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In the second of a series of three RealTime with... interviews, I-Connect007 managing editor Nolan Johnson received knowledgeable and informative answers from John Bushie, director of technology at American Standard Circuits, and Anaya Vardya, president and CEO, to his questions on the unique challenges of RF and microwave PCBs.
Asked by Johnson for a definition of what was currently considered RF, Bushie responded that there was no clear boundary. But as data rates continued to increase, high-speed and ultra-high-speed digital circuits progressively took on analogue properties so that they began to require performance characteristics traditionally associated with RF and microwave circuitry.
Referring to observations noted in their recent book, Johnson remarked that designers had appeared more concerned about material selection than considerations of manufacturability. Bushie’s comment was that in reality the performance achieved resulted from a combination of material factors with fabrication techniques and tolerances, and it was a challenge for designers to find a realistic balance between cost and performance. Attitudes toward material cost ranged from “performance at any price” to “cost is everything.” At the latter end of this spectrum there was potential for using lower-loss variants of more traditional materials. And properties like mechanical strength and dimensional stability were other factors to be taken into account. Bushie stressed the importance of getting the fabricator involved as early as possible in the design process to help optimise the balance between material selection and design for manufacturability, in order to achieve cost-effective and functional designs which might combine different material types. An example was a multilayer stack-up using a core of FR-4 laminates and prepregs in combination with PTFE outer layers.
Anaya Vardya added that some of the rigid-flex designs discussed in the previous session could incorporate RF materials: constructions were becoming increasingly hybridised.
There were no actual boundaries, but it remained critical to be aware of the processing requirements of the different materials to avoid unintentionally, but mistakenly, creating designs that were effectively unmanufacturable. It was also important to understand the consequences of specifying custom design features and selective finishes on cost-effective manufacture.
Johnson enquired whether there was an easy way of avoiding misunderstanding, in response to which Bushie re-emphasised the importance of involving the fabricator as early as possible at the design stage, for expert guidance on materials, constructions and special features. He illustrated and discussed many examples of details to be considered, with an impressive air of calm professional experience and capability that instilled confidence in his wisdom without over-dramatising the potential pitfalls.
Johnson commented that copper surface roughness had become a much-discussed topic in the context of high-frequency performance and asked Bushie for a view. That was a more complex subject than could be explained in-depth within the timeframe of this interview but put simply, roughness caused small phase changes in signals travelling in or near the surface, resulting in increased losses. Bushie discussed the attributes of some of the smoother-finish copper foils now available, and made the point that choice of foil was yet another element to be taken into consideration when specifying the material stack-up. Fortunately, RF designers had become increasingly aware of foil effects on edge-coupled and broadside-coupled features.
Vardya reminded viewers of the amount of detail information available in the numerous webinars that ASC had published and were freely available on their website.
He summed up by saying that if a designer took nothing else away from this discussion, it was the principle of approaching a PCB fabricator as early as possible in the design phase of a complex circuit board. Doing so could add a lot of value and potentially save significant time and expense when presenting the design for manufacture. ASC has accumulated an abundance of experience over many years of assisting designers to understand the realities of PCB materials and processes and the benefits of effective design for manufacture. He gave his assurance that any discussion held with ASC would be in commercial confidence
This RealTime with... video was a valuable reminder of the multiplicity of factors to be taken into consideration when designing PCBs for RF and microwave applications, and the generosity of American Standard Circuits in sharing their knowledge and experience is to be commended. I was particularly impressed and reassured by John Bushie’s relaxed and unpretentious presentation style.
Twenty minutes well spent!
Pete Starkey is an I-Connect007 technical editor. This article originally appeared in the November 2021 issue of Design007 Magazine.
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