Guideline

# Step-by-Step Guide to Calculate Microstrip Trace Impedance

Controlled impedance is commonly used in PCBs with high-speed signal transmission. It is achieved by specially designed copper trace width and spacing, copper thickness, dielectric thickness, and the Dk value for the dielectric of the PCB material. Controlled impedance can be divided into Microstrip and Stripline according to the mode of transmission. In this article, we will talk about what is microstrip, how to design and calculate trace impedance, and the impacts of failure to calculate microstrip trace impedance.

## What is Microstrip Trace Impedance

Here, we are talking about single-ended microstrip transmission lines that show as connecting two devices. In this arrangement, a conductor represents the link between the source and the load. With this reference ground plane acts as the road for the signal return. Various factors, including the geometry of the trace, dielectric constant, and the thickness of the dielectric material, should be considered concerning impedance.

Here “W” refers to the trace width. We specify the width nearest to the upper surface, while “T” refers to the width closest to the lower surface. The microstrip is located over the surface of the dielectric that refers to “H”. And below this ground plane is showing. The image below is a microstrip transmission line structure.

#### The Formula for Microstrip Impedance

After we know about the definition of microstrip impedance, we will talk about how to calculate microstrip trace impedance. Please refer to the following formula. In fact, there are many microstrip impedance calculators.

Z0 = Single-ended Impedance of trace
Zd = Differential Impedance of trace
d = Trace Separation (distance between the trace)
w = Trace Width
t = Trace Thickness
h = Dielectric Thickness
εr = Relative Dielectric Constant

#### Points in Microstrip Trace Impedance Design

It is evident that a signal runs on the board through this, called microstrip. Typically, the board in signal is 50E, but we need to ensure its impedance for the high-speed trace.

That can be done by etching away some material from the copper layer until it becomes thin enough. Another terminology for this is called stripline, which appears between the circuits, sandwiched by dielectric and ground planes.

Compared with stripline trace with the same impedance, a microstrip trace allows more precise impedance control due to absolute value differences during fabrication. For instance, if a 25-mil wide microstrip trace is over etched by just one mil, there will be less noticeable percentage change than when a 10-mil stripline trace is reduced to 8 mils. When reduced to 24 mils, the characteristic impedance increases to about 60.5 ohms, equivalent to a rise of around 5.8%. On the other hand, a ten-mil stripline trace that has been reduced by one mil to nine mils shows an increase in characteristic impedance of approximately 12.3%.

Microstrip transmission lines face drawbacks in high-density and mixed-technology designs. Their wide size and surface orientation limit component placement, making them less suitable for compact designs. Furthermore microstrip lines tend to emit radiation, which can result in interference (EMI) and crosstalk. It necessitates greater spacing and circuit components, reducing the routing density and potentially impacting the effectiveness of microstrip lines in situations where compactness and high routing density are crucial.

In the realm of microstrip designs, external shielding is common. It leads to increased costs and complexities. Particularly in devices such as phones, shielding close to the board surface can influence the capacitance of the microstrip transmission line, thereby affecting its impedance. When utilizing microstrip lines, careful consideration is paramount, especially when it comes to impedance modeling. Adjusting the width of the transmission line may be necessary for distances if a trace needs to pass through an external shield wall via a “tunnel” near the board surface. It is crucial to take into account these factors when determining the height of the tunnel within the shield.

Simple solder mask and other surface coatings can modify the impedance of microstrip transmission lines. It has variations based on manufacturers.

These are the terms required to analysis while designing the microstrip in the circuits and hope to calculate trace impedance.

### Impacts on Failure to Calculate Microstrip Trace Impedance

What would be the implications if we were not to calculate the microstrip trace impedance? Neglecting to determine the impedance of a microstrip transmission line could cause unexpected problems in the functionality and construction of your circuit. Below are the possible outcomes if we fail to calculate trace impedance.

#### Mismatched Impedance

The characteristic impedance of the transmission line affects how signals propagate along it. If the actual impedance deviates significantly from the desired or expected value, it can lead to signal reflections, distortions, and a mismatch between the source and load impedances. This can result in signal degradation and poor overall performance.

#### Signal Integrity Problems

Signal distortion, ringing, and overshoot are part of signal integrity issues that can arise from impedance mismatch. These problems could impair the overall reliability of the circuit and result in data transmission mistakes.

Increased electromagnetic interference (EMI) and crosstalk between adjacent traces can be attributed to incorrect impedance. It may make the circuit less electromagnetically compatible (EMC) overall and cause signals from several transmission lines to couple.

#### Losses and Reflections

The transmission line’s capacity to convey signals effectively and without appreciable losses or reflections is affected by microstrip impedance. The circuit may suffer from higher signal losses and reflections due to improper impedance calculation and control.

#### Problems with Manufacturability

In high-frequency applications, impedance management is essential in the process of manufacturing. The precision and dependability of the finished product may be subject to fabrication problems resulting from a failure to compute and follow the stipulated impedance.

#### Difficulty in Troubleshooting

Troubleshooting issues in the circuit are tricky when the predicted impedance is unknown. If the impedance is not managed properly, it becomes difficult to identify and fix problems with signal integrity or EMI.

#### Propagation Delay of Signal

High-speed signal transfer is very critical in microstrip. If one diff pair is long, make sure all other signals match the particular signal.

#### Bandwidth & Rise Time

Rise time is a very important factor and it is critical. It needs to maintain exact time. Otherwise, it produces distortion in signal and disturbing the data transfer.

#### Handling Capacity of Power

It needs to maintain the microstrip lines which can manage all power levels of a circuit. If we talk about high-power applications, the important thing is to reduce the losses and be immune from damage through excess power.

#### Temperature

Obviously, temperature impacts the microstrip transmission line.