Rigid Flex PCBs

Printed circuit boards (PCBs) are at the core of electronic technology and its expansion, and they are used in nearly every electronic device on the planet. They connect different components of a device together via copper conductors that enable the device to perform a given function. These circuit boards fall into different types, each meeting different technical needs. Rigid PCBs and flex PCBs are two of the most common, allowing engineers to create complex circuit configurations in more strenuous or compact environments.

For many years, engineers had to choose between rigid PCBs and flex PCBs, even in situations where neither was optimal, but that choice is no longer necessary. The rigid-flex PCB combines these two types of circuit boards into one, harnessing both the reliability of rigid PCBs and the versatility of flex PCBs. As a result, rigid-flex PCBs can be arranged in configurations and board shapes that the other types can’t. In this article, we will examine the many applications of rigid-flex PCBs and how they function.

What Are Rigid-Flex PCBs Used For?

Rigid-flex PCBs can be used in several unique ways, in terms of both design and application. Broadly speaking, however, their job is to wire components together in an electrical circuit and send messages through that circuit. The circuit then works with other circuits to power and facilitate the functions of devices of all shapes and sizes. Smartwatches, for example, rely on the versatility and power of rigid-flex PCBs to run their many functions while the watch moves and flexes.

Other uses of rigid-flex PCBs include:

• Powering the mobility and functionality of robots, sensors and other automated technology that relies on motion and durability.
• Operating advanced driver-assistance and infotainment systems in modern vehicles.
• Powering compact personal devices at risk of warping, bending or breakage.
• Facilitating the miniaturization and operation of implantable medical devices and diagnostic tools.
• Operating and holding up under the extreme conditions that come with the systems and equipment that support the aerospace and defense industries.

These technologies rely heavily on rigid-flex PCBs since they can flex and compact without compromising overall durability or malleability. However, the greatest strength of rigid-flex PCBs may be the possibilities for three-dimensional design that they open up.

How Complex Are Rigid-Flex PCBs?

Designing rigid-flex PCBs and integrating them into broader systems is typically more complex than working with rigid or flex PCBs. This is because rigid-flex PCBs can break into the third dimension in a way that rigid and flex PCBs can’t. That complexity is only exacerbated by the unique layer stacking options and board shapes rigid-flex PCBs allow. When working with them, engineers have to consider:

• Bend radii.
• Layer stack-up.
• The thermal expansion coefficient of the material the rigid-flex PCB is made of.
• Substrate flexibility.
• Signal proximity.
• Connector placement.
• Layer transitions.
• Heat dissipation.

All of these factors can complicate the assembly and soldering processes. Not considering them can cause malfunctions or create unusable PCBs. As such, carefully testing and inspecting these PCBs throughout the assembly process is vital to ensure they function properly.

Some examples of rigid-flex PCB design styles include:

• Odd and even layer count designs.
• Flex layer builds with air gap constructions.
• Asymmetrical builds.
• Shielded flex layer designs.
• Blind and buried via builds.
• Varying flex layer count designs.
• Multiple rigid air area thickness builds.
• Integrated zero insertion force (ZIF) tail designs.

What Are Rigid-Flex PCBs Made Of?

Like other circuit boards, rigid-flex PCBs are made up of numerous components, from semiconductors and flex ribbons to plane layers and I/O pins. Each component leverages unique materials in its construction. Rigid-flex PCB design further complicates this construction with its unique balance of firm and flexible components.

That said, many materials are incorporated consistently across these components thanks to their unique properties. The most common rigid-flex PCB materials include:

• Woven fiberglass: Most circuit boards are constructed using woven fiberglass as a base. It is durable and flexible and provides the board on which the other components rest. In most cases, it is treated with epoxy resin to increase its strength, but other substances are also used to preserve the board within more motion-prone environments.
• Epoxies and adhesives: Rigid-flex PCB components often need to bend and twist, and epoxies and adhesives play a vital role in keeping these components rooted to the circuit board itself. Polyimide and polyethylene terephthalate (PET) are two of the most common adhesives, but acrylic adhesives are also used due to their chemical and heat resistance properties.
• Copper: PCBs would be powerless without conductive materials, and the most common and accessible of these materials is copper. To date, over 2.8 billion tons of copper have been discovered, and billions of tons more are speculated to exist within the Earth. While copper is renowned for its workability and conductivity, it may be supplemented with zinc to lengthen its lifespan. It may also be shaped into either electro-coiled copper or rolled copper foil, depending on the circuit board’s needs.

• Protective coatings: Contaminants like dust and oil can hamper or shut down circuit boards if precautions aren’t taken. Protective coatings are the first line of defense between the surface of the board and these contaminants, and they significantly increase the circuit board’s lifespan and durability. Cover lays and cover coats are the standard approach to protective coatings. They involve creating a film or layer of liquid between the circuit board and its environment.

What Are the Rigid-Flex PCB Design Guidelines?

Different types of circuit boards come with different limitations within PCB design programs. Rigid-flex PCBs can be especially tricky to design since they break into the third dimension and feature a mix of component types. However, over time, some basic guidelines have emerged to make working with these circuits simpler. These guidelines include:

• Utilizing board outlines to govern the early stages of board creation.
• Planning out the rigid and flex regions within the stack to make sure all connections and boards are compatible with each other.
• Deciding which flex regions will be static and which will be dynamic in relation to each board, its location and the stack as a whole.

Partner With Via Technology Inc. for All Your Rigid-Flex PCB Needs

There are as many rigid-flex PCB applications as there are devices. Learning to master that technology can be extremely challenging and costly, but it can be even more rewarding, powering technological and economic transformation.

Via Technology Inc. puts that power in your hands. Our PCB design services encompass PCB testing, layout, design and assembly, meaning we are hands-on with the circuit board throughout the entire design process. We have been working with software and circuit boards since 1992, and that experience shows in every PCB we touch.

To begin your journey with rigid-flex PCBs, fill out our contact form or give us a call at (716) 714-0781.