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LCD touch screen technology | TFT capacitive touch screen

We would introduce the new LCD touch screen technology, projected capacitive touch screen on the following article.

What is tft capacitive touch screen:

TFT capacitive touchscreen is tft lcd panel with capacitive touch screen, the capacitive touchscreen bring the multi-touch function to the tft lcd screen. Different to the traditional resistive touch, the tft capacitive touch screen is multi touch points, while resistive touch only one touch point.

TFT capacitive touch screen

What kinds of interface available for tft capacitive touch screen?

The tft capacitive touch screen can be with IC driver version and without IC driver version. If the IC driver not on the TFT capacitive touch screen, that means the IC driver need to be combined on the main controller system. The mainly tft lcd capacitive touch screen is with capacitive IC driver system, its interface has I2C interface or can be USB interface. To the small sizes capacitive touch screen, its interface is mainly in I2C interface, for larger sizes of capacitive touch, its interface could be I2C interface or USB interface.

How the capacitive touch screen connected to tft lcd screen?

The capacitive touch screen is stacked on the tft lcd display, there are two ways to make it, one is frame bonding, another one is optical bonding. the frame bonding means the capacitive touch screen attached on the tft screen by the double glue tapes via the lcd frame on the four sides. the optical bonding means the full lamination about the capacitive touch screen and tft lcd screen, that is the lamination is via OCA glue, and full lamination about the capacitive touch panel and tft lcd screen.

Which is better for frame bonding and optical bonding?

The frame bonding would have an air layer between the capacitive touch panel and tft lcd screen, while optical bonding the full lamination, it is without air gap between the capacitive touch panel (CTP) and tft screen. so the optical bonding would have better optical performance than frame bonding, because the air gap between the PCAP and tft lcd panel, it would have reflaction to the display light or ambient light to this display, that lead to display blur than the optical bonding screen, especially for sunlight readable display, the optical bonding would have more better than frame bonding capacitive touch screen.

But the optical bonding cost is more higher than frame bonding, the high cost is from the OCA glue cost and the lamination yield ratio, more OCA glue amount in optical bonding, means higher cost in optical bonding.

optical bonding and frame bonding

What kinds of tft panel can be better for optical bonding with CTP?

The IPS screen is more suitable than TN tft lcd panel for optical bonding. because the TN tft lcd cell is sensitive in cell gap changed, a nm grade thickness changed in the TN tft panel, it would lead to display color changed, and display light interfence because of the tft cell gap changed. so when the touch force on the capacitive touch screen and tft cell, the interface light or ring would be happed on the TN tft cell with capacitive touch screen. while IPS tft lcd screen is not so sensitive about the tft cell gap changed, because liquild crystal of IPS tft lcd, it is arrangment in horizontal inside the tft cell, a small cell gap changed would not bring so much interfence to the IPS screen with capacitive touch.

Capacitive touch screen technology:

Capacitive touchscreen displays rely on the electrical properties of the human body to detect when and where on a display the user touches. Because of this, capacitive displays can be controlled with very light touches of a finger and generally cannot be used with a mechanical stylus or a gloved hand. Examples of devices with TFT capacitive touchscreens are the Apple iPhone

Capacitive touch screens distinguish and sense specific touch location based on the electrical impulses in a human body, typically the fingertip. This enables capacitive touchscreens to not require any actual force to be applied to the screen’s surface; at the same time, capacitive screens typically don't respond to styluses or gloved hands due to the lack of electrical impulses generated.

Touch sensor will product a varying capacitance that will interact with a relaxation oscillator circuit.

Capacitive sensors are constructed from many different media, such as copper, Indium tin oxide (ITO) and printed ink. Copper capacitive sensors can be implemented on standard FR4 PCBs as well as on flexible material. ITO allows the capacitive sensor to be up to 90% transparent (for one layer solutions, such as touch phone screens). Size and spacing of the capacitive sensor are both very important to the sensor's performance. In addition to the size of the sensor, and its spacing relative to the ground plane, the type of ground plane used is very important. Since the parasitic capacitance of the sensor is related to the electric field's (e-field) path to ground, it is important to choose a ground plane that limits the concentration of e-field lines with no conductive object present.

Designing a capacitance sensing system requires first picking the type of sensing material (FR4, Flex, ITO, etc.). One also needs to understand the environment the device will operate in, such as the full operating temperature range, what radio frequencies are present and how the user will interact with the interface.

Projected capacitance

Projected capacitive touch (PCT) technology is a capacitive technology which allows more accurate and flexible operation, by etching the conductive layer. An X-Y grid is formed either by etching one layer to form a grid pattern of electrodes, or by etching two separate, parallel layers of conductive material with perpendicular lines or tracks to form the grid; comparable to the pixel grid found in many liquid crystal displays (LCD).

The greater resolution of PCT allows operation with no direct contact, such that the conducting layers can be coated with further protective insulating layers, and operate even under screen protectors, or behind weather and vandal-proof glass. Because the top layer of a PCT is glass, PCT is a more robust solution versus resistive touch technology. Depending on the implementation, an active or passive stylus can be used instead of or in addition to a finger. This is common with point of sale devices that require signature capture. Gloved fingers may or may not be sensed, depending on the implementation and gain settings. Conductive smudges and similar interference on the panel surface can interfere with the performance. Such conductive smudges come mostly from sticky or sweaty finger tips, especially in high humidity environments. Collected dust, which adheres to the screen because of moisture from fingertips can also be a problem.

There are two types of PCT: self capacitance, and mutual capacitance.

Mutual capacitive sensors have a capacitor at each intersection of each row and each column. A 12-by-16 array, for example, would have 192 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus near the surface of the sensor changes the local electric field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or styli can be accurately tracked at the same time.

Self-capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, current senses the capacitive load of a finger on each column or row. This produces a stronger signal than mutual capacitance sensing, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing.

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