Copper trace fatigue models for mechanical cycling, vibration and shock/drop of high-density PWAs

Abstract

When assessing the mechanical durability of electronic assemblies, the focus is generally on the solder interconnects. However, in many package styles, such as ball grid arrays (BGAs), land grid arrays (LGAs), micro lead frame (MLFs) and quad flat no-lead (QFNs), the Cu trace emanating from the solder pad may be the weakest failure site, especially if the solder joint is copper-defined rather than mask defined. This is particularly true in situations with cyclic mechanical loading, such as cyclic quasi-static bending, vibration and repetitive drop/shock. This study focuses on quasi-static mechanical cycling durability of LGA assemblies with copper-defined pads. Specimens were cycled to failure under zero-to-max, three-point bending and failure statistics were collected. The failure mode was confirmed to be fatigue cracks in copper traces emanating from the corner solder pads, just at the edge of the solder mask where the solder joint ends. The cracks were identified by lateral polishing after desoldering the component. The cyclic bending of this assembly was modeled with 3D, elastic–plastic, large deformation finite element analysis. Due to the complexity of the geometry, a global–local approach was used to identify the strain history and the mean stress at the failure site. A generalized strain-based fatigue model was used to characterize these failures, and preliminary model constants were iteratively estimated by ensuring that they were simultaneously compatible with both the durability test data and the copper stress–strain curves used in the FEA (finite element analysis). These preliminary model constants were then refined by separately modeling the initiation and progression history of fatigue damage in the cyclic bend tests. Damage progression was modeled in this study by using a technique of ‘successive initiation’ developed earlier by this research group. In this method, finite element simulations are used to progressively ‘kill’ elements that have accumulated sufficient fatigue damage to lose their load-carrying capacity. When the killed elements span the entire cross-section of the copper trace, the trace is assumed to electrically fail. This calibrated model is then used to demonstrate its ability to predict copper trace failures in other situations such as quasi-static four-point bending of LGA assemblies and vibration of BGA assemblies. More important, we demonstrate its use for re-designing of BGA assemblies to prevent copper trace failures under drop/shock loading. The important impact of this study includes insight into copper trace failures in Printed Wiring Assemblies (PWAs) under mechanical cycling, a quantitative model to predict its occurrence, and validated guidelines to prevent it by design.