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The Ultimate Guide to TFT Display Modules

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If you’re wondering whether a TFT LCD display module is the right choice for your application or not, then you’re in the right place. We prepared a detailed guide to help you make the smartest choice for your business. The guide will help you in better understanding the choices you have in subassemblies of a TFT display module.

Read on to find out everything you need to know about TFT display modules before approaching a manufacturer.

What is a TFT LCD display?

Let’s start with the basics. What does TFT LCD actually mean? TFT stands for “Thin-Film Transistor” and LCD stands for “Liquid Crystal Display.” When put together, a TFT LCD display is a flat-panel display or screen that you may find in computer monitors, TV sets, and mobile devices like smartphones and tablets.

How does a TFT display work?

TFT displays are made of large sheets of transistors, where each transistor of which is controlled independently. In its essence, a TFT screen is an active-matrix screen – each pixel on display is illuminated individually.

Pros of TFT displays

  • Brightness and sharp images – Expect a TFT display to be sharper and brighter than a common LCD display. It also refreshes more quickly than a regular LCD display, showing motion more smoothly.
  • Less energy consumption – TFT displays use more power than regular LCD screens. They not only cost more upfront but are also more expensive to operate.

TFT displays module – key subassemblies

Layer 1a: Cover glass – rigid

The first layer we’re going to examine is the cover glass in its rigid variant. Most of the time, clients choose chemically strengthened glass or Gorilla Glass™ in different impact-resisting thicknesses. The cover glass may come with a gloss or matte anti-reflective finish. Adding static backlit images is an option as well. The cover glass may also have mechanical buttons – fixed capacitive or through-hole mechanical ones.

Let’s dive into the details:

  • Chemical treated glass – This glass is made via a process where the glass is dipped into an ion-exchange solution. As a result, smaller potassium molecules are replaced by larger potassium molecules, reducing surface flaws that might occur in untempered glass.
  • Gorilla Glass™ – A proven brand that provides top-notch protection against high-impact stress. Many major smartphones and wearables use this glass.

Final notes: Glass strength is mostly related to the question of equilibrium. As the market requires thinner and stronger glass options, the manufacturers of protective glass strive to innovate and develop new glass technologies. It’s also likely that the demand for flexible protection choices grows in the future.

Layer 1b: Cover glass – flexible

If you’re looking to develop a TFT LCD resistive touch screen that responds to touch pressure, you need a cover glass that is flexible. To accomplish that, the manufacturer may use the same material as the top layer of a membrane switch – a resistive touch screen can include membrane switch functions.

Let’s dive into the details:

  • Chemical treated glass – This glass is made via a process where the glass is dipped into an ion-exchange solution. As a result, smaller potassium molecules are replaced by larger potassium molecules, reducing surface flaws that might occur in untempered glass.
  • Gorilla Glass™ – A proven brand that provides top-notch protection against high-impact stress. Many major smartphones and wearables use this glass.

Final notes: Glass strength is mostly related to the question of equilibrium. As the market requires thinner and stronger glass options, the manufacturers of protective glass strive to innovate and develop new glass technologies. It’s also likely that the demand for flexible protection choices grows in the future.

Layer 2: Touch sensor
PCAP screens The capacitive touch screen (PCAP) was invented many years before resistive touch screens, but they have become more popular recently with the rise of consumer electronics products like smartphones and tablets. PCAP touch screens offer high sensitivity and respond immediately to touch input. These screens are made of transparent and conductive materials such as ITO that are coated onto the glass material. Contrary to the resistive touch screen we mentioned in the previous point, PCAP screens don’t rely on mechanical pressure. Instead, they take advantage of the electroconductivity of the human body (which is naturally conductive). This is why PCAP screens work only with exposed human fingers or special styluses. Resistive touch screens As we mentioned in the preceding section, a resistive touch screen includes a glass panel and a film screen. Both are covered with a thin metallic layer, which is separated by a small gap. When the user touches the screen and applies pressure, the two metallic layers meet and create an electrical flow. This change in voltage detects the point of contact, converting the voltages into X and Y coordinates that are later sent to the controller. Final notes: It’s important that you select a touch sensor appropriate that matches the requirements of your application – for example, exposure to moisture and humidity or use of gloved hands. Thanks to their durability, resistive touch screens find broad use in manufacturing, ATMs, and kiosks, or medical devices. Since in many industries, users need to wear gloves when using touch screens, the resistive screen is a good solution since it doesn’t require contact with exposed human skin or a stylus with capacitive capabilities. However, thanks to advancements in this technology, PCAP screens are replacing resistive solutions thanks to greater capabilities in handling moisture or gloved hand usage.
Layer 3: TFT display

When it comes to the TFT display itself, you can choose from two technologies: TFT IPS and TFT TN.

In general, many businesses choose IPS and consider it better than the traditional TFT TN display – especially when it comes to the viewing angle and color conversion. IPS offers higher contrast but also consumes slightly more power. This technology is also more costly – an IPS display costs around 30-50% more than a standard TFT TN screen.

TFT IPS display

Pros:

  • The superior clarity of images – they remain stable and clear, not sparkly,
  • Colors are more vibrant and clear,
  • Easy installation on walls thanks to the compact form and low depth,
  • Super IPS screens offer a higher angle (170˚) for improved clarity and wider viewing, especially at night,
  • Longer battery life and screen life (on smaller screens),
  • Lower release of heat is lower,
  • Variety of options.

Cons:

  • High cost,
  • Colors don’t always transcribe correctly or accurately,
  • High resolutions might not always be readily available for personal applications.

