The old route we used to take to the beach was a simple two-lane country road with sharp turns, dips, potholes, and cross-streets that made towing a travel trailer a terrifying experience. Finally, a new highway was constructed that features multiple lanes, wide curves, and isn’t interrupted by uncontrolled intersections. With a pathway designed for clear transmission, everything moves more efficiently. The electronics industry knew this long before the department of transportation.
As signal speeds got faster on printed circuit boards, their potential for emitting or being victimized by noise and interference also increased. Many steps were taken 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 of the noise and interference by routing them on a layer adjacent to a reference plane. Let’s take a closer look at 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. Additionally, the transmission lines can generate noise and interference, which must be controlled. The transmission lines need to be routed on a signal layer immediately adjacent to a reference plane in one of two configurations, microstrip or stripline, to manage these problems. While this layer configuration will help with many aspects of signal integrity, the two main benefits are as follows:
- Controlled impedance routing: Impedance miss-matches 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 can be maintained throughout the length of the line.
- 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 being routed giving the most optimum signal return path.
In the picture below, you can see an example of both microstrip and stripline layer configurations. Microstrip is the configuration used for signal routing on an exterior layer of the circuit board, while stripline is embedded within the board layer stackup. Next, we’ll look closer at the configuration of these layers.
Microstrip vs. Stripline and their Various Configurations
With the microstrip configuration, the routing is on the exterior layer of the board, 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 big 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 are better protected from EMI and noise by the double reference planes. However, comparing the two methods is mostly a moot point because both configurations are typically used in high-density, high-speed designs.
Both microstrip and stripline configurations are often used on the same PCB design, but there are useful variations of the two methods.
- Microstrip: As we have described, controlled impedance lines routed externally on a circuit board are usually set up and calculated as a microstrip configuration.
- Edged-coupled microstrip: This is a differential pair that is routed on the external layers of a board with their reference plane immediately adjacent in the layer stackup.
- Embedded microstrip: In some cases, additional dielectric materials may be added to the exterior of the board embedding the trace and must be accounted for in the impedance calculations. Soldermask can be one of the dielectric materials that must be calculated.
- Symmetric stripline: This is the standard form of stripline routing, where the trace is embedded symmetrically in the dielectric between the two reference planes.
- Asymmetric stripline: In this case, the trace is embedded in the dielectric closer to one of the adjacent reference planes than the other.
- Edged-coupled stripline: This is a standard side-by-side differential pair routed between adjacent reference planes.
- Broadside-coupled stripline: This is also a differential pair, but instead of being routed side-by-side, the pair is routed stacked on top of each other between the two reference planes.
Before assigning layers in the PCB stackup, you must determine their configuration.
Determining the Correct Layer Configuration for Your PCB Design
There are different impedance calculators available online that you can use to determine your PCB layer configuration. Some advanced PCB design tools will also perform the calculations for you and automatically set up the layer stackup according to those results. Before you go too far, it is important to include your PCB contract manufacturer in the planning stage. Their experience will help with your layer configuration by advising 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 30 years and understand the questions and challenges you face. Our engineering staff will go through your design to see what board materials and fabrication method will best suit it, and help with design questions that could impact its manufacturability.