Mastering Controlled Impedance: Essential Techniques for Reliable PCB Design

Aug 31,2018

Miniaturization is a hot trend in the PCB industry. There is a huge demand for compact sized and fast-paced PCBs. Although an asset in many electronic devices, there are many components in these fast PCBs, which may not work as expected, and lead to failure. Hence, the PCB manufacturers know the importance of slowing down certain circuits to enable the specific functioning of components. This is where controlled impedance can help. What is a controlled impedance? What is the importance of this factor? What are the determining factors of controlled impedance? Where is the controlled impedance used? Are these questions conjuring your mind? If yes, this post will help you find answers to all your questions.

An Introduction to Controlled Impedance

To understand controlled impedance, it is important that you understand impedance. Impedance is only defined for high-frequency circuits. It is a combination of inductance and capacitance of the circuit. The value of impedance always varies between the two points, if not controlled properly. At high frequencies, PCB traces do not behave as expected, and the quality of signals may degrade at various points. The controlled impedance helps ensure the efficient propagation of signals, without disturbance and distortion. The impedance (Zo) of a circuit is an AC characteristic and is measured in Ohms (Ω). Target value of impedance is 25 Ω - 125 Ω.

Factors that Determine Controlled Impedance

The impedance of a PCB trace is determined by factors, such as resistance, capacitive reactance, and conductance. What contributes to these factors? Well, the answer is, these factors are decided by elements such as physical dimensions of the trace, dielectric thickness, and the dielectric constant of the substrate material used in the PCB. The value of impedance is decided by the following factors:

• Thickness and width of the copper signal trace at its top and bottom
• Dielectric constant of the prepeg material and core
• Thickness of the prepeg material and core on both sides of the copper trace
• Distance of the trace from other copper features

Types of Controlled Impedance

Characteristic impedance is one of the most common types of controlled impedance. The following are a few more important types referred in the market:

• Input impedance
• Image impedance
• Wave impedance

Typical Applications of Controlled Impedance

Controlled impedance is considered ideal for high-frequency digital applications:

• RF Communications
• High-Quality Analog Videos
• Computing above 100 MHz
• Telecommunication Equipment
• Low Noise Amplifiers

Common Manufacturing Mistakes to Avoid While Creating Circuits with Controlled Impedance

The following are some common mistakes to avoid while producing controlled impedance circuits:

• Length Mismatches: Length mismatches between differential pairs will not only lead to signal distortion but also increase the error rates. Hence, the lengths must be matched within +/-5 mils. Also, the mismatched ends can be closed using serpentine routing.
• Traces without Reference Ground Planes: The high-speed signals must be routed to the bottom or the top layer, and a complete ground reference plane must be given on the adjacent layers. The impedance may rise, if there are no adjacent planes.
• Multiple Types of Prepegs: It is always better to avoid using more than three types of prepegs in the circuit board. Also, care should be taken to keep the dielectric thickness of the prepeg below 10 mils. The prepegs with high glass and low resin content should be avoided.

All this means, you should consider purchasing these controlled impedance circuit boards from reliable and experience manufacturers only. Creative Hi-Tech is one of the popular manufacturers of controlled impedance circuit boards in the USA.