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Driving Innovation: Mechanical and Optical Processes During Rigid-flex Production

Rigid-flex printed circuit boards are a highly effective solution for placing complex circuitry in tight, three-dimensional spaces. They are now indispensable across a range of industries, from medical devices and aerospace to advanced consumer electronics, helping designers make the most efficient use of available space. However, their unique construction—combining rigid and flexible materials—presents a fundamental challenge for PCB manufacturers. The mix of materials creates a host of complications for mechanical and optical processes, requiring a unique blend of expertise and specialized machinery. This article explores how these material combinations influence critical manufacturing processes and the specific technologies that are helping process engineers successfully produce these advanced boards.

Alignment
The most critical topic in rigid-flex production is alignment. Getting the registration right is a two-step challenge that begins before lamination. Alignment is especially complex because of the flexible portion of the board, which can bend and move easily. Alignment systems, such as post-etch punch machines, must be designed to work with these delicate materials.

It's also crucial to remember that flexible and rigid materials behave differently under thermal stress—stretching and shrinking at different rates during lamination and prior processes. The first step in managing this is to gather statistical data about these deformations using X-ray systems. Production can then perform one of two compensation types, or preferably both for complex boards:

• Compensate during direct imaging: Each inner layer can be compensated individually during the direct imaging step by making adjustments for material type, copper density, layer position, and other factors.
• Compensate during drilling: Compensation can also be applied during the drilling operation, which is especially easy if the drilling machine is equipped with CCDs and individual tables.

This standard approach, also vital for HDI boards, is sometimes more critical in rigid-flex production because of the very different behaviors of the rigid and flex layers.

Optical Operations
Engineers must carefully adapt optical processes, including direct imaging and laser cutting, for rigid-flex materials. For direct imaging, machines need to handle thin flex layers without causing damage. While manual loading is an option, automated machines require highly adaptable, adjustable grippers that can handle such delicate materials without stress. 

After lamination, these mixed-material boards are never perfectly flat, so the machine must be equipped with an auto-focus system and Z-axis control for each print head to adapt to the uneven surface and maintain a sharp exposure.

At the same time, it is often necessary to cut coverlays or contour the final shape of the flexible layers. Typically, a laser cutting machine equipped with a UV-nanosecond or a pico green laser source does this. While a UV-nanosecond laser is often sufficient for special cases where cutting quality is critical, the pico-green source provides the best results with minimal carbonization. The science behind this is that a laser beam is pure energy, and energy has two ways to dissipate: heat or ablation. The shorter the pulse, the more energy goes directly into ablation and the less into heat. Using a pico-green laser source, we see significantly less carbonization and a cleaner, more precise cut.

Mechanical Operations
Every material has a different recommended cutting speed, and this is a significant challenge when drilling a mixed-material PCB. The process engineer’s goal is to find the "perfect recipe" by slightly varying the settings and performing micro-sections to identify the limits of the tools and set a proper hit count. Working with mixed materials, the need for different cutting speeds forces a compromise, making the process engineer’s job even more critical. In such cases, simply following the material and tool manufacturers’ recommendations may not be enough; dedicated time must be spent finding the ideal drilling parameters for that specific combination.

Depth routing for rigid-flex is a critical mechanical process. After lamination, the board is entirely rigid, but at the final stages, a precise depth-routing operation removes the unnecessary rigid parts to free the flex layer—a process often called "cup elimination." If this depth routing is even slightly overdone, the flexible part would be damaged, and the entire PCB scrapped. To prevent this, it is critical to control the depth routing with a second measuring system (like a pressure foot). Process engineers must adjust the depth and choose the proper inserts for the pressure foot, making this a highly technical and precise operation.

Summary
I often tell people at PCB shows that I love this industry because it's not simply about the machines. It's about people, processes, materials, and the environment. Many factors influence the final result, and in the end, machines are simply tools in the operator's hands. Some people can produce complex PCBs with simple tools, but to achieve a stable and repeatable serial production, one needs good tools. At the same time, when working with specialized PCBs like rigid-flex, the influence and expertise of the process engineer is even more critical.

This column originally appeared in the October 2025 issue of PCB007 Magazine.