Issue link: https://iconnect007.uberflip.com/i/1539283
SEPTEMBER 2025 I DESIGN007 MAGAZINE 11 design of transmission lines will help to mitigate and control the characteristic capacitance, induc- tance, and impedance of the line. This serves two beneficial purposes. First, a con- trolled impedance is necessary to help control and minimize reflections, overshoot, undershoot, and other signal integrity issues. Second, by properly designing and controlling the transmission line structures, the designer/engineer can better under- stand and model the performance of the PDN. Remember that all traces on the PCB will behave as transmission lines even if not designed for a specific impedance. In the case of power traces, the same tool that models the impedance (Z 0 ) of the transmis- sion line will also typically report the capacitance (C 0 ) and inductance (L 0 ) of the transmission line. How can material choice help mitigate these issues? Material choice also plays an important role in the design of the PCB. When it comes to signal integ- rity and power integrity, material selection will be critical to the successful implementation of the design. The dielectric constant, for example, can cause a major change to the performance of the PCB transmission lines. Consider the first order equation for the impedance of a stripline trace (single internal trace evenly spaced between two planes), if we want a single-ended impedance of 50 Ω ±10%, with a 6-mil height to the planes, and a 1-mil trace thickness, and if we select a material with a dielectric constant of 3.6, we will have a target impedance of 50.5 Ω with a 5-mil trace width. However, if the dielectric constant is changed to 4.2, which can easily happen within a given resin system based on the resin content/construction of the layer, this same structure will now have a target impedance of 46.8 Ω. This new value is almost at the lower acceptable limit of the required impedance. This means there is a high probability that the transmission line will be out of tolerance during fabrication resulting in a high scrap rate. In addition, this larger variance from tar- get impedance leads to increased impedance dis- continuity resulting in great overshoot, undershoot, or reflections compared to a design that meets the target impedance. The selection of the material is also critical to the design of the planar capacitance necessary for the proper performance of the PDN.