Revolutionizing PCBs for Flexible Displays

What are Flexible Displays and Foldable Devices?

Flexible displays and foldable devices have been making the rounds of the market in recent years. Numerous phones, monitors, and other devices are now available incorporating this new and innovative technology. Flexible screens are a new spin on the traditional rigid flat-screen display—it is possible to flex and bend them. Foldable devices incorporate flexible displays to give you an appliance that the user can fold and unfold like paper.

Flexible screens are not a new idea. In 1974, Xerox PARC developed a flexible electronic display, which they called Gyricon or electronic paper. Since then, the development of display technology has progressed with leaps and bounds. Possibly the biggest development is OLED technology, making modern flexible screens a reality. This is because OLED screens do not require a separate backlight to function, thus allowing them to be thin enough to bend and flex.

Mobile phones with big screens are attractive and easier on the eyes, but difficult to handle and manage. Therefore, smartphone manufacturers have come up with the idea of a folding phone. The ability to fold a phone in half gives the user the advantage of a big screen when open while making it easier to pocket when folded.

Laptops with foldable screens are also making their debut, proving that folding screens aren’t solely limited to smartphones. At present, these laptops look more like a tablet with a bundled keyboard. Eventually, 20-inch laptops with this technology are likely to appear.

However, flexible displays and foldable devices require PCBs or printed circuit boards to make them function as intended. The nature of the devices requires the PCB inside to also fold and bend. Therefore, these PCBs are different from the regular rigid boards that are common in electronic equipment. In this article, we, at Rush PCB Inc., explain all about the flexible PCB that is the foundation of this new technology.

How Do Flexible Displays Work?

To understand flexible PCBs, it is necessary to first understand how a flexible display works. All displays, whether flexible or rigid, curved or flat, folding or rollable, work roughly the same way. They collate millions of specks of color to form the images we see on the screen. All the specs of color exist on a layer of material, also known as the substrate. Traditionally, this substrate was a thin glass sheet, rigid and fragile.

Over the last decade, display manufacturers changed over to display substrates made of flexible plastic that could bend without breaking. Such flexible plastic-based displays were the basis of phones with curved displays.

With the advancement of technology, display manufacturers found ways of increasing the amount of flexibility of their screens. They also made the screens more durable, while allowing them to bend thousands of times without breaking. Eventually, they arrived at folding screens that can fold almost like a sheet of paper.

However, the flexible substrate is only the beginning of the challenge for engineers and scientists. They also had to solve the problems of manufacturing the substrates to withstand years of mechanical stress and ensure that all the folding and bending did not affect image quality over time. Then they needed to add an equally flexible protective layer on the screen, making sure that all the other technology, like flexible PCBs that help the display operate, still works. Others had to devise methods of putting the flexible display, flexible PCBs, and components into a folding phone, all the time maintaining the high standards customers expect from electronic equipment.

It is important to note that most folding screens on the market today are of the OLED variety. This is because OLED screens do not need a backlight as LCDs do. Rather, OLED pixels emit light whenever they receive power. Therefore, OLEDs can be about 30% lighter and thinner than LCDs. Although OLED is the first choice of manufacturers for screen for foldable displays, flexible LCD screens also exist.

While the devices available on the market have only one fold, manufacturers have demonstrated devices that can fold over twice or even more.

How Flexible PCBs Help?

All electronic devices require PCBs or printed circuit boards to function. When there is adequate space inside the equipment, it is common to use a rigid PCB. However, with electronic equipment trending towards miniaturization and flexibility, the PCB inside must also follow suit. Here, the flexible PCB stands out as a groundbreaking innovation, introducing a design that integrates seamlessly and functionally with the flexible design.

The concept of a flexible PCB has made it possible to develop innovative designs like wearable technology, flexible medical sensors, and foldable smartphones.

How Are Flexible PCBs Different?

Rigid PCBs are composed of solid substances like epoxy and fiberglass, making them inflexible. They maintain a fixed form, offering the electronic assembly the necessary stability and durability.

