The Ultimate Guide to TFT Display Modules: Part 1

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.

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 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).

Stay tuned for the next part of this series, where we continue to explain all the different subassemblies that are part of TFT display modules.

And if you need expert advice, please get in touch with our consultants. At Melrose, we have the know-how and expertise you need to make the best decisions for your application.