Considerations for Rigid Flex Circuits

Rigid flex circuits are a solution for system miniaturization, signal integrity, and reliability in critical environments. They’re commonly used in military and aerospace electronics, but more and more consumer products are now using them as well due to their weight reduction and ability to withstand harsh environments.

Unlike standard printed circuit boards, which consist of two layers of copper bonded to a FR-4 substrate, rigid flex circuits have three or more copper layers. This allows for denser device populations and finer line widths and spacing, reducing package size and weight and freeing up space for additional product features.

The flexible areas in rigid flex circuits are typically routed with a larger bend radius than those in a standard PCB to prevent stress on conductors and damage to the board. As a result, you must carefully design the layout to allow for these large bending radii. A good rule of thumb is to ensure that the bending radius is at least twice as large as the board’s thickness. This will help you avoid causing unwanted stress and reducing the lifetime of your rigid flex circuit design.

Thermal Considerations for Rigid Flex Circuits

In addition to ensuring your rigid flex has enough bending stiffness, you should also pay attention to the positioning of your ground plane. If it’s too close to your pads on the rigid flex, it can cause unwanted thermal gradients that lead to solder fatigue failure and potentially short circuits. To avoid this, make sure your rigid flex design has at least 0.6mm between the ground plane and any pads.

While flex circuits have a lot of advantages, they aren’t without their challenges. Rigid flex circuits are more expensive to fabricate than their traditional counterparts, and they can be difficult to assemble and test. As a result, it’s important to work with your manufacturer during the design phase to make sure you’re implementing all of the best practices for designing a rigid flex circuit and that it’s being fabricated according to industry standards.

Rigid flex PCBs are ideal for applications that require significant flexing or vibration, such as cameras of all types and medical devices. They’re also a great option for applications that have to deal with extreme temperatures, such as aircraft and satellite components.

Another advantage of rigid flex circuits is that they can be used for both static and dynamic flexing applications. Static flexes are flexed once or twice during installation and operation, while dynamic flexes are flexed continuously throughout the life of the product.

Lastly, rigid flex circuits can be panelized to make them easier to assemble with the pick and place and wave soldering processes. This saves time and money, especially during design iterations and prototyping stages. However, it’s important to understand that panelizing rigid flex can increase manufacturing costs and the overall cost of your product.

Rigid flex circuits are drilled, plated through, and chemically etched in much the same way that rigid PCB cores are. However, the etching process can vary depending on whether you choose to use “button plating,” in which copper is only deposited on vias/pads (and not on the entire surface), or conventional through-hole plating.