The effective thermal conductivity κ of multilayer printed wiring boards (PWBs) has been measured for heat flowing in a direction either parallel (κ) or perpendicular (κ) to the plane of the board. The conductivity of the glass/epoxy insulating material from which the boards are manufactured is anisotropic (κge ≈ 3 × κge) 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 ~100 or more. The value of κ 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.

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