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Rigid-Flex PCBs Offer The Best of Both Worlds

Rigid-flex PCBs combine the advantages of rigid and flex circuits in a single package

The history of electronics is one of massive reductions in size and increasing power for several decades. Yet, as the pace of manufacturing improvements and technological advancements have slowed, electronic design has had to accommodate new challenges, namely fitting electronic systems into enclosures and designs that would be impossible with traditional rigid boards. While flex offers unparalleled bending, some stackups need to find a middle ground between the density of rigid boards and the mechanical abilities of flex. Rigid-flex PCBs operate within this space, offering a best-of-both-worlds scenario for printed circuit manufacturing.

Advantages and Disadvantages of Rigid-Flex Printed Circuits
It can combine the benefits of discrete rigid boards and flex circuits into a unified package while cleverly balancing their constraints.

It may be more applicable than a pure flex circuit due to circuit density in a rigid board.
The interface between the flex and rigid portions of the board is highly susceptible to failure.

Rigid-flex is a significant cost adder to the fabrication process, and there are additional considerations during design/system integration to consider.

Constraints of Rigid-Flex: When Is It a Design Requirement?

Rigid-flex boards are a hybrid between standard PCBs and flex printed circuits, offering advantages of both at the cost of a highly intricate manufacturing process. In terms of operations, rigid boards are the go-to option when design factors are not limiting, making the board production routine and predictable, while flex offers improvements to the thickness and applicability (i.e., wearables) of printed circuits; in comparison, rigid-flex combines the flexibility of latter with the stabilization of the former. There’s an inherent difficulty in matching flex and rigid materials’ physical and electrical characteristics, mainly as the transition between the material types results in significant manufacturing hurdles.

Notably, the rigid portion of the stackup must be balanced (a symmetrical arrangement of materials layer-by-layer about the center), whereas this is only a preference for the flex section of the rigid-flex PCB. Designers and manufacturers must take the extra step of ensuring the copper is also balanced between the layers: copper acts as a dimensional stabilizer, lending rigidity to the overall structure that the flex portion of the PCB lacks. Large differences in the copper layer distribution can also reduce the board’s thermal routing capabilities. However, the most notable issue manufacturing must account for is the tendency for stress to accumulate at the rigid-flex junction, resulting in strain, deformation, and eventual mechanical failure. Careful manufacturing techniques can reinforce this weak point, moving the stress away from the edge and into the rigid region that is more resilient.

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Also worth observing is the general increase in complexity with rigid-flex as the layer count increases. While a rigid PCB can easily reach layer counts well into the double digits, multilayer rigid-flex becomes a much more daunting production. Multiple breakout routes are one method to meet the high-density requirements of a rigid-flex while still maintaining manufacturability. In these designs, the rigid-flex is more akin to a wire harness with discrete layers or layer groupings used to route connections between different areas of the system physically. For example, the rigid section of the board may account for layer pairs on the top and bottom sides of the stackup, with inner flex layer pairs routed to a corresponding system area. While this is technically a multilayer stackup (and relatively complex in its own right), realizing rigid-flex designs of this caliber is much more reasonable.

How Rigid-Flex Impacts Design Constraints

To account for the bending of the flex layers, manufacturers will need to vary the lengths of the flex layers between rigid sections. During bending, the flex layers on the inside half of the bend experience a shortening compressive force, while those on the outside half of the bend undergo a lengthening tensile force. Suppose the rigid-flex board can only flex in one direction during operation. In that case, the easy solution is lengthening the necessary flex layers accordingly to avoid excess stress on the outside bend. Many boards are bent bidirectionally depending on the operational forces applied. In those cases, lengthening the outside flex layers on both sides (to account for either bend direction) by approximately 1.5x the layer thickness should withstand the full range of bending motions (the exact bend radii will help guide this decision).

The thickness of the conductor layers will also play a significant role in the long-term functionality of the flex portion of the circuit. As thickness in the flexible region grows, flexibility decreases; the ideal solution for sections of the flex circuit meant to undergo repeated, dynamic flexing during its surface life is a single conductive layer. Static bends are less stressful on the material and can accommodate a greater thickness, provided an appropriate bend radius supports the flex material. Prototyping is essential to stress-test the materials according to the design parameters of the circuit; designers can abide by a flex bend radius “rule of thumb” to ballpark these values.

Bend Radii By Thickness
Metal LayersBend Radius
13~6x thickness
27~10x thickness
Multilayer15~20x thickness
Dynamic flex (single layer)20~40x thickness

Drilling is more feasible in the rigid sections of the board, but flex drilling is possible. While manufacturers may recommend avoiding flex drilling due to long-term reliability concerns, design intent may be the final arbiter. Manufacturers will add anchoring to the pads, providing additional surface contact for the weak adhesion between the conductor and outer flex layers.  Like a sequential lamination for microvias, flex drilling will require supplementary and separate drill-then-plate steps; as a result, per-board fabrication costs and processing time will increase appreciably.

Your Contract Manufacturer Prioritizes Flexibility in Design

Rigid-flex PCBs provide a novel solution to many issues rigid boards or flex-printed circuits face. However, the fabrication requires additional considerations (not to mention cost) that make the decision nontrivial. As always with DFM, designers should speak with their manufacturers to determine the best path to meet design intent while optimizing cost. Here at VSE, we’re a team of engineers committed to building electronics for our customers that satisfy all performance requirements while maintaining long-term reliability. Together with our manufacturing partners, we’ve been realizing life-saving and life-changing devices for over 40 years.

If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.

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