While no circuit material is immune to the PIM of a passive circuit, such as a patch antenna, some circuit materials do better with PIM than others. PTFE-based high-frequency circuit materials have traditionally been known for low PIM levels, although such materials can require specialized processing steps and can be expensive choices for fabricating PCB antennas. Some non-PTFE-based thermoset circuit materials have fared well in PIM-sensitive designs, including as PCB antennas. Early concerns with the use of thermoset materials in such applications was that they lacked the low-loss (low Df) performance of PTFE-based circuit materials. But as the values in Table 1 show, a number of non-PTFE hydrocarbon circuit materials have been developed for PCB antennas with low Df loss and with PIM performance on par with PTFE-based circuit materials. The materials listed in Table 1 feature reasonably good TCDk values for stable performance with temperature, where |50| ppm/°C or less is considered good.
PIM can be caused by any number of variables in a system, including types of cables, loose connectors, and dirt on conductive surfaces. Materials with ferromagnetic properties are generally known to be potential sources of PIM. Ferromagnetic content of even 100 ppm or less in a dielectric material can elevate the PIM levels of a circuit material. For printed circuits, a number of factors can contribute to PIM performance, including the type of dielectric material, the quality of the etched circuitry, the quality of plated through holes (PTHs), the cleanliness of the PCB, soldermask, assembly issues, even the roughness of the copper conductor.
In fact, it is the surface roughness of the copper at the copper-substrate interface which has been found to be critical to PCB PIM performance. For the same material and transmission line, for example, smoother copper at this interface will result in lower levels of PIM. Table 2 lists the results of an internal study performed at Rogers Corp. which details the correlation between the root-mean-square (RMS) surface roughness of the copper at the copper-dielectric interface and the average PIM level. For the four items listed, the same substrate material, same dielectric thickness, same copper thickness, and same circuit design were used, with the only variation being the type of copper which had different amounts of surface roughness.