Study of Metal Contact Resistance and its Statistical Correlationship with Passive Intermodulation

The passive intermodulation (PIM) effect, one of the major reliability problems in communication systems, is mainly affected by the electrical contact characteristics of the system components. One of the major issues is that the tested PIM level is usually fluctuant when the system components are assembled repeatedly. In this paper, the statistical characteristic of contact resistance (R_c) is revealed by repeatedly measuring an AI-AI contact and the relationship between PIM and contact pressure is quantitatively investigated. The frequency polygon of the measured R_c under three different contact pressures indicates that the R_c has statistical character and the most probable R_c value decreases with the contact pressure increasing. Then a near-field-coupling PIM test system is specially designed to measure the PIM level. The PIM measurement results indicate that the PIM level decreases with the contact pressure increasing. Further, a Monte Carlo method based on the Greenwood-Williamson statistic model and Holm R_c model of single asperity is conducted to verify the statistical characteristic of R_c. Finally, an analytical equation is presented to illuminate the relationship between PIM and contact pressure and a PIM calculation method based on CST and Matlab is implemented. The equation also indicates that if the contact load is constant and the R_c is low, the PIM product will be reduced by 60 dB while the R_c is reduced by one order of magnitude. Both the experimental and the theoretical results demonstrated that the fluctuation of R_c and PIM originates from the randomness of asperities' height distribution.