Background
Several schemes have been proposed for calculating effective conductivity, including what is generally considered to be the lower and upper bounds of effective conductivity, namely, the series or ``cross-plane conductivity'' and the parallel or ``in-plane conductivity'', respectively. A simple average of the upper and lower bounds on thermal resistance can provide an accurate measure of effective conductivity when the difference between the bounding solutions is small, but this is generally not the case in PCBs where the ratio between the effective conductivity based on the parallel and series paths is typically of order 25:1 or greater. Other averaging schemes, such as the harmonic or geometric mean of the two limiting solutions can be used but these methods tend to favor one of the limiting conditions more than the other and the calculated effective thermal conductivity may be appropriate for certain design conditions but not for others.
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