A trace is a general term for all copper or transmission lines used in PCBs. However, high-frequency circuit boards require unique transmission lines that minimize losses. Striplines and microstrips are the most typical high-frequency transmission lines, and we will do a stripline vs. microstrip comparison to determine the best applications for each one. Take a look!
What is a Stripline?
A stripline sits inside the substrate material with two ground planes sandwiching it and the substrate. This impregnation implies the transmission line can only exist in internal PCB routing layers (three board layers minimum). Therefore, striplines are prevalent in multi-layer circuit boards, and the arrangement shields the stripline while minimizing emissions.
A radio frequency and microwave PCB
Stripline Routing Styles
Striplines have the following routing techniques.
Also known as a plain stripline, the symmetric stripline features a signal trace routed between two reference planes. Its impedance calculation model uses the trace's width & thickness and dielectric substrate type & thickness as parameters.
This stripline resembles the symmetric stripline structurally, but the trace is not in the middle/centrally placed between the ground planes.
A high-power radio frequency PCB
This technique routes the striplines as pairs in the internal layer, with one line above the other in parallel.
Instead of placing the traces in parallel vertically, edge-coupling places them side by side in parallel. They can be symmetric or asymmetric.
Stripline Design Considerations
Striplines have three conducting layers, with the sandwiched signal conductor embedded in an isotropic and homogeneous dielectric featuring a dielectric constant Er.
In a defined time and cross-section, the electric field lines for the fundamental TEM mode in striplines move outwards from the signal conductor to the ground planes. On the other hand, magnetic field lines revolve around the inner conductor.
In TEM (Transverse Electromagnetic) mode, the signal/inner conductor is at the same potential (equipotential). The region between the outer conductor and the stripline has a single medium. Therefore, the fundamental TEM's characteristic impedance and phase velocity do not vary with frequency.
Advantages of Strip lines
- Shields and protects the signal layer
- Low impedance (reduces crosstalk and emissions)
- Better EMI characteristics
- Noticeable improvement above 50MHz
Limitations of Strip lines
- Difficult to decouple
- Low impedance for proper matching
- Tough to debug embedded traces during prototyping and troubleshooting
Losses in Strip lines
- Magnetic resonance
- Finite conductivity of the conductors
- Limited dumping phenomenon and resistivity of the dielectric
What is a Microstrip?
A microstrip is a planar transmission line that sits outside the dielectric material. The PCB material insulates and separates the trace from its single ground plane, meaning the line has air above it. Therefore, the transmission line can only exist on the upper and lower PCB layers. And the dielectric constant of air and the board material affect the trace impedance.
A microstrip RF transmission line in a microwave PCB
Microstrip Routing Styles
Microstrips have the following routing styles.
These signal traces sit on the exterior layers of PCBs, and their impedance calculation model depends on the trace width & thickness and dielectric type & thickness.
This transmission line resembles the edge-coupled stripline but with a single reference plane and external routing for the two traces.
An embedded microstrip is similar to a regular microstrip transmission line, but the trace has some dielectric material above it. The additional dielectric material above the strip could be a solder mask.
Propagation Mode in Microstrips
The dominant signal propagation mode in microstrips is Quasi-TEM (Transverse Electromagnetic). The phase velocity, field variation, and characteristic impedance (in the transmission line's cross-section) are frequency-dependent in TEM mode.
An RF PCB on a Smith chart for impedance matching
Advantages of Microstrips
- Easy to connect to components
- Creates minimal crosstalk when packed with multiple channels in a high-density design
- Minimally affected by PCB manufacturing process tolerances
- Faster propagation time
- Provides mechanical support and heat sinking simultaneously
Losses in Microstrips
- Radiation effects, depending on the dielectric substrate thickness, dielectric constant, frequency, and circuit geometry
- Dumping phenomenon and the substrate's finite resistivity
- Magnetic losses relevant to the magnetic substrate
- Line conductor's finite conductivity
How To Minimize Losses in Microstrips
The most effective way to reduce microstrip losses is to suspend the substrate in the air. This suspension increases the separation between the signal layer and ground path, enhancing the trace's tendency to radiate at the discontinuities.
How does it work?
The air between the ground plane and the substrate bottom contains an electromagnetic field. Since air has zero dielectric losses compared to standard PCB substrates, it minimizes the insertion losses.
On top of that, the conductive trace's width increases due to the reduced effective dielectric constant. A narrow trace width increases losses, so the broader, the better.
A high-frequency PCB
However, this air suspension technique is effective only up to a few GHz.
Stripline vs. Microstrip
Stripline vs. Microstrip: Which Transmission Line is Better for Routing?
One of the parameters required to calculate routing is the dielectric constant of the dielectric layer used in the PCB.
Routing microstrips above and below the board minimizes dielectric losses more than routing strip lines because air has negligible losses. Additionally, it is simpler to fabricate microstrips because the conductor strip is on the outer layer.
However, microstrips radiate more energy due to exposure to air. The dielectric constant inside the PCB layers is usually 1.4, while that of air is 1. This variation explains the impedance value differences between strip lines and microstrips.
Exposed microstrips on an RF PCB
On the other hand, stripline routing has the advantage of sandwiching the signal traces between two reference planes. This arrangement allows the fabrication of narrower signal lines for the same impedance values required for microstrip routing. Also, the sandwiching creates better insulation by the dielectric material. So you can compress the routing.
Stripline vs. Microstrip: Which One Should You Use?
Each PCB transmission line has its benefits and drawbacks. Most modern high-frequency circuit boards with high-speed designs also require high circuit densities. So you will need a combination of both traces.
An RF microwave mixer evaluation PCB
Modern PCB tools can help simplify the microstrip and stripline routing configurations. Impedance calculators can also help you determine the best trace width and layer configurations, while stack-up editors assist in plugging in the correct board widths and materials.
These tools combined can help you determine the best stripline or microstrip routing method and layer to configure with your routing layers.
As you can see, striplines and microstrips have a simple structural difference that varies their properties greatly. Each transmission line excels in certain areas, and you might have to incorporate both into your high-frequency PCB to minimize losses. That's it for this article. If you have any questions or comments, leave a message, and we'll get back to you asap.