Signal integrity starts at the drill
On a 25+ Gbps channel, an uncontrolled stub is a reflection. The back-drill removes the unused via barrel, but only if the back-drill sheet is flat, consistent and reserved on margin so the bit stops exactly where it should.
Three things break SI on 5G and server boards:
- Stub variation — a soft or uneven sheet lets the bit dive past target.
- Location drift on the fine BGA vias feeding the channel.
- Exit burr that later resonates or fails continuity test.
The high-speed combination
- Back-drill aluminum HW900 — one-piece, margin-reserved, stub CpK ≥1.67. The default for server and base-station channels.
- Lubricant aluminum — coated first-entry keeps the BGA microvias located before the back-drill pass.
- UV Melamine 950 — the backup behind 12–24 L RF stacks, holding exit burr low; our recommended default.
Grade matrix
| Board |
Grade |
Role |
CpK / burr |
| Back-drill |
HW900 |
Stub control |
CpK ≥1.67 |
| Back-drill |
HW600 |
Cost-sensitive links |
CpK ≥1.33 |
| Entry |
Coated Al |
BGA location |
— (entry) |
| Backup |
UV Melamine 950 |
12–24 L RF |
≤0.05 mm (default) |
Services for signal-critical programs
Stub capability reports (CpK) ship with samples. A side-by-side against your current back-drill grade is provided on request. Lots are traceable for audit.
📖 Further reading: Back-Drilling Material Selection Guide · Back-Drill Depth Control: Tolerance, Margin & One-Piece Design · Why Back-Drilling? Stub Removal for 25G/56G SerDes · Glossary: stub definition →
FAQ
Q: HW600 or HW900 for a 25 Gbps link?
A: HW900 for the specified channel; HW600 where margin is looser and cost is driving.
Q: Does coated entry help back-drilling?
A: Indirectly — it locates the BGA vias so the back-drill hits the same centre.
Q: Can you report stub CpK?
A: Yes. HW900 is qualified to CpK ≥1.67 and the report ships with the sample lot.
