PCB back-drilling is a secondary drilling operation that removes the unused portion of a plated through-hole — the via stub — to eliminate signal-reflecting resonances on high-speed channels. It is the standard method for preserving signal integrity on boards operating above 5 Gbps.
Back-drilling (also called controlled-depth drilling) is a secondary drilling operation performed after the PCB has been plated. A drill bit slightly larger than the original hole diameter enters from the side of the board that does not need the via connection and removes the copper barrel down to a target depth. The segment of barrel that remains carries the signal; the segment removed would have been the stub.
The operation differs from standard through-hole drilling in three ways:
A via that connects signal layers 2 and 12 of a 14-layer board does not need to extend through layers 13 and 14. But in a standard plated through-hole, the barrel is continuous from top to bottom. The unused segment is the stub, and at high frequencies it acts as a quarter-wave resonator that reflects signal energy back toward the source.
The resonance condition is: fres = c / (4 × Lstub × √εr)
For a typical FR-4 board, a 1.0 mm stub resonates at ~37.5 GHz. A 2.0 mm stub resonates at ~18.8 GHz. The longer the stub, the lower the frequency where it interferes — which is why back-drilling becomes mandatory above 5 Gbps.
Industry thresholds:
Unlike standard drilling where the backup board controls exit burr, the back-drill sheet's primary job is depth control. It provides a predictable, flat mechanical stop that ensures every drill bit stops at the same Z-height across the panel.
The industry standard is a one-piece, margin-reserved aluminum sheet:
Back-Drill Aluminum Overview — HW600 and HW900 one-piece sheets.
| Attribute | HW600 | HW900 |
|---|---|---|
| Stub CpK | ≥1.33 | ≥1.67 |
| Thickness tolerance | ±0.05 mm | ±0.03 mm |
| Flatness | ≤0.15 mm | ≤0.10 mm |
| Best for | Cost-sensitive, ≤12 Gbps | High-speed, ≥25 Gbps |
For a detailed comparison, see back-drilling principle and stub CpK guide.
Back-drilling is one part of the high-speed PCB production flow. It relates to:
Back-drilling removes the unused portion of a plated through-hole via barrel — the stub — to prevent signal reflections on high-speed channels above 5 Gbps.
A dedicated consumable placed under the board during the back-drill pass. Its primary job is providing a flat mechanical stop for depth control. One-piece margin-reserved aluminum sheets are the standard.
HW600 delivers CpK ≥1.33 for cost-sensitive links up to 12 Gbps. HW900 delivers CpK ≥1.67 with tighter tolerance for high-speed channels above 25 Gbps.
No. Back-drilling is a separate second-pass operation with a larger bit, depth control, and a dedicated entry sheet. Standard drilling uses a backup board and entry board for through-hole formation.
Need to qualify a back-drill sheet for a new high-speed program? Send your stack-up, target stub length and CpK requirement — our engineers will recommend the grade and reserve margin within one business day and ship a sample kit within 5–7 days. Reach Mr. Lucky Xu on WhatsApp or email.