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Structure of Capacitive Touch Screen

A capacitive touch screen typically consists of a single layer of acrylic substrate, with a transparent conductive film uniformly coated on both the inner and outer surfaces. At the four corners of the outer conductive layer, elongated tapered electrodes are embedded.

Working Principle

When a finger touches the surface, a high-frequency signal is transmitted across the working surface. The finger and the touch panel surface form a coupling capacitor, acting as a conductor. This causes a tiny current to be generated at the point of contact. This current flows to the electrodes at the four corners. The amount of current flowing through each electrode is proportional to the linear distance between the finger and each corner. By comparing the current ratios, the system calculates the precise coordinates of the touch point.

Layered Structure

A capacitive touch screen can be considered as a four-layer composite panel:

Top Layer: Protective glass (tempered) or PET (Polyethylene Terephthalate)
- PET is thinner and more cost-effective than traditional plastic or glass, while maintaining transparency and flexibility.
Second Layer: Transparent conductive layer (typically ITO – Indium Tin Oxide)
Third Layer: Insulating substrate (non-conductive glass)
Fourth Layer: Internal conductive layer (also ITO), serving as the shielding layer to protect from internal electrical noise.

The central conductive layer is the key to the capacitive touch function. Electrodes are routed from the corners or edges of this layer to detect the touch location.

Material Details and Electrical Characteristics

Insulating layer: Can be made of glass (0.4–1 mm), organic film (10–100 μm), adhesive, or an air gap.
ITO Layer (Indium Tin Oxide):
- Typical thickness: 50–100 nm
- Sheet resistance: 100–300 Ω/square
- The 3D structure of the ITO layer is crucial to performance, affecting:
  - Mutual capacitance (between upper ITO and the finger)
  - Parasitic capacitance (between ITO layers and between ITO and the LCD module)

Double Glass Design

In most designs, a double-glass structure is used:

A transparent conductive film is coated onto a glass substrate.
A protective top layer (glass or PET) is added to shield the sensing layer.
This enhances durability, improves light transmission, and supports multi-touch functionality.
The electrode strips along the screen’s borders generate a low-voltage AC electric field.
When touched, the body’s electric field causes a local disturbance, creating a capacitive change.

Touch Detection Mechanism

The human finger acts as a coupling capacitor. The current emitted from the electrodes at the screen edges is drawn toward the touch point. The closer the finger is to an electrode, the stronger the current it draws. A controller behind the screen analyzes the current distribution, allowing precise calculation of the touch coordinates.

Even when the surface is contaminated with dirt, dust, or oil, the capacitive screen remains functional and can still accurately detect touch inputs.

Notes on Performance

Since capacitance varies based on contact area and the dielectric constant of the material, capacitive touch screen may experience drift under certain environmental conditions.
Therefore, this type of screen is often more suitable for development and testing phases, or environments where periodic recalibration is acceptable.

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