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Design007-Aug2018

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42 DESIGN007 MAGAZINE I AUGUST 2018 It's a Material World Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Years ago, when clock frequencies were low and signal rise times were slow, selecting a dielectric material for your PCB was not diffi- cult; we all just used FR-4. And we didn't really care about the properties of the materials. However, with today's multi-gigabit designs and their extremely fast rise times and tight margins, precise material selection is crucial to the performance of the product. Materials used for the fabrication of the multilayer PCB absorb high frequencies and reduce edge rates, and that loss in the transmission lines is a major cause of signal integrity issues. But we are not all designing cutting-edge boards and some- times we tend to over-specify requirements that can lead to inflated production costs. Over the years, a huge range of materials have been developed for multilayer PCB fabri- cation. In fact, to give you an idea, iCD now has a choice of over 700 series of dielectric materi- als from more than 60 different manufacturers, in its dielectric materials library. When each material is used for the right target application, the resultant PCB will have the lowest possible cost while still satisfying the design and perfor- mance goals of the project. There's no doubt that we live in a material world and selecting the best material for an application is often a daunting task. In this month's column, I will look at how to quickly sort through the vast array of choices to make an informed decision. The electrical properties of a dielectric mate- rial can be described by two terms: 1. The dielectric constant (Dk) or relative permittivity (Er) is the ratio of the amount of electrical energy stored in a material by an applied voltage. It describes how the material increases the capacitance and decreases the speed in the material. 2. The dielectric loss–dissipation factor (Df) or loss tangent (tan d) is a parameter of a dielectric material that quantifies its inher- ent dissipation of electromagnetic energy.

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