Controlled impedance

At high speeds a PCB trace behaves as a transmission line. If its characteristic impedance doesn't match the driver, connector and receiver, part of the signal reflects back, causing data errors and extra emissions. Impedance depends on trace width, spacing, copper thickness and the distance and material to the nearest reference plane, so it is set jointly by layout and stack-up.

Each interface specification sets its own target: RF systems are commonly built around 50 Ω, Ethernet twisted pair around 100 Ω, and USB and other high-speed buses define their own differential values. The fabricator adjusts trace widths for its materials and measures test coupons to confirm.

Where it bites: impedance control adds cost and must be called out explicitly in the fabrication notes. Forgetting it, or routing a high-speed pair over a gap in the ground plane, causes intermittent link failures and EMC problems.