As signal speeds increase on printed circuit boards, their potential for emitting or being victimized by noise and interference intensifies. Layout steps are necessary to clear the signal’s pathways, including arranging the board stackup in microstrip or stripline configurations to achieve the best signal integrity. This setup protects sensitive high-speed signals from much noise and interference by routing them on a layer adjacent to a reference plane. Take a moment to consider microstrip vs. stripline layer configurations and how they may apply to your next design.
Understanding Microstrip and Stripline in Printed Circuit Boards
High-speed transmission lines in printed circuit boards can suffer signal integrity problems if their signals are degraded or victimized by noise and interference. The transmission lines can also generate noise and interference, which can cause a board to fail electromagnetic compatibility (EMC) testing. The transmission lines must be routed on a signal layer immediately adjacent to a reference plane in one of two configurations, microstrip or stripline, to manage these problems. Microstrip is the configuration used for signal routing on an exterior layer of the circuit board, while stripline embeds within the board layer stackup. While this layer configuration will help with many aspects of signal integrity, the two main benefits are as follows:
- Controlled impedance routing: Impedance mismatches in high-speed transmission lines can send signal reflections back through the line, degrading the signal’s quality. By carefully calculating the width and thickness of the high-speed trace, along with spacing to other signals and the thickness and dielectric constant of the dielectric material, the same impedance remains constant throughout the line’s length.
- Clear signal return path: High-speed transmission lines must have a clear path for their signal returns; otherwise, they will generate noise. With the microstrip or stripline configuration, the reference plane is immediately adjacent to the trace, providing the most optimum signal return path.
Microstrip vs. Stripline and Their Various Configurations
With the microstrip configuration, the routing is on the board’s exterior layer, where it is surrounded by air and will, therefore, have less dielectric loss than stripline. It is also an easier structure to calculate and fabricate since it is on the board’s surface. On the other hand, with the traces exposed on the board’s exterior, the microstrip configuration loses some of the built-in protections of the stripline and can radiate more energy.
The stripline’s primary advantage is the sandwich configuration of the traces between two reference planes. With this layer stackup, narrower traces can achieve the same impedance values as the thicker traces required on the exterior layers. This stackup allows for greater circuitry density, and the signals have better protection from EMI and noise with the double reference planes. However, comparing the two methods is mostly a moot point because combining both configurations in HDI/high-speed boards is common
| Variations of Microstrip vs. Stripline | |
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| Microstrip | Stripline |
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Determining the Correct Layer Configuration for Your PCB Design
There are different impedance calculators available online that you can use to determine your microstrip vs. stripline characteristic impedances. Some advanced PCB design tools will also perform the calculations for you and automatically set up the layer stackup according to those results. Before going too far, including your PCB contract manufacturer in the planning stage is worthwhile. Their experience will help you configure your layer by advising you on the appropriate PCB materials and fabrication processes for your design.
At VSE, we have been helping designers like you configure their printed circuit board designs for well over forty years and understand the questions and challenges you face. Our engineering staff will review your design to determine what board materials and fabrication methods will best suit it and help with design questions that could impact its manufacturability.
