Controlled impedance ensures signal integrity in high-speed PCB design.

Key Concepts:
- 50 ohm is standard for RF and high-speed digital.
- Microstrip: trace on outer layer over ground plane.
- Stripline: trace embedded between two ground planes.

What Affects Impedance:
1. Trace width: wider = lower impedance
2. Dielectric thickness: thicker = higher impedance
3. Dielectric constant (Dk)
4. Trace height above plane

When You Need It: DDR, high-speed serial (USB, PCIe), RF traces, clocks above 50MHz.

Work with your PCB fab for accurate stackup calculations.