One of the unique processes in flexible printed circuit board (FPC) manufacturing is the processing of the cover layer. There are three primary methods for processing the cover layer: covering film, screen printing, and photocoating. Recently, updated technology has broadened the range of options available.
The processing of FPC cover films is divided into three categories:
1. Screen printing of the FPC overlay
2. FPC cover film
3. FPC photocoating layer
1. **Screen printing of the FPC overlay**
The mechanical properties of a missing cover layer are inferior to those of a laminated cover film, but material and processing costs are lower. This method is commonly used in consumer products and automotive flexible printed boards that do not require repeated bending. The process and equipment are similar to those used for solder mask printing on rigid boards, though different inks are required. Inks suitable for flexible boards include UV-curable and heat-curable types. UV-curable inks have a short curing time but typically offer poor mechanical and chemical resistance, which may be unsuitable for bending or harsh chemical environments. Additionally, electroless gold plating should be avoided, as the plating solution can penetrate and cause the cover layer to peel. Thermosetting inks require 20-30 minutes to cure, necessitating longer drying tunnels for continuous curing, with intermittent ovens being more common.
2. **FPC cover film**
1. Cover film is the earliest and most widely used technology for flexible printed board cover layer applications. The film, which serves as the base for the copper-clad laminate, is coated with the same adhesive as used in the copper-clad laminate, resulting in a semi-cured adhesive film. This film is produced and supplied by the copper-clad laminate manufacturer. Upon delivery, a release film (or paper) is applied to the adhesive film. Since the semi-cured epoxy resin adhesive will gradually solidify at room temperature, it should be stored in a low-temperature environment. Before use, the printed circuit manufacturer should store it in a refrigerated warehouse at approximately 5°C or have it shipped by the manufacturer. Typically, material manufacturers guarantee a use period of 3 to 4 months, though it can be extended to 6 months under refrigerated conditions. Acrylic adhesives, on the other hand, rarely solidify at room temperature. Even if not refrigerated, they can remain usable for more than six months. However, the lamination temperature for these adhesives must be quite high.
2. One of the key challenges in processing cover film is managing the adhesive’s fluidity. Before leaving the factory, the material manufacturer adjusts the adhesive’s fluidity to a specific range. When stored under appropriate refrigerated conditions, a service life of 3 to 4 months can be assured. However, within this period, the adhesive’s fluidity decreases over time. Initially, when the cover film is shipped from the factory, the adhesive is very fluid, which can lead to flow-out during lamination and contamination of the terminal parts and connection plates. As the adhesive approaches the end of its service life, its fluidity diminishes significantly or may even become negligible. Without high lamination temperature and pressure, achieving a cover film that adequately fills pattern gaps and maintains high bonding strength can be challenging.
3. The cover film needs to be processed by opening windows, but it should not be processed immediately after being removed from refrigeration. Especially in high ambient temperatures with significant temperature differences, condensation may occur on the surface, which can absorb moisture and affect subsequent processes. Therefore, roll cover films are generally sealed in polyethylene plastic bags. The sealed bag should not be opened immediately after removal from the refrigerator but should be left in the bag for several hours until it reaches room temperature before removing the cover film for processing.
4. The laminating method greatly influences the adhesive state between the lines and the bending resistance of the finished flexible printed board. Laminated materials are generally available as commercial products, but for cost-effective mass production, many flexible board manufacturers create their own laminated materials. The material and structure of the laminated board vary depending on the FPC structure and materials used.
5. FPC photocoating layer
6. The basic process for the photocoating layer is similar to that of the photoresist film used for rigid printed boards. Both dry film and liquid ink types are used. Despite differences between solder mask dry films and liquid inks, the same equipment can be utilized for exposure and subsequent processes, though specific process conditions will vary. The dry film must be applied first, covering all circuit diagrams. The conventional dry film method can result in air bubbles between lines, so a vacuum film applicator is often used.
7. The ink type involves coating the circuit pattern with ink using screen printing or spraying methods. Screen printing is a common technique, similar to that used in rigid printed board processes. However, due to the circuit orientation, the thickness of the ink from a single print can be inconsistent, often resulting in thin areas of approximately 10–15µm, and occasionally, missing prints. To improve reliability, it is necessary to change the printing direction and perform a second print. The spraying method, though relatively new in PCB printed board processes, allows for adjustable thickness through the nozzle. It provides a uniform coating with minimal uncoated areas and supports continuous application, making it suitable for mass production.
