An adaptive Cu trace fatigue model based on average cross-section strain
Abstract 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. To be consistent with the failure criterion defined above, the strain and stress values are averaged over the entire cross-section. 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 finite element analysis (FEA). To minimize the computational burden, a Response Surface (RS) was created for the actual iteration. The key RS variables and their functional inter-relationships were determined from a fractional factorial FEA parametric study. This study is important because it provides insight into copper trace failure in PWAs under mechanical cycling, a quantitative model to predict its occurrence, and validated guidelines to prevent it by design.