Impedance Control Rigid-Flex PCB: The Ultimate Guide

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Oakley Mae

So why would you care about impedance control rigid-flex PCB?

Ensuring the board offers the best performance is in your best interest when working with complex PCBs.

Therefore, the impedance of the circuit board is one crucial consideration.

Surprisingly, controlling the circuit board impedance improves its functionality. 

This article considers everything relating to impedance control, including controlled impedance types and design considerations for reliable impedance control.

Let’s get started!

Table of Contents

What’s Controlled Impedance?

(An engineer working on a PCB) 

Generally, controlled impedance is just a way of determining a circuit’s resistance to current flow.

It involves measuring trace impedance after manufacturing your rigid-flex circuit board to guarantee it is within limits set by the designer.

The impedance arises from both the circuit reactance and resistance.

Surprisingly, controlled impedance affects the integrity and timing of critical circuit functions. 

In most cases, you control the impedance by changing certain circuit board components’ physical structures.

Moreover, controlled impedance is always a circuit board consideration for high-speed digital and high-frequency analog circuits. 

Some of the products that feature controlled impedance include: 

  • Motor control units
  • Tablets, mobile phones, and computers
  • Video signal processing
  • Digital cameras, TVs, and web boxes
  • Digital and analog communications

How Does Impedance Control Work?

The primary function of circuit boards, including rigid-flex boards, is to allow signals to pass through to different connecting parts.

It’s common knowledge that electronic devices work only because of these signals.

And the signals can only travel through the traces available in the rigid-flex circuit board or any other board type. 

But the signal doesn’t travel freely in their direction as they take some time to reach the destination.

The time delay can reduce or increase based on several factors, including but not limited to the material type.

Therefore, impedance control is all about controlling or manipulating the resistance a trace exerts on a signal.

The end goal is to design a product with effective performance. 

Why Control Impedance on Rigid-Flex PCB?

 flexible circuit board

(A flexible circuit board)

Any time a signal requires a specific impedance level to function well, controlled impedance should be a top priority.

For example, high-frequency applications require a constant impedance throughout the rigid-flex circuit board to guarantee signal clarity and prevent data damage. 

Therefore, higher frequencies and longer traces require a stable impedance adaptation. 

Moreover, it becomes difficult to analyze uncontrolled impedance after you mount the circuit board.

We can attribute this to the temperature variations and different tolerance capacities that translate into malfunctions.

And when such malfunctions occur, you might be in a hurry to replace the whole device, not realizing that the problem originates from the trace. 

The solution checks the trace tolerances and impedances early in the design stage.

Furthermore, it would be best to always work closely with your manufacturers to ensure the component values comply with the required standards. 

Controlled Impedance Types

There are two controlled impedance types, as we’ll see below: 

Single-Ended Impedance

This is the impedance of a single trace.

In most cases, this impedance is affected by the coating dielectric, coating thickness, substrate dielectric Er value, substrate height, trace thickness, and trace width.

However, the trace width and dielectric Er value are the main factors affecting it. 

Differential Impedance

This represents the impedance between two traces when driven by opposite but equal polarity signals.

Also, all the factors that affect the single-ended impedance above will affect the differential impedance.

However, when managing differential impedance calculations, the main considerations include the dielectric Er value, substrate height, trace spacing, and width. 

Finally, you can use the differential and single-ended impedance to hatch rigid-flex circuit board impedance. 

Factors Affecting Impedance Control During Rigid-Flex PCB Design

A closer look at a circuit board

(A closer look at a circuit board)

Factors affecting impedance control during rigid-flex circuit board design include dielectric constant, dielectric thickness, trace width, and copper thickness.

But what effects do they bring? Well, check out below:

  • Trace width: This represents the copper foil and coating width. Generally, impedance reduces as the trace width increases. Surprisingly, when you develop specifications for your circuit board, you should base the trace width on factors like temperature rise and capacity. But if you have a specific impedance, go ahead and design it. 
  • Copper thickness: Also, the impedance reduces as the copper thickness increases. And to increase it, reduce the copper thickness and weight.
  • Dielectric constant: This represents the ratio of a material’s electric permittivity to that of a vacuum. In the case of rigid-flex circuit boards, the dielectric constant is inversely proportional to the frequency. Therefore, a circuit board having a low dielectric constant is perfect for controlled impedance and high-frequency applications. 
  • Dielectric thickness: This represents the thickness of the material between traces. The relationship between impedance and dielectric thickness is logarithmic. Therefore, a significant increase in dielectric thickness translates into a smaller increase in impedance. 

Design Considerations To Get Good Impedance Control

Specific design rules are needed to achieve rigid-flex circuit board impedance control.

Check them out below: 

Choose The Signals

As a designer, you must indicate the exact signals that need impedance control.

This is where the component datasheet is handy since it offers detailed signal class information and the corresponding impedance values.

Moreover, designers should specify whether the signal is single-ended or differential. 

Keep Traces Separated

All traces with controlled impedance should be well separated.

The recommended space is 2W, where W represents the width of the track under impedance control. Even better, you can use a spacing of 3W. 

Surprisingly, to eliminate crosstalk, you must abide by the 2W and 3W rules.

However, for the case of high-frequency signals, increase the minimum distance to 5W. 

Reduce The Use of Bypass Capacitors and Vias

A photo of capacitors

(A photo of capacitors)

Generally, bypass capacitors and vias usually create discontinuities in the controlled impedance value.

Therefore, avoid placing bypass capacitors and vias between different signal pairs unless you want signal integrity problems, which we doubt. 

Trace Length Matching

The aim is to ensure the signal maintains a constant propagation speed throughout the trace.

Therefore, the circuit board is unlikely to experience propagation delays. 

FAQ

What factors affect PCB impedance?

The impedance of a circuit board is a combination of its resistance, inductance, and capacitive reactance.

Therefore, we have several factors that affect a PCB impedance and including: 

  • Copper trace thickness and width
  • The dielectric constant of the prepreg or core material
  • The thickness of the prepreg and core material
  • The distance from additional copper features

How do you reduce impedance in rigid-flex PCBs?

Generally, to reduce impedance in your rigid-flex circuit board, start by increasing the copper thickness.

Also, increasing the circuit board’s trace width would be best.

Finally, it would be best if you reduced the dielectric thickness. 

Conclusion

As we highlighted, impedance control involves manipulating a circuit board trace’s resistance to a signal.

Generally, it guarantees you design a product that maintains signal integrity and offers high performance.

Therefore, to acquire excellent impedance control, keep the signal separated, the trace length matching, and eliminate bypass capacitors and vias.