Designing High-Frequency RF Boards: A Practical Guide
Trace impedance, dielectric choice, and via stitching for 5G mmWave front-ends — what actually matters at 28 GHz.
Layer-Up Engineering·4/27/2026At 28 GHz the parasitic inductance of a single via can absorb your entire link budget. Here's how we approach RF stack-up at Layer-Up.
1. Material selection
FR-4 stops being credible above ~5 GHz. For mmWave we standardize on Rogers RO4350B (Dk 3.48, Df 0.0037 at 10 GHz). The tighter Dk tolerance over temperature is what saves you when your customer flies the device from Helsinki to Dubai.
2. Trace geometry
Microstrip is fine to ~20 GHz. Above that, grounded coplanar waveguide (GCPW) with stitched vias every λ/20 is the safer call. We auto-flag any trace narrower than 4 mil for engineering review.
3. Stitching and shielding
The biggest wins come from not being cheap on stitching vias around RF traces. 8 mil pitch is cheap insurance against radiated coupling.
4. Surface finish
ENIG is the default; immersion silver wins on insertion loss but oxidizes if your assembly partner has loose handling discipline. Pick your battle.