Manufacturing Flex PCBs
A flexible printed circuit board (PCB) is a thin copper foil substrate with conductor patterns that create an electrical pathway over the surface. These pathways allow flex circuits to bend, fold and withstand the dynamic movements of the assembly during use. Flex PCBs are used in a variety of applications where flexibility and weight savings outweigh the cost and complexity of rigid or hand wiring.
The process of manufacturing a flex pcbs begins with a design using computer-aided CAD software to layout the circuit and determine the components needed. The design will also include the specific bending and flexing requirements of the assembly.
Next, the appropriate substrate material is selected. This usually involves either a polyimide or polyester (PI and PET) film. These films offer good insulation and high-temperature resistance. A photoresist is applied to the film and then exposed to UV light through a mask with the desired circuit pattern. Then, etching is applied to remove the areas that will not be copper plated, leaving behind the designed conductive pathways or traces.
Depending on the application, a coverlay is added to protect the conductor pattern and the underlying substrate from harsh environmental conditions such as harsh chemicals and solvents. A solder mask is also applied to the conductive pattern for insulation and anti-oxidation. For through-hole connections, immersion tin is typically used as it offers good solderability and lower costs than gold.
Rigid sections are then integrated with the flex section to form a hybrid PCB known as a rigid-flex circuit. These are useful in applications where the circuit needs to be folded or bent for a dynamic application such as a portable device that flexes during operation, or to replace a wire harness.
The Process of Manufacturing Flex PCBs
Conductor traces and pads can be patterned on a rolled flex circuit to create ribbon cables for interconnecting displays, PCBs and other modules over hinges and slides. Membrane circuits incorporate conductor traces and spacers to create ultra-thin touch sensitive control switches that can conform to surfaces.
The final step in the manufacturing process is cutting the flex circuit to size. This is commonly done with a hydraulic punch and die set for high-volume production runs. For low-volume and prototypes, a blade called a blanking knife is used.
Once the flex PCB is cut to the proper size, SMT (surface mount technology) component attachment can be done in a variety of ways including surface mount adhesive placement (SMA) where an adhesive paste or film is screen printed and cured with heat or UV. Another option is dispensing liquid adhesive where a syringe or needle dispenses the adhesive directly to the component terminals. Finally, through hole components can be attached to the flex board with adhesive placed under the terminals or preapplied to the component itself. The resulting assembly is ready to be shipped to the customer.
The manufacturing process for rigid PCBs is well-established, with automated machines capable of producing them in large quantities. This level of automation and established production methods keeps the costs low, especially for simple designs. Flex PCBs, however, require more intricate processes such as the use of multiple layers, special adhesives, and precise handling of delicate materials. These additional steps lead to higher labor costs, specialized equipment requirements, and longer lead times. Moreover, flex PCBs often require more thorough testing to ensure reliability due to their dynamic nature (i.e., bending and flexing), which adds to their cost.
