Flex PCB Production Handles Multilayer Designs
Multilayer flex circuits offer flexibility and durability for dynamic, space-constrained applications. They are a mainstay of many modern technologies, and are poised to continue to grow as miniaturization and weight reduction trends move forward. They can be used in a variety of industries, but they are especially popular for medical devices, aerospace and automotive components, and high-performance mobile devices.
Flex PCBs are typically constructed from a rigid base and a flexible core layer, with one or two coverlays on top. The flex section is made of an unclad PI film or a single- or double-sided copper-clad laminate (CCCL). The coverlay is bonded to the flex section using either acrylic adhesive or an epoxy resin. The flex circuit is then drilled, plated through, and etched in much the same way that a 2-sided flex core would be processed.
The cladding process can be either adhesive-based or adhesiveless, depending on the materials used for the flex and core sections. The cladding technique also impacts the final dimensional stability of the multilayer flex circuit. Adhesive-based multilayer flex pcb production have good dimensional stability at high temperatures, while adhesiveless ones have a low sensitivity to temperature.
During fabrication, the flex section of the PCB is inserted into a routed slot in a rigid board, which can be made of MDF, plywood, Teflon, or other material that can handle bending and compression without deforming or damaging the flexible circuits. For high-volume production, a hydraulic punch and die set can be used to cut the flex circuits from the backing boards, with a hydraulic press ensuring that no creases or kinks form in the material. For smaller run sizes, a blanking knife is used instead.
How Flex PCB Production Handles Multilayer Designs
To prevent damage to the flex section, the drill-to-copper distance must be kept as small as possible, around 8 mil. This ensures accurate layer alignment and helps to eliminate issues like via flaking and cracking. It’s also important to use teardrop-shaped vias, as they reduce the risks associated with hole misregistration, and to keep the annular rings large enough to strengthen connections.
The layout of the traces and routing should be carefully planned at this stage, with collaboration between the PCB designer and the mechanical engineer to ensure that the board will fit into the final product. In addition, the conductive pattern and impedance should be optimized, and the layers should be arranged in such a way as to minimize flex ribbon bend areas where solder lands must be placed. It’s often a good idea to place power and ground planes on the signal layers, as this can simplify the manufacturing process and improve impedance.
Once the layout is designed, the PCB manufacturer will use an etching and imaging system to generate the circuit patterns on the FCCL and PI/PET films. They will then cut the FCCL and PI/PET to size, and process them for layer alignment and lamination, circuit testing, surface finishing, silkscreen printing, baking, and cutting edges. As with rigid-flex, the manufacturing processes for multilayer flex circuits are based on a highly automated process. The resulting product is a fully functional, reliable circuit board that can be assembled into the final product with little to no downtime.
