Back-drill stub accuracy depends on entry material, not just machine depth. Dedicated back-drill aluminum sheets (HW600 and HW900) give a stable Z-reference; lubricant or coated aluminum cools and centres the bit. YUESHAN's two grades differ by CpK for stub-length control, holding the tight tolerance 25 Gbps links demand.
When a 25 Gbps link fails its eye diagram, the post-mortem usually points to back-drill stub length. The recommended fix is almost always "tighter depth control." But depth control is not just a machine setting — it is a materials decision that starts with the entry sheet on top of the panel.
This article is a practical guide to the two entry-material categories that determine back-drill stub accuracy: dedicated back-drill aluminum sheets (HW600 / HW900) for Z-reference stability, and lubricant/coated aluminum sheets for resin smear reduction and thermal control. We will cover what each does, when to use which, and how to specify them in your fab documentation.
Key Takeaways
- HW600 (0.16 mm, CpK ≥ 1.33) is the cost-optimized choice for standard back-drill up to 25 Gbps.
- HW900 (0.19 mm, CpK ≥ 1.67) delivers the tighter stub control needed for 56/112 Gbps high-speed digital.
- Lubricant/coated aluminum sheets reduce drill tip temperature by 30–50°C, cutting resin smear by up to 80%.
- One-piece + margin-reserved construction eliminates the bond-line variation of laminated sheets.
- Specifying the right entry material in your Gerber notes is the most cost-effective way to improve back-drill yield.
HW600 vs HW900 — Which Grade for Which Job?
YUESHAN produces two grades of dedicated back-drill aluminum sheet, differentiated by process capability (CpK) for stub length control:
| Parameter | HW600 | HW900 |
|---|---|---|
| Base thickness | 0.16 mm | 0.19 mm |
| Construction | One-piece coated, margin-reserved | One-piece coated, margin-reserved |
| Stub CpK | ≥ 1.33 | ≥ 1.67 |
| Best signal range | ≤ 25 Gbps / PCIe 4.0 | 25–112 Gbps / PCIe 5.0–6.0 |
| Z-reference stability | Good | Excellent |
| Cost position | Cost-optimized | Premium |
The CpK number tells you how reliably the sheet holds stub length within spec across a production lot. CpK ≥ 1.33 means approximately 99.73% of holes will meet stub target under a normal distribution. CpK ≥ 1.67 moves that to 99.994% — the margin needed for 112 Gbps PAM4 where even a single failing lane can scrap the board.
When to choose HW600: Standard back-drill applications where cost is a primary consideration. Typical use cases include automotive radar PCBs, 10G communications boards, and general server motherboards running PCIe 3.0/4.0.
When to choose HW900: High-speed digital designs where signal integrity margin is critical. 5G massive-MIMO, 112 Gbps switch/routing platforms, AI accelerator boards, and any design where stub failure would cause an SI compliance failure. If your fab is quoting ±0.1 mm tolerance and you need ≤ 0.10 mm residual stub, HW900 is the spec to write.
For the detailed spec sheet, sizes, and packaging, see our back-drill aluminum sheet product page.
One-Piece + Margin-Reserved — Why It Matters for Stub Accuracy
Both HW600 and HW900 use one-piece + margin-reserved construction. Here is why that matters:
In a laminated entry sheet (two layers bonded together), the bond line creates a thickness step — the two layers are never perfectly matched, so the Z-zero reference drifts as the bit moves across the seam. The bonded edge can also lift, burr, or delaminate, especially under the heat of high-speed drilling.
A one-piece sheet removes the seam entirely. The coating and aluminum base form a single continuous structure — no glue line, no step, no edge delamination. The reserved margin is the uncoated border around the working area that handles registration and prevents edge damage from reaching the drill zone.
The result: the drill bit meets the same consistent entry plane at every hole, across every panel. This consistency is what turns a theoretical "back-drill tolerance" into a measured, repeatable number.
For a deeper comparison of one-piece vs laminated construction, see Back-Drill Depth Control: Tolerance, Margin & One-Piece Design.
Lubricant Aluminum for Resin Smear Control
Back-drilling is a high-heat process. The drill bit enters the panel a second time, cutting through cured resin and glass at a controlled depth. Because the hole is already partially drilled, heat dissipation is poorer than first-pass drilling. Drill tip temperatures can exceed 200°C — enough to soften the epoxy resin system.
When softened resin spreads across exposed inner copper layers, it creates resin smear. In severe cases, smear can bridge between adjacent copper planes, creating intermittent shorts. In mild cases, it degrades the CAF (conductive anodic filament) resistance and long-term reliability of the board.
