signal integrity

Microstrip PCB Impedance

Calculate the characteristic impedance, capacitance, propagation delay, and inductance per unit length of a microstrip PCB trace.

Microstrip transmission lines are routed on the external layers of a PCB, always above a large reference plane on the adjacent layer. As the name suggests, microstrip lines are used to convey microwave-frequency and RF signals.

This online calculator helps you compute the impedance, capacitance, propagation delay, and inductance of a Microstrip Line PCB. Understand the characteristics of your electrical transmission lines and optimize your circuit design.

Understanding Microstrip PCB Impedance

The characteristic impedance of a flat wire hanging above a ground plane with a dielectric separating them is referred to as microstrip PCB impedance. This structure is typically built on printed circuit boards (PCBs), although other materials can also be used as long as a conductor is isolated from a ground plane by a dielectric substance.

Calculating Microstrip Impedance

The characteristic impedance of a microstrip can be estimated using a variety of online calculators, taking into account variables such as the dielectric constant, trace width, trace thickness, and trace-to-ground plane distance. However, these results are limited to crude estimation and approximation. For precise impedance calculations, it is advised to use specialist software such as Polar Instruments.

Impedance Control in PCB Design

To attain impedance control in PCB design, adhere to these five essential guidelines:

Step	Guideline
1. Understand the Fundamentals	Become familiar with characteristic impedance, transmission lines, and impedance matching
2. Choose Appropriate Materials	Select the right dielectric material and trace width to achieve the required impedance
3. Consider the Stack-up	Engineer the PCB stack-up to minimize impedance variances throughout
4. Apply the TDR Method	Use Time-Domain Reflectometry (TDR) to find potential impedance differences across a broad range of frequencies
5. Verify with Measurements	Measure the real impedance of the PCB with specialized equipment to confirm impedance control

Types of Microstrip Traces

Two primary varieties of microstrip traces exist:

Type	Description
Uncoated Microstrip	Lacks a soldermask coating above the trace
Coated Microstrip	Features a soldermask coating above the trace, which affects the characteristic impedance

A microstrip's characteristic impedance is determined by the dielectric constant, trace width, thickness, and the distance between the ground plane and the trace.

Formulas

$Z = \frac{87}{\sqrt{e_r + 1.41}} \times \ln\left(\frac{5.98h}{0.8w + t}\right)$

$P = 3.333 \times \sqrt{0.475e_r + 0.67} \times k$

$C = \frac{P}{Z}$

$L = \frac{P \times Z}{1000}$

$R = \frac{17.2}{w \times t \times k^2} \times 1000$

where:

$Z$ = Single-Ended Impedance (Ω)
$e_r$ = Relative Dielectric Constant
$h$ = Trace Height (above ground plane)
$w$ = Trace Width
$t$ = Trace Thickness
$P$ = Propagation Delay
$C$ = Capacitance
$L$ = Inductance
$R$ = Resistance
$k$ = Dimension Factor — $k = 1$ if unit is mm, $k = 0.0254$ if unit is mils

Inputs

Relative Permittivity

Relative permittivity of the dielectric — FR4: ~4.5, Rogers 4003: ~3.55

Height of Dielectric(mm)

Height of the dielectric substrate in millimetres

Width of Trace(mm)

Width of the PCB trace in millimetres

Trace Thickness(mm)

Thickness of the PCB trace in millimetres

Results

Characteristic Impedance125.37ΩCharacteristic impedance in ohms

Capacitance Per Unit Length4.454e-11F/mDistributed capacitance per unit length in farads per metre

Effective Propagation Delay5.882e-9s/mEffective propagation delay per metre in seconds

Inductance Per Unit Length7.000e-7H/mDistributed inductance per unit length in henries per metre