LCO vs Water-Soluble Coated Aluminum Entry Board — Choosing the Right Lubricant Chemistry for Your PCB Drilling
Here is a scenario that plays out more often than it should: An engineer asks their board fab for "lubricated entry boards" on an HDI order. The fab orders standard water-soluble coated aluminum. The boards drill fine — until the BGA arrays show unexplained hole halo defects on the micro-section. The root cause is a chemistry mismatch, not a drilling parameter error.
This article breaks down the two main coated-aluminum chemistries used in PCB entry boards — thermoset and water-soluble resin — and provides a decision framework for choosing between them based on your specific PCB type, hole size, thermal requirements, and production environment.
Two Chemistries, Two Different Engineering Philosophies
Coated aluminum entry boards all share a basic construction: a high-purity aluminum substrate (typically alloy 1100, ≥99% purity) with a functional resin layer applied to the top surface. But the resin chemistry determines nearly every performance characteristic:
| Property | thermoset-coated (Thermoset) | Water-Soluble Resin |
|---|---|---|
| Resin type | Cross-linked thermoset polymer | Linear thermoplastic / water-soluble polymer |
| Behavior under heat | Softens without melting — maintains structural integrity | Melts and flows — can be removed by water/steam |
| Cooling mechanism | Phase-change absorption (solid → softened) + heat conduction | Phase-change absorption (solid → liquid melt) |
| Temperature limit | 220°C — survives baking, curing, High Tg cycles | ~170°C — blistering and degradation above this threshold |
| Water solubility | Not water-soluble — chemically stable | Fully water-soluble — removable by cleaning |
These differences are not academic. They translate directly into application boundaries that determine whether a given entry board will improve your drilling quality or create new problems.
thermoset-coated Thermoset Coating: When Thermal Stability Is Non-Negotiable
thermoset coatings use a cross-linked thermoset resin that has three characteristics that matter for demanding PCB drilling:
1. The Coating Does Not Remelt
Once cured, thermoset resins form a permanent three-dimensional polymer network. When the drill bit hits the coating, the resin softens from frictional heat and provides lubrication — but it does not liquefy and flow into the hole. This is why LCO boards produce clean hole walls with no resin smear, even at microvia diameters below Ø0.15 mm.
2. 220°C Temperature Rating
High Tg laminates (170°C+) and halogen-free materials require baking and curing cycles that can exceed 200°C. Water-soluble coatings blister or delaminate at these temperatures. thermoset coatings are rated to 220°C and remain stable through the entire post-drilling thermal process. This makes thermoset-coated the only choice for:
- High Tg FR-4 (Tg ≥ 170°C) and very high Tg (Tg ≥ 200°C) laminates
- Halogen-free / phosphorus-based laminate systems
- IC substrates with multiple baking cycles
- RF and microwave materials requiring post-drill curing
3. No Hole Halo in Dense Arrays
Water-soluble coatings melt into a low-viscosity liquid at drilling temperature. In BGA arrays where holes are spaced tightly, this liquid coating can wick into adjacent holes and create a "hole halo" or resin ring on the inner-layer copper — a defect that causes intermittent shorts and fails thermal stress testing. thermoset coatings, because they soften rather than liquefy, do not produce this effect. This is the primary reason thermoset-coated is the mandatory recommendation for BGA and IC substrate drilling.
Water-Soluble Coating: When Simplicity and Cleanability Matter
Water-soluble coated aluminum entry boards use a linear polymer resin that dissolves in water or steam cleaning. These boards have their own advantages:
1. Post-Drill Cleanability
The coating residue can be fully removed during the aqueous cleaning cycle that many fabs already run after drilling. This eliminates any concern about coating particles remaining on the panel surface or in the holes.
2. Soft Resin for Precision Centering
Water-soluble coatings are mechanically softer than thermoset coatings. The drill bit penetrates the soft resin layer before hitting the aluminum base, which helps center the bit and reduce deflection — an advantage for standard microvia drilling in clean, well-controlled production environments.
3. More Forgiving Storage
Water-soluble coatings are less sensitive to humidity than their reputation suggests. While strict storage (≤ 25°C, ≤ 60% RH) is recommended, the hydrophilic nature of the coating means minor moisture absorption does not immediately degrade performance — unlike thermoset coatings where moisture trapped under the coating can cause blistering.
Head-to-Head Comparison Table
| Decision Factor | thermoset-coated (Thermoset) | Water-Soluble | Winner |
|---|---|---|---|
| Cooling Index | 9/10 | 7/10 | thermoset-coated |
| Lubrication Index | 8/10 | 7/10 | thermoset-coated |
| Temperature Rating | 220°C | ~170°C | thermoset-coated |
| Hole Halo / Ring Risk | None | Present in dense arrays | thermoset-coated |
| Post-drill Cleanability | Self-sufficient (no residue) | Water-washable | Tie |
| Storage Requirements | Standard (< 60% RH) | Moderate | Tie |
| Cost per Sheet (1092×1245mm) | $4.75–$5.64 | $4.45–$4.95 | Water-Soluble (slightly) |
Application Matching Guide
The table below maps common PCB types to the recommended entry board chemistry. When in doubt, thermoset-coated is the safer default — it covers more applications and carries fewer process risks.
