Impedance control means setting trace width, dielectric thickness and material so a transmission line has a specified characteristic impedance, commonly 50 ohms single-ended or 85, 90 or 100 ohms differential, usually within plus or minus 10 percent. The designer specifies targets per layer, the fabricator tunes widths and stackup to its materials, and test coupons on the panel are measured with a TDR to prove it. It matters once edge rates are fast enough for a trace to behave as a transmission line.
Note impedance control on layers 1, 3, 6 and 8, 90 ohm differential for the USB, and ask for the TDR report.
Also heard as
- controlled impedance
- impedance-controlled routing
Related terms
Stackup
Ordered arrangement of copper layers, cores and prepregs that makes up a multilayer board, with each thickness.
Dk
Dielectric constant: how much a board material slows signals and sets trace impedance, relative to vacuum.
Differential pair
Two traces carrying equal and opposite signals, routed tightly together at matched length and impedance.
Coupon
Small test pattern built on the production panel and measured or sectioned to prove the boards meet spec.