Lightweight fibreglass cloth, viscous epoxy resin, and lustrous copper foil—these seemingly disparate raw materials undergo a series of meticulous processes to be refined and fused, ultimately transforming into FR4 copper-clad laminate (CCL) that underpins the operation of electronic devices. This metamorphosis from basic materials to functional substrates holds the key to FR4 CCL quality and represents an indispensable critical link in the electronics industry supply chain. Precise control at every stage of the process not only determines the full release of raw material properties but also directly impacts the efficiency of subsequent PCB manufacturing and the reliability of end products.
The production transformation of FR4 copper-clad laminates commences with rigorous selection and pre-treatment of core raw materials. These include glass fibre cloth, epoxy resin, curing agents, copper foil, and various auxiliary additives. The quality of glass fibre cloth and epoxy resin directly determines substrate performance, while copper foil influences the subsequent circuit’s electrical conductivity stability. The core objective of pre-treatment is to enhance material compatibility, paving the way for smooth progression through subsequent processes.
The pre-treatment of glass fibre cloth constitutes the crucial first step. E-type alkali-free glass fibre cloth is commonly employed in FR4 pcb. The selected cloth must meet requirements for uniform warp and weft density, absence of broken yarns, and freedom from impurities. Common specifications include 1080, 2116, and 7628 types. Pre-treatment primarily comprises two processes: degreasing and coupling agent coating. The degreasing process employs high-temperature baking (200°C–220°C) to remove textile oils from the fabric surface, preventing residual oils from impairing epoxy resin impregnation. Coupling agent coating involves uniformly spraying silane-based coupling agents onto the fabric surface. High-temperature curing (120°C–140°C) enables coupling agent molecules to bond with surface hydroxyl groups, forming an active film layer. This film significantly enhances interfacial adhesion between the glass fibre fabric and epoxy resin, reducing subsequent delamination risks. Experimental data indicates that pretreated glass fibre cloth exhibits over 25% higher peel strength with epoxy resin.
The formulation of the epoxy resin system is equally critical. The epoxy resin system employed for FR4 is based on bisphenol A epoxy resin, combined with phenolic resin curing agents, imidazole accelerators, flame retardants, fillers, and other auxiliary components, mixed and stirred in precise proportions. The mixing process requires controlled stirring speed (300–500 rpm) and temperature (25–30°C) to ensure uniform dispersion of all components and prevent lump formation. Following preparation, viscosity testing of the epoxy resin system is mandatory. Standard FR4 production requires viscosity control within 2000–3000 mPa·s. Excessively high viscosity compromises glass fabric impregnation, whilst excessively low viscosity may increase post-curing board porosity.
Copper foil pretreatment focuses on enhancing adhesion strength with epoxy resin. FR4 copper-clad laminates commonly use electrolytic copper foil or rolled copper foil, with thicknesses specified as 1oz, 2oz, etc. Pre-treatment comprises surface roughening and oxidation resistance treatment: Surface roughening employs electrochemical etching to create microscopic microstructures on the copper foil surface, increasing contact area with the epoxy resin. Oxidation resistance treatment forms a dense passivation layer on the foil surface, preventing oxidation during subsequent processes and ensuring stable electrical conductivity. The bond strength between pretreated copper foil and epoxy resin can exceed 1.8 N/mm², meeting subsequent PCB board processing requirements.
Impregnation and Drying
Following completion of the pre-treatment, production proceeds to the first step of the core conversion stage: the impregnation and drying of glass fibre cloth. This ultimately yields the critical intermediate product for FR4 pcb, the ‘prepreg sheet (PP sheet)’. The essence of this process lies in ensuring the epoxy resin system uniformly and thoroughly impregnates the glass fibre cloth while simultaneously removing excess solvent and moisture, thereby laying the foundation for subsequent lamination moulding.
The impregnation process is executed using a continuous impregnation machine, with core parameters including impregnation speed, temperature, and viscosity of the epoxy resin system. The glass fibre cloth traverses the impregnation tank containing the epoxy resin system at a constant speed (1-3m/min), ensuring complete immersion of the cloth surface within the resin. To enhance impregnation efficiency, guide rollers and pressure rollers are installed within the tank. Adjusting the pressure roller force (0.3–0.5 MPa) enables thorough resin penetration into the warp and weft gaps of the glass fibre fabric, preventing the occurrence of ‘dry spots’ (unimpregnated areas). Different specifications of glass fibre cloth require distinct impregnation parameters. For instance, Type 7628 roving glass fibre cloth necessitates reduced impregnation speed and increased pressure roller force to ensure resin penetration into the deeper layers of the cloth. Conversely, Type 1080 filament glass fibre cloth permits a slightly higher speed to prevent excessive resin accumulation.
