In high-frequency and high-speed PCB design, choosing the right transmission line structure—stripline or microstrip—is pivotal for ensuring signal integrity, minimizing interference, and balancing cost-effectiveness. Both structures serve as fundamental building blocks for RF/microwave circuits and high-speed digital systems, but their distinct geometries and electromagnetic behaviors dictate their suitability for specific applications. This article dissects the core differences between stripline and microstrip, exploring their designs, electrical properties, and practical implications for PCB engineers.
What is Stripline and Microstrip?
Stripline:
A stripline is an embedded transmission line with a central conductive trace sandwiched between two parallel ground/power planes, fully enclosed by a uniform dielectric material. The trace is routed on an internal PCB layer, shielded from external interference.
Microstrip:
A microstrip is a surface-mounted transmission line consisting of a single conductive trace on the PCB’s top/bottom layer, a dielectric substrate, and a single ground/power plane beneath the substrate. The trace is partially exposed to air (or covered by solder mask), creating a hybrid dielectric environment.
Structural Foundations and Propagation Modes
Microstrip: Surface-Mounted and Quasi-TEM Propagation
A microstrip consists of a conductive trace on the PCB’s outer layer, separated from a single ground plane by a dielectric substrate (e.g., FR4). Its structure exposes the trace to two dielectric environments: the substrate below and air above. This asymmetry gives rise to a quasi-transverse electromagnetic (quasi-TEM) mode, where electric fields are partially confined to the substrate and partially radiate into the air. As a result, the effective dielectric constant (εeff)—a weighted average of the substrate’s permittivity (εr) and air’s permittivity (εr=1)—dictates the phase velocity and impedance, making microstrips frequency-dependent and dispersive.
Structure:
Propagation Mode:
Quasi-TEM mode (quasi-transverse electromagnetic), where electric fields exist in both the dielectric and air, leading to frequency-dependent dispersion.
Stripline: Shielded and TEM Propagation
A stripline is embedded between two parallel ground planes, fully enclosed by a dielectric material. This symmetric structure supports a pure TEM mode, with electric and magnetic fields entirely confined within the dielectric. Unlike microstrips, striplines exhibit no dispersion because the uniform dielectric environment ensures frequency-independent phase velocity and impedance. The absence of air exposure also eliminates radiation losses, making striplines inherently shielded and suitable for high-interference environments.
Structure:
Propagation Mode:
Pure TEM mode (transverse electromagnetic), with fields entirely confined within the dielectric, ensuring no frequency dispersion.
Types of Stripline and Microstrip
1. Microstrip Line Subtypes
Microstrips are categorized by their geometric configurations, dielectric environments, and application-specific requirements:
a. Basic Microstrip
b. Embedded Microstrip
c. Differential Microstrip Pair
d. Coplanar Microstrip
e. Suspended Microstrip
2. Stripline Subtypes
Striplines are classified based on symmetry, dielectric layers, and routing complexity:
a. Basic Stripline
b. Symmetric Stripline
c. Asymmetric Stripline
d. Differential Stripline Pair
e. Coplanar Stripline
f. Wideband Stripline
3. Hybrid Variations
Some designs combine microstrip and stripline elements for specific needs:
a. Microstrip-to-Stripline Transition
b. Embedded Stripline with Microstrip Overlay
4. Contrast with Other Transmission Lines
While the focus is on microstrip and stripline, other types are mentioned in the documents:
| Type | Structure | Key Feature |
|---|---|---|
| Coplanar Waveguide | Trace with side-by-side ground planes on the same layer | Easy impedance tuning for RF circuits; natural shielding |
| Slotline | Signal propagates through a slit in a ground plane | Used in microwave antennas and balanced circuits |
| Parallel-Plate Waveguide | Two parallel conductive plates with a dielectric in between | Broadband, low-loss performance; high-power applications |
Key Takeaways
Electrical Characteristics: Impedance, Loss, and Speed
Characteristic Impedance
Microstrip Impedance:
Where:
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Microstrips offer a wider impedance range (20–120 Ω) but require broader traces for the same impedance compared to striplines.
Stripline Impedance:
Where:
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Striplines support higher impedances (35–250 Ω) with narrower traces, thanks to the fully dielectric environment.
