants are iteratively estimated by ensuring that they are simultaneously compatible with both the durability test data and the copper stress-strain curves used in the FEA (finite element analysis). To make better use of time, a Response Surface (RS) was created after a study determined certain key constants varied reasonably linearly with each other from model to model. This RS was used for the actual iteration. The important impact of this study includes insight into copper trace failures in PWAs under mechanical cycling, a quantitative model to predict its occurrence, and validated guidelines to prevent it by design.