Cover film is the earliest and most widely used technology for FPC cover layer applications. The process involves coating the same adhesive used in copper clad laminate onto the same film as the copper-clad laminate base film to create a semi-cured adhesive film. This film is typically provided by the copper clad laminate manufacturer with a release film (or paper) attached for protection during delivery. The semi-cured epoxy resin adhesive will gradually solidify at room temperature and should therefore be stored in low temperature refrigeration. Prior to use, it should be kept in a refrigerated warehouse at around 5°C or sent by the manufacturer shortly before use. Manufacturers typically guarantee a shelf life of 3 to 4 months, with refrigerated storage allowing for extended use up to 6 months. Acrylic adhesives, on the other hand, do not cure at room temperature and can be stored for longer periods even without refrigeration. However, a high lamination temperature is required for this adhesive to cure properly.
Effective management of adhesive fluidity is crucial for processing cover film. Prior to leaving the factory, the material manufacturer adjusts the adhesive’s fluidity to a specific range. Proper temperature refrigeration storage conditions can maintain a service life of 3 to 4 months. However, over time, the adhesive’s fluidity gradually decreases, affecting its performance. When freshly shipped from the factory, the adhesive is highly fluid and prone to leaking during lamination, potentially contaminating the terminals and connection plates. As the adhesive approaches the end of its shelf life, its fluidity diminishes significantly, hindering the formation of a high bonding strength covering film.
When processing cover film for window openings, it should not be done immediately after removal from the refrigerator to prevent moisture absorption. The film should be allowed to reach room temperature inside a sealed plastic bag before processing. High-speed CNC drilling and milling machines or punching machines are used for window opening, with caution taken to prevent the adhesive from adhering to the drill bit. For batch hole processing, simple punching dies are employed for holes smaller than 3mm in diameter, while larger holes necessitate the use of punching dies or combined CNC drilling and punching.
After removing the release film, the cover film with window holes can be affixed to the etched circuit substrate. Before lamination, the circuit surface must be thoroughly cleaned using chemical methods to remove contamination and oxidation. The cover film, now resembling a skeletonless film with various holes, poses challenges during alignment with the circuit pattern. Manual alignment and stacking are commonly employed in mass production, where the operator carefully positions the cover film window holes and circuit connections before fixing them temporarily.
To fix the cover film in place, manual methods such as using an electric soldering iron or simple pressing are utilized. The fixed cover film is then heated and pressurized to ensure complete curing and integration with the circuit. Various heating methods, including steam, thermal media, electric heating, and silicone oil, can be used with a heat press for efficient processing. The use of a vacuum press is ideal for fully filling the adhesive into circuit pattern gaps, though equipment cost and longer pressing cycles should be considered.
The lamination method significantly impacts the adhesive state between lines and the bending resistance of the finished flexible printed board. While commercially available laminate materials are cost-effective, many flexible board factories opt to produce their own based on the specific requirements of their boards and materials used.
Effective management of adhesive fluidity is crucial for processing cover film. Prior to leaving the factory, the material manufacturer adjusts the adhesive’s fluidity to a specific range. Proper temperature refrigeration storage conditions can maintain a service life of 3 to 4 months. However, over time, the adhesive’s fluidity gradually decreases, affecting its performance. When freshly shipped from the factory, the adhesive is highly fluid and prone to leaking during lamination, potentially contaminating the terminals and connection plates. As the adhesive approaches the end of its shelf life, its fluidity diminishes significantly, hindering the formation of a high bonding strength covering film.
When processing cover film for window openings, it should not be done immediately after removal from the refrigerator to prevent moisture absorption. The film should be allowed to reach room temperature inside a sealed plastic bag before processing. High-speed CNC drilling and milling machines or punching machines are used for window opening, with caution taken to prevent the adhesive from adhering to the drill bit. For batch hole processing, simple punching dies are employed for holes smaller than 3mm in diameter, while larger holes necessitate the use of punching dies or combined CNC drilling and punching.
After removing the release film, the cover film with window holes can be affixed to the etched circuit substrate. Before lamination, the circuit surface must be thoroughly cleaned using chemical methods to remove contamination and oxidation. The cover film, now resembling a skeletonless film with various holes, poses challenges during alignment with the circuit pattern. Manual alignment and stacking are commonly employed in mass production, where the operator carefully positions the cover film window holes and circuit connections before fixing them temporarily.
To fix the cover film in place, manual methods such as using an electric soldering iron or simple pressing are utilized. The fixed cover film is then heated and pressurized to ensure complete curing and integration with the circuit. Various heating methods, including steam, thermal media, electric heating, and silicone oil, can be used with a heat press for efficient processing. The use of a vacuum press is ideal for fully filling the adhesive into circuit pattern gaps, though equipment cost and longer pressing cycles should be considered.
The lamination method significantly impacts the adhesive state between lines and the bending resistance of the finished flexible printed board. While commercially available laminate materials are cost-effective, many flexible board factories opt to produce their own based on the specific requirements of their boards and materials used.
