The treatment at the copper-substrate interface is not always considered in circuit material PIM studies, but it is generally not purely copper at that interface but a metal alloy used as a treatment to enhance the bond between the copper and the substrate material. The treatment also acts as a thermal barrier to the formation of copper oxide and to ensure good thermal robustness in the interface structure for the temperatures that will be used in processing the circuit material. While most copper treatments do not degrade PIM performance, some can and it must be considered as a possible factor that can contribute to circuit material PIM. For example, a nickel alloy copper treatment was found to have deleterious effects on PIM performance, since nickel is a ferromagnetic material.
Testing for PIM
As Table 2 indicates, small differences in versions of PCB materials can result in differences in PIM performance for printed antennas and other high-frequency circuits. With the low levels of PIM that are being evaluated, however, testing materials for PIM can be extremely challenging. No standard test method exists for evaluating circuit materials for PIM performance, although a test method has been developed at Rogers Corp. using a commercial PIM tester from Kaelus (www.kaelus.com) based on the behavior of a 300-mm-long 50-Ω microstrip transmission line circuit when fabricated on a circuit material of interest. With over a decade of collecting PIM data on circuit materials, Rogers has found that the test results using this microstrip test method can at best be held to a tolerance within ±6 dBc. With considerable experience in testing, it has been found that PIM varies with time and can vary even during the process of making the measurements on the microstrip transmission lines. Even to achieve that ±6 dBc measurement tolerance, it is important to ensure that the test equipment and test environment is stable before conducting the PIM measurements. Figure 3 shows an example of results as a function of time for a microstrip transmission-line circuit being tested for PIM.
As the plot shows, the level of PIM is relatively stable for a relatively long test period. The test data were collected over a period of 55 seconds, with 10 data points for every second of test time. In this case, testing was of a known good PIM circuit material with a thickness of 60.7 mils (1.54 mm). It has been found that if a thinner substrate of the same material is tested, the PIM performance will degrade slightly. This is likely due to the fact that a thinner 50-Ω microstrip transmission-line circuit will contain a narrow conductor with a resulting higher power density.