stress migration modeling

**Stress migration modeling** is the **prediction of thermomechanical driven vacancy transport in metal interconnects even when no electrical current flows** - it captures voiding risk from temperature cycling and material mismatch that can silently reduce via and line reliability. **What Is Stress migration modeling?** - **Definition**: Model of metal mass transport induced by mechanical stress gradients instead of electron wind. - **Primary Drivers**: Thermal expansion mismatch, process-induced stress, and repeated thermal excursions. - **Failure Signatures**: Void nucleation near vias, open circuits, and intermittent resistance jumps. - **Model Inputs**: Temperature history, material properties, geometry, and stress relaxation constants. **Why Stress migration modeling Matters** - **Hidden Reliability Risk**: Stress migration can damage interconnect in low-current but high-thermal-cycling blocks. - **Package Interaction**: Assembly and board-level thermal expansion affects on-die stress state. - **Design Rule Guidance**: Keep-out zones and via topology choices depend on stress migration sensitivity. - **Failure Isolation**: Distinguishing stress migration from electromigration avoids incorrect fixes. - **Lifetime Confidence**: Model-based prediction improves robustness for long service products. **How It Is Used in Practice** - **Thermomechanical Simulation**: Compute stress evolution across process and operational thermal cycles. - **Model Correlation**: Validate predicted voiding locations against FA data from stress experiments. - **Mitigation**: Adjust stack materials, via arrays, and thermal ramp profiles to lower stress gradients. Stress migration modeling is **critical for complete interconnect lifetime analysis** - reliable products require control of both current-driven and stress-driven metal degradation paths.