Copper-clad laminate, commonly abbreviated as CCL in the industry, is one of the most essential structural base materials in the modern electronics industry. As the name implies, it is produced by bonding electrolytic copper foil to one or both sides of an insulating substrate (such as glass fiber cloth, polyimide film, ceramic composites, or metal core layers) through a high-temperature, high-pressure lamination process, resulting in a composite sheet material where the conductive and insulating layers work in tandem.
Concept and Classification of Copper Clad Laminates
Copper-clad laminates are widely used across multiple end-use sectors. We manufacture these sheet-shaped materials by taking petroleum wood pulp paper or glass fiber fabric as reinforcing materials, soaking the reinforcements in resin, covering one or two sides with copper foil, and carrying out hot pressing. As a core material for manufacturing printed circuit boards (PCBs), copper-clad laminates primarily serve functions such as conductivity, insulation, and structural support. These materials greatly affect circuit signal transmission speed, energy loss and electrical resistance. Manufacturers widely adopt them for many terminal fields such as consumer electronics, computers, communications and automotive electronics.
Based on mechanical properties, copper-clad laminates can be classified into rigid and flexible types.
- We produce rigid copper-clad laminates with strong bending resistance, stable hardness and good toughness. We categorize these laminates into glass fiber substrates, paper substrates, composite substrates and metal substrates, and FR-4 stands as our most mainstream product at present.
- Flexible copper-clad laminates are manufactured using flexible reinforcing materials coated with electrolytic copper foil or rolled copper foil. Their bendable nature facilitates the assembly of electrical components.
We classify copper-clad laminates into conventional rigid grades (standard FR-4, lead-free compatible variants, halogen-free lead-free boards, and more), high-speed substrates, automotive-grade laminates and IC packaging substrates according to their environmental properties and practical application scenarios.
Performance Metrics of Copper-Clad Laminates
The performance metrics of copper-clad laminates cover multiple dimensions, including physical, chemical, electrical, and environmental properties. We regard electrical performance as the core indicator for these materials. Meanwhile, we must also attach great importance to flex resistance life and temperature resistance to fit the unique service conditions of FCCL products.
In terms of electrical performance, FCCL has requirements similar to those of rigid copper-clad laminates:
The telecommunications sector requires a low dielectric constant (Dk < 3.0) to reduce signal delay, while data centers require a low dielectric loss factor (Df < 0.005) to minimize signal loss. We need to guarantee steady electrical performance after bending owing to the inherent properties of FCCL flexible substrates. We must limit the dielectric constant variation rate to less than 5% following 10,000 bending cycles.
In terms of physical performance, bend life is a core competitive advantage of FCCL:
FCCL for wearable devices must withstand more than 100,000 bends (at a 5 mm radius), while that for automotive electronics must withstand more than 50,000 bends (at a 10 mm radius). We control the thickness tolerance of FCCL within ±5%, a stricter standard compared with the ±10% thickness tolerance we apply to rigid copper-clad laminates.
In terms of environmental performance, FCCLs face stricter temperature resistance requirements: standard FCCLs must withstand temperature cycling from -55°C to 125°C, those used in automotive engine compartments must withstand -40°C to 150°C, and those used in medical devices must even withstand high-temperature sterilization at 180°C.
Performance requirements for FCCL vary significantly across different sectors: foldable smartphones prioritize bend life and thin profiles; new energy vehicles emphasize resistance to high and low temperatures as well as vibration; and medical devices must balance biocompatibility with high-temperature resistance.
Core Functional Properties of CCL
Functional Property 1: Electrical Conductivity (Copper Foil Layer)
Copper foil serves as the conductive layer of CCL, providing a low-impedance current path for signal traces and power/ground planes on the PCB. The thickness of the copper foil determines its resistivity and current-carrying capacity: 12-μm copper foil (commonly used for fine circuits on HDI boards) has relatively limited current-carrying capacity, while 105-μm-thick copper foil (used for high-power power supply boards) can carry currents of tens of amperes.
