Predicting thermal contact resistance in circuit card assemblies

On a typical conduction-cooled Circuit Card Assembly (CCA) there is a temperature rise caused by the resistance between component die, Printed Circuit Board (PCB), thermal frame (heat sink), and chassis. While the resistance at the component junction, through the PCB, and through the thermal frame can be readily calculated from material properties, modeling the thermal frame and card clamp to chassis interface can be difficult. For CCA power below approximately 25 watts the loss at the interface is usually less than 4°C (0.30°C/W for bare aluminum). As the thermal dissipation on a CCA reaches 50-75 watts (>1 watt/in² on VME 1101.2 format), it becomes necessary to model this interface because of its increasing influence on the overall temperature rise. A method utilizing several Unigraphics GRLP routines, SINDA thermal analysis software, and empirical data has been developed that predicts the thermal interface performance between both the chassis and thermal frame, and the chassis and card clamp to within 30 percent of measured values