limitations of flex circuit
A flex circuit is a hybrid printed circuit board with rigid sections integrated with flexible ones to create a PCB that can bend, twist and conform to the shape of the product it’s installed in. This allows for flexible and dynamic bending which is impossible with rigid boards and opens up new design possibilities in the form factor of the final product.
However, a flex circuit does have some limitations that you should be aware of before you start designing and ordering for your next project. These limitations relate to layer stack design, component placement and the fact that a flex circuit absorbs moisture during lamination, which can affect the performance of solder joints. Luckily, these problems can be mitigated by following some dos and don’ts of designing for a flex circuit.
Do: Stagger copper layers across adjacent layers. This reduces tension stress on the traces as they bend and helps to distribute stresses more evenly. Don’t: Avoid using stacked traces or plated copper in the center of a bend radius. Both of these can put undue stresses on the copper and cause it to break or crack over time. It’s also important to follow proper bend guidelines so that your flex circuit can withstand the forces it will be subjected to during use.
What are the limitations of flex circuit?
It’s also recommended to use a thinner copper foil than the standard copper used on rigid PCBs to get better conductivity. This is especially critical for high-speed flex circuits. You should also consider the types of coverlays, flexible solder masks, stiffeners and adhesives used on a flex circuit. These will differ from those used on rigid PCBs and may increase costs or change the thickness of the final product.
A flex circuit is also not designed to support high-powered components, like processors and GPUs, due to its bending and twisting characteristics. In these cases, it’s best to use a rigid-flex PCB or a combination of rigid and flex circuits to accommodate these higher power requirements.
For lower powered applications, polymer thick film (PTF) flex circuits are an option. These are single conductor layer structures that print onto a plastic film with insulating layers between each printed metal layer. While this type of flex circuit is not as flexible as other types, it can still be folded and bent with minimal damage to the components and solder joints.
Flex circuits are a cost-effective alternative to wire cable and can simplify system assembly. This is because a flex circuit eliminates or reduces wiring, which can save you the expense of creating numerous purchase orders and the cost of receiving, inspecting, kitting and assembly. It can also reduce the number of parts and levels of interconnection, reducing the chance for human error.
It’s important to keep in mind that the flex circuit materials must be able to work reliably together to meet the demands of your application. The z-axis expansion coefficients of the adhesives, the lower adhesion of copper to the PI substrate and coverlay, and the metallurgical properties of rolled annealed (RA) base copper all affect how well your circuit can perform under bending.
