High-Speed Vias: Why Solid Ties Beat Thermal Reliefs
- Francesco Poderico
- 7 days ago
- 2 min read
Thermal-relief “spokes” help solderability, but they add inductance and cut slots in your reference plane—exactly where high-frequency return current needs a short, wide path. For HDMI, PCIe, and other multi-GHz links, prefer solid connections to ground on anything that carries RF return (stitching vias, connector grounds, shields, EP/PowerPAD vias). Keep thermals only where assembly truly demands them.
Why I avoid thermals on RF returns
I don’t skip thermals because I’m lazy; I skip them because I’ve seen the surprises they cause below 1 GHz and well into the GHz range. A thermal relief connects a pad to a plane with a few narrow spokes. Those spokes add series inductance, and the clearance ring carves a slot in the plane. Measured/modelled work puts a typical thermal’s inductance in the tens to a few hundred picohenries; tiny in DC terms, but at GHz it becomes ohms of reactance. Example: 200 pH at 3 GHz ≈ 3.77 Ω; 0.1 nH at 6 GHz ≈ 3.77 Ω; 1 nH at 1/3/6 GHz ≈ 6.28/18.85/37.70 Ω—enough to force return current to detour and radiate.
High-frequency impedance is dominated by XL=2πfL, not DC resistance. Shrink L, and the return current hugs the plane right under your trace.
The slot-resonance trap (it’s not just vias)
Thermals don’t only choke vias; they also create short slots that can excite slot-wave resonances in launch/connector pads (think SMA/HDMI shells), producing the classic “mystery notch” in IL/RL. Best practice: avoid thermals in launch/return pads, or make the slot so short its resonance sits well above your signal bandwidth.
Why this really matters at HDMI/PCIe speeds
HDMI 2.0 : up to 6 Gb/s per channel (18 Gb/s aggregate).
HDMI 2.1 : up to 12 Gb/s per lane (48 Gb/s aggregate).
PCIe: Gen-4 16 GT/s, Gen-5 32 GT/s, Gen-6 64 GT/s (PAM4). Those lane rates push strong energy into the low/mid-GHz region where thermal-relief inductance hurts most.
When to use solid ties (no thermals)
Use solid connections anywhere the copper’s job is RF return, not “thermal isolation”:
Stitching vias at layer changes. Place two GND stitch vias flanking each differential-pair via pair (≈0.3–1.0 mm away) so the reference hand-off is tight and low-inductance.
Connector grounds & shells. Keep HDMI receptacle grounds, SMA grounds, and shield pads solid-tied to planes to prevent slot modes.
EP/PowerPAD via arrays. Vendors explicitly recommend solid ties to ground planes (no thermal webs) for thermal and EMC performance.
Via fences / ground frames. Close-spaced, solid-tied fences help contain fringing fields and provide many parallel return paths.
DFM note: Most power/ground vias do not need thermals—there’s no pin to solder in them. Use thermals only where a soldered component lead/pad would otherwise be starved of heat.
References & further reading
Thermal-relief impedance (DC→800 MHz): Boesman et al., Modeling the Low- and High-Frequency Impedance of Thermal Reliefs. (KULeuven Lirias; IMEC listing). Linias
Slot-mode resonances & thermals in launch pads: Eric Bogatin, Signal Integrity Journal. signalintegrityjournal.com
Stitching-via guidance & return hand-off: Altium docs/resources. Altium
DFM: vias usually don’t need thermals: Cadence PCB blog. resources.pcb.cadence.com
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