
92 Heat Transfer: Thermal Management of Electronics
this large difference between the thermal conductivity of copper and glass epoxy, the
thermal conductivity of PCB, as a whole, is strongly anisotropic. This is qualitatively
shown in Figure 5.26 where most of the heat that arrives to an area of a copper layer
is transferred in the lateral (or planar) direction into the copper layer itself rather than
directly transferring through the copper and the glass epoxy layers. This shows that
PCBs have orthotropic heat transfer properties, and their thermal conductivities in
planar and normal directions are different.
PCBs are commonly modeled as a ...