Stripline vs Microstrip: What’s the Difference?

Introduction

In printed circuit boards, to create pathways for electrical signals routing technique is used. Stripline and microstrip are two common routing methods. Yet, both methods are used to carry signals, but they differ in many aspects. Read on to know more about stripline vs microstrip.  

What is Stripline?

Stripline is a type of electrical transmission line used in printed circuit boards. It is used to carry electrical signals that have high frequency. You can also hear it as a planar transmission line or PCB transmission line. Striplines consist of:

• A flat conductor. 
• Two parallel ground planes. 

Both mentioned above are embedded within PCB layers like a sandwich. In stripline, the signal path is in the middle, and the ground layers are on the top and bottom of it. This layer helps in keeping the signal clean and stable while flowing in the right direction. 

Structure of a Stripline

Regarding the structure of stripline, it is simple but effective. It involves a flat conductor made of copper. This copper conductor is placed between the two parallel ground planes (one above and one below). This design is used because it helps in controlling the signal flow and reducing interference. 

The copper conductor that is placed between the ground planes itself stays inside a material usually called a dielectric. This dielectric helps in keeping the conductor separate from ground planes at the top and the bottom. It also affects how fast the electrical signals can travel. 

The dielectric that surrounds the copper conductor is commonly made of some specific materials:

• FR-4 with a dielectric constant of around 4.5.
• PTFE with a dielectric constant of around 2.0.

To get the right signal speed and impedance, you have to do the following:

1. Carefully choose each part of the stripline. 
2. Adjust the width of the conductor and the thickness of the dielectric. 
3. Keep the spacing accurate.

How Does Stripline Work?

A stripline works by guiding an electrical signal through a flat copper conductor, as discussed earlier. An electromagnetic field is formed around the conductor when a signal is applied to it. This field interacts with the surrounding material and lets the signal move forward. 

The signal travels in the transverse electromagnetic mode. In this mode, both electric and magnetic fields stay at right angles to the travel direction. Moreover, it protects the signal from high distortion. 

Regarding how fast the signals travel, it is affected by the dielectric material. 

Low dielectric constant = Faster signal speed

The ground planes, both the top and bottom ones, perform two jobs. 

1. They pave the way for the current to return by completing the electric circuit. 
2. They protect traveling signals from external interference, ensuring stability. 

Impedance is important for signal integrity and depends on dielectric properties and stripline size. Usually, the impedance of a stripline is calculated using the following formula. 

Z₀ = (60 / √εᵣ) × ln (4h / πt)

Z₀ = characteristic impedance

εᵣ = relative permittivity (dielectric constant)

h = height of the dielectric

t = Conductor thickness

π = Pi (3.1416 approx.)

Advantages and Disadvantages of Stripline

Below are the advantages and disadvantages of stripline in printed circuit boards.

Advantages

Stripline used in PCB usually has a high frequency (approx. 50 MHz), which makes it a good choice for high-speed circuits. It offers low impedance, which helps reduce signal noise. Additionally, it exhibits low radiation loss and gives strong EMI shielding to improve signal clarity.

Disadvantages

Stripline is harder to design and build, which increases manufacturing time and cost. It is buried inside the board and is hard to troubleshoot or make changes to. 

The traces in stripline are usually narrower than a microstrip. This can cause issues if not handled well. Moreover, the strip conductor may come loose if not placed properly in the ground plane.

Impedance, Capacitance, and Permittivity

Striplines in PCBs depend on three properties, including:

• Impedance
• Capacitance
• Permittivity

These properties help you in designing circuits that can have high speed and low noise. 

Impedance of Stripline

Regarding the impedance of a stripline, it further depends on two things:

• Width of the conductor
• Space between ground planes
• Dielectric material

As the conductor width increases, the impedance goes down. The formula below is used for calculating the impedance of a stripline in PCB. 

Z₀ ≈ (30π / √εᵣ) × [ b / (Wₑ + 0.441b) ]

In the above formula:

Z₀ is the characteristic impedance (usually measured in ohms)

b is the spacing between the two ground planes

Wₑ is the effective strip width

εᵣ is the dielectric constant

Capacitance of Stripline

Capacitance – It tells the capacity of electric charge a stripline can store or handle. This capacity depends on two things, i.e., the dielectric material and the shape of the conductor. 

To calculate the capacitance, the following formula is used:

C₀ = [1.41 × (εᵣ)] / ln [3.81 × H / (0.8 × W + T)]

Where:

C₀ = Capacitance per inch (pF/in)

εᵣ = Dielectric constant

H = Height between strip and ground plane

W = Width of the strip

T = Thickness of the strip

Regarding permittivity, it is referred to as the dielectric constant and affects both impedance and capacitance. Materials having permittivity values between 2.2 and 10.8 (e.g., FR-4, Teflon, and Rogers) help maintain signal speed and quality.

What is Microstrip?

