1. Core functions and roles Line protection and insulation: As a Coverlay, its main function is to isolate the copper foil from the external environment, prevent oxidation, corrosion and erosion by moisture/chemical substances, and at the same time provide electrical insulation to avoid short circuits between conductors and ensure stable signal transmission. Mechanical support and durability: Enhance the mechanical strength of FPC to meet dynamic bending requirements (such as millions of bends), and improve the tensile and impact resistance of local areas by combining PI or PET substrates with adhesive layers. Solder mask and process control: In SMT (Surface Mount Technology), prevent solder from flowing to non-soldering areas to ensure soldering accuracy; To meet the requirements of welding, testing or heat dissipation through window design (pads, vias, heat dissipation areas), it is necessary to precisely control the laser cutting or gradient window opening process to avoid window deviation, peeling or poor welding. Environmental adaptability: Resistant to high temperatures (approximately 150℃ for bonding and curing), resistant to chemical corrosion, and suitable for high-temperature and high-humidity environments such as etching, electroplating, and reflow soldering in FPC processing.
2. Material systems and structures Substrate and coating Insulation layer: Polyimide (PI, temperature resistance ≥200℃) or polyester (PET, temperature resistance 180-200℃) film, thickness options include 0.5mil, 1mil, 2mil, etc., featuring both flexibility and dimensional stability. Adhesive layer: Epoxy resin or acrylic adhesive system, thickness 15-50μm, providing bonding strength and curing performance; UV anti-sticking, anti-static or conductive coatings can be added for special requirements. Release paper: Protective adhesive layer during the production process, stripped after lamination. Color and Customization: The standard colors are yellow, white and black. Special requirements (such as rainbow color, green) require additional coating, which may increase the thickness. Transparent color is used in scenes that require visual inspection. Reinforcing materials: PI reinforcing (on the back of the gold finger), FR-4 reinforcing (for chip /IC support), metal reinforcing (steel/aluminum/copper sheet, with both heat dissipation and grounding functions), thickness 0.05-1.5mm, suitable for connector terminals or high-reliability scenarios.
3. Processing technology flow General process for single/double-sided panels: material cutting → baking (120℃,2 hours) → dry film application → exposure → development → etching → film removal → pretreatment → cover film application → lamination (high temperature and high pressure) → curing (150℃,1 hour) → surface treatment (gold plating/gold plating) → electrical testing → assembly → text printing → shape cutting → final inspection → packaging. Key process control Pressing and curing: Achieve a tight bond between the covering film and the substrate through a fast press or oven. Control the temperature, pressure and time to avoid delamination or bubbling. Window opening and laser cutting: Precisely position the pad/via area, use laser cutting or gradient window opening, reserve tolerances and buffer zones to prevent delamination or poor welding. Surface treatment: Gold plating or gold immersion enhances the solderability and corrosion resistance of the pads, while chemical cleaning or sandblasting ensures the cleanliness of the board surface. Special processes: Black holes (conductive carbon powder deposition) and VCP (vertical continuous electroplating) are used for metallizing through holes in multi-layer boards. Rigid-flex boards (R-FPC) require additional bonding layers (such as PP) and lamination processes.
4. Quality Control and Inspection Raw material inspection: Test the thickness of the base material (PI/PET), surface quality (no bubbles/scratches), adhesive layer adhesion and curing performance; The size and flatness of the reinforcing material. Process control Etching and line accuracy: Line width/line spacing should meet the design requirements to avoid short circuits/open circuits. The control of etching solution concentration, temperature and time ensures neat edges. Lamination and thickness: Interlayer bonding strength, thickness uniformity (±5% deviation) and flatness detection. Window opening and welding: Window opening position accuracy, pad oxidation inspection, welding reliability testing (such as tensile testing). Reliability test Environmental tests: Temperature cycling (-40℃ to 125℃), thermal shock, salt spray test (ISO 9227), moisture resistance (85℃/85% RH,500 hours). Mechanical properties: Bending test (millions of dynamic bends), tensile test, vibration and shock test. Electrical performance: open and short circuit testing, impedance matching, signal integrity analysis. Final inspection and packaging: Visual inspection (no scratches/stains), size measurement, clarity of marking; Vacuum/anti-static packaging, storage environment dry and free of corrosive gases.
5. Application Scenarios and cases Consumer electronics: Flexible connection cables for mobile phone motherboards and tablet computers, which require high-density circuits and thin and light designs. Automotive electronics: On-board sensors and control modules need to be heat-resistant, vibration-resistant and resistant to electromagnetic interference (EMI shielding film). Medical devices: Wearable devices, implantable medical electronics, which require biocompatibility and high reliability. Industrial automation: Robot flexible arms, industrial control boards, adaptable to complex environments and dynamic movements. Aerospace: Satellite antennas, aircraft control boards, require extreme environment tolerance and lightweight design.
