On predicting the operational thermal resistance of electronic components

The thermal resistance of electronic components is known to often differ considerably between standard test conditions and those found in service. One way to correct for this is to use multi-parameter thermal resistances. Another, presented here, is to adjust the junction-to-ambient thermal resistance to take account of operational conditions. For forced convection, two factors are proposed to take account of these conditions; the first is due to any upstream aerodynamic disturbance, and the second is due to any purely thermal interaction. Thus, for an upstream powered component, the two factors are combined. It is shown that both factors may be quantified in terms of readily measured temperatures and then used as coefficients to adjust the standard thermal resistance to operational conditions. To overcome current problems with the definition of an electronic package thermal resistance, it is redefined to include both the resistance of the package and its thermal footprint on the board. New data is presented for an array of board mounted 160-lead devices, showing how the factors vary with component position, nondimensional power distribution and Reynolds number