Rigid-Flex PCB Originates from the Past
In electronics, we sometimes encounter new technologies that seem to originate from the past. Rigid-flex PCB technology dates back about 50 years to replacing wiring harnesses in spacecraft.
Including the first commercial mobile computer also used rigid-flex technology.
Today, laptops, wearable technology, medical equipment, test equipment, and satellites are just a few of the applications that rely on rigid-flex PCBs.
What is a rigid-flex PCB?
For rigid-flex PCBs, the flexible circuit substrate and the rigid circuit substrate are laminated together. Rigid and flexible PCBs straddle the boundaries of traditional rigid PCBs and the unique properties of flexible circuits lithographically etched onto flexible insulating films using highly ductile electrodeposited or roll-annealed copper conductors.
Flexible circuits include laminates made of flexible polyimide such as Kapton or Norton, and copper laminated together by heat, acrylic adhesive, and pressure.
As with traditional PCBs, you can mount components on both sides of a rigid board. Rigid-flex designs do not use connectors or connecting cables between sections because integration occurs between rigid and flexible circuits. Instead, flex circuits electrically connect the system together.
Several things can be accomplished without connectors and connecting cables:
Improve the ability of circuits to transmit signals losslessly
Adapt to controlled impedance
Eliminate connection issues such as cold splices
reduce weight
Free up space for other components
Each rigid-flex PCB is divided into regions with different materials and different numbers of layers. Rigid regions may have more layers than flexible regions, and the material transitions from FR-4 to polyimide in the transition region.
Complex designs often go from rigid to flexible and back to rigid multiple times. As these intersections occur, the overlap of rigid-flex materials requires keeping the holes away from the transition zone to maintain integrity.
Additionally, many rigid-flex designs include stainless steel or aluminum reinforcements to provide additional support for connectors and components.
Different Design Rules Apply to Rigid-Flex PCB Design
Different challenges offset the versatility and flexibility that allows you to build 3D designs and products. Traditional rigid-flex PCB designs allow you to mount your product’s components, connectors, and chassis to the physically stronger, rigid portion of the component.
Also, with traditional designs, flex circuits are used only as interconnects, while reducing mass and improving vibration resistance.
New product designs combined with improved flex circuit technology have introduced new design rules for rigid-flex PCBs.
Components can now be freely placed on the flex circuit area. Combining this degree of freedom with a multi-layer approach to rigid-flex design enables more circuits to be built. However, gaining this freedom adds some challenges in routing and vias.
Flex circuits always have bent wires that affect wiring. Components or vias cannot be placed close to bend lines due to potential material stress.
Even when components are positioned correctly, bent flex circuits can create repetitive mechanical stress on surface mount pads and vias. These stresses can be reduced by securing the pads using through-hole plating and reinforcing the pad support with additional overlays.
When designing trace routing, follow practices to reduce circuit stress. Use shaded polygons to maintain flexibility when carrying power or ground planes on flex circuits. Curved traces should be used instead of 90° or 45° angles, and a teardrop pattern should be used to vary the trace width.
Teardrop Pattern for Trace-to-Trace Connections
These practices reduce stress points and weaknesses. Another best practice is to distribute stress across the traces by staggering the top and bottom traces of a double-sided flex circuit. Offset traces prevent traces from overlapping each other in the same direction and strengthen the PCB.
It should also be routed perpendicular to the flex line to reduce stress. When going from rigid to flexible and back to rigid, the number of layers may vary from one medium to another. You can use traces to increase the stiffness of the flex circuit by offsetting the routing of adjacent layers.
Electromechanical Factors Affect Design
When designing a rigid-flex PCB, consider the electromechanical factors that affect flex circuits and rigid boards. When building your design, pay attention to the ratio of bend radius to thickness.
For flexible circuits, tight bends or increased thickness in the flex area can increase the chance of failure. Manufacturers recommend keeping the bend radius at least ten times the thickness of the flex circuit material and building a “paper doll” of the circuit to determine where the bend occurs.
Stretching the flex circuit along the outer bend or compressing it along the inner bend should be avoided. Increasing the bend angle beyond 90° increases tension at one point and compression at another point on the flex circuit.
Another key issue for rigid-flex reliability is the thickness and type of conductors in the flex region. Thickness and mechanical stress can be reduced by reducing the amount of plating on the conductors and using pad-only plating. Using thick copper, gold or nickel plating reduces flexibility in bends and allows mechanical stress and fracture to occur.
Rigid-Flex Pcb Design Requires Teamwork
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