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12 DESIGN007 MAGAZINE I FEBRUARY 2019 Not All PCB Substrates Are Created Equal Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA PCB substrates are all around us in every gadget we use whether it's a computer, smart- phone, or simple child's toy. The substrate may be rigid or flexible, or a combination of both. It is a carrier for the electronic devices and the signal and power interconnects and is usu- ally planar in structure with conductors sepa- rated by insulating dielectric materials. How- ever, each product has a specific performance requirement and may need a distinct type of substrate to comply with the product's speci- fications—particularly for high-speed designs. Because the impedance of transmission lines is a function of substrate dielectric constant, essential requirements for a multilayer PCB dielectric material is extremely tight tolerance and consistency in the dielectric constant and thermal coefficient of dielectric constant (the amount of change in the dielectric constant as a function of temperature). In addition, the coefficient of thermal expansion (CTE)—espe- cially in the z-axis (through the thickness of the material)—is of particular importance in mul- tilayer designs because plated through-holes (PTHs) are used to make connections between different layers of the stackup. The CTE is a yardstick for expected PTH barrel reliability. A wire suspended in free air has impedance in the order of 330 ohms. This impedance may have been viable in the old vacuum tube days but is not a good starting point for any digi- tal design. A twisted pair cable with an outer shield has 100 ohms differential impedance. A coax cable with an inner wire surrounded by dielectric material and an outer shield is typi- cally 50–75 ohms. As we add a coupled return path close to the signal conductor in the pres- ence of a dielectric, the impedance reduces.