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Although flex and rigid-flex technology has been around for many years, it is only in recent years that it has come into its own. The reason for the increased requirements for the flex and rigid-flex technology is simple: Devices are getting smaller. We are packing increasingly more computing power and hence capabilities in smaller packages thus creating a need for smaller but more powerful and more flexible circuitry. Flex circuitry is in virtually every new product today from cellphones to medical devices, to scanners for ultrasound wands, industrial, and new defense/aerospace products.
The advantages of using flex technology are many. One of the most important is reliability; flex circuits are far more reliable over a much longer period than the old wire harnesses they are replacing. With flex circuits, for example, there is much less risk with the solder connections when you are using flex circuits to connect directly into the components.
There are several types of flex and rigid-flex boards. First, there are simple flex boards that are basically connector PCBs connecting one rigid PCB to another, as when you have two rigid boards positioned 90 degrees to one another. There is also what we call dynamic flex boards; these are robust flex circuits designed to be subjected to numerous flex cycles in devices such as laptops and notebooks and any other devices that are hinged and open and close many times. Consider this, just about every electronic device can be improved by using flex and rigid-flex boards. Think of a plane or even your car today—their functionality has been greatly improved by the incorporation of flex and rigid-flex PCBs.
The design and lay-out of flex circuit is critical so it is very important that the designer and the PCB fabricator communicate with one another as it is being designed. The designer must consider: What environment will the flex PCB be in? How much flexibility will be needed and how often? Will the board be put in place one time to connect two rigid boards or will the device need ongoing flexibility like with a laptop?
With this information, the designer can decide what flex material needs to be specified. The flex radius of the board will also need to be considered. How the board will be designed also depends on the components that are required in the final product. They need to get right down to specifying the copper grain direction in the lay-out. This is especially important when designing dynamic flex boards. When it comes to a multilayer flex board with more than one flex layer, the designer must decide if the two or more layers of flex will be bonded together or kept separated.
Therefore, we in the fabrication end of the business must work closely with the flex board designers to ensure that what they want is achievable. The fabrication of a flex and especially a rigid-flex PCB is vastly more complicated than that of a rigid PCB. There are typically more press cycles, more CNC routing, and the a much larger variety of materials. The migration or movement of the material is particularly critical when fabricating flex circuits as achieving good layer-to-layer registration is much more challenging.
Of course, thermal management is always an issue in all PCB fabrication and is even more so with flex and rigid-flex boards. Whenever so many different types of materials are used one must always be aware of the CTE (coefficient of thermal expansion) compatibility of these different laminates.
With that said, flex and rigid-flex technology is not only here to stay but demand is growing. As I said earlier, the advantages of this technology are so great that more companies are turning to it and incorporating it into their new products. We can expect to see many more requirements for flex and rigid-flex technology in the future.
I would advise anyone who requires rigid-flex today and in the future to find a good PCB shop partner—someone they can trust to handle their needs today and keep up with them in the future. If you’re smart, you’ll include this fabricator in the entire process of designing your flex boards from the material selection to cost considerations to manufacturability. Find a house with experts who know what they’re doing. There is no room for amateurs when it comes to flex and rigid-flex technology.
Dave Lackey is vice president of business development at American Standard Circuits.