On the other hand, flexible PCBs use flexible substrate materials like polyimide. This flexibility allows the board to fold, twist, or bend. This feature makes it suitable for applications with dynamic form factors like wearable technology, medical sensors, and foldable displays. There are three key differences between rigid and flexible PCBs:

● Material and Structure — Rigid PCBs use inflexible materials like epoxy and fiberglass, that, while offering stability, limit their flexibility. In contrast, flexible PCBs make use of pliable substrates like polyimides, allowing them to conform to various shapes and sizes,

● Design Flexibility — While rigid PCBs are confined to fixed shapes, thereby limiting design options, flexible PCBs, being highly pliable, offer unparalleled design flexibility. They allow designs with space-effective and innovative layouts, adaptable to unique products.

● Suitability for Applications — Rigid PCBs are suitable for applications demanding stability. In contrast, flexible PCBs excel in dynamic environments where adaptability and flexibility are crucial. Industries like automotive interiors, wearable tech, and medical devices take advantage of the unique benefits of flexible PCBs, enhancing their functionality and usefulness.

What Types of Flexible PCBs are Available?

In general, there are three types of flexible PCBs available:

● Single-Sided Flexible PCBs — These boards consist of a single conductive layer on one side of a flexible substrate. These are suitable for simple electronic applications where flexibility of the board is essential, but complex circuitry is not a primary requirement.

● Double-Sided Flexible PCBs — These boards consist of two conductive layers on both sides of a flexible substrate. These are suitable for electronic applications that require a more complex circuit design, requiring higher levels of connectivity and functionality.

● Multilayered Flexible PCBs — These boards consist of multiple conductive layers separated by insulating layers. The technique allows the creation of dense and highly intricate circuits. This makes multilayered flexible PCBs ideal for electronic devices with advanced and complex requirements.

What are Flexible PCBs Made of?

A flexible PCB typically requires two materials:

● Base Material — In flexible PCBs, the base material is crucial for providing flexibility, insulation, and strength. Typically, manufacturers use Polyimide, a high-performance polymer, for the purpose. Apart from the necessary flexibility, it has excellent thermal stability and chemical resistance.

● Conductive Material — Flexible PCBs typically use conductive material like copper, on account of its exceptional electrical conductivity. Conductive traces patterned onto the flexible substrate allow mounting components and interconnecting them.

What are the Challenges in Making Flexible PCBs?

There are two major challenges in making flexible PCBs:

● Selecting the Material — Material selection is an important factor in achieving flexibility in these PCBs. Although the basic material is polyimide, choosing the right blend for achieving durability, flexibility, and thermal stability is a critical factor. A proper balance is necessary to ensure the flexible PCB can bend without compromising its electrical performance or structural integrity.

● Manufacturing Processes — There are significant differences between the manufacturing processes for flexible PCBs and rigid boards. Precision is paramount for flexible PCBs, necessitating the use of lasers for drilling, and additive methods for creating conductive traces. It is necessary to ensure reliability and consistency in these processes.

Is It Possible to Combine Rigid and Flexible PCBs?

The combination of rigid and flexible PCBs is aptly called a rigid-flex PCB. These combine the benefits of flexibility and rigidity. The flexible PCB is sandwiched between rigid sections, allowing a versatile design with the capability of withstanding folding and bending while maintaining structural stability.

Are there any Design Considerations for Flexible PCBs?

Apart from the design considerations necessary for regular rigid boards, there are primarily two design considerations specific to flexible PCBs:

● Flexibility Requirements — The designer must begin with a clear and comprehensive understanding of the requirements of the application’s flexibility. This is because the degree of flexibility required guides the decisions on:

○ Material Selection

○ Circuit Layout

○ Overall Design Strategy

● Additional factors that play a crucial role in the design phase are:

○ Intended Functionality

○ Range of Motion

○ Environmental Conditions

● Folding Considerations and Bend Radius — A critical aspect of the design involves establishing the bend radius, this being the minimum radius the PCB can withstand without damage. The designer must carefully consider the intended use of the PCB along with its form factor, and determine the appropriate bend radius to achieve optimal functionality.

Conclusion

Flexible PCBs have brought a revolution in the electronics industry, by offering a wide range of design and application possibilities. This includes flexible displays and foldable devices like wearable electronics and automotive systems, ranging from consumer devices to aerospace. Key design considerations require an effective handling of bend radius, trace width, material selection, component placement, and environmental conditions.

At Rush PCB Inc., we understand the importance of flexible PCBs in the ever-widening field of electronics. With our expertise in PCB manufacturing and assembly, we offer high-quality flexible PCB solutions for the unique needs of our customers.