Following impregnation, the glass fibre fabric must immediately enter the drying oven for processing. The oven employs segmented temperature control, comprising a pre-drying section (80°C–100°C), main drying section (120°C–140°C), and cooling section (40°C–60°C). The pre-drying section primarily removes low-boiling-point solvents and surface moisture from the resin. The main drying section promotes initial cross-linking between the epoxy resin and curing agent (Phase B reaction), transforming the resin from liquid to semi-solid state. This ensures the prepreg possesses adequate rigidity while retaining flowability for subsequent lamination. The cooling section rapidly reduces the prepreg’s temperature to prevent over-curing due to high heat. Drying duration requires adjustment based on prepreg thickness. Standard-specification prepregs typically undergo 3–5 minutes of drying, ultimately controlling volatile content below 0.5% and resin content (resin mass as a percentage of total prepreg mass) between 50%–60%.
Dried prepregs undergo rigorous quality inspection, with acceptance criteria including: smooth fabric surface free of wrinkles, absence of bubbles or pinholes, uniform resin distribution (resin content deviation not exceeding ±2%), and compliant volatile content. Non-compliant prepregs must be re-impregnated and dried, or scrapped outright, to prevent compromising subsequent copper-clad laminate quality. Approved prepreg sheets are sorted by specification and stacked, awaiting entry into the lamination process.
Laminating Process
Laminating constitutes the core stage in FR4 copper-clad laminate production and represents the pivotal step transforming ‘prepreg + copper foil’ into finished laminates. This process employs high temperature and pressure to fully integrate multiple layers of prepreg with upper and lower copper foils, enabling the epoxy resin to undergo final cross-linking and curing. This yields structurally dense, performance-stable FR4 copper-clad laminates.
The pre-lamination stacking process must be strictly executed according to design specifications. According to the required thickness of the FR4 laminate, the appropriate number of prepreg sheets are stacked. Pre-treated copper foil is applied to both upper and lower surfaces, forming a sandwich structure: ‘copper foil – prepreg stack – copper foil’. This lamination must be performed within a cleanroom environment (cleanliness grade ≥ Class 1000) to prevent contamination by dust or impurities, which could cause internal defects in the laminate. Upon completion of lamination, the laminate must be placed into a mould coated with release agent to prevent adhesion between the cured laminate and the mould.
The lamination process is executed using a hot press, with core parameters including heating rate, lamination temperature, lamination pressure, and holding time. These parameters require precise adjustment based on the prepreg specifications and laminate thickness. The standard lamination profile for conventional FR4 copper-clad laminates is as follows: – Temperature ramp rate controlled at 2–3°C/min, gradually heating to 160–180°C – Pressure application commences at 120°C, with final pressure maintained at 2.0–3.0 MPa – Holding time at 160–180°C with sustained pressure: 60–90 minutes This ensures complete cross-linking and curing of the epoxy resin (Phase C reaction). During lamination, the partially cured resin within the prepreg reactivates, flowing to fill microscopic voids within the laminate stack while tightly bonding with the roughened surface of the copper foil to form a robust interface.
Following lamination completion, a controlled cooling and pressure release process is required. The cooling rate must be strictly controlled (1-2°C/min) to prevent thermal stresses within the laminate, which could cause warping, cracking, or other defects. Only when the laminate temperature falls below 60°C should pressure be released and the laminate removed. At this stage, the laminate possesses basic rigidity and insulation properties but requires subsequent post-processing to further enhance performance stability.
Post-Processing and Quality Control
Following lamination, FR4 copper-clad laminates undergo post-processing and comprehensive quality control to eliminate surface defects and stabilise performance parameters, ensuring the final product meets industry standards. This stage serves as the ‘final line of defence’ for FR4 quality, directly determining market eligibility.
