Fighting stray frequencies in PCBs

Michael Miller

Isola Group

To meet the constant consumer demand for higher bandwidth in their wireless devices, companies have developed ultra-wideband communications equipment and advanced wireless techniques, such as frequency-hopping spread spectrum transmission, in which radio signals are rapidly switched between frequencies. The same technique can also guard against eavesdropping. With those advances, however, comes a growing challenge for manufacturers of the high-performance printed circuit boards aimed at the wireless communications market. That challenge lies in controlling a phenomenon known as Passive Intermodulation.

Passive Intermodulation, or PIM, can reduce wireless coverage area, cut data rates, and increase the potential for dropped calls. When two or more signals are transmitted through a system that has non-linear characteristics, they can combine to create new frequencies. If the frequency of one of these products of PIM falls within the receiver band or up-link of a base station, for example, it can create interference and cause noise, resulting in degraded performance. Sources of these non-linearities can include contamination—such as dirt, moisture, or oxides—on electrically conducting surfaces, inconsistent metal-to-metal contact, the presence of ferromagnetic materials, and the presence of other oxides.

In PCBs in particular, PIM often arises from characteristics of the copper and the dielectric resin composite matrix used to build the boards. Copper that has a lower surface roughness and a finer crystalline structure yields higher PIM performance; experts believe that a rough surface can trap contaminants more easily, as well as lead to the creation of mechanical defects and voids that cause non-linearities. The quality of the finished copper after etching also plays a role. The etched tracks must be consistent and free of edge fractures. Characteristics of the PCB’s dielectric material are also important. Materials with lower dielectric loss produce lower PIM. Finally, using a resin material that absorbs little moisture also helps with PIM performance.

One way PCB manufacturers have responded to the need for better PIM behavior is by introducing promising new thermoset materials. Isola, for example, offers IS680, IS680AG, I-TeraMT RF, and AstraMT. Components built from these laminates have shown PIM measurements from -155 dB to -165 dB while retaining the mechanical, thermal, reliability, and processing advantages of a thermoset system.

By addressing the various sources of defects and contamination, PCB manufacturers can ensure that their products have the lower PIM, and thus the higher bandwidth, that wireless communications equipment will continue to require.