When it comes to printed circuit boards, rigid-flex PCBs can be one of the most complicated. Many describe rigid-flex as being more akin to two boards in one, both in processes and materials. This can make for a difficult time in designing the board, setting standards, and even selecting materials.
Materials for a circuit board alone can be a complex array of different brands, types, usages, and many other properties. This alone can complicate the process, making an already difficult specification process even more difficult.
In this post, we will be covering the materials for the key elements of a rigid-flex design, ranging from the solder mask and coverlay selection to the materials that make up the dielectrics themselves.
Silkscreen, Solder Mask, and Coverlay
Solder Mask
Starting from the top for the rigid section, the solder mask serves as the barrier protecting the first copper layer. Material selection in solder mask is easy enough, where the industry has an understood definition of solder mask. In essence, the solder mask is more a question of color than material. Thankfully, solder mask colors can be chosen based on usage, with many customers opting for their conventions of defining colors and their meanings for the parts. Commonly, green ends up being seen as a production color indicator, with other colors denoting non-production or prototype boards.
Rigid-flex PCB with a green solder mask.
Silkscreen
Silkscreen follows similarly to solder mask. Just defining the material as “silkscreen” is enough for all manufacturers to understand the material needed and use the most common form: a non-conductive epoxy ink that can be cured in an oven. Beyond this, the selection is similar to the solder mask, where the main choice is color. Usually, the default is white since these contrast almost all solder masks, an important element of silkscreen for readability. Otherwise, the choice is mostly for cosmetic purposes.
Coverlay
In the flex regions of a rigid-flex, therein lies two choices, flexible solder mask or coverlay. The choice here is relatively simple. If the flex portion of the board receives components and requires thin webs, the choice is flexible solder mask. If it’s simply the flex entirely being covered, it would be coverlay instead. This choice can be a difficult one and can change from manufacturer to manufacturer, so the best practice is to contact the circuit board manufacturer before determining which is best suited for your part.
Dielectrics
Among the most complicated of the elements of a stack-up are dielectrics. There are two main categories in the rigid-flex stack-up: the rigid element and the flexible element.
With the rigid element, there are two main dielectrics commonly chosen, either polyimide or FR4. Beyond those, materials start to become specialty use or holding special properties for a particular design. When these are chosen, the important aspect of the dielectric is for the prepreg to have a no-flow or low-flow variant, where the prepreg cannot actively flow out onto the flexible portion during lamination. The most important aspect of rigid dielectrics would be that it is best to consult a manufacturer first for what materials they can and cannot support, as most manufacturers cannot support beyond polyimide or FR4.
With the flexible portion of the board, the most common and standard material is polyimide. The only decision for polyimide lies in the thickness, which can be determined based on bend radius, thickness requirements, cost restrictions, and more.
Copper
Among all types of circuit boards, the common piece to them all is the copper within each layer. In essence, there are no choices for materials for copper. The only choices come from the type of copper, where flexible portions of rigid-flex designs ‘rolled-annealed’ can be used, which is better suited for high bend/flexing operations. Beyond the type, the other main choice is the thickness itself. These tie into the various elements of the board such as current carrying requirements, flexibility when it comes to the flexible section, and the overall thickness of the board itself.
Stiffeners
While this is a rare element to a rigid-flex design, stiffeners can still be an important consideration for a rigid-flex PCB. Stiffeners most commonly are in two forms, either as a reinforcement for a component or a method to ‘thicken’ up a portion of the board for applications such as ZIF connectors. As such, depending on the use case, the stiffener used is either a polyimide or FR4 stiffener, with polyimide stiffeners used for thinner applications or for ZIF connectors, and FR4 stiffeners used for components in flexible sections that require reinforcements.
Adhesives
Much like many other forms of materials in a stack-up, the main aspect of an adhesive choice relies on the use case. The different choices boil down to three options: acrylic, epoxy, and polyimide-based adhesives. While acrylic and epoxy are nearly standard, it’s important to remember that a polyimide-based adhesive is incredibly rare, costly, and requires specialized processes to use in a construction. Opposite to this, epoxy and acrylic are incredibly common, with both providing nearly the same amount of accessibility. The only difference beyond those is finding which better suits your use case, which can best be determined by working with a manufacturer.
Summary
A rigid-flex design is complex enough without adding all the “bells and whistles” in it. But while it can be difficult, it’s important to not forget the basics, such as the materials that go into the stack-up for a rigid-flex design. From the solder mask color to the copper thickness to the adhesive types, everything should be considered during a construction. And, if you cannot decide, Epec is always happy to help determine what materials would best go into your design and to help you figure out if the default materials suit your application or if you require something specialized.
Key Takeaways
- Material Selection Challenges: Rigid-flex PCBs combine rigid and flexible materials, making material selection complex. Early collaboration with manufacturers ensures the right choices for both performance and manufacturability.
- Solder Mask and Silkscreen Options: Solder mask and silkscreen primarily involve cosmetic choices like color, with green and white being common for production and readability.
- Coverlay vs. Flexible Solder Mask: Coverlay is best for protecting flex sections without components, while flexible solder mask suits flex areas requiring thinner designs or components.
- Dielectrics for Rigid and Flex Sections: Rigid sections typically use FR4 or polyimide with low-flow prepregs to prevent issues during lamination. Flex sections use polyimide, with thickness depending on bend radius and cost considerations.
- Copper Selection: Copper is standard across PCBs, with rolled-annealed copper preferred for flexing applications. Thickness depends on current requirements, flexibility, and board specifications.
- Stiffener Applications: Polyimide stiffeners suit thin or ZIF connector applications, while FR4 stiffeners reinforce components in flex areas.
- Adhesive Choices: Epoxy and acrylic adhesives are common and cost-effective, while polyimide-based adhesives are rare and expensive, reserved for specialized needs.
- Manufacturer Collaboration: Consult with manufacturers like Epec early in the design process to ensure the right material selections and optimize your rigid-flex PCB for cost, performance, and manufacturability.
