In high-speed PCB designs, transmission lines are core structures that must ensure signal integrity is maintained. Most errors on the transmission line in electronic circuits propagate through interaction, loss, and distortion, which occur through interference from today's fast-moving digital and communication systems. Among the most common types of transmission lines in PCB designs are two defining types: stripline and microstrip.

But precisely what are stripline and microstrip, and how do they affect PCB performance? This article will examine these two transmission lines, break down their structures, working principles, and certain advantages, and find a definitive conclusion by the end as to when to use microstrip rather than stripline for optimum signal performance and better manufacturing efficiency.

What is Stripline?

It is one of the convenient transmission lines on a PCB, which can carry high-frequency signals without interference. The last part of the word is more of an ordinary word since it is nothing but the path for buried electrical signals in the PCB layers. This makes it preferable in multilayer boards where the integrity of the signal is an extreme aspect.

It is commonly used in applications where circuits are high-speed, and communication systems are used. Due to its characteristic feature along with shielding, the stripline allows support to very complex signals without loss or interference. For this reason, it has found a broad application within sectors requiring precision and stability.

Structure of Stripline:

The stripline configuration is simple but highly efficient. Two layers are formed by the conductor and covered with dielectric. Between the two layers, there exist two ground planes; one sits above the structure and the other at the bottom. These are the ground planes. They act as shields so that outer noises do not manage to strike the signal. It makes the signal travel quite fast across the board without interference from outside noises.

Working Principle of Stripline:

Signals carried by stripline are contained in a fully shielded environment. The ground planes shield the conductor; therefore, it has minimal interference with other components. In this configuration, impedance is constant and essential for signal integrity. Signal integrity is maintained with stable signal conditions at high frequencies.

Advantages of Stripline:

1.   Shielding against Other Interferers: The best advantage of a stripline is its excellent shielding. Since the conductor is fully enclosed, maximum protection is provided from electromagnetic interference. Any circuit running at high speed needs immunity against interference to high levels since minor interference can affect the circuit's performance.

2.   Balanced Impedance: As it is a sandwich construction from inner to inner, the stripline will likely have a balanced impedance. This means the signal should be fully un-perturbed while being transmitted along the PCB with the minimum possibility of distortion or signal loss.

Disadvantages of Stripline:

1.   Complexity in fabrication: The microstrip would be the most complex to fabricate compared to the stripline. An increased number of layers, along with the presence of a conductor close to the ground plane, increases complexity and, therefore, leads to an increase in cost.

2.   Planar Constraints: Because the stripline is striated, it is more bulky on a PCB than, for example, a coaxial cable. This, in turn, would tend to make boards thicker overall on average, which may become a problem in compactness-driven designs.

What is Microstrip?

The microstrip transmission line has been widely used in PCBs because of its simple and efficient high-frequency design. Unlike the stripline, a microstrip lies on the surface of the PCB and hence easy to make and deploy. Further, it is widely used in RF circuits and microwave devices as its design is simple and economical.

It involves low-cost design, less space, and is easy to produce; it is therefore widely used. It carries a high-frequency signal and is very popular in communication systems, radar, and high-speed electronics.

Configuration of Microstrip:

This is much less complex in its layout than stripline. In microstrip, one conductor is placed on a dielectric surface layer while touching the ground plane below only with one of its sides. The microstrip has an open configuration wherein the top side of the conductor is exposed to air, unlike every other transmission line. This inverted ground plane does steer the signal but increases exposure to this side, which gives the signal far less shielding from possible external noises.

Working Principle of Microstrip:

In the case of the microstrip, the signal travels along the upper surface of the conductor. Due to the presence of one ground plane only at the bottom, the electromagnetic fields surrounding the conductor are open to interference from the outer environment. The signal enters the air through partial dimensions and thus has some exposure to interference sources. However, with high-frequency operation and low costs, the simplicity makes such a design possible.

Advantages of Microstrip:

1.   Economical: Stock of all raw materials and layers is saved so that the cost of production comes down.

2.   Available for Manufacturing: Because there are fewer layers needed, microstrip is less complicated and more accessible to manufacture compared with stripline.

Shortcomings of Microstrip:

1.   Highly Susceptible to Interference: Microstrip is highly susceptible to interference from the outside since it has almost negligible resistance.

2.   Poor balance: This cable needs to be appropriately shielded. So, impedance is not constant. Hence, the quality of this signal is going to reduce.

Microstrip line vs Stripline: A Comparative Analysis

Route Differences

Microstrip Routing Techniques

Stripline Routing Styles

Design Issues for Stripline and Microstrip

Impedance Control Factors

Minimization Technique Loss

Multilayer PCB Design Considerations

This would, therefore, require management of signal quality in the stripline and microstrip of a multi-layered PCB. Quality may be managed through trace widths, layer stacks, and impedance matching.

Conclusion

There are two of them: the stripline, better designed and more complex for fabrication but with greater signal integrity if used as a shield, and the microstrip, which is more straightforward and much more inexpensive but with significantly greater susceptibility to interference. This would depend on your application, cost, and performance.