Design Tips for Rigid-Flex PCBs

PCB designers are always looking for the next idea to save time, money, and effort. By exploring recent developments in design standards, it is possible to find new ways of overcoming common problems in PCB design. One example comes from a rigid flex circuit board. These products represent a hybrid of rigid flex PCBs, combining the best benefits of both types of products. They are used in applications that require the design flexibility offered by flex boards with the high-frequency performance and high-quality long-term reliability of rigid boards.

Rigid flex PCBs are a great option for anyone looking to add a complicated, thin-profile circuit to their design. However, these boards can be tricky to work with at first, and you need to know the best way to handle them. Follow this blog post to learn some tips and tricks on how rigid flex PCBs work and what you should consider when designing with this type of board.

Technical Design Tips for Rigid Flex PCBs

Here is a rundown of the most common design tips for rigid flex circuit boards:

1. When designing rigid flex PCBs, consider that the bending radius of rigid parts should be larger than that of flexible parts. Otherwise, the flexible part may be broken during board assembly or operation.
2. To increase mechanical strength and reduce crack propagation on rigid flex PCBs, add more copper plating layers on areas with high mechanical force and choose thicker base materials such as FR4 or Rogers RO4350 having high thermal conductivity to improve heat dissipation performance.
3. Make sure that the flex traces are thick enough. The flex trace thickness can be calculated by dividing the total length of the flex trace by its width. For example, if you have a 10 cm long flex trace with a width of 0.1 mm, it should have a thickness of 0.1/10 = 0.01 mm.
4. Since rigid flex boards flex more than traditional PCBs, there is more stress on solder joints and traces when the board bends — so use larger pads and vias than you would otherwise use on an FR4 or GFR4 board. The extra copper helps spread the stress from bending and makes soldering easier. In fact, pre-tinned pads should be used whenever possible to increase solder adhesion and reduce voiding during reflow soldering processes.
5. Ensure that there is enough clearance between the top of the rigid portion and the bottom of the flex portion. This will prevent shorts between the two parts when they are assembled.
6. Consider using a ground plane to provide a low impedance path for heat dissipation to reduce thermal impedance and avoid large temperature gradients that lead to solder fatigue failure. However, if your ground plane is too close to the pads on your rigid flex circuit board, it can cause problems with signal integrity. To prevent this from happening, make sure that there is at least 0.6mm between your ground plane and any pads on your rigid flex circuit board layout.
7. Use an adequate number of vias to provide good thermal conduction from one side of the board to another, especially on high-speed boards where large currents can cause thermal resistance problems. However, avoid using vias that cross over traces and pads too closely together and use via arrays that are too close together. This will help reduce the risk of cracks forming due to movement during flex circuit board fabrication or use in harsh environments.
8. The core layer is in the center of the board, where all the traces connect. It should be made from a rigid material such as copper-clad laminate (CCL) because it combines all the traces together and provides good electrical conductivity. This is especially important if you are using an air gap instead of a solder mask or want to make connections without soldering them directly onto the surface of your board, such as through-hole mounting.
9. Flexible interconnects are often used on rigid flex boards due to their ability to withstand wear and tear while remaining functional over time. However, it is important to choose the right type of flexible interconnects for your application based on its environment. For example, if a device will be used outdoors or in harsh environments, selecting an epoxy over a conductive adhesive would be beneficial because epoxy has higher resistance than conductive adhesives do against moisture and humidity and thus better long-term performance.
10. If you are designing a system with components such as antennas or RF transceiver chipsets, you need to make sure that they are mounted on a rigid flex circuit board with enough bending stiffness to not break off during use. Otherwise, your product design might not be as effective as you would like it to be.
11. Take care when designing over large areas of copper. This is because it is more difficult for the rigid flex circuit board manufacturer to apply solder paste across large areas of copper than it is on a standard FR4 PCB. If possible, try to avoid large areas of copper in your design.
12. When designing with multiple layers of materials with different thermal conductivities, try to put your high current traces on top where they will be cooled by airflow from fans or other cooling devices rather than having them underneath any different layers. This is to avoid overheating issues caused by high current paths passing through low thermal conductivity layers.
13. Take advantage of bend radii and layer stacks. You can use these features to place components closer together than they would otherwise be able to fit on a single layer of the PCB. This will reduce the amount of space taken up by trace routing and improve electrical performance when bending occurs in certain directions, such as moving from left to right.

Final Word

Designing for Rigid Flex PCBs can be tricky, but with a little foresight, you will be able to conquer the design task and save time, especially if you follow Hemeixin’s guidelines on how to make your PCB flex well and look nice. This article is one such guide. If you have ever had an experience with rigid flex circuits, it will teach you new ways to improve production and troubleshoot future designs. If you are just getting started working with them, it is an indispensable guide that could help you solve problems before they happen.