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.
PCB back-drilling (also called controlled-depth drilling) is a secondary drilling operation performed after the PCB has been plated. A slightly oversize bit enters from the side of the board that does not need the via connection and removes the unused copper barrel — the via stub — down to a target depth. The stub acts as a quarter-wave resonator that reflects signal energy back toward the source, which is why back-drilling becomes mandatory above 5 Gbps and is the standard method for preserving signal integrity on high-speed channels. Depth control is the critical parameter: over-drill kills the connection, under-drill leaves too much stub, so the back-drill entry sheet's primary job is a predictable, flat mechanical stop — the industry standard being a one-piece, margin-reserved aluminum sheet. The one-piece back-drill aluminum sheet is the entry-side consumable behind that mechanical stop, and 5G and server back-drilling is where the requirement bites first.
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-drill entry sheet specifications are documented per grade: base material, coating weight, thickness tolerance and moisture class. The product datasheet lists qualified diameter windows and stack counts; MOQ is normally one pallet per grade, and sheet thickness options are documented alongside each grade rather than mixed across them. Request the specification sheet together with the price so quotation and qualification reference the same document set.
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.
Knowing what back-drilling is does not tell you what to put on the drawing. The antipad on every plane the tool passes, the keepout nobody draws, the depth reference surface and the stub budget are all fixed before the panel reaches the drill — that set is in the back-drill design rules. If the open question is which entry sheet to run on the second pass, the back-drill aluminum sheet range covers the specification.