Signal Loss and Velocity
Microstrip Signal Velocity:
Where:
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Stripline Signal Velocity:
Losses are primarily from conductor resistivity and dielectric damping, with no radiation loss. The fully dielectric environment slows signal velocity but ensures consistent performance.
Where:
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Design Considerations and Manufacturing
Layer Stackup and Cost
Impedance Control and Crosstalk
Applications and Practical Guidelines
When to Use Microstrip
When to Use Stripline
Applications and Practical Guidelines
Case Study 1: Microstrip in 5G Antenna Arrays
Scenario: A 5G base station antenna array requires compact, low-cost transmission lines for phase-shifting networks.
Solution: Microstrips are used for radiating elements and feedlines due to their exposed structure, which enables coupling with air for radiation. For example, Samsung’s 5G Massive MIMO Antenna employs microstrips with a Rogers RT/duroid 5880 substrate (εr=2.2) to achieve 28 GHz operation. The quasi-TEM mode allows engineers to adjust impedance (50 Ω) via trace width (w=0.3 mm, h=0.762 mm), balancing radiation efficiency and cost.
Advantage: Microstrips eliminate the need for expensive shielded layers, reducing weight and fabrication complexity in antenna arrays.
Case Study 2: Stripline in High-Speed Data Centers
Scenario: A 400G Ethernet switch motherboard demands low-latency, EMI-free signal routing for backplane connections.
Solution: Striplines are embedded in the motherboard’s inner layers to route differential pairs (e.g., PCIe 5.0, 32 GT/s). For instance, Intel’s Ice Lake Server Chipset uses striplines with an FR4 core (εr=4.4) and 100 Ω differential impedance. The dual ground planes suppress crosstalk between 100+ high-speed channels, ensuring BER < 10^-12 at 25 GHz.
Advantage: Striplines’ TEM mode minimizes dispersion, critical for maintaining signal integrity over 30 cm backplane traces.
Case Study 3: Hybrid Microstrip-Stripline in Automotive Radar
Scenario: A 400G Ethernet switch motherboard demands low-latency, EMI-free signal routing for backplane connections.
Solution: Striplines are embedded in the motherboard’s inner layers to route differential pairs (e.g., PCIe 5.0, 32 GT/s). For instance, Intel’s Ice Lake Server Chipset uses striplines with an FR4 core (εr=4.4) and 100 Ω differential impedance. The dual ground planes suppress crosstalk between 100+ high-speed channels, ensuring BER < 10^-12 at 25 GHz.
Advantage: Striplines’ TEM mode minimizes dispersion, critical for maintaining signal integrity over 30 cm backplane traces.
Case Study 4: Stripline in Medical MRI Equipment
Scenario: A 3T MRI scanner’s transceiver coil demands ultra-low loss and EMI isolation to detect faint biological signals.
Solution: Striplines with a PTFE dielectric (εr=2.1) are used for RF coil connections, enclosed in copper-clad layers to prevent interference with the magnetic field. GE Healthcare’s SIGNA™ MR System employs 50 Ω striplines with h=0.5 mm and w=0.1 mm, achieving Q-factors > 1000 at 128 MHz.
Advantage: Striplines’ shielding eliminates crosstalk between 32 receiver channels, critical for high-resolution imaging.
Case Study 5: Microstrip in Consumer Wi-Fi Routers
Scenario: A Wi-Fi 6 router (802.11ax, 2.4/5 GHz) requires cost-effective signal routing for multiple antennas.
Solution: Microstrips on a 4-layer FR4 PCB (εr=4.4) connect the SoC (e.g., Qualcomm IPQ8074) to surface-mounted diplexers and antennas. The traces (w=1.2 mm, h=1.6 mm) achieve 50 Ω impedance with <0.5 dB loss at 5 GHz.
Cost Savings: Microstrips reduce layer count by 50% compared to striplines, cutting PCB costs from $25 to $15 in high-volume production.
Conclusion
Stripline and microstrip represent two pillars of PCB transmission line design, each optimized for distinct scenarios. Microstrips excel in simplicity, cost-effectiveness, and moderate performance, while striplines deliver superior shielding, signal integrity, and high-frequency capability. By evaluating factors like frequency, impedance requirements, and environmental constraints, engineers can select the optimal structure to balance performance and practicality in modern electronics.
Final Tip: For hybrid designs, combine both structures: use microstrips for surface-mounted components and striplines for internal high-speed routing, leveraging their complementary strengths.
