Rigid-Flex PCB Materials Selection
Designing a Rigid-Flex pcb is delicate work. With the specific intended use in mind, you also have to consider the electrical characteristics you want in the circuit, as well as the mechanical implications of the design. It used to be a hassle, especially when it came to dimensional testing and mechanical design integration of a rigid-flex, but new design software have made the process relatively easier.
But one part of a rigid-flex that many designers don’t give as much attention as they should is the material selection. This is a mistake because even the most intricately designed rigid-flex will fall short of its capabilities if one of the materials used to fabricate it is not the right match.
Rigid-Flex PCB Materials
Rigid-flex circuits are usually manufactured with the flex part acting as the core throughout the length of the circuit. But the substrate isn't the only material that has an impact on the electrical and mechanical properties of a rigid-flex.
Copper is the most commonly used and dependable conductor layers used in rigid-flex PCBs. It can be used as a thin flat layer, or as a hatched-polygon form for better mechanical flexibility in the flex part of the circuit. The copper layer can be very thin (for flexibility) and relatively thicker, especially on the hard parts, for high-current applications.
How the copper is deposited should also be considered. The simplest method is to electro-deposit a uniform copper layer. But for dynamic rigid-flex PCBs where the flex part has to be bent repeatedly, Rolled Annealed (RA) foils are preferred. They are a higher grade material, and they are relatively more expensive to include in the circuit, but they have a lot of tolerance against fabrication fatigue and repeated stress.
Copper deposits can weigh anywhere from a quarter of an ounce to ten ounces and have thicknesses ranging from 9 micrometers to 356 micrometers.
Usually, the rigid PCBs use FR-4 as the base material. But for a rigid-flex circuit, the most commonly used substrates are Polyimide and in second place, PET (Polyester). FR-4 might be used in rigid-flex with a very simple design and static applications (where no bending is required apart from the time of installing it).
It’s the ideal material for rigid-flex circuits because of its incredible flexibility, toughness, and heat-resistance. The last trait comes in very handy because that means the rigid-flex can go through multiple lamination cycles without any adverse effect. This makes it perfect for high layer counts in a rigid-flex. High tensile strength makes Polyimide much more durable than other substrates. Good chemical resistance makes it a favored material for applications where exposure to elements is inevitable. It comes in films as thin as 12.5 micrometers, allowing you to design a much more dimensionally compact circuit.
Another very desirable variety within the polyimide substrate is the adhesive-less PI (by removing the adhesive layer). It allows for a much thinner rigid-flex circuit, better thermal stability, and greater circuit reliability. Copper is directly deposited on the PI film.
The second most commonly used material for rigid-flex is PET. It’s very flexible, resistant to chemicals, which make it suitable for industrial application where it might be exposed to toxic elements. But it isn’t as tolerant to heat as Polyimide. This makes it hard to solder components into the substrate, and usually, pressure or low-temperature soldering techniques are used to mount the components on the PET-based circuit. It’s relatively cheaper.
While using an adhesive-less core is usually desirable, it’s not an option in some cases and not economical in others. But one major problem with some adhesives is a high coefficient of thermal expansion. So they might expand and create a mechanical strain on vias in the rigid part of the circuit if it is used in a high-temperature environment or goes through multiple lamination steps in fabrication.
Polyimide based adhesives are the best inline, and as the substrate material, they are also the most heat resistant. They are used in circuits that are deployed in harsh environments, or in high-heat places. But they are also very costly.
Acrylic adhesives are very commonly used. They are relatively cheaper, readily available, and can be used to create very thin adhesive layers (0.5 micrometers).
In rigid-flex manufacturing, adhesives have a job beyond that of joining two layers together. The rigid to flex transition area of a rigid-flex board is especially susceptible to tears and mechanical stress, especially in dynamic flex applications. Protective beads of epoxy resins and silicones are sometimes added to the rigid to flex transition area (on the outside), where the flex “patty” sticks out from the rigid “bun” in the rigid-flex board. This cushions the flex part’s movement. Pressure-sensitive adhesives are another form of adhesive that is popular for rigid-flex that have to be fabricated in low-heat situations (less soldering and substrates like PET).
For FR-4 based substrates, prepregs (preferably no-flow prepregs) act as adhesives.
A lot of designers confuse cover-lay with solder masks, considering them the flex alternative of solder masks. But there is a major difference between the two: State. Cover-lay is a solid film, whereas solder masks are liquid.
For rigid-flex, the most commonly used cover-lays are the Polyimide or PET film. The film is pasted on the outer layers of the board using adhesives. This protects the circuit, especially the flex part, from many unwanted exposures, while maintaining its flexibility and mechanical integrity.
Another method is called the cover coating. In it, acrylate-ed epoxy and acrylate-ed polyurethane (in liquid state) is layered on the board and then thermally cured.
There are a lot of material characteristics and considerations that one has to make before choosing the right materials for a Rigid-Flex PCB. You must understand that choosing one right material isn't enough. You have to pick the combination that works best. For that, it’s better to work with fabricators that not only have a wide range of materials available but also have years of experience backing them up so they can guide you about the best material selection based on your circuitry needs.