The performance and reliability of FPC circuit boards depend not only on design but also on manufacturing processes. Especially for high-end FPCs, processes such as blind and buried vias, precision lamination, and electroplating directly determine product quality. Many designers focus solely on design while neglecting process feasibility, resulting in designs that cannot be implemented or have extremely low yields.

First is the blind and buried via process, which is the core technology of high-density FPCs and a representation of process complexity. Ordinary through vias penetrate the entire FPC layer, occupying significant space and affecting wiring density. Blind vias only connect the surface layer to inner layers, while buried vias connect only between inner layers, saving over 30% of space and making FPCs thinner and lighter. However, the blind and buried via process is extremely challenging. The first step is laser drilling, which requires high-precision laser equipment to control the depth and diameter of the holes accurately, with an error margin of less than 5μm. The second step is hole metallization, where copper is plated onto the tiny hole walls to ensure conductivity. If the holes are too small, uneven plating may occur, leading to open circuits. The third step is alignment and lamination, where the alignment accuracy for blind and buried vias is critical. A deviation exceeding 10μm can cause misalignment and circuit failure.

The more layers involved in blind and buried vias, the more complex the process. For example, the difficulty of blind and buried vias in a 4-layer FPC is twice that of a 2-layer FPC, and the cost is 50% higher. During design, blind and buried vias should be rationally planned based on wiring density requirements to avoid overuse. For simple FPCs, through vias are sufficient, while for high-density FPCs, blind vias should be prioritized, and buried vias should only be used when necessary to reduce process complexity.

Next is the precision lamination process. FPCs are multilayer structures that require lamination to bond substrates, copper foil, coverlays, and stiffeners together. The quality of lamination directly affects the flexibility and reliability of the FPC. The key to lamination lies in controlling temperature, pressure, and time. If the temperature is too low, the adhesive cannot flow adequately, resulting in weak bonding and delamination. If the temperature is too high, the substrate may deform, and circuits may shift. Insufficient pressure fails to expel air bubbles, causing delamination and bulging. Excessive pressure can crush circuits and lead to uneven thickness.

For high-end FPCs, vacuum laminators are used to perform lamination in a vacuum environment, preventing air bubbles. The lamination temperature is typically controlled at 180–200°C, pressure at 10–15 MPa, and time at 60–90 minutes. Parameters must be precisely adjusted for different materials. For instance, the lamination temperature for PI substrates differs from that for PET substrates and cannot be interchanged. Additionally, during multilayer FPC lamination, precise alignment of each layer is essential, with deviations not exceeding 5μm; otherwise, circuit misalignment or short circuits may occur.

Then there is the electroplating process, which determines the conductivity, corrosion resistance, and bend resistance of FPCs. Common electroplating types include copper plating, tin plating, and gold plating. Copper plating is fundamental for circuit and via conductivity, requiring uniform copper layers with strong adhesion to avoid pinholes or depressions. Tin plating is used for pads to facilitate soldering, requiring uniform thickness and no peeling. Gold plating is applied to high-frequency signals and contact terminals to enhance conductivity and corrosion resistance. A gold layer thickness of 0.05–0.1μm is sufficient; thicker layers increase costs, while thinner layers may wear out easily.

The challenge in electroplating lies in uniformity. FPCs are thin, lightweight, and irregularly shaped, leading to uneven current distribution during electroplating. This can result in localized plating that is too thick or too thin. For example, edges may have thicker plating while the middle has thinner plating, causing inconsistent circuit resistance and signal transmission issues. Solving this problem requires optimizing electroplating fixtures and adjusting current density to ensure uniform plating. Additionally, pre-electroplating cleaning is crucial to remove surface oils and impurities; otherwise, the plating layer may have poor adhesion and peel off easily.

Another critical process is coverlay lamination. The coverlay acts as a protective layer for the FPC, and its lamination quality directly affects circuit protection. The coverlay must bond tightly to the substrate without bubbles or wrinkles, as these can lead to short circuits or oxidation. For high-end FPCs, vacuum laminators are used to apply the coverlay in a vacuum environment, preventing bubbles. The lamination temperature is controlled at 150–170°C, with pressure at 5–8 MPa, ensuring full adhesion between the coverlay and the substrate. Openings must be precisely aligned with the pads, with deviations not exceeding 5μm; otherwise, soldering may be affected.

The complexity of FPC processes is the core distinction between high-end and ordinary products. Processes such as blind and buried vias, precision lamination, and electroplating require meticulous control at every step, leaving no room for error. Designers must consider process feasibility during the design phase and communicate with manufacturers about their process capabilities to avoid designs that cannot be implemented. Only when design and process work in perfect harmony can high-quality FPC circuit boards be produced.