Factors affecting the operational thermal resistance of electronic components

The thermal resistance of electronic components is known to often differ considerablybetween, standard test conditions and those found in service. One wcy to correct for thisis to use multi-parameter thermal resistances. Another, presented here, is to adjust thejunction-to-ambient thermal resistance to account for operational conditions. Forfbrcedconvection applications, two factors are proposed the first accounts ftir any upstreamaerodynamic disturbance and the second addresses purely thermal interaction. Thus if anupstream powered component interacts with a downstream component, the rtro factorsare combined. It is shown that both factors may be quantified in terms of readily mea-sured temperatures and then used as coefficients to adjust the standard thermal resistancedatafbr operational conditions. To overcome the misconception that the currently pub-lished single-value thermal resistances are solely a property of the electronic package, thermal resistance is redefined to include both the resistance of the package and the partof the printed circuit board (PCB) covered by the components thermal footprint Thisapproach is applied to a symmetrical array of board mounted 160-lead devices and datashowing how the factors vary with component position, nondimensional power distribu-tion and Reynolds number is presented Based on data a new method of generatingoperational component thermal resistances is proposed.