Stripline vs Microstrip
Updated: 2 hours ago
1. Introduction: Transmission Lines in PCBs
The high-speed signal on the PCB behaves like a transmission line, which is not just the wires. The increase in edge rate, dielectric height, trace geometry, and reference planes determines impedance, losses, EMI, and crosstalk in the PCB systems. So that the PCB traces must be treated as transmission lines with a defined impedance. Controlled-impedance traces can reduce reflections and maintain signal integrity in PCBs. Stripline and microstrip are the most common impedance-controlled structures used for high-speed RF and microwave signals.
2. Microstrip Structure and Behavior
Microstrip is a trace-routed impedance-controlled structure on the outer layer of PCBs, referred to as the ground or power plane. The basic layout of the microstrip structure includes a copper trace on the surface, a dielectric below, air above, and a ground plane on an inner layer of the PCB. The electromagnetic field propagates equally in the dielectric and in air, making it a quasi-TEM structure. The impedance is controlled by tracing the width and height of the dielectrics. This impedance control structure is easy to probe and tune. The rework can be done easily, and most of its components are exposed to external noise and EMI.
3. Stripline Structure and Behavior
A stripline is a buried trace structure between two reference planes within PCB systems. The signal trace is on the inner layer sandwiched between two parallel planes, typically the ground planes. The electromagnetic field is fully confined within the dielectric, which forms the ideal TEM mode in the PCB system. The impedance control is determined by the trace width, the dielectric thicknesses above and below, and the dielectric constant. It can be repeatable due to the uniform environment. This kind of structure provides better EMI shielding.
4. Key Differences in Performance
The following table provides the key performance differences between microstrip and stripline.
5. When to Use Each
Microstrip structure is used for tuning and rework on PCB systems, such as RF front ends and lab prototypes. This structure is suitable for systems that require moderate shielding. The microstrip is also used when board costs are limited. It can be used for systems that need short trace lengths and a moderate frequency.
Moreover, stripline is used when excellent isolation is required. The tighter crosstalk and EMI margins in the PCB system demand the stripline structure. If the PCB stack-up supports the buried high-speed layer with solid planes above and below, then the Stripline system can be used. The highly uniform impedance and timing across the entire production line also required stripline rather than microstrip.
6. Design Trade-Offs
The choice between microstrip and stripline is crucial for PCB transmission. Both have benefits and drawbacks in signal integrity, cost, debuggability, routing freedom, and more.
Generally, the major design trade-offs between choosing microstrip and stripline structures on PCB are given below:
Signal integrity and complexity: Microstrip is simple and reworkable, whereas stripline requires more layers and careful stack-up design, making it more complex. However, stripline offers improved Signal integrity and EMI protection.
Debuggability: Microstrip is easy to access and maintain with probes because its traces are on the outer layer of the PCB. However, stripline is difficult to modify because of its complex design.
Cost: Microstrip is cheaper than stripline because stripline requires additional layers and tighter tolerances, which can increase system cost.
Stack-up constraints: The stripline structure requires dedicated reference planes to implement the correct stripline, which creates power-distribution and mechanical constraints on the PCBs.
Routing freedom: The microstrip structure shares routing space with components, pads, and vias in PCB systems. However, stripline can be used for dense routing between reference planes, reducing routing freedom.
7. Real-World Applications
The microstrip structure is used for RF front-end lines between antennas, filters, and LNAs. Microstrip is also used for short high-speed runs from the connectors to the first IC. It is also used for tuning stubs, impedance-matching networks, and lab test structures where rework can be essential.
Moreover, Stripline is most commonly used for high-speed differential pairs such as PCIe, HDMI, and USB3.x or higher on multilayer boards. It is used in the core clock distribution in complex digital integrated systems. Stripline is also used in systems where EMI and crosstalk are important.
8. Summary Comparison
The several features of the microstrip and stripline structures, as per their uses, design structure, losses, and cost, are presented in the table below:
9. Conclusions
Microstrip and stripline are the two major transmission structures on PCBs. Both have unique strengths. Microstrip is simpler, more accessible, and slightly faster, making it suitable for systems that require rework during maintenance. Stripline offers low EMI, improved shielding, and more uniform impedance, making it suitable for high-speed cores and multilayer boards. The proper selection of these transmission structures in PCBs can help engineers and designers reduce costs and achieve efficient system performance in real-world applications.
10. FAQs
1. Why are microstrip structures preferred in RF tuning?
Microstrip structures are most commonly used for RF tuning because they are readily accessible on the PCB surface, allowing them to be trimmed and modified to control impedance without any rigorous work.
2 Is it okay to mix microstrip and stripline structures in the same design?
Yes, you can mix them and create a hybrid structure in the same design. You can use microstrip as an antenna feed-matching network on the outer layer. You can use stripline for a sensitive high-frequency line to improve shielding and reduce radiation in RF front-end modules.
3. What are the major differences between microstrip and stripline?
Microstrip is easy to access and faster than stripline. However, a stripline offers better EMI shielding and lower radiation, which protects the sensitive components in the PCB system.
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