Flex Circuit Boards
The primary benefits of flex circuits in comparison to rigid PCBs are weight reduction, assembly time, and flexibility in the final product. They can also help to overcome design challenges by providing additional space for components. They can also be fabricated with areas designed for surface mount technology (SMT) which can reduce assembly steps and simplify the placement of components. Using a flexible PCB can also allow you to build more complex assemblies that would not be possible with traditional wire harnesses.
A flex circuit is an insulated,flex circuit board that is typically made of polyimide or epoxy film material that is etched with copper conductors. The resulting traces are then laminated with an adhesive-based polyimide coverlay and then a solderable surface finish, such as Electroless Nickel Immersion Gold (ENIG). The coverlay helps to protect the bare copper from moisture and environmental conditions.
To reduce the amount of stress placed on a flex circuit during bending, it is important to minimize discontinuities, such as vias, cut-outs and slits. This will reduce the risk of stress concentration spots which can lead to failures and malfunctions. It is a good idea to use annular rings instead of circular sections for vias in flex areas to decrease the likelihood of cracking during bending.
How Flex Circuit Boards Impact Assembly Yield
It is also a good practice to stagger the conductors that traverse a flex circuit bend area to distribute the load evenly. It is also a good idea to avoid 90deg bends which cause high strain and can damage the circuit.
Flex circuits are subject to the same mechanical requirements as other PCBs and need to be modeled with accurate material properties to determine their reliability, minimum allowed bending radius, and electrical performance. This includes controlling impedance and signal integrity, determining the appropriate thickness of the copper traces, reference plane layers, and shielding.
To increase the bending life of a flex circuit, it is often helpful to utilize higher grade, rolled-annealed copper foils. This process elongates the grain structure of the copper and makes it more springier in the Z direction, which is the direction that will be most stressed when a flex circuit is being bent or creased. Additionally, it is a good idea to consider using a stiffener in order to mechanically stabilize the SMT, connector and other areas of the flex circuit during bending. The stiffener can be made from a variety of materials, such as ceramic, FR-4 or a thermoset resin. In addition, it can be laser or photo imageable.
It can be a great way to improve the reliability and performance of your flex circuit. Talk to your fab house about the options available for your specific application. They can offer a solution that will meet your design goals, cost and mechanical requirements. They can also provide a mechanical sample to help resolve fit and form concerns prior to manufacturing your flex circuit. This will give you a good understanding of how your flex circuit will appear in the formed installed position and help you to anticipate any interference issues.