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12 The PCB Design Magazine • August 2014 In a previous column, Material Selection for SERDES Design, I pointed out that 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 de- signing cutting-edge boards, and sometimes we tend to over-specify requirements that can lead to inflated production costs. In this column, I will look at what types of materials are commonly used for digital design, and how to select an adequate material to mini- mize costs. Of course, selecting the best possible material will not hurt, but it may blow out the costs. Signals propagate in a vacuum or air at ap- proximately the speed of light. But, as the elec- tromagnetic energy is enveloped in a dielectric material, sandwiched between planes in the PCB medium, it slows down. Figure 1 illustrates a signal propagating through a curved wave- guide. This is representative of a typical strip- line configuration of a PCB. What needs to be understood is that the signal traces on a PCB simply guide the signal wave, as the electro- magnetic energy propagates in the surrounding BEyoND DESIGN feature column by Barry olney in-CiRCuiT Design PTy lTD Material Selection for Digital Design