High-speed circuits need clean signals.
No distortion and no interference. That’s why PCB designers use striplines. A stripline is a type of
transmission line embedded between PCB layers. It keeps signals isolated and
protected from external noise. That’s a big deal in RF, microwave, and
high-frequency digital circuits.
But how does it work? Why choose it over
a microstrip? And what should you know before designing one? Let’s find out in
this blog.
What Is a PCB Stripline?
A stripline is a conductor placed between two ground
planes inside a PCB. It’s completely buried within the board-- unlike
microstrips, which are placed on the outer surface.
This design makes striplines
electromagnetically shielded. Signals travel with minimal interference.
Consequently, less crosstalk and more consistency.
Here’s a simple way to understand it:
• A microstrip is like
a road—open and exposed to everything.
• A stripline is like a subway—protected, isolated from the outside
world.
Striplines are commonly used in:
• High-speed PCBs for
better signal integrity.
• RF and microwave
circuits.
• Aerospace and
defense electronics.
• Multi-layer PCBs with dense routing.
Why Use a Stripline?
So, why go for a stripline instead of a microstrip?
1. Less Noise and EMI
Striplines sit between two ground planes.
This acts like a shield, blocking interference from outside signals.
2. Better Signal Integrity
In high-speed designs, signals can
degrade without hassle. Striplines offer a consistent impedance that reduces
distortion.
3. No Radiation Loss
Since striplines are buried inside the
PCB, they don’t radiate as much energy as microstrips. That means fewer losses.
4. Compact Routing
In dense PCBs, routing signals on the
surface can be tricky. Striplines help by utilizing inner layers and keep the
board layout clean.
However, there are trade-offs. Striplines
are not easy to manufacture. They require precise layer stacking and dielectric
control. They also have higher losses due to the surrounding dielectric
material.
That’s why they are not always the best
choice. It depends on the design.
Design Considerations
When designing with striplines, a few
things need your attention. Let’s break them down.
1. Material Choice: The material
that surrounds the stripline is crucial. It’s called the dielectric material.
Different materials have different properties, such as the dielectric constant.
This affects how fast the signal travels through the line. You’ll need to
choose a material that matches your specific needs for speed and signal
quality.
2. Trace Width: The width of the
copper trace in the stripline determines how much resistance it has. A wider
trace means less resistance, which can help maintain the strength of the
signal. On the other hand, a narrower trace increases the impedance. Balancing
the trace width is key to getting the right impedance for your circuit.
3. Dielectric Thickness: The
thickness of the dielectric material between the trace and the reference plane
matters, too. Thicker dielectrics can slow down the signal, while thinner
dielectrics speed it up. Finding the right balance is important for optimizing
both signal speed and integrity.
4. Layering: The stripline is fully enclosed. Therefore, more layers are required
in the PCB. This adds complexity to the design but provides better performance,
especially in high-frequency applications. More layers mean more work in terms
of manufacturing, but it’s often worth it for the benefits.
Striplines have their advantages, but
they require careful planning. The materials, trace width, and thickness all
play important roles in how the stripline behaves. Once you get these details
right, your design will perform much better, reducing noise and ensuring the
signal travels cleanly.
Stripline Impedance Formula
Getting the impedance right is critical
in PCB design. If it’s off, signals can reflect and cause problems. Striplines
follow a specific formula for impedance calculation. Here’s a simplified
version:
Where:
• Z0 is the characteristic
impedance (measured in ohms).
• εr is the dielectric constant of
the material.
• h is the distance between the
trace and the ground plane.
• w is the trace width.
• t is
the trace thickness.
This formula helps engineers design
circuits with the right impedance. Matching impedances reduces signal loss and
improves reliability. If this seems complicated, don’t worry. Many PCB design
tools have a built-in stripline
impedance calculator to handle the math for you.
How to Use a Stripline Impedance Calculator
If manual calculations are not your
thing, don’t worry. Many online tools can do the math for you. Here’s how they
work:
1. Enter the material properties. Most calculators ask for the dielectric constant (εr) and layer
thickness.
2. Input the trace width and thickness. These values affect impedance directly.
3. Set the spacing between layers—the distance between the trace and reference plane matters.
4. Click
calculate. The tool will give you the exact impedance
value.
Using a stripline impedance calculator saves time. It also
reduces errors. Many PCB design software programs include built-in calculators
for this purpose.
Stripline vs. Microstrip: What's the
Difference?
People often compare stripline vs. microstrip
because they serve similar functions. But they have key differences.
1. Placement
• Striplines are
buried inside the PCB, surrounded by dielectric material.
• Microstrips sit on
the surface of the PCB and are exposed on one side.
2. Shielding
• Striplines are fully
enclosed. This protects signals from external noise.
• Microstrips are more
vulnerable to interference since one side is exposed.
3. Impedance Control
• Striplines provide a
more stable impedance. This helps in high-speed circuits.
• Microstrips can be
affected by external factors, making impedance harder to control.
4. Manufacturing Cost
• Striplines require
more PCB layers. This increases cost and complexity.
• Microstrips are simpler and cheaper to produce.
So, which one is better? It depends. A PCB stripline is ideal
for high-speed and low-noise applications. Microstrips work well for simpler
designs that don’t need as much shielding.
Conclusion
Striplines are essential in modern PCB
design. They provide excellent signal integrity, impedance control, and noise
protection. Unlike microstrips, they are fully enclosed, reducing interference.
However, they require more layers and precise calculations.
When designing with stripline, remember to
choose the right dielectric material. Adjust trace width and thickness
carefully. Plus, use an impedance calculator to get accurate values.
Striplines may take extra effort, but
they improve performance. If your project involves high-speed signals, they’re
worth considering. Want a hassle-free approach? Many PCB design tools simplify
the process. Use them to optimize your PCB stripline layout and get the best results.
