Millimeter Wave Specific Challenges mmWave Design Challenges Informational

What is the effect of PCB via inductance on millimeter wave circuit performance?

Q: Can I use standard through-hole vias at 28 GHz?

Only with great care: (1) For signal vias: a standard through-hole via in a 1.6 mm board (L ≈ 0.7 nH) is problematic at 28 GHz (Z = 123 ohms). The via must be backdrilled to remove the stub, and the antipad must be optimized by EM simulation. Even with optimization: the insertion loss per via transition is 0.3-0.8 dB at 28 GHz (significant in a system with multiple transitions). (2) For ground and power vias: through-hole ground vias are acceptable (the low impedance of the via is actually beneficial for grounding; you want the ground connection to be as low-impedance as possible). Multiple parallel ground vias: even better (lower total inductance). (3) Best practice at 28 GHz: use blind microvias for all signal transitions. Use through-hole vias only for ground stitching and power connections. Backdrill any through-hole vias that must carry signals.

Q: How many vias in parallel to reduce inductance?

The inductance of N parallel vias: L_total ≈ L_single / N × (1 + (N-1)×M/L_single), where M is the mutual inductance between vias. For vias spaced > 3× their length apart: M is small and L_total ≈ L_single / N. For closely spaced vias (< 1× length): M ≈ 0.3-0.5 × L_single. L_total ≈ L_single / N × (1 + (N-1) × 0.4) = L_single × (0.4 + 0.6/N). For N = 2: L_total ≈ 0.7 × L_single (30% reduction, not 50%). For N = 4: L_total ≈ 0.55 × L_single. For N = 8: L_total ≈ 0.48 × L_single. Diminishing returns above 4-6 parallel vias due to mutual inductance. For ground vias: use 4-6 parallel vias for the best practical inductance reduction.

Q: What about coaxial via structures?

A coaxial via: a signal via surrounded by a ring of ground vias, forming a coaxial transmission line through the PCB stack-up. The ground via ring creates a controlled-impedance environment around the signal via. Design: signal via: centered. Ground vias: 6-8 vias in a ring at a radius that provides the desired impedance: Z = (60/sqrt(epsilon_r)) × ln(R_ring/R_signal). For 50 ohms on FR-4: R_ring / R_signal ≈ 5.5. If the signal via pad = 0.4 mm diameter (R = 0.2 mm): R_ring = 1.1 mm (ground vias at 1.1 mm radius = 2.2 mm diameter ring). The coaxial via provides: 50-ohm impedance through the transition (matched), excellent shielding (the ground ring isolates the signal via from adjacent circuits), and predictable, simulatable performance. Used in: mmWave PCB designs, high-speed backplanes, and precision RF interconnects. The trade-off: the coaxial via structure consumes significant PCB area (2.2 mm diameter per signal transition).

PCB via inductance becomes a dominant parasitic at millimeter-wave frequencies, creating impedance discontinuities that cause reflections, reduce gain, and limit bandwidth. Via inductance: a standard through-hole via (0.2-0.3 mm drill, 1.6 mm PCB) has L_via ≈ 0.5-1.0 nH. At 28 GHz: Z = 2×pi×28e9×0.7e-9 = 123 ohms. This 123 ohms in series with a 50-ohm line creates massive mismatch (VSWR > 3:1, return loss < 6 dB). At 77 GHz: Z = 339 ohms (essentially an open circuit). Even short vias have significant inductance: a 0.2 mm via (blind via, connecting adjacent layers) has L ≈ 0.05-0.15 nH. At 28 GHz: Z = 9-26 ohms. At 77 GHz: Z = 24-73 ohms. Still significant compared to 50 ohms. Effects on mmWave circuits: (1) Matching network detuning: the via inductance adds to the matching network elements, shifting the match frequency. For a quarter-wave transformer at 28 GHz: a 0.1 nH via adds 18 ohms in series, detuning the transformer and increasing the return loss. (2) Interconnect loss: the via mismatch reflects power back and forth, creating standing waves in the transmission line. The multiple reflections increase the effective insertion loss beyond the simple mismatch loss. (3) Resonances: the via inductance combined with pad/antipad capacitance forms an LC resonator. Near the via SRF: the insertion loss peaks. The SRF of a typical through-hole via: f_SRF = 1/(2×pi×sqrt(L_via × C_pad)) ≈ 20-50 GHz (depending on geometry). If the SRF falls within the operating band: the circuit performance is severely degraded. Mitigation: (a) Use blind/buried vias (shorter = lower L). (b) Use multiple parallel vias (L_total = L_single/N approximately). (c) Design the via with a controlled antipad size to manage the capacitance and match the via impedance closer to 50 ohms. (d) Avoid vias in the signal path entirely: use edge-launch connectors and horizontal transitions wherever possible.

