Question 1

What is controlled impedance?

Accepted Answer

Controlled impedance refers to PCB traces designed to have a specific characteristic impedance (typically 50Ω or 100Ω differential). This is critical for high-speed signals to minimize reflections and signal integrity issues. The impedance is controlled by precisely managing trace width, dielectric thickness, and material properties.

Question 2

When do I need controlled impedance?

Accepted Answer

Controlled impedance is needed when: 1) Signal frequency exceeds 100MHz, 2) Rise times are under 1ns, 3) Using high-speed interfaces like USB 3.0, HDMI, PCIe, DDR4/5, or Ethernet, 4) Trace length exceeds 1/6 of the signal wavelength. When in doubt, consult with your fab house or a signal integrity engineer.

Question 3

What is the difference between microstrip and stripline?

Accepted Answer

Microstrip is a trace on an outer layer with a ground plane below - it's easier to route but has more EMI radiation. Stripline is a trace between two ground planes on inner layers - it offers better EMI shielding and more consistent impedance, but is harder to route and has higher propagation delay.

Question 4

How accurate is this calculator?

Accepted Answer

This calculator uses standard IPC-2141 formulas and provides estimates within ±5-10% for typical geometries. Actual manufactured impedance depends on material tolerances, etch factors, and plating thickness. For production boards, always verify with your PCB manufacturer's impedance modeling tools.

Question 5

What factors affect impedance the most?

Accepted Answer

The main factors are: 1) Trace width - wider traces = lower impedance, 2) Dielectric height - thicker dielectric = higher impedance, 3) Dielectric constant - higher εr = lower impedance, 4) Copper thickness - thicker copper slightly lowers impedance. Trace width and dielectric height have the most significant impact.

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