The assembly of flex-rigid PCBs is a specialized manufacturing process that combines the assembly techniques of rigid PCBs and flexible PCBs, focusing on precision component mounting, reliable interconnection between rigid and flexible sections, and protection of flexible layers during handling. Unlike rigid PCB assembly (which uses standard pick-and-place and reflow soldering), flex-rigid assembly requires additional steps to address the unique challenges of flexible layers—such as preventing damage during bending, ensuring consistent solder joint quality, and securing components in dynamic areas. This process is critical for ensuring the functionality, durability, and reliability of flex-rigid PCBs in applications like medical devices, automotive electronics, and consumer wearables.

A key step in flex-rigid PCB assembly is pre-assembly preparation, which involves stabilizing the flexible sections to enable accurate component placement. Flexible layers are often mounted on rigid carriers (made of aluminum or FR-4) using temporary adhesives, which prevent the flexible sections from bending or shifting during the pick-and-place process. This ensures that surface-mount components (SMDs) are placed with high precision (±0.1mm) on both rigid and flexible sections. For example, a flex-rigid PCB for a hearing aid has a flexible section (mounted on a carrier) where tiny SMD microphones (0.5mm x 0.5mm) are placed accurately, ensuring they align with the device’s sound ports.

Component placement and soldering are performed using specialized equipment tailored for flex-rigid designs. Pick-and-place machines with vision systems are used to place components on both rigid and flexible sections, with adjustable vacuum pressure to handle delicate flexible layers without causing damage. Reflow soldering is conducted in ovens with precise temperature profiles—optimized for the solder paste and component types—with the rigid carriers protecting flexible sections from excessive heat or warping. For rigid sections, standard reflow profiles (with peak temperatures around 250°C) are used, while flexible sections may use lower peak temperatures (230°C) to avoid damaging the polyimide substrate. For instance, a flex-rigid PCB in a car’s infotainment system has ICs soldered to the rigid section at 250°C, while passive components on the flexible section are soldered at 230°C, ensuring both sections have reliable solder joints.

Interconnection between rigid and flexible sections is a critical assembly step, as this area is prone to mechanical stress. The bonding between rigid and flexible layers is reinforced using additional adhesives or stiffeners (made of FR-4 or metal) to distribute stress and prevent delamination. In some cases, plated through-holes (PTHs) or blind vias are used to connect layers across rigid and flexible sections, ensuring electrical continuity. For example, a flex-rigid PCB in a drone’s flight controller uses blind vias to connect the rigid section’s processor to the flexible section’s motor drivers, creating a reliable electrical path while maintaining flexibility for the drone’s moving parts.

Post-assembly processes include inspection, testing, and protection of the flex-rigid PCB. Automated optical inspection (AOI) is used to check for solder joint defects (like bridges or cold joints) and component placement errors. Electrical testing (such as in-circuit testing or functional testing) verifies that all connections and components work correctly. Flexible sections are often coated with conformal coatings (acrylic, silicone, or urethane) to protect against moisture, dust, and mechanical wear. For example, a flex-rigid PCB in a medical implant undergoes rigorous functional testing to ensure it meets safety standards, and its flexible sections are coated with a biocompatible silicone to prevent bodily fluid ingress. With precise pre-assembly preparation, specialized component placement, reinforced interconnections, and thorough testing, flex-rigid PCB assembly ensures the production of reliable, high-performance hybrid boards for demanding applications.