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Impedance Control: When Your 50Ω Lines Aren't Actually 50Ω

Impedance Control: When Your 50Ω Lines Aren't Actually 50Ω

Stack-up tolerance, copper roughness, and the 5% number that determines whether your radio works.

Layer-Up Engineering·4/27/2026

You asked the fab for 50Ω. They built 47Ω. Your eye diagram closes. Here's what went wrong.

Sources of impedance error

  • Dielectric thickness tolerance: typically ±10% on standard FR-4.
  • Copper trace etching tolerance: ±0.5 mil on the trace width.
  • Solder mask presence: drops Z₀ by 1–2Ω.
  • Copper surface roughness (the Hammerstad-Jensen correction nobody applies).

How to actually get ±5%

Pay for impedance control as a line item. The fab pulls TDR samples from the panel and adjusts trace width on the next iteration. Without that test pad, you're rolling dice.