  Passive intermodulation (PIM) is a niggling phenomenon that debilitates
performance of modern communications and navigation systems. PIM products
interfere with the information signals and cause their nonlinear distortion.
The sources and basic mechanisms of PIM were studied in literature but PIM
remains a serious problem of signal integrity. In this paper, the main sources
and mechanisms of PIM generation by joints of good conductors are discussed. It
is shown that the passive electrical, thermal and mechanical nonlinearities are
intrinsically linked despite their distinctively different time scales. The
roughness of the contact surfaces plays an important role in PIM generation by
conductor joints. A review of the PIM phenomenology at contacts of the good
conductors suggests that novel multiphysics models are necessary for the
analysis and reliable prediction of PIM products generated by several
concurrent nonlinearities of diverse physical nature.
