Number of Layers in a Rigid Flex Circuit

Rigid flex circuits combine the advantages of rigid and flexible PCB technologies. This allows them to flex, bend and fold into different shapes without losing structural integrity or performance. It also eliminates the need for extra connectors, cables and other wire assemblies, making it easier to assembly and more cost effective.

This type of circuit is used in a wide range of consumer electronics from smartphones to laptops. It is also common in medical and military equipment, as it can help fit more functionality into smaller spaces. Rigid flex circuits provide higher levels of reliability and allow for denser device populations, lighter conductors, and smaller hole sizes.

In addition to the increased flexibility, these rigid flex circuits can save space and reduce weight by allowing multiple systems in rigid boards to be combined into one single component. The resulting product is less expensive and weighs less, which is an important factor in many of today’s electronic products.

Limitations to the Number of Layers in a Rigid Flex Circuit

The limit to the number of layers in a rigid-flex circuit is dependent on the complexity of the design, but typically no more than six. This is because the etching process becomes more difficult as the number of layers increases. Additionally, the rigid-flex circuit needs to be thick enough to withstand its mechanical bending requirements.

As a rule, the conductive layers in the flex area should be kept to a minimum and the space between traces minimized to avoid signal interference. It is recommended to place the signal layers in the center of the board, and the power and ground planes on either side. In this way, the traces can be routed in a circular or teardrop shape that will not interfere with the stiffeners and provide greater mechanical flexibility.

When routing traces, care must be taken to avoid the flex areas around bends, as they may have high stresses in them. In addition, the traces must be placed in perpendicular to the bends to prevent them from getting damaged. Finally, if possible, it is advisable to stagger traces across the flex section instead of arranging them in a line along the axis of the bend.

To increase the flexibility of a rigid-flex circuit, it is often necessary to use S curves. This helps reduce the stress on the flex areas, and it can also help to reduce the size of the solder pads and tracks.

Another way to improve flex-ability is to add hashed ground planes in the flex areas. This will help to filter out EMI from the signal lines, allowing for better flexibility. In addition, it is a good idea to use a lower flow prepreg than the one used for rigid-only boards in order to ensure that the prepreg will not extrude out of the corners of the flex area during lamination. If you are designing a rigid-flex circuit, be sure to consult with an experienced designer to make sure that the stack-up and construction is suitable for its intended application.