22 May Flexible PCB Design Considerations
Flexible circuits are all around us – in cell phones, cameras, and laptops – and also in us – in pacemakers, cochlear implants, and defibrillators. Over the years, as devices have become smaller and wearable and implantable medical devices have become more prevalent, the flex circuit industry has boomed. Key Tech has also been involved in this trend. We’ve designed a number of flex and rigid-flex PCBs for various applications, and along the way we’ve picked up a few tips.
When compared to typical rigid PCBs (the green, stiff boards you see in most electronics), flex PCBs offer many other benefits in addition to being flexible. Flex circuits are made of layers of polyimide instead of FR-4 (fiberglass reinforced epoxy laminate), making the board much lighter. The polyimide layers are also thinner allowing the board size to be thinner than typical rigid PCBs. Flex PCBs can eliminate the need (and cost) for connectors and cables improving connection reliability and reducing assembly time, assembly cost, and overall size of the device. Additionally, when two rigid PCBs connected by a cable can be optimized to one Flex PCB, this simplifies the design to one Bill of Material, one set of design files to manage, and one PCB fabrication and assembly step.
If the benefits are so remarkable, why aren’t all PCBs of the flex variety? One little dirty secret of flex is that it’s not always that flexible. The bending portions become increasing less flexible as the number of copper layers increase. There are a number of good rules of thumb to estimate bend radius, one of the more conservative being the minimum bend radius should be greater than 10x the overall thickness for multilayer boards. From a design perspective, significantly more communication is required between the designer and board house to finalize materials stackup and address DFM considerations. Also, the manufacturing lead time is longer and more costly than that of rigid PCBs. Though flex PCB manufacturing has existed for a number of years, it is a newer art and therefore there are fewer manufacturers. And, unfortunately, snafus in manufacturing are more common for flex PCBs and do occur from time to time pushing back delivery lead times.
In summary, flexible PCBs have been opening the door to exciting new applications and devices that weren’t possible with rigid PCBs. However, in the early stages of product development the team must carefully consider the pros and cons of creating a flex vs. rigid PCB before jumping into a flex design.
