Understanding the Different PCB Layer Configurations for Microstrip vs Stripline
I’m a back roads kind of a guy. If given a choice I would rather drive all over the countryside instead of using the interstate to go directly to my destination. If I had been born 50 years earlier, I would have been the poster child for driving on route 66. My passion for taking the more challenging route in order to enjoy some breathtaking scenery does have a downside however. Those long diversions really annoy my family. I have to admit that they are right when they complain about my preferred driving routes not being the fastest way to get from point A to point B.
Taking time to enjoy the scenery is a great thing while traveling in the car, but it is definitely not acceptable when routing high speed transmission lines on a printed circuit board. In high speed design it is imperative that we route our traces to get from point A to point B in the precise amount of time as required by the operating characteristics of the signal. To do this means that we have to configure our board layer stackup and how we route in order to give the signals the clearest path on their journey. This requires using a microstrip or stripline routing methodology, and there are different board layer configurations for each. Let’s take a quick look at the differences between these microstrip vs stripline routing configurations.
Understanding the Basics of Microstrip and Stripline Configurations
For a high speed signal to operate most efficiently, it must have a clear return path adjacent to it for the signal to return on. This is usually accomplished with a ground plane immediately above and or below the signal layer in the board layer stackup. These planes must be clear of obstructions that would block the return path such as cutouts, voids, or splits for other power and ground nets. The more obstructions there are on the return path, the more possibility there is for noise and crosstalk to occur.
The microstrip configuration of routing is where the signal line is routed on an exterior layer of the board. Because of this its ground plane return path is only below or above it depending on whether the routing is on the top or bottom layer of the board. The Stripline configuration is where an internal signal layer is sandwiched between two ground planes. In both microstrip and stripline configurations, the amount of metal, type of dielectric material, and width of the layers in the stackup all have to be calculated in order to configure the routing to provide the proper amount of impedance in the routed traces. Which method you choose to route with will be dependent on what you are trying to accomplish as there are advantages and disadvantages to both configurations.
Advanced PCB design tool capabilities can help with microstrip and stripline routing
Which is Better for Routing; Microstrip vs Stripline?
One of the components of calculating the routing is the dielectric constant (Dk) of the dielectric materials used in the construction of the PCB. Microstrip routing on the top or bottom of the board can result in less dielectric loss than stripline since a portion of the microstrip is surrounded by air which has negligible dielectric loss. In addition, microstrip is easier to fabricate since it is on an exterior layer and its configuration is simpler with only a conductor, dielectric layer, and a ground plane. On the other hand, microstrip will radiate more energy due to its exposure on an exterior layer to air. The dielectric constant of air is 1,and the dielectric constant inside the pcb stackup is usually 1.4, that's why there is a difference in impedance values of microstrip and stripline.
Stripline routing has a big advantage in being sandwiched between two ground planes. This configuration allows for narrower traces to be used for the same impedance values as what would be required in microstrip routing. With stripline routing being isolated within the board layers, the signals are better insulated by their dielectric materials and the routing can be more compressed.
As to which method of routing is better, microstrip or stripline, it is mostly a moot point. Today’s high density, high speed designs are going to require multiple board layers which by nature will force a combination of both methods. Fortunately there are calculators and tools that can help you to decide how best to configure your routing layers and with what type of microstrip or stripline routing methods you will need to use.
Microstrip and stripline routing examples
Examples of Microstrip and Stripline Layer Configurations
Here are the definitions of the microstrip and stripline routing examples shown above, and how their configuration will impact their impedance value calculations:
Microstrip: These are signals that are externally routed on a PCB. Their calculation model is based on the thickness and width of the trace, the thickness of the substrate, and the dielectric type and thickness.
Edge-Coupled Microstrip: External layer differential pairs will be routed using this methodology. It is the same configuration as regular microstrip routing, but the calculation is more complex with the additional differential pair trace.
Embedded Microstrip: This configuration is similar to regular microstrip except for additional dielectric material above the signal trace which must be included in the calculation. Soldermask is one of the sources of additional dielectric material.
Symmetric Stripline: Signal traces that are routed between two ground planes on an internal layer are commonly referred to as stripline routing. This calculation model is also based on the thickness and width of the trace, the thickness of the substrate, and the dielectric type and thickness just as with microstrip. The difference is that the calculation is modified for the dual ground planes with the trace embedded between them.
Asymmetric Stripline: This is very similar in configuration to the regular symmetric stripline model, but the calculation accounts for the signal trace that is not positioned symmetrically between the two planes.
Edge-Coupled Stripline: This configuration is used for routing differential pairs on internal layers. The calculation for this stripline is more complex with the addition of the additional trace for the differential pair.
Broadside-Coupled Stripline: Instead of routing differential pairs side by side, this method routes the pairs stacked on top of each other.
Today’s PCB design tools can offer you a lot of help when it comes to routing microstrip or stripline routing configurations. Impedance calculators will help you to determine your line width and layer configurations while board layer stackup editors will help you to plug in the correct PCB materials and widths. Complimenting these features are various differential pair routers, high speed design rules, and much, much more to help you get the job done.
The key is to use a PCB design system that has all of the features that we’ve talked about here in order to give you the maximum amount of design power. OrCAD PCB Designer from Cadence has all of these capabilities, and is the kind of design tool that you need for success.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.