What is PCB Impedance and Why is it Important?

Printed Circuit Board (PCB) impedance refers to the opposition that a circuit presents to a current when a voltage is applied. In other words, it is a measure of how much the circuit impedes the flow of current.

Impedance is made up of two components:
– Resistance (R) – opposition to DC current flow
– Reactance (X) – opposition to AC current flow

The impedance of a PCB trace is determined by several factors:
– Trace width
– Trace thickness
– Dielectric constant of the PCB material
– Thickness of the dielectric material
– Frequency of the signal

Matching the impedance of transmission lines on a PCB is critical for several reasons:
1. Signal integrity – Impedance mismatches cause reflections which distort the signal
2. Power efficiency – Reflections due to mismatches waste power
3. Electromagnetic Compatibility (EMC) – Mismatches cause radiation of EM noise
4. Prevents damage to ICs – Reflections can cause overvoltage that damages components

Therefore, in high-speed digital and RF circuits, the impedance of PCB traces must be tightly controlled. The target impedance is typically 50Ω for RF and 100Ω for differential digital lines.

How to Calculate the Impedance of a PCB Trace

The characteristic impedance of a PCB trace can be calculated using formulas that take into account the trace geometry and PCB material properties. The two most common types of PCB transmission lines are:
1. Microstrip – a trace on the outer layer, above a ground plane
2. Stripline – a trace embedded in the inner layer, between two ground planes

Microstrip Impedance Formula

The characteristic impedance (Z0) of a microstrip trace can be calculated using this formula:

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

Where:
– ϵr = dielectric constant of PCB material
– h = height of substrate (dielectric thickness)
– w = width of trace
– t = thickness of trace

All dimensions should be in the same units, typically mils (1/1000th of an inch).

Stripline Impedance Formula

For a stripline trace, the impedance formula is:

Z0 = (60/√ϵr) • ln(4h/0.67πw)

Where the variables are the same as for microstrip.

Example Impedance Calculation

Let’s calculate the impedance of a microstrip trace given:
– ϵr = 4.5 (typical for FR-4 PCB material)
– h = 5 mils
– w = 10 mils
– t = 1.4 mils (1 oz copper)

Plugging into the microstrip formula:

Z0 = (87/√(4.5+1.41)) • ln(5.98•5/(0.8•10+1.4))
= 55.5 • ln(2.1)
= 55.5 • 0.74
= 41Ω

So this 10 mil wide trace over a 5 mil substrate has a characteristic impedance of approximately 41Ω. To get to the target 50Ω, we would need to adjust the trace width and/or the substrate thickness.

Impedance Calculator for PCB Design

Rather than doing the calculations by hand, it is much more convenient to use an impedance calculator tool. Many PCB design software packages include trace impedance calculators.

Here are some free online impedance calculators for PCB design:
– Saturn PCB Design Toolkit
– EEWeb PCB Trace Width Calculator
– All About Circuits Microstrip Impedance Calculator

To use these calculators, simply input the trace geometry and material properties and the tool will output the characteristic impedance. You can then adjust the parameters to optimize the impedance as needed.

How to Use the Saturn PCB Toolkit Impedance Calculator

Let’s walk through an example of using the Saturn PCB Toolkit to calculate microstrip impedance:

1. Go to http://www.saturnpcb.com/pcb_toolkit.htm
2. Under “Calculators” select “Edge Coupled Microstrip Impedance”
3. Input the following parameters:
4. Dielectric Constant (Er): 4.5
5. Trace Thickness (T): 1.4
6. Substrate Thickness (H): 5
7. Trace Width (W): 10
8. Trace Spacing (S): 10
9. Select “Compute Impedance”

The calculator outputs:
– Characteristic Impedance (Z0): 40.83 ohms
– Odd Mode Impedance (Z0o): 60.1 ohms
– Even Mode Impedance (Z0e): 30.11 ohms
– Effective Dielectric Constant (Eff Er): 3.23

This matches closely with our earlier hand calculation. The calculator also provides the odd and even mode impedances which are relevant for differential pair routing.

Tips for Controlling PCB Trace Impedance

Achieving precise impedance control requires careful PCB design practices. Here are some tips:

1. Use controlled impedance pcb materials
2. Fr-4 is most common but has variations in Er
3. Higher-quality laminates like Rogers provide tighter Er tolerance
4. Set trace width and spacing to hit target impedance
5. Consult impedance calculators early in design
6. Smaller traces yield higher impedance
7. Keep dielectric thickness consistent
8. Specify controlled dielectric thicknesses
9. Avoid using multiple dielectrics if possible
10. Minimize discontinuities that cause reflections
11. Avoid sharp bends, stubs, vias if possible
12. Use tapered bends and transitions
13. Provide reference plane for return current
14. Microstrip needs uninterrupted reference plane
15. Route over voids or gaps in ground/power planes
16. Consider dielectric losses at high frequencies
17. Fr-4 loss tangent increases above a few GHz
18. Use low-loss materials for multi-GHz applications
19. Perform PCB impedance testing to validate
20. Time-domain reflectometry (TDR) is common technique
21. Validate test traces & coupons along with actual PCB traces

PCB Impedance Control Standards

There are several standards that provide guidance on specifying and measuring PCB impedance:

• IPC-2141 – “Controlled Impedance Circuit Boards and High-Speed Logic Design”
• IPC-TM-650 2.5.5.7 – “Characteristic Impedance of Lines on Printed Boards by TDR”
• IPC-6018 – “Microwave End Product Board Inspection and Test”

These standards define impedance classes (e.g. Class 1, 2, 3) with associated tolerance bands (e.g. ±10%). They describe material, design, manufacturing, and testing requirements to meet those classes.

Designers should specify the relevant standard and class on their fabrication drawings to communicate impedance requirements to the PCB manufacturer. Many PCB shops are certified to these IPC standards.

Impedance Calculator PCB FAQ

What is the typical impedance of a PCB trace?

50Ω is most common for single-ended traces in RF and high-speed digital designs. 100Ω differential is typical for differential pairs. The actual target impedance depends on the specific application and I/O standards involved.

What affects the impedance of a PCB trace?

PCB trace impedance is mainly determined by:
– Trace width and thickness
– Dielectric material type and thickness
– Presence of reference planes (power/ground)
– Proximity to other traces (coupling)

How do you match the impedance of a trace to a source?

To match a trace to a source impedance:
1. Determine the source impedance (e.g. 50Ω)
2. Use a PCB impedance calculator to set trace parameters
3. Adjust trace width and/or dielectric height to match source
4. Minimize discontinuities that cause reflections
5. Perform validation testing to confirm match

What happens if PCB traces are not impedance controlled?

Signal integrity issues may occur such as:
– Reflections causing ringing, overshoot, undershoot
– Increased crosstalk between traces
– Inconsistent rise/fall times, timing errors
– Increased electromagnetic radiation (EMI)
– Reduced power efficiency due to reflections

The severity depends on the degree of mismatch and edge rates of the signals. Low-speed designs are more tolerant of mismatches than high-speed designs.

How do you test PCB impedance?

The most common method is Time Domain Reflectometry (TDR):
1. Inject fast rise time pulse into trace
2. Measure reflected pulse amplitude vs. time
3. Compute impedance from pulse amplitudes
4. Scan probe along trace to measure Z vs. position

TDR provides an impedance profile along the length of the trace to pinpoint any discontinuities. VNAs (vector network analyzers) are also sometimes used to measure S-parameters and compute impedance across frequency.

I hope this in-depth article helps explain how PCB trace impedance works, how to calculate it, and how to control it. Let me know if you have any other questions!