Impedance control in PCB

PCB manufacturer is given a set of patterns - copper patterns, hole patterns, ink patterns, which are combined into a single circuit board with all the pattern sizes and positions within certain tolerances. These patterns are generated from line, drill, printing process. Failure to meet a certain size or position with the specified tolerance can be cause for the circuit board to be rejected. The tolerance requirement for PCB equipment is relative high. If a trace has been defined as an impedance control trace, it is not the trace size which is strictly defined, but rather the impedance. While a nominal trace size will be provided in the Gerber layer, it is understood the circuit board manufacturer can vary trace width, height, and dielectric thickness as long as the final impedance is within tolerance. Normally, if a PCB layout wants to control impedance for a specific line, it should be indicated in the Gerber file. So the PCB manufacturer can make the adjustments.

There are 3types of impedance control in printed circuit board

1. No impedance control. The impedance tolerance is loose enough that simply making a design with no extra precautions will result in the correct impedance as long as the design is made correctly within the standard specifications. This is the fastest and least expensive option since it places no extra burden on the circuit board manufacturer.

2. Impedance watching. The designer indicates the impedance control trace. The PCB provider adjusts the (W)width of the trace and (H)height of the dielectric and gets approval on the proposed specifications before starting manufacturing. A TDR (Time Domain Reflectometry) test can be performed to confirm the impedance for an additional cost.

3. Impedance control. Usually reserved for high-end designs containing either an odd design that doesn't fit the usual microstrip configuration or a tight tolerance. With manufacturing capability limits approaching the dimension requirements, confidence is not high the target impedance will be achieved on the first pass. The circuit board manufacturer first makes the board, getting as close to the target impedance as possible. Next a TDR test is done to determine if the impedance is within specification and adjustments are made as necessary.