![]() IPC-2141 microstrip and stripline impedance formulas ![]() The most cited formulas for microstrip impedance and stripline impedance are the IPC-2141 formulas: The equations for trace impedance must account for everything from the substrate dielectric properties to the geometry of your traces. PCB Impedance Calculations With IPC-2141 FormulasĪ number of equations are used to define and calculate various design parameters in PCBs, just like any other area of engineering. Unfortunately, online PCB calculators and some analytical formulas cannot account for these points, so you won't have the ability to adjust for roughness, dispersion, or etching in these models. These tools should also have utilities to model copper roughness and dispersion. Edge-coupled, embedded, and offset trace arrangements all have variations in their impedance values, and you’ll need design software that can account for the impedance of these different geometries. The impedance of the traces in your PCB depends on their geometry, the PCB core/prepreg material used in the stackup, and the layer arrangement. How to Calculate PCB Trace Impedance in Your Stackup ![]() You can control the impedance of all these trace by designing the right stackup and trace geometry. If you have a PCB impedance calculator integrated into your design tools, you can quickly determine impedance for any trace geometry as you create your stackup and start your PCB layout. Although the layer arrangement in the PCB stackup is critical for ensuring impedance goals are met. There are calculators on the internet that can give you some accurate results, but they cannot account for dispersion throughout the signal bandwidth, nor can they consider things like copper roughness or etching along the surface of a copper trace. Since impedance control is so important, designers need to perform impedance calculations when they design the PCB stackup. Once the requirement for specific impedance values was realized, engineers would simply demand the fabricator deliver PCBs with a known impedance, usually 50 Ohms. Few of the early engineers designing with TTL and CMOS had any understanding of how to design a PCB with controlled impedance. ECL was the first logic family that made these problems prominent. Today, logic families in ICs are fast enough that problems such as reflections and crosstalk are prominent in any high-speed board unless an impedance profile is properly engineered for interconnects. The goal was the lowest priced PCB that would stand up to the soldering process. ![]() The glass cloth in the laminate and prepreg did not matter, nor was the resin system important. All a designer needed to do was provide enough signal layers for all wires and enough copper in power paths to deliver DC power with minimum sag or droop. In the early days of circuit board design and fabrication, logic circuits were so slow that the only concerns were making connections between logic or discrete parts and providing a path for DC power to each part. If you're ready for a simplified, seamless PCB impedance calculator for your high-speed design, look to the Layer Stack Manager and routing features in Altium Designer.Ī rules-driven PCB design platform with a built-in PCB impedance calculator for high-speed PCB design. The integrated field solver in Altium Designer's layer stack manager and routing features can help you manage impedance throughout your board. Instead of using an online PCB impedance calculator to determine the impedance of each trace in your PCB, Altium Designer incorporates PCB impedance calculations as part of your stackup design process. While you can work out many of these details by hand or with a circuit design program to model impedance in your board, the best PCB design tools give you more sophisticated ways to design for controlled impedance. ![]() Impedance in your traces becomes a critical parameter to consider during stackup design for high-speed PCBs, but the stackup will also influence crosstalk susceptibility and sheet capacitance between ground and power planes. High-speed designs carry a requirement for controlled impedance, crosstalk control, and the need for interplane capacitance. There are many demands placed on PCB stackup design. ![]()
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