Drawback to conventional methods
Conventional schemes used to calculate effective conductivity are only applicable if boundary conditions are uniformly applied and spreading resistance can be neglected. Consequently the calculated value of effective conductivity is based only on the resistance of the bulk materials and all other intrinsic resistances between the source and the sink are excluded. Unfortunately, the mixed boundary value problems typical of printed circuit board applications do not lend themselves to these idealized boundary conditions. The spreading resistance, which is often of similar or greater magnitude than the bulk resistance, must be accounted for in the calculation of effective conductivity, and any estimates of conductivity based solely on series and parallel resistor networks can lead to significant miscalculations of temperatures in PCBs.
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