CCL Kraft Paper: Core Role – Stability & Layer Adhesion

What is CCL and its layered structure?
CCL (Copper Clad Laminate) is a PCB raw material made by bonding copper foil to an insulating substrate with adhesive. Its core layered structure (from surface to core) is: copper foil → adhesive → kraft paper substrate/reinforcement layer → adhesive → copper foil. In multilayer CCL, 1–5 layers of kraft paper can be stacked. This layered structure requires tight adhesion and zero deformation; otherwise, PCB traces may misalign. Kraft paper, as the core or reinforcement layer, plays a critical role in structural stability.

Core role of kraft paper in CCL: the structural backbone
In CCL, kraft paper mainly serves as the core substrate or reinforcement layer, providing three key benefits:

• Structural support: Offers a rigid backbone to prevent warping or deformation after lamination.
• Even pressure distribution: Spreads pressure during lamination to ensure tight adhesion between copper foil and adhesive, minimizing air bubbles.
• Insulation enhancement: Works with adhesive to improve overall insulation, preventing interlayer leakage.

The 3 Core Benefits: Stability & Strong Layer Adhesion

Improve CCL Lamination Precision and Reduce Production Defects

• Kraft paper made from pure wood pulp maintains a thickness tolerance of ≤±1 μm, ensuring uniform layer thickness during lamination. This allows the total CCL thickness tolerance to stay within ±0.03 mm, compared to the industry average of ±0.05 mm.
• Its structural stability prevents local shrinkage during lamination, reducing copper foil and substrate misalignment. As a result, trace alignment precision improves by 20%, lowering the risk of short circuits during PCB etching.

Enhance CCL Processing Tolerance for Complex Operations

• CCL often undergoes drilling, cutting, and etching. A stable kraft paper substrate can:
  • Resist tearing during drilling, thanks to tightly interwoven long fibers. Drill walls remain smooth and burr-free, achieving a 99.5% hole quality rate (compared with 97% for standard kraft paper CCL).
  • Cut cleanly without delamination or debris, preventing separation of copper foil and substrate, and improving processing efficiency by 15%.

Ensure Long-Term CCL Reliability and Extend PCB Lifespan

• Environmental adaptability: In humidity ranges of 20%-80% RH and temperatures from -40 to 85 °C, pure wood pulp kraft paper CCL retains ≥90% interlayer bonding strength (standard kraft paper CCL ≤75%), preventing delamination over time.
• Stable insulation: Dense structure reduces moisture penetration, keeping CCL insulation resistance above 10¹² Ω long-term, eliminating PCB leakage risks, and extending electronic device lifespan—for example, increasing home appliance PCB lifespan from 8 to 12 years.

Specifications: Grammage Of Boosts CCL Stability

Specification	Parameter Range	Application Scenario
Basis Weight	80 g/㎡, 100 g/㎡, 120 g/㎡, 150 g/㎡	80–100 g/㎡: Thin CCL (e.g., consumer electronics PCBs); 120–150 g/㎡: Thick CCL (e.g., industrial control PCBs)
Size	Roll: width 1220 mm / 1530 mm, length 500–1000 m; Sheet: 1220×1530 mm / 1020×1220 mm	Roll: suitable for automated CCL production lines; Sheet: for small-batch customized CCL production
Color	Natural wood (brown-yellow)	For CCL applications without special appearance needs; natural wood color is free of dyes, preventing impurities that affect insulation performance
Other Key Parameters	Tensile strength: ≥3.5 kN/m (MD), ≥2.0 kN/m (CD); Thickness tolerance: ≤±1 μm; Resin uptake: 18–22 g/㎡	Meets CCL lamination requirements for strength, precision, and resin bonding

Material Comparison: Boosts CCL Stability & Adhesion

Comparison Dimension	Pure Wood Pulp CCL Kraft Paper	Regular Kraft Paper (Mixed Pulp)	Recycled Kraft Paper
Fiber Quality	Long conifer fibers (2.8–4.2 mm), purity 99.8%+	Hardwood short fibers (1.2–2.0 mm) + small amount of softwood pulp	Short fibers (1.0–1.8 mm), with impurities (plastic/ink)
Tensile Strength	MD ≥3.5 kN/m, CD ≥2.0 kN/m	MD ≤2.5 kN/m, CD ≤1.5 kN/m	MD ≤2.0 kN/m, CD ≤1.2 kN/m
Thickness Tolerance	≤±1 µm	±3 µm	±5 µm
Moisture Deformation Rate (24h soak)	≤0.1%	≥0.3%	≥0.5%
High-Temperature Shrinkage (150℃ / 1h)	≤0.05%	≥0.15%	≥0.25%
CCL Compatibility	Perfect fit, no warping/bubbles	Tends to cause warping, bubble rate 1.2%	Not suitable, CCL defect rate exceeds 5%

Stability & Adhesion: Controlled in Key Processes

Structural stability is the defining property of kraft paper for CCL, as it directly affects lamination accuracy. This stability is achieved through strict control across three stages—raw material selection, papermaking process, and resin impregnation.

Raw Material Selection: Pure Wood Pulp as the Foundation

• Main Material: 100% conifer pulp (70%+ pine, 30% spruce), with no mixed or recycled pulp.
• Why It Matters: Conifer fibers are 2.8–4.2 mm long—twice that of hardwood pulp—interweaving into a dense, high-purity (ash ≤0.05%) network. This three-dimensional mesh resists breakage and prevents structural looseness.
• Excluded Materials: Hardwood pulp (short fibers prone to deformation) and recycled pulp (impurities and weak fibers) reduce stability and are not used.

Papermaking Process: Precision Control for Dense Structure
By fine-tuning refining, forming, and drying parameters, kraft paper achieves uniform fiber distribution and tight structure:

Step	Controlled Parameter	Stability Principle	Advantage Over Standard Process
Refining	28–35°SR (low)	Preserves long fibers with minimal cutting, fewer weak points	Standard 40–50°SR shortens fibers, weakens structure
Forming	Long-wire machine, vacuum –0.07 MPa	Even dewatering, uniform fiber layout, thickness tolerance ≤±1 μm	Standard forming ±3 μm, uneven density
Drying	80–110℃ gradient for 20 min	Gentle drying ensures even shrinkage, avoids deformation	Fast 120–140℃ drying causes fiber brittleness

Resin Impregnation: Reinforcing the Structure
Phenolic resin impregnation (solid content 45–50%) further locks in stability:

• Resin Load: 18–22 g/m² (dry), penetrating fiber pores and forming a rigid resin network.
• Curing Parameters: 130–140℃ for 30 min, ensuring full crosslinking and preventing shrinkage during CCL lamination.
• Core Effect: After impregnation, tensile strength rises by 40% (≥3.5 kN/m), while moisture deformation drops to ≤0.1%, doubling overall structural stability.