Construction of Flexible PCBs
The raw materials used to construct flexible pcb have a direct impact on the performance and cost of the finished product. Choosing the right material depends on the design requirements and manufacturing process. For example, PCBs with welding requirements require a material that is capable of handling high temperatures and reflow during assembly. This is why we recommend using a polyimide material for these applications.
The leading conducting material in flex PCBs is copper foil, but sometimes an alloy like aluminum or nickel is also utilized. The insulator is usually a film material, such as PI or PET. In some cases, a glass fiber or epoxy substrate is used as well. These materials are laminated to the flex circuit with acrylic thermally-cured adhesives or epoxy resin. Stiffeners are sometimes added to the flex circuit board in select locations to add rigidity. Materials like FR4 and Kapton are frequently employed as stiffeners in flex PCBs, but aluminum or stainless steel is also sometimes utilized. These stiffeners can provide strain reduction, weight balance, and help in heat dissipation.
A flex PCB must be durable enough to withstand repeated bending and rolling movements. To minimize the risk of damage, manufacturers use a special material called “rolling annealed copper.” This higher-grade copper is much more resilient and can stretch before fatigue cracking begins. Additionally, it’s springier in the Z deflection direction – perfect for flex circuits.
Materials Used in the Construction of Flexible PCBs
To insulate the copper conductor and protect it from corrosion, manufacturers use a layer of coverlay that’s often made of PI. In addition to covering the conductor, this layer provides a solderable surface that’s compatible with most components and soldering techniques. The final coating is a solder mask that’s applied in the same way as a conventional solder mask.
In some cases, a protective layer of tin or gold is added to the copper in order to prevent corrosion and improve conductivity. This is especially important for flex PCBs that will be subjected to harsh environments and chemical interactions.
Lastly, the surface of the conductive copper must be protected with a finish that’s resistant to oxidation and can tolerate high temperatures. The most popular finish for flex PCBs is Electroless Nickel Immersion Gold (ENIG), which is used on about 80% of all boards.
Once the layout has been analyzed, it’s time to create the physical circuit board. This involves routing the traces, identifying and labeling components, and drilling holes. This stage is also the time to incorporate any reference designators and markers into the layout.
The resulting files can then be sent to the PCB layout provider who will place the components and drill holes according to the design guidelines set by the customer. Once the layout is ready, the manufacturer will apply the various layers to the PCB and perform any necessary soldering. After that, the flex PCB will be ready for use.
