THERMAL-CONDUCTIVITY MEASUREMENTS IN PRINTED WIRING BOARDS

effective thermal conductivity kappa of multilayer printed wiring boar ds (PWBs) has been measured for heat flowing in a direction either par allel (K-parallel to) or perpendicular (K-perpendicular to) to the pla ne of the board The conductivity of the glass/epoxy insulating materia l 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 ani sotropy in PWBs that contain continuous layers of copper. For such boa rds, values of the board-averaged conductivity in the two directions c an differ by a factor of similar to 100 or more. The value of kappa(pa rallel to) is found to depend on the ratio of the total thickness of c ontinuous layers of copper to the total thickness of glass/epoxy, whil e it depends hardly at all on the amount of copper circuitry visible o n the surface.