Microstrip in PCB – It is an electrical transmission line that carries signals having high frequency. It has a single flat conductor that is placed on the top of a dielectric material. Also, there is a ground plane on the opposite side of it. 

Microstrips are often used in RF and microwave circuits. 

Why microstrip?

Because they are easy to design and manufacture. This makes them popular for many electronic devices. Since one side of the conductor is exposed to air in a microstrip, the signal partly travels through both the dielectric and the air. This helps control signal speed and performance effectively.

Structure of a Microstrip

A microstrip has a simple structure that is composed of a flat conductor placed on top of a dielectric layer and a ground plane below it. That flat conductor is made using copper because copper can carry signals better.   

The multilayer dielectric in microstrip keeps the conductor and ground plane separate. It also affects how fast the signals can move. Moreover, the ground plane that lies under the dielectric provides the return path for the current. 

Unlike stripline, microstrip has only one ground plane. The thickness of the conductor and dielectric, along with the width of the strip, control the impedance and signal speed. Wider strips reduce impedance, while thicker dielectric layers slow the signal. 

Another parallel construction is the coplanar waveguide (CPW) that has the ground planes on each side of the conductor and minimizes radiation loss.

Factors and Formulas for Microstrip Design

 To design a microstrip line, several factors come under consideration. These include:

• Width of the conductor (w)
• Thickness of the copper (t)
• Dielectric thickness (h)
• Dielectric constant (εr)

All these factors have a certain impact on the impedance, capacitance, and performance of a microstrip. 

Regarding the impedance following formula is used for calculating it:

Z₀ = (87 / √(εr + 1.41)) × ln((5.98 × h) / (0.8 × w + t))

For calculating capacitance, the formula is:

C₀ (pF/in) = (0.67 × (εr + 1.41)) / ln (5.98 × H / (0.8 × W + T))

Note: frequency can affect the effective dielectric constant. So, always adjust your design for your signal frequency. 

Advantages and Disadvantages of Microstrip

Below are the advantages and disadvantages of microstrip in printed circuit boards.

Advantages

Microstrip can be built on a single-layer PCB, which lowers material and production costs. In microstrips, all the components and traces are on the same surface. The need for vias, avoiding excessive inductance, is almost none. 

The wider trace in microstrips helps in maintaining the intended impedance. In microstrip, you can control impedance better. It performs well in high-frequency circuit designs. Moreover, the layout of microstrips is flexible. You can easily add bends, corners, or route them across different layers. 

Disadvantages

Microstrip needs more surface area, which reduces space for other components on the board. Moreover, they are prone to external interference and radiation. This affects the quality of electrical signals. In addition, microstrip often needs shielding, which sometimes impacts the cost. 

Furthermore, in microstrips, surface coatings such as solder mask are used. They affect the impedance of the microstrip in PCB. Plus, high frequency in them is also prone to signal dispersion. 

What Are the Types of Wiring for Stripline And Microstrip?

PCB designers/engineers use several types of wiring to ensure signals are routed efficiently, both in stripline and microstrip. Here are some of the commonly used types:

Edge-Coupled Microstrip 

In this type, two microstrips are placed side by side. The purpose is to carry out differential pair routing.

Embedded Microstrip

It is used in HDI PCBs where the need is for low-speed signals. It is usually impacted by solder mask effects. 

Symmetric Stripline

The stripline is placed between two ground planes with the trace centered in between.

Asymmetric Stripline

It is somewhat similar to symmetric, but the trace is off-center between the ground planes.

Edge-Coupled Stripline

In this type, parallel striplines are used for differential signals. They can either be symmetric or asymmetric.

Broadside-Coupled Stripline

Here, the differential pair is routed one above the other in between ground planes rather than side by side. 

Difference Between Stripline and Microstrip

The table below presents the key difference between the stripline and microstrip used in printed circuit boards. 

Stripline Vs Microstrip: What’s Better?

Well, the answer to it depends on your circuit needs. In case your need is for better speed, noise-sensitive signals, and low radiation loss, stripline is recommended. But know that it is a bit hard to design and is also expensive. 

However, if you are looking for an easier-to-build and cost-effective solution, go for microstrip. It is generally considered for applications requiring surface-level routing. But remember that the microstrip is prone to external interference. 

Conclusion

Stripline and microstrip both have their own advantages and disadvantages. What to use will be determined by your requirements, i.e., design, speed, cost, and signal strength. The best way to get the best with your PCB is always to plan it out carefully. To get professional assistance in the manufacturing of PCBs, contact PCBMay, a leading PCBA service provider in China.

FAQs

Can you combine Microstrip and Stripline on the same PCB?

Yes, you can combine both in one PCB. You can use microstrip for surface routing and stripline for internal high-speed signal layers. 

Is Stripline always better for high-speed signals?

Not always. Stripline offers better shielding and less noise, which is good for high-speed signals. But it depends on the design needs and cost limits.

Why is Stripline more expensive?

Stripline is more expensive because it is buried inside the board. It requires additional layers and design, which delays the time as well as the cost of manufacturing.