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Designing a flex printed circuit board requires a slightly different approach than rigid PCBs. While flex PCBs can provide major savings in manufacturing cost as well as reduced space consumption and lower weight compared with rigid, their design must be optimized for their materials and use cases. A well-designed flex PCB will be lightweight, durable, easy to install, and suitable for demanding applications such as wearable devices and satellites. Indeed, the physical advantages for flex is that it offers an improved resistance to vibrations and movement, and it is easier to prepare for harsh environments.
There are plenty of ways to ensure a high-quality flex PCB design. Let’s look at five things you should know going into your first attempt at flex PCB:
It is vital to know two things in relation to bend: how many times it will be bending, and what it can bend. The amount of times it can bend, or application, determines whether the board will be a static or dynamic board. A static board is considered bend-to-install, and will flex less than 100 times in its lifetime. A dynamic board’s design needs to be more robust in nature, as flexing will be done on a regular basis—and will need to withstand tens of thousands of bends.
Bend radius—the minimum amount of bendiness for the flex area—must be properly identified early in the design. This ensures your design can allow for the necessary number of bends without damaging the copper. The figure below will help determine how thick you can make your circuit.
When laying out the bend areas, avoid 90-degree bends that cause high strain. Plated through-holes should be avoided in the bend area, conductors should be staggered in multilayered circuits (for greater effectiveness), and conductors smaller than 10 mils should be placed within the neutral bend axis where there is no tension or compression during flexing.
Flex PCBs usually require looser outline tolerance than other boards. This is because their materials have less dimensional stability than rigid ones. Depending on the profile tolerance, you may also require a hard tool or laser cutting, which can be expensive.
Also, note that flex PCB materials may contain acrylic adhesives. Since these chemicals can become soft when heated, it is important to make your pads as large as possible. Using spurs, anchors, and/or teardrops in your design can help stabilize the outer layer and reduce stress. Multi-layer designs are also a way around adhesive issues.
Circuitry layout makes or breaks a PCB. Going back to the bend radius, a large radius is preferable here to the sharp angles that shorten a board’s lifespan. Moreover, it is best to avoid I-beaming so as to minimize the stress that can thin out copper circuits.
Curved traces cause lower stress than angled ones. Traces should also be kept perpendicular to the overall bend and, if placed on a flex PCB with two or more layers, staggered on the top and bottom.
For rigid-flex PCBs, Sierra processes the flex layer as a two-layer board, laminates it between the rigid layers, and mills it so the flex is visible. Putting flex layers on the inside of the stack-up provides protection from exposure to outer-layer plating. This placement also simplifies manufacturing and improves impedance and control in the flex area.
The flex layer can be etched away from the design as part of a separate process, allowing for more protection.
It is possible for vias to crack or break peel flex PCB designs. To prevent them from doing so, make sure that vias are tear-dropped, that anchors and or tabs are added, and that the annular rings are as large as possible.
