Coverlay Quality Assurance

1. Raw Material Inspection

• Material Composition Analysis: When procuring coverlay materials, such as polyimide (PI), a stringent examination of their composition is imperative. Employing advanced analytical instruments like Fourier Transform Infrared Spectrometer (FT - IR) is essential to ascertain that the material composition adheres to the stipulated standards. For instance, the PI material should possess the designated chemical structure, devoid of impurities or anomalous constituents. Any impurity could potentially compromise its insulation performance and adhesion to the copper foil.

• Thickness Tolerance Control: High-precision thickness gauges are utilized to measure the thickness of the coverlay. In diverse application scenarios, specific thickness requirements for coverlay are mandated. For example, in FPCs with high-density circuitry, the thickness tolerance might be restricted within ±5μm. An excessive thickness could impede the flexibility of the FPC, while a meager thickness might fail to furnish adequate insulation protection.

  
  

2. Processing Control

• Lamination Process Optimization: During the lamination of coverlay onto the FPC circuitry, temperature, pressure, and time are the cardinal parameters. For instance, in the thermal compression lamination process, the temperature is typically regulated between 180 - 220°C. An appropriate temperature ensures the sufficient flow of the coverlay adhesive, facilitating a firm bond with the circuitry. The pressure usually ranges from 3 - 5MPa to preclude the presence of bubbles and voids between the coverlay and the circuitry. The time, contingent upon the material and FPC dimensions, generally spans from 30 - 90 seconds. By precisely calibrating these parameters, the issue of delamination due to suboptimal lamination can be effectively averted.

• Curing Treatment Monitoring: If a curing process is requisite subsequent to coverlay lamination, meticulous surveillance of the curing temperature and time is obligatory. For example, in the case of ultraviolet (UV) curing, the intensity of the UV lamps and the irradiation time must conform to the material requisites. Insufficient curing could result in the incomplete reaction of the coverlay adhesive, leading to inadequate adhesion; over-curing might render the coverlay brittle, thereby diminishing its flexibility and durability.

  
  

3. Quality Inspection Phase

• Visual Inspection: Both visual inspection and optical microscopy are employed to scrutinize the coverlay surface for any scratches, wrinkles, bubbles, or other defects. In FPCs with aesthetic or functional requirements, such as those used in the exterior components of electronic products, even the minutest blemish could potentially mar the final product quality. For example, a scratch might rupture the insulation layer of the coverlay, augmenting the risk of short circuits.

• Adhesion Force Testing: Professional tensile testers are deployed to assess the adhesion force between the coverlay and the copper foil circuitry. The test samples should be representative and selected in accordance with specific sampling protocols from the production batch. The adhesion force ought to comply with the product design specifications. Generally, the adhesion strength should surpass a certain threshold value (e.g., 0.5N/mm) to ensure the absence of delamination during the FPC's operational lifespan.

• Insulation Performance Testing: Insulation resistance testers and withstand voltage testers are utilized to appraise the insulation performance of the coverlay. The insulation resistance should attain the prescribed value (e.g., greater than 10^12Ω), and the withstand voltage capacity must also meet the requirements of the product's operating environment. For example, in FPCs deployed in high-voltage environments, the coverlay must be capable of withstanding elevated voltages without experiencing breakdown.

1. Raw Material Screening

• Material Selection and Validation: When choosing protective film materials, such as polyethylene terephthalate (PET), the selection should be predicated on the characteristics and application exigencies of the FPC. For FPCs necessitating long-term storage, a PET material with excellent weatherability should be opted for. The procured materials are subject to validation, with their mechanical and optical properties inspected to ensure compliance. For example, tensile tests are conducted to examine their tensile strength, guaranteeing that the material does not rupture readily during stretching.

• Antistatic Performance Detection: In FPCs with antistatic requirements, professional electrostatic testers are employed to evaluate the antistatic performance of the protective film. The surface resistance should fall within the prescribed antistatic range (e.g., 10^6 - 10^9Ω) to preclude electrostatic damage to the FPC components.

  
  

2. Film Application Management

• Film Application Environment Control: The film application workshop environment must be maintained in a pristine and stable state in terms of temperature and humidity. Dust and other contaminants could adhere to the protective film and FPC surfaces, thereby compromising the film application quality. Fluctuations in temperature and humidity might affect the adhesiveness of the protective film. For example, an excessive temperature could lead to a decline in the adhesive viscosity of the protective film, resulting in inadequate adhesion. Generally, a temperature range of 20 - 25°C and a relative humidity of 40% - 60% are deemed suitable.

• Film Application Precision Control: High-precision film application equipment is utilized to ensure the accurate attachment of the protective film onto the FPC surface. The positional deviation of the film application should be confined within a minuscule range (e.g., ±0.5mm) to guarantee the effective protection area of the FPC. Simultaneously, the formation of wrinkles during the film application process must be obviated by regulating the film application speed and tension.

  
  

3. Quality Inspection Measures

• Peel Strength Testing: Peel strength testers are employed to measure the peel strength of the protective film from the FPC surface. The peel strength should be optimized to permit facile removal when necessary, while ensuring that it does not detach spontaneously during normal usage. For example, the peel strength is typically controlled within 3 - 5N/25mm.

• Transparency Inspection: In scenarios where visual inspection of the FPC is requisite, photometers and other apparatuses are used to assess the transparency of the protective film. The transparency should meet specific standards, such as a light transmittance exceeding 90%, to ensure a clear visualization of the FPC surface condition.