Post-processing primarily comprises trimming, sanding, and ageing treatment. The trimming process employs CNC trimming machines to remove burrs and irregularities from the board edges, ensuring dimensional accuracy (typically within ±0.1mm tolerance) while preventing edge burrs from causing injury to personnel or damaging equipment during subsequent operations. The sanding process involves lightly grinding the laminate surface to remove oxidation layers and residual impurities from the copper foil. This enhances surface roughness, laying the foundation for photoresist adhesion during subsequent PCB circuit formation. Ageing treatment constitutes a critical post-processing stage. The laminate is placed in a constant temperature and humidity chamber (23°C ± 2°C, 50% ± 5% humidity) for 24-48 hours. This allows internal stresses within the laminate to fully dissipate, stabilising performance parameters and preventing warping or deformation during subsequent processing.
Quality control encompasses the core performance metrics of FR4 laminates, including electrical properties, mechanical properties, thermal resistance, and visual quality. Electrical property testing encompasses dielectric constant (Dk), dielectric loss factor (Df), insulation resistance, and breakdown voltage. Standard FR4 laminates require a dielectric constant controlled between 4.2–4.8, a dielectric loss factor below 0.02, and insulation resistance exceeding 10¹³ Ω·cm. Mechanical property testing encompasses flexural strength, tensile strength, and interlaminar peel strength. Flexural strength must exceed 300 MPa, while interlaminar peel strength should not fall below 1.5 N/mm. Thermal resistance testing primarily evaluates the glass transition temperature (Tg). Standard FR4 laminates require a Tg ≥ 130°C, whereas high-Tg variants demand ≥ 170°C. Visual quality inspection employs a combination of manual inspection and AOI (Automated Optical Inspection) equipment to detect surface defects such as bubbles, pinholes, scratches, and copper foil detachment. The area proportion of visual defects must be less than 0.1%.
Beyond finished product inspection, FR4 laminate production implements comprehensive quality control throughout the manufacturing process. This includes incoming material testing (glass fabric composition and strength, epoxy resin purity, copper foil thickness and conductivity), semi-cured sheet production monitoring (resin content, volatile matter content), and real-time lamination parameter surveillance (temperature, pressure, time). This comprehensive control ensures consistent quality for every FR4 laminate, with batch-to-batch performance variations maintained within permissible limits.
As electronic technology advances, diverse application scenarios demand enhanced FR4 laminate performance. Conventional production processes require targeted optimisation to establish specialised manufacturing pathways. These enhancements primarily focus on raw material selection and core process parameter adjustments, ensuring products meet specific environmental requirements.
For low-loss FR4 laminates targeting 5G mid-to-low frequency communication applications, production optimisation centres on raw material selection: employing low-loss epoxy resin (dielectric loss factor ≤0.012) and fine-weave glass fibre cloth (e.g., 1080 type) to minimise signal transmission losses; During lamination, temperatures are appropriately elevated (170°C–190°C) and dwell times extended (90–120 minutes) to ensure complete epoxy curing and reduce internal porosity. For high-Tg FR4 copper-clad laminates targeting automotive electronics’ high-temperature applications, optimisation centres on selecting high-Tg epoxy resin (Tg ≥ 180°C) and moisture-resistant coupling agents. Laminating pressure should be increased to 2.5–3.5 MPa, while ageing treatment time extended to 48 hours, enhancing panel stability in high-temperature, high-humidity environments. For high-insulation FR4 copper-clad laminates in high-voltage new energy equipment applications, increased flame retardant dosage (halogen-free flame retardants constituting 15%-20% of resin mass) is required. High-purity copper foil (conductivity ≥99.9%) must be selected, with post-lamination high-voltage testing (test voltage ≥5kV) implemented to guarantee insulation safety.
The transformation journey from glass fibre cloth to FR4 copper-clad laminate represents a precise interplay of ‘raw material selection, process coordination, and quality control.’ Every parameter adjustment in each process step and every precise raw material ratio contributes to the laminate’s quality. It is this end-to-end precision control that endows FR4 pcb laminates with outstanding electrical performance, mechanical strength, and stability, establishing them as the mainstream substrate material in the PCB industry.
Against the backdrop of continuous technological evolution in electronics, the manufacturing processes for FR4 laminates are undergoing constant optimisation and advancement. The progression from conventional to specialised techniques, coupled with the shift from manual oversight to intelligent production, is propelling FR4 laminates towards greater efficiency, enhanced stability, and improved suitability for specialised applications.
