Thermal design aspects of an avionic module including fully populated PCBs housed in a sealed enclosure have been studied by means of computational fluid dynamics. Effect of power distribution between the sides of a double-sided PCB on the case temperature of surface-mounted components has been investigated within a proposed simulation strategy. Simulation-based guidelines have been developed for thermal design of avionic modules, regarding preferable power configuration on a double-sided PCB, representing an alternative approach to thermal management, as compared to introducing additional cooling devices.

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 acommercially 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.

A flexible experimental setup enabling power control of a fully populated double-sided PCB has been realized, and is described in detail. A CFD model of a double-sided PCB housed in a sealed enclosure has been validated in a 19/spl deg/C environment by means of temperature and flow measurement. The difference between simulated and measured component temperatures has been within 10%. Potential errors both in the model and in the experiments have been discussed and their impact on temperatures has been numerically evaluated.

Anexperimental procedure for investigating the effect of power distribution onthe cooling of a double-sided PCB is implemented. A numberof computational fluid dynamics (CFD) models are validated by laboratoryexperiments performed in 19.5°C temperature environment. Case temperatures of surface-mountedcomponents fully populating the PCB sides are measured and monitoredin simulations. Different combinations of power distribution with other coolingmethods, such as a heatsink tooled on a sealed oropen enclosure, at natural or forced convection, are studied. Thermallyefficient uniform and non-uniform power configurations are determined on adouble sided PCB. It is concluded that managing power distributionon a double-sided PCB can be considered as a measureto improve the thermal performance of electronic modules.