Control impedance in PCBs

The electrical properties of the traces carrying signals between devices become increasingly more important. The impedance of a PCB trace is controlled by its configuration dimensions (trace width and thickness and height of the board material) dielectric constant of the board material.

As with a cable, when the signal encounters a change of impedance arising from a change in material or geometry, part of the signal will be reflected and part transmitted. These reflections are likely to cause aberrations on the signal which may degrade circuit performance (e.g. low gain, noise and random errors). In practice board designers will specify impedance values and tolerances for board traces and rely on the PCB manufacturer to conform to the specification.

Testing the PCB.

Most controlled impedance PCBs undergo 100% testing. However, it is not uncommon for the actual PCB traces to be inaccessible for testing. In addition, traces may be too short for accurate measurement and may well include branches and vias which would also make exact impedance measurements difficult. Adding extra pads and vias for test purposes would affect performance and occupy board space. PCB testing is therefore normally performed, not on the PCB itself, but on one or two test coupons integrated into the PCB panel.

Impedance testing machine

The coupon is of the same layer and trace construction as the main PCB and includes traces with precisely the same impedance as those on the PCB, so testing the coupon affords a high degree of confidence that the board impedances will be correct. Need impedance control PCB? SysPCB is your best choice, our PCB impedance tolerance less than 50 ohm, +/-5%, less than 100ohm +/-10%, call us today, to get quotation with lower Shenzhen China PCB manufacturer price.