Category: Millimeter Wave Specific Challenges

Updated: April 2026

Product Tie-In: mmWave Components, Substrates, Packaging

Via Inductance at mmWave

At millimeter-wave frequencies, the via is not just a simple connection between layers; it is a complex electromagnetic structure that must be designed as carefully as any other circuit element.

Common Questions

Frequently Asked Questions

Can I use standard through-hole vias at 28 GHz?

Only with great care: (1) For signal vias: a standard through-hole via in a 1.6 mm board (L ≈ 0.7 nH) is problematic at 28 GHz (Z = 123 ohms). The via must be backdrilled to remove the stub, and the antipad must be optimized by EM simulation. Even with optimization: the insertion loss per via transition is 0.3-0.8 dB at 28 GHz (significant in a system with multiple transitions). (2) For ground and power vias: through-hole ground vias are acceptable (the low impedance of the via is actually beneficial for grounding; you want the ground connection to be as low-impedance as possible). Multiple parallel ground vias: even better (lower total inductance). (3) Best practice at 28 GHz: use blind microvias for all signal transitions. Use through-hole vias only for ground stitching and power connections. Backdrill any through-hole vias that must carry signals.

How many vias in parallel to reduce inductance?

The inductance of N parallel vias: L_total ≈ L_single / N × (1 + (N-1)×M/L_single), where M is the mutual inductance between vias. For vias spaced > 3× their length apart: M is small and L_total ≈ L_single / N. For closely spaced vias (< 1× length): M ≈ 0.3-0.5 × L_single. L_total ≈ L_single / N × (1 + (N-1) × 0.4) = L_single × (0.4 + 0.6/N). For N = 2: L_total ≈ 0.7 × L_single (30% reduction, not 50%). For N = 4: L_total ≈ 0.55 × L_single. For N = 8: L_total ≈ 0.48 × L_single. Diminishing returns above 4-6 parallel vias due to mutual inductance. For ground vias: use 4-6 parallel vias for the best practical inductance reduction.

What about coaxial via structures?

A coaxial via: a signal via surrounded by a ring of ground vias, forming a coaxial transmission line through the PCB stack-up. The ground via ring creates a controlled-impedance environment around the signal via. Design: signal via: centered. Ground vias: 6-8 vias in a ring at a radius that provides the desired impedance: Z = (60/sqrt(epsilon_r)) × ln(R_ring/R_signal). For 50 ohms on FR-4: R_ring / R_signal ≈ 5.5. If the signal via pad = 0.4 mm diameter (R = 0.2 mm): R_ring = 1.1 mm (ground vias at 1.1 mm radius = 2.2 mm diameter ring). The coaxial via provides: 50-ohm impedance through the transition (matched), excellent shielding (the ground ring isolates the signal via from adjacent circuits), and predictable, simulatable performance. Used in: mmWave PCB designs, high-speed backplanes, and precision RF interconnects. The trade-off: the coaxial via structure consumes significant PCB area (2.2 mm diameter per signal transition).

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What is the effect of PCB via inductance on millimeter wave circuit performance?

Via Inductance at mmWave

Frequently Asked Questions

Can I use standard through-hole vias at 28 GHz?

How many vias in parallel to reduce inductance?

What about coaxial via structures?

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