TFT NT display

Pros:

  • Lower energy consumption in bigger screens,
  • Lower upfront and operation costs,
  • Excellent visibility – no geometric distortion,
  • Good response time and physical screen design.

Cons:

  • Poor viewing angles that might create distortions,
  • Static resolution (the resolution can’t be changed – however, newer models deal with this issue efficiently),
  • The accuracy of the display colors might not be perfect (especially strong blacks and bright whites).

How does this glass layer work in a resistive touch screen? A resistive touch screen is an overlay that uses physical pressure to detect any touch input. This type of touch screen consists of two layers – a flexible outer layer and a glass inner layer. These two layers are separated by an air gap maintained by microdots. This layer configuration can process only single touch events, reducing the transmittance (brightness) of the display underneath by 20% to 30%.

Layer 4: Backlight to illuminate TFT
What is a backlight, and why is it important in a TFT display? A backlight is a form of illumination used in LCD displays because these displays don’t produce light by themselves. In order to produce a visible image, they need some form of illumination – for example, ambient light or a special light source. A backlight can illuminate the LCD from the side or back of the display panel. Backlights are most often used in smaller displays to increase their readability in low light conditions. Backlights can be made up of:
  • Light-emitting diodes (LEDs)
  • An electroluminescent panel (ELP)
  • Cold cathode fluorescent lamps (CCFLs)
  • Hot cathode fluorescent lamps (HCFLs)
  • External electrode fluorescent lamps (EEFLs)
Only ELP can give off uniform light over the entire display surface. Other backlights – like LEDs – need to use a diffuser. This is how they can provide even lighting coming from an uneven source. Backlights come in many colors. Monochrome LCDs typically use yellow, green, blue, or white backlights. Color displays employ white backlights that cover most of the color spectrum. What is an NIT? It’s a unit used to measure a backlight’s brightness. As a general rule, one nit equals the light produced by one candle. The majority of LCD display backlights are LEDs. In most cases, they require the same power, which is applied to the LCD component. Manufacturers use two common methods to make the LED backlight brighter – they modify the current limiting resistor or integrate a transmissive polarizer. Current limiting resistor LED backlights need a current limiting resistor to reduce the driving current that reaches the backlight. The lower the value set for the resistor, the brighter the backlight will be. By reducing the resistor value, manufacturers also shorten the half-life of the LED backlight. The normal half-life can reach 50-70K hours; when overdriven, it can drop to 20K hours or less. Note that the half-life of the LED is the amount of time measured in hours for the LED to become half as bright as when it was first turned on. This might not become an issue if the product in question has a short lifespan – or if the backlight will be rarely on. The customer needs to decide on the tradeoff between making the LED backlight brighter and dealing with a shorter half-life. Transmissive polarizers The other option to increase the LED brightness is by replacing the transflective polarizer with a transmissive polarizer. Every LCD includes two polarizers:
  • the front polarizer (facing the user), which is always transmissive,
  • and the rear polarizer (selected by the user).
The transmissive polarizer allows for more light to pass from the backlight without decreasing the half-life of the LED. However, using a transmissive polarizer might make the display less readable when the backlight is off. Transmissive isn’t recommended for battery-powered projects since, in this case, the backlights need to always be on. Final notes: The backlight can be engineered to be bright enough for outdoor signage or office viewing or include other optical light filters.

Electronic interface to the main system

Another thing to consider is the three aspects of the electronic interface connecting the main system:

Interface 1: Display electronic interface

Interface 2: Touch electronic interface

Interface 3: Backlight electronic interface to PWM

Importance of assembly

Another key aspect is the physical connection between the touch panel and the LCD. When connecting the LCD to the touch panel, it’s important to remember the laws of physics. One cost-efficient solution is bonding by a double-sided adhesive gasket. The thickness of the gasket will depend on the size of the module.

The advantage of this type of bonding is its tightness – the air gap between the touch panel and the LCD display is free from any pollution and provides a clear picture. The tight assembly also protects the module from the humidity and even chemicals dissolved in the 

air that can present a danger to the device mechanics.

Testing

EMI emissions

To obtain certification and launch a product on the European market, the company needs to meet specific requirements of electromagnetic compatibility (EMC). Every electronic component emits some electromagnetic radiation, and that includes TFT LCD displays.

Electromagnetic Compatibility (EMC) is the ability of an electronic device to work properly in a specific electromagnetic environment without emitting any noises (EM Susceptibility) that might interfere with other devices that are located nearby.

Electromagnetic interference might be caused by the radiation emitted by the device, as well as the environment, but the compatibility is a characteristic for the entire device, not just its components like the display module. That’s why appropriate testing is essential before releasing the project to mass production.

Effects of ESD

Electrostatic discharge (ESD) refers to the sensation of an electric shock when walking across a carpet or opening a car door. The ESD definition offered by https://www.esda.org describes it as “the rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field.” Manufacturers employ specific strategies to limit the effect of ESD.

Shock and vibration testing

Thanks to these standard testing procedures, the manufacturer can ensure that a TFT display is durable and reliable. They allow detecting problems and solving them to avoid costly failure in mass production.

  • Vibration test – this test checks the mechanical resonance in all three directions (X, Y, and Z). Mechanical engineers use its results to optimize the design and improve the product.
  • Shock test – the purpose of the shock test is to verify how rugged the design is in all three directions (X, Y, and Z) to optimize it.

Other issues to consider

Another key problem to consider is the startup process of the display that includes the initialization and requires a backlight inrush current.

When Requirements Become Real Prototype

Talk with the Melrose engineers to layout a prototype you wanted to launch. Get your customized product with a 100% satisfaction rate.

Melrose has superior knowledge in using the right technology, meeting every requirement consideration. Call us for any clarification.

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