The physical mechanism of the experimentally observed dependence of passive intermodulation (PIM) in printed circuits on conductor surface roughness is studied. It is shown that electrothermal (ET) nonlinearity, arising due to heating of imperfect conductors by high-power carriers in a multicarrier system, is correlated with conductor surface roughness and has a unique signature. Carriers modulate the conductor resistivity, skin depth, and surface impedance which generate PIM products. The detailed analysis demonstrates that ET-PIM depends on the conductor resistivity, shape, and roughness profile and also on the electric and thermal properties of the substrate. Their effects on PIM are illustrated by examples of uniform microstrip lines with different conductor and substrate materials, and periodically perturbed and meandered microstrip lines.

The effect of conductor surface roughness on electro-thermally (ET) induced passive intermodulation (PIM) products is analysed and discussed. The analytical model combined with the commercial simulator SIMBEOR is used to examine the products of ET nonlinearity, which is significantly influenced by the surface topography of the conductor. It is further shown that, in spite of the rise of ohmic losses with frequency, conductor roughness may somewhat allay the ET-PIM growth due to concurrent increase of additional heat-sinking provided by the interiors of dendrites and conductor cladding.