Thermal conductivity measurements in printed wiring boards

effective thermal conductivity kappa of multilayer printed wiring boards (PWBs) has been measured for heat flowing in a direction either parallel (K-parallel to) or perpendicular (K-perpendicular to) to the plane of the board The conductivity of the glass/epoxy insulating material from which the boards are manufactured is anisotropic (K-parallel to(ge) approximate to 3 x kappa(perpendicular to)(ge)) and nearly three orders of magnitude smaller than the conductivity of copper This large difference between glass/epoxy and copper produces extremely high anisotropy in PWBs that contain continuous layers of copper. For such boards, values of the board-averaged conductivity in the two directions can differ by a factor of similar to 100 or more. The value of kappa(parallel to) is found to depend on the ratio of the total thickness of continuous layers of copper to the total thickness of glass/epoxy, while it depends hardly at all on the amount of copper circuitry visible on the surface.