Impedance Control Work in PCB Fabrication
Impedance control is a crucial step in the PCB fabrication process, especially for high-speed digital applications. Impedance control ensures that traces present anticipated impedance values, allowing necessary termination resistors to damp signals and prevent over/under loading which can cause unwanted signal reflections – all of which can contribute to electromagnetic interference (EMI).
Specifying the correct impedance at the design stage is critical for successful project execution. When the designer specifies the desired impedance value, the fabricator is able to match the impedance requirements in the laminate and copper foil. This is particularly important when working with a high-speed circuit, as impedance changes rapidly at higher frequencies.
The impedance value is determined by the combination of the resistance (real part) and capacitance (imaginary part). These elements are controlled during the etching process, which results in trace widths that are closely matched to the desired impedance. Impedance control is critical to high-speed performance because it helps to minimize the effects of reflections and ringing caused by mismatched transmission lines.
How Does Impedance Control Work in PCB Fabrication?
Traces that are not properly matched in length with a differential pair can lead to an unacceptable delay in the transfer of the positive and negative signals across the board. To reduce this delay, a technique called serpentine balancing is used to compensate for the longer trace length of the differential signal. This technique requires the insertion of serpentine-shaped traces that run over, under and alongside the differential pairs to provide an even path for current.
One of the most common mistakes in specifying impedance is using inconsistent track widths for a given layer. For example, if you have a number of traces that require 5mil trace widths to achieve the required impedance and another set that needs only 3mil wide tracks, it will be very difficult for the fabricator to identify these traces during routing.
Another factor that can lead to a mismatch between designed and actual impedance is the difference between the dielectric constants of different pcb board fabrication materials. If a designer chooses a material for the PCB that is slightly different from the manufacturer’s standard FR4 stock, it will be very difficult to achieve their desired impedance goals.
To avoid these issues, it is essential that the designer work closely with the PCB fabricator to discuss the specifics of the desired impedance and stackup for each layer. It is also wise to include a summary of the impedance requirement as a note in the stackup table, so that the fabricator can easily reference this information when creating the PCB’s fabrication drawings. This helps to eliminate confusion, and can save valuable time that would otherwise be spent reworking the board design to meet the fabricator’s impedance specifications. If you are interested in learning more about how to create a detailed stackup table that will help you to specify the proper impedance, reach out to the experts at Sierra Circuits today!