Rigid Flex PCB Process
Rigid flex is a circuit board that incorporates both rigid and flexible substrate materials, resulting in an assembly with both high performance and a compact form factor. These boards are used in many electronics devices due to their usefulness, reliability and flexibility. The manufacturing process of a rigid-flex PCB requires special consideration to manage the multi-layer integration and connections between rigid and flexible sections.
Rigid-flex PCBs are often made of a combination of FR-4 and conductive copper foil. The conductive copper is typically in the form of electro-deposited or rolled copper coils, which are then laminated with an adhesive or non-adhesive material like polyimide or fluoropolymer. The cladding is then treated to reduce bond degradation, enhance adhesion, and resist oxidation. The cladding is then laminated to the flexible and rigid sections of the circuit board.
A rigid flex PCB’s underlying structure is a 2-layer flex core with an outer layer of FR4 coverlay. This is followed by the patterned copper on both sides of the core and then a layer of solder mask over the copper pattern. The final step in creating the patterned copper is to etch the circuit pattern with either chemical or laser techniques. This etching is done to create pads and vias for component connection points on the rigid-flex circuit board.
The Rigid Flex PCB Process
Next, holes with the required specifications are drilled through the pads and vias of the patterned copper on both the rigid-flex and the FR-4 core. These holes must be plated with copper and then chemically deposited to ensure proper pad to pad and layer to layer electrical interconnection. The through-hole plating step is especially critical in flex circuits because these holes must be able to support the full design load and maintain stability during flexing, which can be quite substantial.
The next steps in the rigid-flex circuit board fabrication process are to laminate the flexible section with the patterned FR-4 core, as well as the non-patterned flex, and the final layers of copper. The flex and the rigid sections are then drilled for through-holes that are plated from top to bottom and blind vias, if any are needed, which is another time-consuming and detailed process. The final plated through hole and blind vias are then etched as per the design, and the etching process is repeated for both the rigid-flex and the FR-4 layer, which results in the rigid-flex circuit board.
The design of a rigid-flex circuit board is an advanced and challenging task that requires expert engineering skill. PCB software solutions such as Cadence’s allow designers to create accurate representations of a rigid-flex board, including stack up zones, bend areas and surface finishes. In addition, features such as zone aware placements and routing, can help to automatically make route keepouts and prevent traces from being added into a zone where there is no conductor layer. This allows designers to avoid introducing flaws in the layout. The design tools also enable engineers to automatically round off sharp corners on flex circuits, which can be susceptible to tearing during flexing.