Rigid flex printed circuit boards (PCBs) are highly versatile circuit boards that incorporate aspects of both hardboard and flexible circuits. They are typically composed of multiple layers of rigid circuit boards with layers of flexible circuitry buried within the hard boards.
Although rigid flex PCBs come at a higher cost than rigid PCBs, they are more versatile and easier to tailor to the needs of specific applications. Rigid flex PCBs are particularly useful for military, aerospace, and medical applications, where they exhibit resilience to high levels of shock and vibration. They are easier to assemble into precise applications, and their smaller and more flexible configuration makes them well suited to electronics where circuit weight and space are of particular consideration.
When appropriate, cost-saving measures can be exercised to make sure you are still able to get the best value from rigid flex PCBs. The number of circuit layers, materials used, and design processes can all be tailored to meet the needs of your application.
Rigid Flex PCB Cost
Rigid flex PCBs are approximately seven times more expensive than an equivalent rigid PCB, largely due to the raw materials required, and in general lower overall yields.
Rigid flex PCB’s use materials that are common to hard board and flexible circuit PCB manufacturers – core, prepreg, copper foil, flexible laminates, coverlayers and bondplies. But some are unique to rigid flex manufacturers and cost more than traditional PCB materials.
One of the most critical components to successful rigid flex manufacturing is the use of no or low flow prepregs. No flow prepregs are necessary to prevent the flow of epoxy or polyimide resin out onto the flexible sections of the boards. They flow enough to go to the edge of the rigid, without flowing out onto the flex arms.
No flow prepregs generally cost about 10X traditional FR4 and polyimide prepregs. Additionally, they are only available in two glass fabrics – 106 and 1080, which are very thin. Rigid flex manufacturers do not have the option of using some of the lower cost, and thicker fabrics, such as 2113, 2313, 2116, 1652 and 7628 because they are not available in no flow variants. Consequently, rigid flex manufacturers use a more expensive ply of prepreg, and at the same time need to use more sheets to glue the final composite package together.
Also, because it is a no flow resin, it is generally risky to use a single ply of prepreg, typically utilized by hard board manufacturers to reduce board costs. Two plies are usually essential to assure adequate encapsulation of the internal circuitry – another factor that increases cost of rigid flex designs.
One cost driver in rigid flex manufacturing is the flexible copper clad laminates within the package. There are many different types of flexible laminates in the World – polyester, PEN, polyimide, and even paper, etc. Some are made with adhesive systems to bond the copper onto the base film, and some are not. Rigid flex manufacturers to improve yield, use almost exclusively adhesiveless laminates with polyimide base materials – generally the most expensive flexible laminates available. Additionally, controlled impedance designs, generally require thicker polymide films, to make the impedance circuits function properly. Those materials come at an exponentially higher cost than the thinner materials.
Rigid flex boards are built in separate components and then assembled into the final board – adding processing steps and complexity, which increases cost. In general, slower, more conservative and intentional manufacturing methods are used by rigid flex fabricators to achieve better manufacturing yields.
The last thing that impacts the cost of rigid flex boards is overall manufacturing yield. Rigid flex boards combine materials with very dissimilar dimensional stability characteristics. Getting those dissimilar materials to register to one another requires experience, specialized equipment and software tools. Even with those though, yields in rigid flex manufacturing are inherently lower than hard boards or conventional flexible circuits.
The overall cost of rigid flex PCB manufacturing can, therefore, be reduced by choosing materials carefully and tailoring the material set to the specific needs of the application. The less material used, the greater the reduction in manufacturing costs. We recommend working closely with your rigid flex PCB manufacturer to find out if raw material costs can be streamlined for your specific needs.
For a good starting point, for designing low cost rigid flex designs, consult our Valu Build Brochure that is available here. The Valu Build program gives material layups and suggestions to provide you with optimized cost solutions for your design.
Minimizing Rigid Flex PCB Design Costs
You can further lower the cost of rigid flex PCBs by tailoring the material to minimize the layers, thickness, and overall costs. Some of the major considerations that will affect the cost of your PCB design include:
Keep overall layers to a minimum: Reducing the number of layers in your design, reduces the number of plies of prepreg required to bond your board together. At the same time, fewer layers optimize the ability of the manufacture to improve manufacturing yields – both of which reduce your overall cost.
Keep flexible layers to a minimum: The flexible laminate is more expensive than the rigid laminates. Limiting the number of layers of flexible circuits, reduces your overall cost for the board. The flexible layers are constructed separately from the final rigid flex board, which also adds to their cost. Reducing the layers of flexible circuitry on your design, lowers your overall package cost.
Use rigid board laminates to achieve overall thickness: If you are attempting to achieve a specific overall thickness, try to do so using the rigid board laminates, rather than additional plies of no flow prepreg or flexible laminates. The rigid laminates are the lowest cost material in the construction.
Limit controlled impedance requirements: In high speed designs, it is tempting to define all the impedance values that you wish to achieve. And this is fine, and you should employ your fabricator to help model all the impedance values you desire. However, the print should list only the impedance traces that you want tested. With rigid flex designs and test coupons required for the flexible sections as well as the rigid sections, the impedance coupons can get very large very quickly, removing parts from the production panel. It is wise to model and test only those impedance values your design truly needs.
Have all flexible arms in the design terminate in rigid boards: Often rigid flex designers want one or more arms to end in a flex cable, typically for mating with a zif connector, or other device. They do not desire the thickness or rigidity of the hard board in these sections. To achieve this feature though, the rigid flex manufacturer must incorporate a technique called “pouching” to protect the flex arm, during outerlayer manufacturing. Pouched rigid flex boards require a lot of extra hand processing to build them successfully and should have strain relief beading applied to the rigid to flex transitions area – a subsequent hand applied material. Having all flexible arms terminate in rigid boards affords you the lowest possible cost for your design.
Specialized processing: There are techniques that are very common to rigid board designs that are much more difficult to achieve reasonable yields in rigid flex designs. Because they are difficult to achieve, they create lower overall manufacturing yields, that increase the cost of your part. Some of the most common elements are via in pads requiring filled vias, dual surface finishes such as ENIG and electroplated nickel gold or hot air leveled solder, buried, blind and laser vias, innerlayer copper thicknesses greater than three ounces, blue and black soldermask, and V scoring can all present challenges for your fabricator. Consult your fabricator for advice on what they would suggest for improving the cost of your design.
Manufacturing Cost Considerations for Rigid Flex PCBs
In addition to optimizing material usage and design parameters, the configuration of the PCB can be simplified to further reduce manufacturing costs. The size, shape, complexity, and configuration of a board will affect the cost of materials and assembly. Any additional plated slots, edges and other customized design requirements will reduce yield and further inflate the cost.
Early involvement of the fabricator in the product design process allows the fabricator to produce PCBs optimized for the end application, without the need for costly adjustments or redesigns. A full assessment of available manufacturing options can be conducted at the beginning of the design process. This careful planning will ensure the design is optimized for use in the desired applications, a successful first time build, while accounting for expected product variations.
Affordable Rigid Flex PCBs From Printed Circuits
Although more expensive than traditional rigid counterparts, the manufacturing costs of rigid flex PCBs can be significantly reduced using thoughtful engineering that reduce unnecessary complexity and material usage. In addition, the savings in product design, logistics, and assembly make rigid flex PCBs a cost effective option.
For more than 40 years, Printed Circuits has been providing cutting-edge PCB technology for customers around the world. We are dedicated to providing the highest quality rigid flex and flexible circuits in the industry. To learn more about reducing rigid flex PCB costs, contact us today.