How do flexible screens really work?

Ryan Haines/Android Authority

Folding screens have become commonplace in recent years, thanks in large part to Samsung’s two lines of foldable phones, the Z Flip and Z Fold. As other manufacturers join in, prices for foldable devices are falling rapidly, promising a new era of personal computing. But how do foldable screens really work?

Whether you’ve always been curious about how foldable screens work or haven’t considered it yet, we’ll give you a crash course on foldable screens and the cool technology that makes them possible.

Also read: Display specs and terms explained: resolution, contrast, color gamut and more

Screens: the basics

All screens, rigid or flexible, flat or curved, rollable or folding, work in roughly the same way.

Simply put, millions of specks of color combine to form the images we see on the screen. There are different ways to achieve this, resulting in the various screens you see, including LCD, OLED, and more recently micro-LED and mini-LED.

All of those flecks of color sit on top of a layer of material called the substrate. For many years, the substrate has been a thin sheet of glass: rigid, brittle glass that can only be bent until it breaks.

Then, over the last decade, display manufacturers have produced display substrates made of flexible plastic that can be bent without breaking. Plastic screens made it possible to create the first phones with curved screens, like the Galaxy Note Edge from 2014.

The flexible substrate is only part of the equation. Scientists and engineers had to solve some ridiculously difficult problems.

As technology advanced, display manufacturers discovered ways to increase the amount of flexibility they could safely build into a display. Crucially, they also solved the problem of durability, allowing screens to be bent thousands of times without breaking. Eventually, this path led us to today’s foldable screens, which can be folded almost like a sheet of paper.

Manufacturers have been teasing foldable screens for over a decade, but the first foldable phones only came out in 2019. There’s a reason foldable screens took so long to mature, or more accurately, there are plenty of reasons for this.

The flexible substrate is only part of the equation. Scientists and engineers had to solve ridiculously difficult problems, such as making substrates that are light and flexible but can withstand years of mechanical stress; make sure that all the bending and folding do not affect the quality of the image over time; create an equally flexible protective layer for the screen; and make sure all the other technology that goes into the display still works. When all this was done, other smart people had to come up with ways to incorporate flexible screens into foldable phones while maintaining the incredibly high standards we’ve come to expect from our electronics. Hard work indeed.

A closer look at how folding screens work

Before we discuss the individual components of a foldable screen, it’s important to note that all of the foldable screens you see on the market today are of the OLED variety. OLED displays do not have a backlight like LCDs; instead, the pixels themselves emit light when power is applied to them. Because of this, OLEDs can be made 30% thinner and lighter than LCDs. Along with other benefits over LCD, OLED is the first choice for flexible screens, but flexible LCD screens do exist.

flexible oled

To understand how foldable OLED screens work, it’s helpful to visualize the screen as a very thin (and probably not very tasty) layer cake. Each layer of this high-tech cake has a specific function. These layers are laminated together into a very thin package that is fractions of a millimeter thick. Let’s review them.

  • substrate layer — Also called the board, this is the very base of the screen, which supports all the other layers. In a flexible display, the substrate is made of plastic or, less commonly, metal. Most of today’s flexible display devices use a substrate made from a plastic polymer called polyimide (PI). In addition to being flexible and insulating, polyimide has high mechanical resistance and thermal stability.
  • TFT layer — Applied on the flexible substrate, the TFT (Thin Film Transistor) layer controls the power delivery to each pixel. Think of it as an “electrical network” that connects all the pixels on the screen. In an OLED display, unlike an LCD, each pixel can be individually controlled, allowing for high contrast rates and lower power consumption.
  • OLED layer — The light-emitting layer, made up of individual pixels, each consisting of red, green, and blue sub-pixels. Each pixel can achieve a certain color and brightness by varying the amount of energy its sub-pixels receive. In turn, the pixels combine to form the image we see on the screen. The OLED layer is made up of several sublayers, including a cathode, an anode, and a layer of organic light-emitting material sandwiched between them.
  • covering layer — Also called the encapsulation layer, this is the layer that seals and protects the other layers. It is also the layer that users touch when interacting with foldable screens. In terms of materials, the cheapest option is polyimide (same as substrate), while more recently we have seen manufacturers adopt ultra-thin glass (UTG). UTG is stronger than plastic and feels more like normal glass, while still being able to bend. UTG is what Samsung has been using on the latest Z Flip and Z Fold.
royole 2 stretchable micro led screen

What else should I know about how folding screens work?

Folding screens can be folded in or out. On a flip screen (eg Galaxy Z Flip 3), the screen is hidden inside the device when folded, which helps with durability, but tends to create a slight crease in the screen. On a foldable screen (like the Huawei Mate XS 2), the screen folds around the outside of the device when it’s folded. That leaves it open to scratches, but provides a wrinkle-free look.

Also read: The best flip phones you can get

The foldable screen devices we’ve seen so far only have one fold, but manufacturers have shown concepts of devices that fold twice or even more. Here are some Samsung designs that fold twice into an “S” or “G” configuration.

Not all flexible displays fold. We’ve seen devices with roll-up screens that roll up and disappear inside the body of the device. Examples include the Oppo X 2021 rollable phone or LG’s crazy OLED R rollable TV.

The screen is a key aspect of how folding screens work, but it’s not the only one. The hinge can be just as important to the user experience. Manufacturers have invested a great deal of resources to ensure that the hinges on their folding products work smoothly and consistently, have the correct amount of “click” and provide a smooth surface for the screen to sit on.

Another key factor is durability. By definition, foldable screens have moving parts, which opens up the possibility of water, dust, and other contaminants getting into the device. In fact, we’ve seen issues with debris building up under the screen on some devices, which ruins the user experience and can damage the screen.

Also read: What is screen burn and how can you prevent it?

Numerous manufacturers have already released or at least teased foldable-screen products, including phones, laptops, and even televisions. It’s easy to imagine a future where tablets, handheld devices, gaming consoles, and even home appliances have screens that bend. Innovation will also come from stretchable, wearable and even skin-embeddable screens. In the meantime, as more resources are poured into the technology, foldable screens will only get better.

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