What Techniques Are Used For Impedance Control in PCB Assy Design?

Used For Impedance Control in PCB Assy Design

A PCB with high-speed signals requires an impedance control technique to prevent signal distortion. When a signal encounters an unmatched impedance, it is reflected back and lost, which can cause jitter and delay in the transmission of data. Fortunately, a few techniques are used for impedance control in pcb assy design to ensure that the signals travel smoothly across the board.

Characteristic impedance matching refers to the balance of a signal’s relationship to ground. This is important for ensuring that a signal doesn’t get thrown off, causing unwanted noise like static on a radio. This is particularly important for critical signals, such as those on a DDR or Gigabit Ethernet circuit. It also helps reduce cross-talk between adjacent signals, which can disrupt the transmission of data.

Impedance matching is accomplished by adjusting the copper trace geometry, choosing the right dielectric material, and optimizing the layer stack-up design. It is especially important for high-frequency signals, where the characteristic impedance can change based on a number of factors including temperature rise, copper thickness, and capacity. In addition, it is necessary to make sure that the signal traces are a constant distance apart from each other, following 3W rules.

What Techniques Are Used For Impedance Control in PCB Assy Design?

To calculate the impedance for a particular trace, the PCB layout designer should use specialized software or online options that provide the necessary data. These will calculate the trace width and spacing based on the characteristics impedance value of the signal and the circuit’s dielectric constant. This information should be included in the fab drawing by the designer to help the manufacturer meet the required impedance specifications.

During the manufacturing process, the PCB manufacturer will use the specified impedance values and tolerances to build a stack-up with the appropriate dielectric thickness and layers. They will then follow the fab drawing notes provided by the PCB designer and verify that the traces meet the target impedance requirements through testing with a tool called time-domain reflectometry (TDR).

The most important thing to remember when designing for controlled impedance is to communicate the proper dimensions and tolerances with the manufacturer. This includes identifying which signals require CI and specifying their characteristic impedance value, trace width, and differential pair spacing. In addition, it is a good idea to include a detailed impedance table in the fab drawing for each layer of the board, which can be used by the manufacturer as reference during construction to ensure that they are meeting the required ohms value. It is also recommended to keep a separate stack-up for the CI signal traces, and route them with extra precautions to minimize distortion. This way, the manufacturer will be able to deliver a quality product with consistent impedance.

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