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Passive intermodulation (PIM) is a circuit anomaly that has been occurring in cellular base station antennas, causing tremendous frustration and cost to antenna OEMs. As the supplier of the materials being used by these OEMs, Rogers Corp., and more specifically, Product Manager Tony Mattingly, must come up with a solution. I-Connect007 Editor Stephen Las Marias and I sat down with Tony to learn more about PIM and how Rogers is combating this issue for their customers.
Barry Matties: Tony, for a little context, start with a bit about what Rogers does.
Tony Mattingly: Rogers Advanced Connectivity Solutions is a market leader of high-frequency PCB materials. We supply markets such as automotive, aerospace and defense, and wireless communications. I manage the antenna grade materials for base station infrastructure, and that has to do with cellular infrastructure related to macro towers, small cells, and so on. We're also starting to explore enterprise Wi-Fi.
Matties: You were just explaining to us about passive intermodulation, or PIM. I think the question is, why is PIM referred to as a passive issue when it causes everyone that works with it to be aggressive? [Laughs]
Mattingly: That’s definitely a funny way of putting it. PIM is such a problem for our customer base. Fabricators are trying to understand what drives PIM and performance issues in their circuit boards. The antenna OEMs are trying to understand when they have PIM failures in their antennas what the source is behind that. There are so many variables that impact PIM. From cables to connections, to solder mask to etch resolution at the board shops, and all the way down to, “Hey, is the core laminate that I'm buying free of ferromagnetic materials? Are you using copper profiles that are in control?” In any case, whenever there's a PIM failure at the antenna OEM, you're potentially talking about millions of dollars of antennas that aren't being put on the base station towers, and those deployments are time critical. It's frustrating and it costs people a lot of money.
Matties: Does it really start with the base material?
Mattingly: It is one of the constituents of building a good PIM-performing antenna out of PCB material. It's where it all begins.
Matties: Why don't you just give us some of the basic concepts of PIM? How does it work and why is it a problem?
Mattingly: Passive intermodulation affects systems that use a single antenna for both transmitting and receiving signals on multiple carrier frequencies. Slight non-linearities in the input and output relationship in the system can generate additional signals that are in-band and at very low power levels; this increases the noise-floor. Increased noise degrades system performance and results in operational problems like choppy transmissions or dropped calls.
Rogers recently published a PIM brochure called “PIM and PCB Antennas.” For anyone interested in an even more technical explanation of PIM, that brochure would be helpful.
Matties: What materials and properties influence it?
Mattingly: It has been demonstrated that low-profile copper foils provide lower PIM response, and ferromagnetic contamination leads to higher PIM response of PCBs. For core laminate suppliers, these two variables must be controlled in order to successfully provide antenna materials to antenna customers looking for the best PIM performance possible. Rogers builds all laminate materials within a cleanroom environment free of such contamination, and offers two types of antenna grade materials. A RO4000® hydrocarbon resin system laminate using a developed LoPro® technology, combining reverse treat foil and proprietary adhesive for improved PIM and peel strength, and an AD series™ PTFE based resin system with reverse treat copper.
Matties: What is Rogers doing to address the problem?
Mattingly: We are determined to building our PIM test capabilities and understanding in order to remain a leading laminate materials supplier for PIM sensitive designs. We developed a PIM test method working with Summitek and various base station antenna OEMs.
Within the last year we have invested in additional PIM testing equipment for the U.S. and Asia operations. This has allowed us to further characterize our materials, and gain additional insights into normal PIM variance, and to understand what the normal range of PIM performance is for each material, considering lot-to-lot variations.
Matties: You have to, right? It's all got to be tested. The cost of failure is so dramatic.
Mattingly: It's tricky, too. PIM is not a base laminate attribute. It's a circuit so there are all these variables. To PIM test our material, you have to build a circuit, so you're introducing fabricator-level variables. There is a large degree of variability at fabrication and final assembly levels for finished antennas that impact PIM performance. Solder mask finish, interconnects, conductor surface roughness, joints with dissimilar metals, loose connections, and the use of ferromagnetic metals are all things impacting PIM. Base laminate is considered one of many important constituents to understanding PIM.
Stephen Las Marias: Is the PIM issue only prevalent or disruptive in RF applications? What about for other segments?
Mattingly: I have not heard of PIM being an issue in other segments, but that doesn’t mean it isn’t.
Matties: Thank you very much, Tony.
Mattingly: Thank you.