Impedance Control Techniques for PCB
The power delivered to a circuit board’s components depends on the board’s impedance. The board’s traces act like transmission lines at high frequencies, and any mismatch in their impedance will reflect the signal, disrupting it. This is a problem in both digital and analog devices because it introduces distortion into the signal that interferes with its operation. This is why controlled impedance is essential for modern products.
PCB fabricators need precise specifications to ensure that a board meets its target impedance profile. A design with loose tolerances may not need special impedance control techniques, but tight tolerances require the manufacturer to carefully monitor the process and run a time-domain reflectometry (TDR) test to verify the final product meets the specifications.
Impedance control is most commonly used for higher-speed traces, which must be properly routed to avoid unwanted reflections. Differential pairs should be routed symmetrically and kept at least 30 mils away from other signals to prevent signal integrity problems.
A variety of factors affect the impedance of a pcb traces and the overall system. The copper thickness of the traces and dielectric layer are both significant for controlling impedance. Copper thickness increases the trace’s impedance, but at a logarithmic rate, so large increases in copper thickness cause only modest increases in impedance. The thickness of the insulating layer is critical for achieving the desired impedance, and is dependent on the type of semicured sheet material used, its resin content and the lamination procedure.
Impedance Control Techniques for PCB Design
Trace width is also important for achieving impedance control. The thinner the trace, the lower its impedance; the wider the trace, the greater its impedance. Trace widths must be etched precisely to avoid etch undercut, lithographic errors and pattern transfer distortion, all of which can affect impedance.
Another factor is the trace’s capacitance, which increases with its length and decreases as its frequency increases. To keep the traces’ capacitance within acceptable limits, a designer can limit their length or increase their pitch.
In addition to trace dimensions, other factors that influence impedance are the thickness of the dielectric layer and the track’s tracking resistance. Both the thickness of the dielectric layer and the tracking resistance are influenced by the type of semicured sheet material used, the resin content of that sheet and the thickness of the press plate it is fed through for lamination.
For accurate impedance calculations, the designer must account for the loss of the laminate, copper roughness and dispersion. Using PCB design tools that provide an integrated field solver to calculate impedance and capacitance makes this easier. The resulting simulations can help designers determine the ideal dielectric thickness, copper weight and trace width for their application. In addition, they can use the simulation results to verify that the board will meet its impedance goals before submitting it to fabrication. By taking the proper steps, PCB designers can achieve the highest quality boards with tightly controlled impedance. This helps minimize the potential for EMI problems and improves the reliability of the final device.