The processing of FPC cover films is divided into three categories:
1. Screen printing of the FPC overlay
2. FPC cover film
3. FPC photocoating layer
1. **Screen printing of the FPC overlay**
The mechanical properties of a missing cover layer are inferior to those of a laminated cover film, but material and processing costs are lower. This method is commonly used in consumer products and automotive flexible printed boards that do not require repeated bending. The process and equipment are similar to those used for solder mask printing on rigid boards, though different inks are required. Inks suitable for flexible boards include UV-curable and heat-curable types. UV-curable inks have a short curing time but typically offer poor mechanical and chemical resistance, which may be unsuitable for bending or harsh chemical environments. Additionally, electroless gold plating should be avoided, as the plating solution can penetrate and cause the cover layer to peel. Thermosetting inks require 20-30 minutes to cure, necessitating longer drying tunnels for continuous curing, with intermittent ovens being more common.
2. **FPC cover film**
1. Cover film is the earliest and most widely used technology for flexible printed board cover layer applications. The film, which serves as the base for the copper-clad laminate, is coated with the same adhesive as used in the copper-clad laminate, resulting in a semi-cured adhesive film. This film is produced and supplied by the copper-clad laminate manufacturer. Upon delivery, a release film (or paper) is applied to the adhesive film. Since the semi-cured epoxy resin adhesive will gradually solidify at room temperature, it should be stored in a low-temperature environment. Before use, the printed circuit manufacturer should store it in a refrigerated warehouse at approximately 5°C or have it shipped by the manufacturer. Typically, material manufacturers guarantee a use period of 3 to 4 months, though it can be extended to 6 months under refrigerated conditions. Acrylic adhesives, on the other hand, rarely solidify at room temperature. Even if not refrigerated, they can remain usable for more than six months. However, the lamination temperature for these adhesives must be quite high.
2. One of the key challenges in processing cover film is managing the adhesive’s fluidity. Before leaving the factory, the material manufacturer adjusts the adhesive’s fluidity to a specific range. When stored under appropriate refrigerated conditions, a service life of 3 to 4 months can be assured. However, within this period, the adhesive’s fluidity decreases over time. Initially, when the cover film is shipped from the factory, the adhesive is very fluid, which can lead to flow-out during lamination and contamination of the terminal parts and connection plates. As the adhesive approaches the end of its service life, its fluidity diminishes significantly or may even become negligible. Without high lamination temperature and pressure, achieving a cover film that adequately fills pattern gaps and maintains high bonding strength can be challenging.
3. The cover film needs to be processed by opening windows, but it should not be processed immediately after being removed from refrigeration. Especially in high ambient temperatures with significant temperature differences, condensation may occur on the surface, which can absorb moisture and affect subsequent processes. Therefore, roll cover films are generally sealed in polyethylene plastic bags. The sealed bag should not be opened immediately after removal from the refrigerator but should be left in the bag for several hours until it reaches room temperature before removing the cover film for processing.
4. The laminating method greatly influences the adhesive state between the lines and the bending resistance of the finished flexible printed board. Laminated materials are generally available as commercial products, but for cost-effective mass production, many flexible board manufacturers create their own laminated materials. The material and structure of the laminated board vary depending on the FPC structure and materials used.
5. FPC photocoating layer
6. The basic process for the photocoating layer is similar to that of the photoresist film used for rigid printed boards. Both dry film and liquid ink types are used. Despite differences between solder mask dry films and liquid inks, the same equipment can be utilized for exposure and subsequent processes, though specific process conditions will vary. The dry film must be applied first, covering all circuit diagrams. The conventional dry film method can result in air bubbles between lines, so a vacuum film applicator is often used.
7. The ink type involves coating the circuit pattern with ink using screen printing or spraying methods. Screen printing is a common technique, similar to that used in rigid printed board processes. However, due to the circuit orientation, the thickness of the ink from a single print can be inconsistent, often resulting in thin areas of approximately 10–15µm, and occasionally, missing prints. To improve reliability, it is necessary to change the printing direction and perform a second print. The spraying method, though relatively new in PCB printed board processes, allows for adjustable thickness through the nozzle. It provides a uniform coating with minimal uncoated areas and supports continuous application, making it suitable for mass production.
