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In ICs, the inductors can magnetically couple to the the substrate and cause eddy current to flow which causes power loss (emf^2/Rsub). To minimize this people put metal (copper, aluminum) shielding below the inductor to prevent magnetic lines of from from reaching substrate. Now the catch is both copper and aluminum have mu_r =1 i.e. they will pass all magnetic lines of force to the substrate. How can they help in shielding magnetic coupling? If iron were used then it might make sense but ICs do not use iron usually. Therefore, what is the mechanism?