Why do you need it? As PCB signal switching speeds increase, the electrical properties of the traces carrying signals between devices becomes increasingly more important. Traces can no longer be considered as simple conductors but must be treated as transmission lines. For stable and predictable high speed operation the electrical characteristics of PCB traces and the dielectric of the PCB must be controlled. (i.e digital edge speeds faster than 1ns or analog frequencies greater than 300MHz).
How is impedance controlled in a PCB?
The PCB will frequently be multi-layer in fabrication and the controlled impedance can be constructed in several ways. However, whichever method is used the value of the impedance will be determined by its physical construction and electrical characteristics of the dielectric material:
- The width and thickness of the conductor
- The height of the core and pre-preg material on either side of the trace
- The configuration of traces and planes
- Dielectric constant of the core and pre-preg material
Board designers often will specify impedance values and tolerances (typically +/- 10%) for board traces and rely on the PCB manufacturer to conform to the specification. Board manufacturers work with the stack up, material properties, and raw material tolerances to predict the final outcome. If tighter tolerances are required, work with your fabricator upfront. It may be possible after a first run of a prototype to determine how the stack up presses during manufacturing for fabricators to dial in the process on future runs.
Almost all controlled impedance designs require 100% testing. In nearly all cases the traces are inaccessible for testing and testing is normally performed on one or two test coupons designed into the PCB panel. The coupon is on the same layer, has the same trace construction as the PCB, and includes traces with exactly the same impedance as those on the PCB. Testing of these coupons provides a high degree of confidence that the PCB impedances will be correct.
Impedance measurements are typically done with TDR (time domain reflectometer). This applies a fast voltage step to the coupon via a controlled impedance cable and probe. This test is able to identify the location and scale of any incontinuity and provides a graphical representation of the trace characteristic impedance.
Work with your fabricator early in the design. If you are new to controlled impedance, unsure of the dielectric properties of the materials or would like a review of your current design your PCB fabricator will be happy to assist with stack up and impedance control reviews!
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