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18 DESIGN007 MAGAZINE I MAY 2023 In the past, selecting a dielectric material for PCB fabrication was a no-brainer because we all just used FR-4. Clock frequencies were low and signal rise times were slow, so sub- strate performance was not an issue. How- ever, in today's multi-gigabit designs, with their extremely fast rise times and tight timing margins, precise material selection is crucial to the performance of the product. is puts the materials selection process under tighter scrutiny. Materials used for the fabrication of multilayer PCBs absorb high frequencies and reduce edge rates, which is a major cause of signal integrity issues. But we're not all design- ing cutting-edge boards. Sometimes we tend to over-specify requirements, which can lead to inflated production costs. Over the years, a huge range of materials has been developed for multilayer PCB fabrica- tion. To give you an idea, iCD now has a choice in its dielectric materials library of more than 700 series of dielectric rigid/flex materials from more than 60 different manufacturers. When each material is used for the right tar- get application, the resultant PCB will have the lowest possible cost, yet still satisfy the design and performance goals of the project. Electromagnetic energy propagates in a vac- uum or in air at the speed of light. But, as the field is enveloped in a dielectric material in the PCB media, it slows down. e signal trace in a PCB simply guides the wave as the electro- magnetic energy propagates in the surround- ing dielectric material. e signal rides on this carrier wave. It is the dielectric material that determines the velocity (v) of propagation of the electromagnetic energy: c is the speed of light (in free space) and Dk is the dielectric constant of the material (FR-4 is ~4.0). By contrast, the Dk of air is 1. erefore, the velocity of propagation in FR-4 is about half Select Dielectric Material With Precision Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA

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