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MAY 2023 I DESIGN007 MAGAZINE 23 suming. A direct visual comparison of dielec- tric materials—based not only on the manufac- turer's product lines, but, more importantly, on one's preferred fabricator's stock—is undoubt- edly the most efficient approach for material selection. Key Points • Materials used for the fabrication of multilayer PCBs absorb high frequencies and reduce edge rates, which is a major cause of signal integrity issues. • When each material is used for the right target 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 vacuum 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. FR-4 is about half the speed of light, or 6 inches per nanosecond. • e glass epoxy material (FR-4) commonly used for PCBs has a negligible loss for digital applications below 1 GHz. But at higher frequencies, the loss is of greater concern. • A lower Dk material is desirable for high-speed design. Conversely, a high Dk material is very good at condensing electric fields, so having it between the planes increases planar capacitance. • e dielectric constant (Dk) is the amount of electromagnetic energy stored in a material by an applied voltage. • e dissipation factor (Df ) quantifies a material's inherent dissipation of electro- magnetic energy. • Conduction loss is the flow of charge through the material that causes energy dissipation. • Dielectric loss is the dissipation of energy through the movement of charges in an alternating electromagnetic field as polarization switches direction. • Dielectric loss is especially high around the resonant frequencies, causing an interac- tion between the field and the dielectric's polarization that results in heating. • A material with a low Dk has a low Df, a high Tg, and a high Td, which is required for high-speed design. • Matching material performance numbers of the dielectric constant is important. A small difference in the value between materials can impact impedance, line widths/clearances, and thus losses, significantly. • e Dk determines the propagation velocity of the signal in the medium. If Dk values vary on different layers of the substrate, then bus signal timing may also be an issue. DESIGN007 Resources Beyond Design columns by Barry Olney: • Material Selection for SERDES Design • Material Selection for Digital Design • It's a Material World Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board-level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at To read past columns, click here.

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