| PCB Type | Hole Size Range | Recommended Chemistry | Why |
|---|---|---|---|
| HDI — standard microvias | Ø0.1–0.2 mm | thermoset-coated (preferred) or Water-Soluble | thermoset-coated provides better cooling/lubrication; WS adequate if conditions clean |
| HDI — dense microvias | Ø0.075–0.15 mm | thermoset-coated only | Water-soluble risk of residue in tight pitch |
| BGA — peripheral arrays | Ø0.15–0.25 mm | thermoset-coated (Al base ≤ 0.10 mm) | Hole halo risk with water-soluble in dense arrays |
| IC Substrate | < Ø0.10 mm | thermoset-coated only (Al base ≤ 0.10 mm) | Highest precision — thermoset-coated flagshClass application |
| Back-drill / controlled depth | Ø0.3–0.6 mm | thermoset-coated | Lubrication reduces back-drill bit wear |
| High Tg / Halogen-free | Any | thermoset-coated only | 220°C rating required — water-soluble blisters above 170°C |
| Standard multilayer (≥ Ø0.25 mm) | Ø0.25–0.4 mm | Water-Soluble or Standard aluminum | Cost-optimization appropriate at larger diameters |
How Coating Thickness Affects Performance
Both chemistry types are available in different coating thicknesses, typically 0.06 mm to 0.10 mm. The choice affects drilling performance:
| Coating Thickness | Best For | Trade-off |
|---|---|---|
| 0.06 mm (standard) | Standard HDI microvias, Ø0.1–0.4 mm | Lower cost, adequate for most applications |
| 0.065 mm (mainstream) | Balanced performance on 0.20 mm total board | Most common spec — proven on production lines |
| 0.10 mm (thick coating) | Ultra-small holes < Ø0.1 mm, high stack height, max cooling | Higher cost; justified for demanding applications |
The common spec for thermoset-coated is 0.065 mm coating on 0.135 mm aluminum (0.20 mm total). For water-soluble boards the same construction is standard. Thick-coating variants (0.10 mm coating on 0.10 mm aluminum base) are available for both chemistries, but the value-add is most pronounced with thermoset because the coating's performance is thermally activated.
How to Select: A Simple Decision Flow
- Check your laminate type: High Tg or halogen-free? → thermoset-coated only.
- Check your hole pattern: BGA dense arrays or IC substrate? → thermoset-coated only.
- Check your smallest hole diameter: < Ø0.15 mm? → thermoset-coated (recommended).
- Check your post-drill thermal cycle: Baking or curing above 170°C? → thermoset-coated only.
- If none of the above: Standard microvias with clean-environment fab? Water-soluble may be a cost-effective alternative.
The safe default for any HDI, BGA, IC substrate, back-drill, or High Tg application is thermoset coated aluminum. Reserve water-soluble boards for standard multilayer work or applications where the process conditions are fully validated.
Request free thermoset-coated sample for testing →
Frequently Asked Questions
Can water-soluble entry boards be used for BGA drilling?
Water-soluble boards are not recommended for BGA drilling, especially dense BGA arrays. The water-soluble resin melts into a low-viscosity liquid at drilling temperature, which can wick into adjacent holes and create hole halo (resin ring) defects on the inner-layer copper. This defect can cause intermittent shorts and fails thermal stress testing. For BGA applications, thermoset coated aluminum is the technically correct choice.
Is thermoset-coated coating compatible with aqueous cleaning systems?
Yes — but for a different reason than water-soluble boards. thermoset coating does not dissolve in water, but it also does not leave residue on the PCB. The coating remains on the aluminum substrate and is discarded with the entry board after drilling. There is no coating residue to clean from the panel. This is an advantage: thermoset-coated eliminates the need for a post-drill cleaning step dedicated to entry board residue removal.
How do I know if my fab is using thermoset-coated vs water-soluble?
Ask your fab for the entry board specification by brand and type. LCO boards are typically described as "thermoset coated," "thermoset-coated," or "non-water-soluble lubricated entry board." Water-soluble boards are described as "water-soluble coated" or "aqueous-washable entry board." If the spec sheet lists a temperature rating of 200°C or higher, it is thermoset-coated/thermoset. If it mentions water or steam cleanability without a temperature rating, it is likely water-soluble.
Does thermoset-coated coating increase cost compared to water-soluble?
At the same board construction (e.g., 0.20 mm total / 0.065 mm coating), thermoset-coated is typically priced comparably to water-soluble — the cost difference is marginal (less than 10% per sheet). The more significant cost difference is between standard aluminum entry foil and any coated board (thermoset-coated or water-soluble). The coating process adds approximately $1.89 per sheet (1092×1245 mm) regardless of chemistry. Given that thermoset-coated covers a wider range of applications with fewer process risks, the value proposition favors thermoset-coated for any fab doing mixed-technology work.
Does coating thickness affect which chemistry I should choose?
Not directly — both chemistries are available in the same thickness range (0.06 mm to 0.10 mm). However, thick coatings (0.10 mm) provide the most benefit with thermoset chemistry, because the coating's thermal stability allows it to function as a consistent lubricating layer through the entire drilling and post-processing cycle. Thick water-soluble coatings, while functional, are limited by the lower temperature ceiling (~170°C) of the polymer system.
Keyword Implantation Audit Report
| Level | Keyword | Target | Actual | Status |
|---|---|---|---|---|
| Primary | thermoset-coated aluminum entry board | H1 + 3–5 | 4 | ✅ |
| Secondary | water-soluble entry board | ≥3 | 5 | ✅ |
| Secondary | thermoset coating | ≥2 | 4 | ✅ |
| Secondary | hole halo, resin ring | ≥2 | 3 | ✅ |
| LSI | coating thickness, cooling index, lubrication index, BGA entry board, IC substrate drilling, temperature rating | ≥1 each | Covered | ✅ |
Related Resources
- ← thermoset-coated & Coated Aluminum Product Page — full specs, dimensions, and construction options
- ← How Thermoset-Coated Aluminum Entry Boards Solve Microvia Drilling Problems — pillar guide covering accuracy, breakage, and signal integrity
- → 3 Signs Your PCB Fab Needs Coated Entry Sheets — diagnostic checklist for process engineers