Reliability in Rigid Flex Circuit Assemblies
Rigid PCBs require a certain amount of space and can have difficulty fitting into small applications. This limitation is overcome by using rigid flex circuits that are both highly flexible and able to provide the electrical functionality required. These hybrid PCBs are often used in automotive, medical, and military applications where reliability is paramount. Rigid flex circuits have the ability to withstand harsh environments including extreme temperatures, moisture, and chemical exposure that could damage other types of PCBs.
The rigid flex circuit panel is typically assembled with components on the flex sections that are then soldered to the remaining rigid areas of the board. In cases where the flex circuit is required to bend at specific points, stiffeners are added to reduce the amount of stress placed on these bending areas. This will prevent the copper traces from failing and damaging the circuit.
It is important to have a good understanding of what the minimum bending radius should be for a given application. By adhering to these guidelines, it is possible to create rigid flex circuits that can withstand sudden shocks and vibrations as well as hundreds of thousands of bending cycles without failure.
How to Ensure Reliability in Rigid Flex Circuit Assemblies
Using the correct materials in both rigid and flexible portions of the circuit is another key factor to consider. Different materials have varying thermal expansion coefficients that can cause signal integrity and electromagnetic interference (EMI) issues. Additionally, the thickness of the rigid and flex sections of the circuit will affect how they are able to bend and the load they can bear.
When a circuit is exposed to high levels of vibration, the copper traces can experience stress that may cause them to lift from the surface. Known as a “trace lifting” issue, this can lead to short circuits or signal disruption. Ideally, it is best to avoid overlapping copper traces on adjacent layers in dynamic double-sided flex circuits. By staggering traces across different layers, this can reduce the overall stress on the copper and allow it to bend more easily during use.
In addition to a proper layout, it is also important to properly size the pads and coverlay openings for maximum reliability. This will ensure that the adhesive has sufficient “squeeze out” when the flex section is subjected to flexing. IPC-2223 recommends 360deg of solder wetting around the pads for high-reliability flex designs and 270deg for moderate-reliability flex designs.
Choosing the right manufacturer for your rigid flex project is critical. You should look for a manufacturer that has extensive experience and a strong understanding of the unique challenges and requirements of rigid flex circuits. Additionally, you should make sure the manufacturer adheres to industry quality standards and has a solid track record of success.
With the help of a reliable PCB designer, you can be assured that your rigid flex circuit assembly will meet and even exceed your expectations. By following these tips, you can be sure that your rigid flex circuit will be able to withstand the most challenging environmental conditions and will be reliable for years to come.