Substrate Pretreatment: Ensuring Stronger Bonding Between Copper Foil and Substrate
While the substrate and copper foil of ordinary rigid PCBs bond tightly, the PI substrate of FPCs has a smooth surface, requiring special treatment to ensure firm copper foil adhesion. During pretreatment, tiny pits are etched into the PI film using a chemical solution (such as sodium hydroxide), followed by an adhesion promoter (similar to "glue"), and finally, hot pressing to bond the copper foil to the substrate. Experiments at an FPC factory show that the adhesion of pretreated copper foil can reach 0.8 N/mm, twice that of untreated foil, making it less prone to detachment during bending.
Precise control of the pretreatment temperature and time is crucial. Excessive temperature (above 120℃) can cause deformation of the PI substrate, while insufficient temperature results in poor treatment performance. A production line stabilized the pretreatment temperature at 100℃±2℃ for 3 minutes, improving the consistency of copper foil adhesion to over 95%.
Circuit Fabrication: Etching Circuits on an "Elastic Film"
Circuit fabrication for FPCs is similar to that of rigid PCBs, involving photolithography and etching, but it is more challenging. Because the PI substrate expands and contracts with heat, vacuum adsorption is needed during exposure to fix the substrate flat and prevent circuit deformation. A high-density FPC (0.1mm line spacing) using a high-precision exposure machine with a vacuum adsorption platform achieved line deviation control within ±10μm, a 60% improvement over the ±25μm of ordinary exposure machines.
During etching, an acidic solution (such as ferric chloride) is used to remove excess copper foil, retaining the required lines. The etching speed for FPCs is 30% slower than for rigid PCBs because excessively fast etching can cause burrs on the line edges. A certain FPC has a line spacing of 0.08mm. By reducing the etching speed (from 1m/min to 0.7m/min), the line edge roughness was reduced from 5μm to 2μm, preventing short circuits between adjacent lines.
Cover Film Lamination: Giving the Lines a "Protective Suit"
Cover film lamination is a critical process unique to FPCs. First, a layer of thermosetting adhesive is applied to the cover film. Then, the cover film is aligned with the lines through positioning holes, and finally, it is pressed together using a hot press (temperature 160℃, pressure 0.5MPa, time 30 seconds). Air bubbles must be avoided during lamination, otherwise, the lines will become damp and oxidize. One FPC manufacturer uses a "step-by-step lamination" method: first, low-temperature pre-pressing (100℃) removes air, then high-temperature lamination reduces the bubble rate from 5% to 0.5%.
The thickness of the cover film is very important. Thin cover films (25μm) offer better flexibility but slightly less protection; thicker cover films (50μm) provide strong protection but increase stress when bent. Smartwatch FPCs typically use 35μm thick cover films to strike a balance between flexibility and protection.
Punching and Forming: Cutting into the Required Shape
FPCs need to be cut into specific shapes according to the device structure, commonly using die punching or laser cutting. Die punching is suitable for mass production with an accuracy of ±0.1mm. For example, die punching is used for mass production of mobile phone FPCs, achieving an efficiency of 50 pieces per minute. Laser cutting is suitable for small batches or complex shapes, with an accuracy of ±0.05mm. For example, irregularly shaped FPCs for medical devices are laser-cut to perfectly match the device's curved structure.
The cut edges must be smooth and free of burrs; otherwise, bending will tear the cover film. The laser cutting parameters of a certain FPC were optimized (power 10W, speed 50mm/s), resulting in an edge roughness Ra≤1μm, a 67% improvement over the unoptimized 3μm.
Surface Treatment: Both solderability and oxidation resistance are required.
FPC solder joints require surface treatment, commonly immersion gold and tin plating. Immersion gold layers are thin (0.05-0.1μm), do not affect flexibility, and are suitable for fine-pitch solder joints (e.g., 0.3mm pitch chips). Tin plating layers are slightly thicker (1-3μm), lower in cost, but may produce tin whiskers (fine tin crystals) when bent. The baking temperature after tin plating (125℃, 2 hours) needs to be controlled to suppress tin whisker growth. A certain automotive sensor FPC was tin-plated, and after baking, the tin whisker length was controlled to below 5μm, meeting automotive electronic safety standards.
Reinforcement Process: Adding a Rigid Plate to Critical Areas
While FPC is flexible, the areas where components are soldered require a certain degree of rigidity; otherwise, deformation will occur during soldering. Therefore, a reinforcing plate (materials can be PI, steel sheet, or epoxy resin board), 0.1-0.5mm thick, is attached to the back of the FPC using adhesive. The positional deviation of the reinforcing plate must not exceed ±0.1mm, otherwise it will obstruct the solder joints. In one smart bracelet, after attaching a 0.3mm thick PI reinforcing plate to the battery solder joints, the soldering yield increased from 90% to 99%.