Via Inductance

Via Inductance

At high frequencies, via inductance matters more than resistance. A via's parasitic inductance can cause signal integrity problems for fast edge rates.

The via inductance formula (Howard Johnson approximation):

L = 5.08 × H × (ln(4H/D) + 1) nH

Where:

• H = via height (board thickness) in inches
• D = via drill diameter in inches
• L = inductance in nanohenries (nH)

Wait — old formulas often use inches. We'll accept mm inputs and convert:

1 inch = 25.4 mm
H_in = H_mm / 25.4
D_in = D_mm / 25.4

Example: Standard 1.6mm board, 0.3mm drill:

• H_in = 0.0630, D_in = 0.01181
• L = 5.08 × 0.0630 × (ln(4×0.0630/0.01181) + 1) ≈ 5.08 × 0.063 × (3.054 + 1) ≈ 1.30 nH

At 1 GHz, Z = 2π × 1e9 × 1.30e-9 ≈ 8.2 Ω — significant for a 50Ω system!

Your Task

Implement viaInductance(H_mm, D_mm) returning inductance in nH.