On Thermomechanical Durability Analysis Combined With Computational Fluid Dynamics Thermal Analysis

Results are presented on durability analysis of an electronic module subjected to thermal and power cycles, and vibration. A hierarchical analysis process for analyzing the durability of the module is described. The initial step is a transient thermal analysis of the unit in which the module is located. The three operating modes of the unit are modeled and analyzed using a commercially available computational fluid dynamics (CFD) tool. The tool generates a time history of the temperature at all points within the unit and module. The second step comprises exporting temperatures from the transient temperature analysis to a durability prediction tool. The temperatures calculated by the global analysis are mapped to the printed wiring assembly (PWA) mounted within the box, yielding the temperature distribution of the PWA as functions of time. The durability tool utilizes a modified Coffin Manson formula together with the transient temperature profile to estimate the durability of each lead and solder joint included in the module. Thermomechanical fatigue level of leads and solder joints within the unit are reported as a cumulative damage index (CDI). The CDI is the ratio of the number of cycles required for the test item to endure under a life time to the number of cycles the item is predicted to sustain before failure. Durability analysis of solder joint due to vibration is performed separately. The environment is specified according to the location where the unit is mounted. CDI due to vibration is added to form an overall CDI based on Miner’s rule.