The surface condition of the copper foil is critical to high-frequency performance. In high-frequency circuits (signal frequency > 1 GHz), current tends to flow within an extremely thin “skin depth” range on the conductor’s surface; the skin depth decreases as frequency increases (the skin depth of copper at 10 GHz is approximately 0.66 μm). If the surface roughness (Rz) of the copper foil is significantly greater than the skin depth, the current path will be greatly extended due to the undulations on the copper foil surface, resulting in additional copper loss (conductor loss) and causing signal attenuation to exceed acceptable limits. This is why M7/M8/M9-grade high-speed CCLs must use ultra-low-profile copper foil with a surface roughness of Rz < 2 μm (HVLP).
Functional Property 2: Electrical Insulation (Base Layer)
The base layer (glass cloth + cured resin, or PI film) provides high-quality electrical insulation between the different layers of the CCL and between the CCL and the external environment. For high-speed signals, the most critical base layer parameter is:
- Dielectric constant Dk (Dielectric Constant / Relative Permittivity): This reflects the degree of polarization of the substrate in an electromagnetic field and determines the propagation speed of electromagnetic waves within the substrate (speed = c / √Dk). The lower the Dk, the faster the signal propagation speed and the lower the transmission delay. The Dk of standard FR-4 is approximately 4.2–4.5 (at 1 GHz), while that of M7-grade CCL is approximately 3.3–3.5, and the Dk of PTFE-based high-frequency CCL can be as low as 2.2.
- Dielectric loss factor Df (Dissipation Factor / Loss Tangent): This reflects the proportion of energy lost by the substrate during each cycle of the electromagnetic field and directly determines the degree of signal attenuation as it passes through the PCB. The lower the Df, the lower the signal transmission loss. For AI server backplanes (where signal transmission distances can reach 50–100 cm), a reduction in Df from 0.020 (FR-4) to 0.003 (M7) means signals can travel longer distances without requiring additional relay amplification. This is the fundamental physical reason why M6/M7-grade CCLs are revolutionary for high-speed digital system design.
- Moisture Absorption: The amount of water absorbed by the substrate in humid environments causes Dk/Df to degrade; CCLs with high moisture absorption rates exhibit significantly poorer performance when operated in high-humidity environments. Low moisture absorption is one of the key evaluation criteria for high-end CCLs.
Functional Attribute 3: Mechanical Support
As the backbone of the entire PCB, copper-clad laminate must maintain dimensional stability and structural integrity under mechanical and environmental challenges such as stress, vibration, and thermal shock. Key mechanical parameters include:
- Glass Transition Temperature (Tg): The critical temperature at which the resin matrix transitions from a glassy state to a rubbery state; it is also the inflection point at which the mechanical properties of the CCL decline sharply. The Tg of standard FR-4 is approximately 135°C. High-Tg FR-4 can reach 170°C, while automotive-grade CCL requires a Tg higher than 170°C to withstand the high temperatures in the engine compartment.
- Coefficient of Thermal Expansion (CTE): The CTE in the Z-axis direction is typically 50–70 ppm/°C (higher than copper’s 17 ppm/°C), while in the X-Y plane it is approximately 15–20 ppm/°C (determined primarily by the glass fiber fabric). CTE mismatch is the root cause of copper plating cracks in via walls (PTH failure) during thermal cycling of PCBs; low-CTE CCL is critical for improving the reliability of multilayer boards.
- Copper Foil Peel Strength: The strength of the bond between the copper foil and the substrate affects chemical resistance and operational reliability during the PCB manufacturing process; the standard requirement is typically ≥1.2 N/mm.
- Bending Strength: This determines a PCB’s resistance to fracture during assembly and use. The aluminum/copper core layers in metal-based CCL significantly enhance the board’s bending strength (by approximately 40%), making it suitable for high-vibration environments such as the automotive sector.
Summary
As a “foundational material” in the electronics and information technology industry, rigid copper-clad laminates support fixed-application scenarios, while copper-clad laminates open up limitless possibilities for flexible electronics. With technological breakthroughs and growing downstream demand—particularly the widespread adoption of FCCLs in new energy vehicles, wearable devices, and 5G applications—the copper-clad laminates industry will see more opportunities, and companies that master core processes will gain a competitive edge.
