A Displays Technology Guide for Electronics Engineers

By Frank Ploenissen   |   January 10, 2025

Displays come in a wide range of technologies with varying performance characteristics. Selecting the right display and ensuring its protection are crucial to a successful end product.

In today's world, displays are omnipresent, from our homes and workplaces to mobile phones, tablets, and large information terminals. For engineers, selecting the right display and ensuring its protection are crucial steps in creating successful electronic products. This comprehensive guide will help you navigate the complex landscape of display technologies, including cover glass options, and integration processes.

When embarking on a display project, the first step is to thoroughly analyze the requirements. Consider the product's environment, as this significantly impacts display selection. For instance, a milling machine in a workshop needs a robust display to withstand scratches and impacts, while a navigation system for a luxury boat must resist UV radiation and water exposure. Display brightness is another critical factor, varying based on usage environment. Outdoor applications in bright sunlight demand high brightness displays, often requiring 1000 nits or more. Indoor uses may require less, typically ranging from 200 to 500 nits. Mobile devices often benefit from adaptive brightness to accommodate various conditions, with brightness levels adjusting automatically based on ambient light sensors. The size and dimensions of the display, typically measured diagonally in inches, can indicate its intended use. Large displays (e.g., 70 inches) are usually for stationary applications like digital signage or conference room displays. Smaller displays, ranging from 1 to 21.5 inches, are suited for mobile devices or specific stationary uses like vending machines or industrial control panels.

Categories of display panels

Displays can be broadly categorized into passive and active types. Passive displays, typically LCD (Liquid Crystal Display) or E-ink, are used in applications requiring low power consumption, offer limited information display and are often monochrome. You'll find these in things like calculators, thermostats, and circuit breakers. Active displays, on the other hand, can show large amounts of information, often in color, and can display photographs and complex graphics. Technologies like OLED (Organic Light Emitting Diode), AMOLED (Active Matrix OLED), and TFT (Thin Film Transistor) fall into this category. These displays offer superior image quality, wider viewing angles, and faster refresh rates, but at the cost of higher power consumption. The viewing angle is another crucial consideration. Some applications, such as vending machines and television sets, require wide viewing angles, while others may only need to be viewed from directly in front. This factor will influence the choice of display technology. IPS (In-Plane Switching) technology offers wide viewing angles, typically up to 178 degrees, making it suitable for multi-viewer scenarios. VA (Vertical Alignment) panels provide high contrast ratios, ideal for applications where image depth is crucial. TN (Twisted Nematic) displays are a cost-effective technology with narrower viewing angles, suitable for applications where the display is viewed head-on.

Cover Glass: Materials, processing, and enhancement

Cover glass plays a vital role in protecting displays while enhancing their functionality and aesthetics. Several types of cover glass are available, each with unique properties. Standard float glass is the most common and cost-effective but has a slight green tint due to iron content. This tint is usually not noticeable in everyday applications but can affect color accuracy in professional settings. Super white glass is ideal for applications requiring true color representation, such as medical imaging or professional photography displays. It achieves its clarity through a reduction in iron content, minimizing the green tint associated with standard float glass. Ultra-thin glass, lightweight yet durable, is designed for mobile applications and can be bent to withstand strong impacts despite its thinness. This glass can be as thin as 0.1mm, significantly reducing the weight of portable devices while maintaining protective properties. Borosilicate glass, with its high heat and chemical resistance, is suitable for laboratory environments and can withstand temperatures up to 500°C and temperature shocks of about 200°C. This makes it ideal for displays in harsh industrial environments or scientific equipment. Cover glass can be enhanced through various processes.

Printing, usually done on the back of the front glass, serves both functional and aesthetic purposes. It can hide internal components, provide additional information about functions or keys, and create illuminated indicators when backlit. Two main types of printing are used: organic printing and ceramic printing. Organic printing offers a wide variety of colors but is susceptible to UV light and heat damage. It's suitable for indoor applications where the display won't be exposed to harsh environmental conditions. Ceramic printing, on the other hand, is more durable and resistant to UV light and scratches. It requires the glass to be tempered, as the ceramic ink is burned into the glass at high temperatures, typically around 600-700°C. This makes ceramic printing ideal for outdoor displays or devices exposed to harsh conditions. Mechanical processing of cover glass offers numerous possibilities for customization. The glass can be cut into different sizes and shapes, including round corners for safety. Edges can be seamed or shaped (e.g., C-shaped) to prevent shattering. CNC (Computer Numerical Control) processing allows for drilling, milling, and creating haptic elements. The glass can even be bent when heated for unique designs, opening possibilities for curved displays or ergonomic device shapes. To enhance the robustness of cover glass, especially for thinner applications, two main methods are employed: chemical toughening and thermal toughening.

