Implement Redundancy in Flex Circuit PCBs
There are many things to keep in mind when designing a flex circuit PCB. These include layer stack design, parts placement, and cutouts. However, there are some gnarly material quirks that are specific to flex circuits and that can throw a wrench into your design plans. These can range from higher z-axis expansion coefficients of adhesives to copper’s work hardening and fatigue. Fortunately, these can be compensated for by following some Dos and Don’ts of flex circuit design.
First, understand the difference between a rigid and flex circuit. Rigid printed circuit boards are constructed from a woven fiberglass impregnated in epoxy resin, and they have a reasonable amount of elasticity to them. They can withstand quite a bit of stress from product movement and are fairly tolerant to solder reflow cycles. Flex circuits, on the other hand, are more flexible and must be able to withstand much greater bending loads. They also require a different set of materials and fabrication processes to ensure the integrity of the circuit.
The main differences between a rigid and a flex circuit pcb are the number of layers and the bending radii. Flex circuits typically have a lower layer count than rigid boards and the bending radius for the traces needs to be much tighter. Keeping the copper layer thickness low and staggered in adjacent layers is essential to improving the bendability of your circuit.
How to Implement Redundancy in Flex Circuit PCBs
Additionally, if your flex circuit requires high speed operation you will need to consider the use of redundancy. This could involve using a spare copy of the circuit that is only brought online by a GPIO ENABLE toggling. This would allow you to verify that the redundant circuit is working before bringing it online and potentially eliminating downtime caused by a component failure.
Another consideration is that flex circuits are prone to more movement during manufacturing and assembly. As a result, you will want to make sure that the drill-to-copper distance is at least 8 mil on all the pads/vias in your flex circuit. This will minimize the chances of a single copper-layer failure causing all the traces to fail in the same region.
In addition, you will want to consider whether to use panel plating (which uses copper on all traces) or pad-only plating (button plating). Button plating is generally recommended for flex circuits because it allows for more precise control of the copper thickness and width and better etch yields in small traces. However, it can be more expensive than panel plating because of the extra production steps involved.
Finally, you will want to make sure that all traces in your flex circuit have a continuous path to the ground plane on each side of the circuit. This will help reduce the possibility of a single point failure and improve signal transmission. Additionally, you will need to make sure that all inside corners on your flex circuit have a radius of more than 1.5 mm. This will prevent tearing of the flex material at the corner.