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Flexible printed circuit boards are highly reliable, exceptionally flexible printed circuit boards manufactured using polyimide or polyester film as the substrate material. They feature high wiring density, light weight, thin profile, and excellent bendability. FPC raw materials include the substrate, cover film, reinforcement, and other auxiliary materials.
Base Material:
1. Adhesive-Backed Base Material
Adhesive-backed base material primarily consists of three components: copper foil, adhesive, and PI. It is categorised into single-sided and double-sided types. Materials with copper foil on one side are single-sided base materials, while those with copper foil on both sides are double-sided base materials.
1.1 Copper Foil: Commonly used copper foil thicknesses include 1 oz, 1/2 oz, and 1/3 oz. A thinner 1/4 oz copper foil has recently been introduced and is already in use domestically for ultra-fine trace applications (line width/spacing ≤ 0.05 mm). As customer demands continue to increase, this specification will see broader adoption in the future.
1.2 Adhesive film is a thermosetting adhesive film suitable for FPC reinforcement lamination. It is made by coating an adhesive onto a substrate such as PET or PI and laminating it with a release paper. This insulating adhesive film exhibits excellent adhesion to interfaces with metals and PI films, along with outstanding insulation and flame-retardant properties.
1.3 PI film boasts outstanding mechanical properties, dielectric performance, chemical stability, and exceptional resistance to irradiation, corrosion, and extreme temperatures. It is recognised as one of the world's most advanced super-engineering polymer materials, earning the reputation as the “golden film.”
2. Adhesive-Free Substrate
An adhesive-free substrate refers to a substrate without an adhesive layer. It is formed by directly laminating copper foil + PI + copper foil through thermal rollers. Adhesive-free substrates are 2-4 times more expensive than adhesive-backed substrates but offer advantages such as thinner profiles, superior dimensional stability, higher heat resistance, enhanced flex resistance, and better chemical resistance. They also boast a longer service life, with a sustained operating temperature up to 150°C. Due to these superior properties, over 90% of current market products require adhesive-free substrates.
Covering Film:
Primarily composed of three parts: release paper, adhesive, and PI. Only the adhesive and PI remain on the final product, as the release paper is removed during production (its function is to protect the adhesive from contaminants).
Core Functional System:
Circuit Insulation and Protection System: The core value of cover films lies in establishing circuit insulation barriers. Through the composite structure of polyimide film and adhesive, they fully encapsulate etched copper conductive patterns, forming an insulation system equivalent to the solder mask layer of rigid PCBs while achieving breakthroughs in material flexibility and bend resistance.
Physical Protection and Mechanical Reinforcement: In dynamic applications like wearable devices, the cover film serves as the primary defence against mechanical damage. Its polyimide matrix exhibits tensile strength exceeding 150 MPa and withstands over 1,000 bending cycles with a 3mm radius, effectively shielding copper conductors from assembly stresses, friction, and abrasion.
Environmental Tolerance and Chemical Protection: The cover film blocks environmental corrosive factors through its fully sealed structure: it reduces copper wire oxidation rates by 90% in 85°C/85% RH humid-heat environments. For scenarios involving oil contamination and flux residues in industrial control equipment, chemical resistance tests show no swelling after 24-hour immersion in a 10% sulfuric acid solution.
Welding Process Optimisation Mechanism: During SMT soldering, the cover film's window precision (±0.03mm) synergises with its masking performance in non-soldered areas: Laser-cut pad windows ensure wettability quality for fine-pitch solder joints below 0.3mm pitch. while the edge sealing design reduces solder bridging risks by over 95%. In multi-pin connector soldering for motherboard FPCs, defect rates can be controlled below 0.1%.
Assembly Surface Engineering Design: The cover film provides a micron-level flat surface (roughness Ra<1μm), laying the foundation for 01005 ultra-micro component placement. Its polyimide matrix exhibits a thermal expansion coefficient (CTE) highly matched to PCB substrates (<30ppm/℃), maintaining interlayer stress equilibrium during 180℃ reflow soldering to ensure long-term reliability of BGA solder joints.
Reinforcement:
Specifically designed for FPC applications, reinforcement materials are applied to targeted areas to enhance structural support and compensate for the inherent flexibility of FPCs.
Currently, the following reinforcement materials are commonly used:
1. FR4 Reinforcement: Primarily composed of glass fibre cloth and epoxy resin adhesive, identical to the FR4 material used in PCBs. Typically applied as backing support for chips or ICs, it effectively increases the mechanical strength of the FPC, enhancing its resistance to bending and tensile forces. Standard thicknesses range from 0.1mm to 1.5mm.
2. Metal Reinforcement: Materials include steel, aluminium, or copper sheets. Their support function is similar to FR-4, but they offer additional advantages such as heat dissipation, grounding capability, and higher flatness. Typically used for high-end chips or ICs, or in specialised applications. Standard thicknesses range from 0.1mm to 0.4mm, with custom thicknesses available.
3. PI Reinforcement: Similar to cover films, it consists of PI and adhesive release paper layers, but with a thicker PI layer. Primarily used on the backside of gold fingers for insertion/removal, it increases FPC thickness and mechanical strength to accommodate connector terminals. Thickness ranges from 0.05mm to 0.275mm.
Other Auxiliary Materials:
1. Pure Adhesive Film: A thermosetting acrylic adhesive film comprising a release liner and adhesive layer. Primarily used for laminated boards, rigid-flex boards, and FR-4/steel reinforcement boards to provide bonding.
2. Electromagnetic Shielding Film: Applied to board surfaces for shielding purposes.
3. Pure Copper Foil: Composed solely of copper foil, primarily used in the production of openwork boards.
The selection of these different FPC raw materials will affect the durability, mechanical properties, and electrical performance of the FPC to varying degrees. Therefore, in practical applications, choices must be made based on specific requirements.