A lubricant / coated aluminum entry sheet addresses this through three mechanisms:
| Mechanism | How It Works | Benefit |
|---|---|---|
| Thermal barrier | The thermoset coating absorbs and dissipates heat before it reaches the drill tip | Drill tip temperature drops 30–50°C |
| Lubricated cutting | Water-soluble or thermoset lubricant reduces friction at the cutting edge | Lower axial force, less heat generation |
| Chip evacuation | Lubricant helps flush debris from the hole during retraction | Cleaner hole wall, less embedded debris |
In a production comparison on 20-layer 5G infrastructure boards, switching from a plain 1100 H18 entry board to a coated lubricant aluminum sheet reduced:
- Resin-smear-related scrap from 18% to 2.8%
- Average drill tip temperature from 215°C to 178°C (measured by embedded thermocouple)
- Microsection reject rate by 74%
For full specifications and available grades, see our lubricant aluminum entry board page.
Pairing Entry Materials for First-Pass and Back-Drill
A well-designed drilling stack uses two different entry materials — one optimized for first-pass drilling (standard 1100 H18 or lubricant aluminum) and one optimized for the back-drill pass (HW600 or HW900 back-drill sheet).
Common pairing strategies:
| Application | First-Pass Entry | Back-Drill Entry | Backup Board |
|---|---|---|---|
| Standard server (≤ 25G) | 1100 H18 plain aluminum | HW600 | HDF or UV Melamine |
| High-speed digital (56G+) | Lubricant aluminum | HW900 | UV Melamine 880/950 |
| 5G / RF infrastructure | Lubricant aluminum | HW900 | UV Melamine 950 |
| HDI / IC substrate | Lubricant aluminum (thin) | HW600 | UV Melamine 840 |
This two-tier approach optimizes each drilling pass independently. The first-pass material minimizes burr and bit wander. The back-drill material maximizes Z-reference stability and stub control. For more on backup board pairing, see our UV Melamine backup board page.
Specifying Entry Materials in Your Fab Documentation
Here is a template you can adapt for your next fabrication drawing or EQ communication:
BACK-DRILL ENTRY MATERIAL SPECIFICATION
- Dedicated back-drill aluminum sheet, one-piece coated, margin-reserved
- Grade: HW600 (standard back-drill) or HW900 (tight-tolerance / high-speed)
- Thickness: 0.16 mm (HW600) / 0.19 mm (HW900)
- Stub CpK requirement: ≥ 1.33 (HW600) / ≥ 1.67 (HW900)
- Lubricant entry (first pass, if applicable): Water-soluble coated aluminum
- TDS/MSDS to be provided with each lot
Adding this to your fabrication notes gives the fab a clear, measurable spec to work to. It also signals that you understand the material side of back-drill performance — which changes the conversation from "can you relax the tolerance?" to "can you meet this material spec?"
For technical support or help writing your spec, contact Mr. Lucky Xu with your stack-up details and target data rate.
FAQ
What is the difference between HW600 and HW900?
HW600 (0.16 mm) delivers Stub CpK ≥ 1.33 for standard back-drill up to 25 Gbps. HW900 (0.19 mm) delivers CpK ≥ 1.67 for tight-tolerance back-drill at 56–112 Gbps. Both use one-piece margin-reserved construction.
Can I use a lubricant aluminum sheet as a back-drill entry?
Lubricant aluminum is primarily designed for first-pass microvia drilling. For the back-drill pass, a dedicated back-drill sheet (HW600/HW900) provides better Z-reference stability. However, lubricant aluminum on the first pass complements the back-drill sheet on the reverse pass — each optimized for its role.
How do I know which grade my line needs?
Match the grade to your fastest serial data rate: HW600 for ≤ 25 Gbps standard back-drill; HW900 for 25G–112 Gbps where CpK ≥ 1.67 is required. When in doubt, request both samples and run a microsection comparison on production panels.
Is one-piece construction always better than laminated?
For back-drill entry where Z-reference stability drives stub accuracy, yes. The one-piece design eliminates the bond-line thickness step and edge delamination risk that add variation in laminated sheets.
Can I get a sample to test on my line?
Yes. Free HW600 and HW900 samples are available with full TDS/MSDS, shipped worldwide within 5–7 days. Contact Mr. Lucky Xu to arrange a side-by-side comparison against your current entry material.
Related Guides
- Why Your Back-Drilled PCB Fails SI — The Material Truth — the full picture of entry material impact on back-drill tolerance.
- Aluminum vs Phenolic Entry Board for Back-Drilling — material comparison for back-drill entry.
- Lubricant Aluminum Entry Board — reduce resin smear with coated entry sheets.
- Back-Drilling Principle Guide — how stub CpK is measured and controlled.