[Rt_design]

Hi everyone,
I work as a thermal design engineer for aircraft and EW applications. and we usually work with multilayered PCB like fr4, rogers etc.

My question
1. is there any hand calculations that thermal design engineers use to get the rough estimate of the effective thermal conductivity of such PCB.
i usually use parallel and series formulae for calculation of effective thermal conductivity. but the results are way off to the practical values i get when being calculated in the software ( i use flotherm XT)

2. How do you model vias under the PCB for doing thermal simulation as the vias will take a lot of time for both to model and simulate.