Key Considerations in Designing Rigid Flex Circuits
Designing Rigid Flex Circuits
Rigid flex circuits are an increasingly popular choice with the miniaturization of technology and the explosion in wearable devices. They streamline the manufacturing process by integrating the rigid areas into the flex area and eliminating the need for additional connectors and cables to connect individual printed circuit boards together. However, they do have their own unique manufacturing process constraints that must be taken into account. Having a thorough understanding of these complexities can help designers ensure their final product meets all required standards and requirements.
One of the key considerations in designing a rigid flex circuits is the materials used for the flex and rigid areas. There are many different mask types, dielectrics, stiffeners and thicknesses of copper layers that can all impact how well a flex or rigid-flex board will bend. Understanding these factors can help PCB designers optimize their designs for maximum flexibility and cost efficiency.
Conductor width and trace spacing should be designed to minimize stress on the copper traces. This can be done by staggering the traces along the length of the flex section or by using large corner radiuses to eliminate tears at the ends of the flex sections. It is also important to use tear guards to prevent the flex section from pulling away from the rigid part of the board during assembly.
Key Considerations in Designing Rigid Flex Circuits
Pad fillets are also important in a flex circuit to improve the etch yield and material strength. These can be designed in-tool or as part of the CAD process to eliminate holes and reduce the number of vias needed. Another option is to use a pad-only plating method (button plating), which will increase the etch yield and improve signal transmission in small etch patterns. This can also be implemented on a rigid-flex PCB to improve impedance control and reduce signal skew.
Stiffeners are also important to consider, especially for the extremity ends of a flex section. Adding a bead of epoxy, acrylic or hot-melt adhesive to these end points can limit movement and protect the integrity of solder joints at these locations. This can be an automated process, reducing assembly time and cost, or it can be hand-applied and cured under UV light. Ideally, these stiffeners are specified in the fabrication drawing so that they do not get overlooked by the manufacturers.
A good place to start is to decide if the flex section needs to be flexible or not. If the flex section will be permanently folded and positioned in a static position – like an ultrasound device – then there are few limitations on layer count, copper thickness or type and other design features. However, if the flex will need to move, bend and roll repeatedly, then there are more restrictions on the design that should be considered up front.
Other important details include the use of a coverlay layer to protect the flex, ensuring there is anchoring at each end of the flex section during assembly and to avoid any unwanted movement. The coverlay can be an additional polyimide film with adhesive or, for a more cost-efficient option, a photo-imageable solder mask.