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44 DESIGN007 MAGAZINE I JUNE 2020 High-frequency circuit materials are used in a variety of diverse applications. Due to this diversity, it is difficult to write an application (app) note for a specific high-frequency circuit material. Combine that with the fact that one design- er can use a different circuit design technique than another designer for the same circuit ap- plication, and that could make one high-fre- quency material a better choice than another based on design technique. Rogers Corporation has app notes, but not as many as one may as- sume, and for a good reason. The app notes for high-frequency circuit ma- terials are typically related to a large-scale top- ic. For example, Rogers has an app note on the copper foils used in high-frequency PCB ap- plications. This app note covers the different manufacturing processes to create a copper foil, different types of copper foils, and prop- erties of the copper foil as they relate to high- frequency performance and measurement val- ues of the critical properties. Copper foil is obviously critical to high- frequency PCB applica- tions, but for some ap- plications, the copper foil properties are less critical than others. High-Frequency Circuit Material App Notes The same circuit application may be very sen- sitive to copper properties at high-frequency, but at lower frequencies, the copper proper- ties may be much less critical. Unfortunately, the topic of critical copper properties for dif- ferent RF PCB applications is a large subtopic and cannot be addressed in a relatively short app note. To expand on the copper comments, the copper surface roughness of a copper foil can be very influential for high-frequency appli- cations. To be more specific, the copper sur- face roughness at the substrate-copper inter- face can have an impact on insertion loss and phase response, which is related to dielectric constant (Dk). Basically, if the copper surface is rougher, the propagating wave will slow down, and the cir- cuit will behave as though it has a higher Dk. This is true regardless of the Dk property of the substrate. Additionally, a rougher copper sur- face will cause increased conductor loss, which is a component of insertion loss. As copper roughness increases, the conductor loss will in- crease, and so will the insertion loss. Howev- er, there is a substrate thickness dependency a n d f r e q u e n c y Lightning Speed Laminates Feature Column by John Coonrod, ROGERS CORPORATION

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