Chemical toughening, also known as ion exchange, creates internal tension to counteract external impacts. This process involves immersing the glass in a potassium salt bath at temperatures around 300-450°C. The potassium ions replace smaller sodium ions in the glass surface, creating a compressive stress layer that significantly increases the glass's strength. Thermal toughening, or tempering, increases robustness through rapid heating and cooling. The glass is heated to about 650°C and then rapidly cooled with air jets. This process creates tensile stress in the center of the glass and compressive stress on the surface, making the glass much stronger than annealed glass. When broken, thermally toughened glass shatters into small, relatively harmless pieces, like car side windows, enhancing safety.

Touch integration and display assembly

Many modern applications require touch functionality. This is typically achieved by placing a transparent touch sensor in front of the display. The most common types are capacitive and resistive touch sensors. Capacitive sensors, the dominant technology used in most smartphones and tablets, detect changes in electrical fields when a conductive object (like a finger) touches the screen. Resistive sensors, often used in industrial applications, rely on pressure to detect touch points but have mostly been replaced by capacitive types. These sensors detect finger position and movement, and software interprets these inputs.

Touch sensors are often paired with cover glass for protection, forming a single unit called a touch panel. Creating a robust, integrated display unit involves processes such as lamination and bonding. Lamination involves gluing the touch sensor to the cover glass, ensuring consistent touch detection and eliminating air gaps that could affect visibility or touch sensitivity. This process requires specialized equipment and is often performed in clean room environments to prevent dust or particles from being trapped between layers. Bonding attaches the laminated touch panel to the display, maintaining consistent alignment between touch input and display output. There are several bonding methods, including air bonding (leaving an air gap), optical clear adhesive (OCA) bonding, and optically clear resin (OCR) bonding. Each method has its advantages in terms of optical performance, durability, and cost.

Optimizing display performance and durability

A significant challenge with cover glass is managing reflections, which can impact display visibility, especially in bright environments. To address this, anti-reflective (AR) coatings can be applied to significantly reduce light reflection off the glass surface. These coatings typically consist of multiple thin layers of materials with alternating high and low refractive indices. When applied correctly, AR coatings can reduce reflections to less than 0.5% of incident light, compared to about 4% for untreated glass. AR-coated glass appears more saturated in color and helps maintain display visibility in bright light conditions. This is particularly important for outdoor displays or devices used in well-lit environments. Another option is etching the glass surface, creating a matte finish that diffuses reflected light. This technique is particularly effective at minimizing the impact of bright, focused light sources, but it can slightly reduce the sharpness of the displayed image.

When designing display assemblies, engineers must also consider environmental factors and electromagnetic compatibility. Rigorous testing, including temperature and humidity cycling, vibration and shock testing, and UV exposure, ensures the display's reliability in various conditions. For instance, automotive displays might need to function reliably from -40°C to 85°C, while consumer electronics typically have a narrower operating temperature range. Additionally, proper shielding techniques and EMI-resistant component selection are crucial for maintaining electromagnetic compatibility in the final product. This might involve using conductive coatings on the inside of the device housing, implementing proper grounding techniques, or using specialized EMI-absorbing materials.

Conclusion

Selecting the right display and integrating appropriate cover glass are essential steps in creating successful electronic products. By understanding various factors such as the operating environment, required functionality, and available technologies, you can ensure that your product has the optimal display assembly. Cover glass not only protects the display but also offers opportunities for enhanced functionality and design. With comprehensive knowledge of display technologies, integration processes, and enhancement techniques, engineers can create robust and efficient display systems for almost any application, ensuring optimal performance and user experience.

Every application is subtly different and that’s where working with a partner like Avnet can accelerate your design cycle, minimize risks, and ensure a cost-effective outcome that delights your customer and the users of their products.

About Author

Frank Ploenissen , Marketing Manager Display Integration, Avnet

After getting his diploma in electrical engineering, Frank entered the world of display solution and...

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