Microstrip vs. Stripline
The importance of PCB interconnect techniques increases with frequency of operation. At higher frequencies, transmission line wiring provides superior performance by minimizing crosstalk, signal distortion and radiation (as compared to ordinary point-to-point wiring). When implementing transmission lines on a PCB, there are two options: microstrip and stripline.
A microstrip transmission line consists of a copper trace separated from a ground plane by an insulating substrate. This configuration is depicted in the image below.
Since one side of the conductor is exposed to air, these transmission lines can only exist on top and bottom PCB layers. The trace impedance is influenced by both the dielectric of the substrate material and the air above it.
Unlike microstrip, stripline transmission lines are fully contained within a substrate – which is sandwiched between two ground planes.
Due to the substrate impregnated nature of stripline, these transmission lines can only exist on internal routing layers and require a minimum of 3 board layers (2 ground planes and a routing layer). Though the image above shows the substrate as a single, contiguous layer, if you’re familiar with the pcb construction process, you’ll know that isn’t the case. The half of the substrate below the trace is part of the PCB laminate while the substrate above the trace is prepreg. The dielectric of both of these materials affects transmission line impedance.
So which one?
Now that you know the physical differences between stripline and microstrip transmission lines, which one should you use in your design? Well, with proper material selection, both techniques can provide excellent performance up through the milimeter wave frequency range. Though, like many engineering decisions, there are tradeoffs associated with each technique. Based on the specifics of your design it’s up to you to make the right choice.
- Dielectric losses are less (when using identical materials)
- Cheaper and easier to manufacture
- Location of traces on top and bottom layers leads to easier debugging
- Greater isolation of transmission lines
- Supports more densely populated designs (traces are smaller, large number of internal layers possible)
- Requires stricter manufacturing tolerances