Controlled Impedance Requirements

Impedance, generally measured in Ohms is a characteristic of the circuit that has to be taken care of during design of the pcb. Resistance and reactance of an electrical circuit have a major impact on functionality, as certain processes must be completed before others to ensure proper operation. This chain of command disrupts a system and leads to a complete failure when these actions aren’t performed as planned. Printed circuit boards with impedance requirements will nullify the changes in voltage occurring and lead to an appliance or gadget that operates as expected.

The pcb design factor’s affecting impedance are: Trace width, copper thickness, dielectric thickness, dielectric constant.

When Is Impedance Most Important?

Operations controlled by impedance printed circuit boards perform faster processing and use less energy. Covering impedance control in PCB design plans helps products perform better for longer periods of time, improving value and controlled reliability.

Controlled impedance is essential during transitions from a lower Ohm to higher Ohm environment WHERE impedance is present. These kinds of transitions can lead to energy reflection in the form of powerful pulses capable of disrupting energy flow. This issue is as critical for those manufacturing high-powered digital devices as it is in RF applications.

How Does PCB Impedance Control Work?

Before you can decide what kind of impedance control service you need for your printed circuit boards, you need to understand impedance control basics. What is impedance control, exactly, and how does it work? There are three basic levels of service when it comes to PCB impedance control. They are:

No Impedance Control: This is a situation where you do not need any extra design elements to ensure correct impedance because you have very loose impedance tolerance. Naturally, this will result in a faster-completed, less expensive board because the manufacturer does not have to include any special measures.

Impedance Watching: What is impedance watching? This is a situation where the designer will outline the impedance control trace and the PCB provider adjusts the trace width and dielectric height accordingly. Once the manufacturer approves these specifications, they can begin to manufacture the board. You can request a Time Domain Reflectometry (TDR) test to confirm the impedance for a fee.

Impedance Control: Actual impedance control is something you will typically only request when your design has tight impedance tolerances that could be tough to hit the first time around. When the capability limits of the manufacturer get close to the dimension requirements, it can be tough to ensure target impedance on the initial attempt.

In an impedance control situation, the manufacturer makes the board doing the best they can to hit the target impedance. They then run a TDR test to see if they are successful. If not, they adjust accordingly and try again until they achieve the desired impedance.

What Is Impedance Control on a Microstrip?

There are four parameters to consider when it comes to impedance for a microstrip:
Dielectric height, or H, which you can adjust incrementally (e.g. +1mil =+2 Ohms)
Material Dielectric, or Er, which will be fixed once you choose your material
Trace Thickness, or T
4. Trace Width, or W
Standard final impedance tolerance is about +/- 10 percent.