Rigid & Flexible PCB Manufacturing: A Process Overview

Introduction

Rigid-flex PCBs (Printed Circuit Boards) represent a sophisticated blend of rigid and flexible circuit technologies, offering a unique combination of durability and adaptability. These boards integrate rigid and flexible layers into a single circuit, providing a versatile solution that accommodates complex designs and space constraints. Rigid & Flexible PCB manufacturing involves a series of intricate steps, from material selection and layer stacking to final lamination and routing. This guide delves into each stage of the process, shedding light on the techniques and considerations that ensure high-quality, reliable rigid-flex PCBs.

Flexible Layer Stacking

At first glance, typical flexible circuits or rigid-flex circuits may appear straightforward. However, their nature requires several additional steps during construction. The starting point for any rigid-flex circuit is always a single-sided or double-sided flexible layer. Manufacturers can begin with pre-laminated flexible foils or uncoated PI (polyimide) films, then laminate or electroplate copper for initial coating. Laminated films require a thin adhesive layer, while adhesive-less coatings need a “seed” copper layer initially deposited using vapor deposition techniques. This seed layer is critical for subsequent chemical copper plating. The drilling, plating, and etching steps for single or double-sided flexible circuits are largely similar to those for typical double-sided inner cores in rigid PCBs.

Flexible Manufacturing Steps

Applying Adhesive/Seed Coating

Epoxy or acrylic adhesives are applied, or a thin copper layer is created for plating using sputtering techniques.

Adding Copper Foil

Copper foil is added by laminating it to the adhesive (the more common method) or chemically plating it onto the seed layer. New manufacturing processes allow for adhesive-less lamination using rolled annealed copper.

Drilling

Holes for vias and pads are typically drilled mechanically. Multiple flexible substrates can be drilled simultaneously by combining them from multiple reels into a work panel. For very small holes, laser drilling is used, although this increases costs significantly as each film layer must be drilled separately.

Through-Hole Plating

After drilling, copper is deposited and chemically plated in a manner similar to rigid PCB inner cores. Flexible circuits require a minimum through-hole plating thickness of 1 mil for mechanical support, compared to the typical ½ mil thickness in low-cost rigid PCBs.

Photoresist Printing

A photosensitive resist is applied to the film’s surface and exposed using the desired mask pattern before the chemical etching of the copper.

Etching and Stripping

The exposed copper is etched away, and the etching resist is chemically stripped from the flexible circuit.

Coverlay Application

The top and bottom areas of the flexible circuit are protected by an overlay, which is cut to shape. Coverlays can also serve as solder masks. Common materials include adhesive-backed polyimide films, but adhesive-less processes using photo-imageable solder masks are also available. Screen printing with UV curing is an option for lower-cost designs.

Cutting the Flexible Circuit

The final step in creating flexible circuits is cutting them out, often referred to as “blanking.” High-volume blanking uses hydraulic presses and die sets, while prototyping and small runs use blanking knives shaped to the flexible circuit’s profile.

Lamination and Routing

For flexible circuits forming part of a rigid-flex stack-up, the process continues with lamination between rigid sections. These flexible circuits made thinner and more flexible by the lack of fiberglass, are laminated with rigid prepreg and inner cores. Each rigid panel is individually routed to allow for flexible circuit bending.

Etching, Plating, Coverlay, and Blanking Flexible Panels

Flexible circuits are laminated into panels with additional adhesives, heat, and pressure. Multiple flexible sections are not laminated adjacent unless designing multilayer flex. Flexible sections are separated by rigid prepreg and inner cores or PI adhesive plates. This results in a laminated rigid-flex PCB, embedding flexible circuits between rigid sections.

Conclusion

Flexible PCB manufacturing, though complex, shares many conceptual similarities with rigid PCB production. By understanding each step—from flexible layer stacking to final lamination and routing—engineers can ensure the creation of high-quality flexible circuits tailored for diverse applications. As technology advances, flexible PCBs will continue to play a critical role in the development of innovative electronic products.