Chapter 4:
Stackups and layouts of capacitive touch sensors
4.1 Common stackups, materials, and structures of capacitive touch sensors
Basic materials used in a touch sensor stackup
The most common materials used as substrates in the touch sensor stackup (also often called “structure”) fall into three major categories:
- A wide variety of polymers, called “Film” materials (e.g. PET, PC, PDMS, FR4)
- The Glass category, including materials with high diversity and different properties (e.g. Corning’s Gorilla Glass™, AGC’s Dragontrail™, SCHOTT’s Xensation™)
- The Adhesives. These materials bond the touch panel to the main liquid crystal display (LCD), and also any protective cover, such as the lens, to the touch panel. They can either be liquid (LOCAs) or non-liquid (OCAs).
Common 0D and 1D touch sensor stackups based on the material stackup used
The stackups used in 0D and 1D touch sensor applications (buttons, sliders, wheels etc) are usually quite simple and standard. Contrary to the 2D touch sensors (screens), there is only a limited range of different stackups.
The most common stackup consists of:
- A top layer (usually called Overlay, or Cover Lens/Glass).
- The conductive elements (Sensors, Vias, Ground/Shielding entities, Traces).
- The PCB layer (usually FR-4 or Flexible Plastic Substrates/FPC)
Figure 1. Most common 0D and 1D Capacitive Touch Sensor stackup. (Source)
Common 2D touch sensor stackups based on the material stackup used
The most common Capacitive Touch Screen stackups used are:
where:
GFF: Glass-Film-Film
G1G: 1 Glass-Glass
GG: Glass-Glass
SITO: Single ITO
OGS: One Glass Solution
GGG: Glass-Glass-Glass
The reason why one setup is chosen over another can be a very complex issue for Touch/Display companies. A few of the things that need to be taken into consideration and depend on the requirements of the final application are:
- Transmittance and Optical performance
- Thickness and Weight
- Border width due to routing (traces)
- Cost & availability of ITO film or deposition
- Lamination experience & yield rates
- Existing equipment and/or method experience
Key takeaways from this section:
Capacitive Touch Buttons mostly utilize a simple material stackup comprised of an overlay (glass or plastic), the electrodes and a bottom PCB substrate.
On the contrary, there is a high diversity in the material stackups of the Touch Screens, with GFF and GG being the most popular ones.
Common touch sensor structures based on the location of the touch sensor (with respect to the display)
This segmentation takes into account the position of the touch sensor electrodes with respect to the complete touch panel. The basic technologies here are:
- Add-On (or Out-Cell) type,
- On-Cell type,
- In-Cell type.
Figure 2. Most common structures based on the position of the touch sensor (Source)
Add-on type positions the touch sensor, which includes touch cover glass, and TX (transmitting) and RX (receiving) electrodes, as an independent module on top of the LCD or OLED panel. These two entities are separated by an air gap to help bonding. The add-on type has been mainly adopted for medium-to-large sized touch applications because of its easy implementation. However, as the size of the screen increases, the Signal to Noise Ratio (SNR) is degraded, which means that special care should be taken to maximize the performance of the sensor.
On-cell type includes the electrodes of the touch sensor between the polarizer and color filter in the LCD display panel, whereas between the polarizer and encapsulation layer in the OLED display panel. The on-cell type was primarily employed for small-to-medium sized mobile touch applications due to its higher transmittance, lower manufacturing cost, less weight, and thinner thickness than the add-on type. However, an extra fabrication process is required, since the TX and RX electrodes must be deposited on the color filter (LCD display panel) or the encapsulation layer (OLED display panel) in two layers or in a single layer with the bridge metal. In addition, compared to add-on structure, the on-cell type touch sensor is located further away from the finger and closer to the display, thus it is more sensitive to the display noise, resulting in a lower SNR than the add-on type.
In-cell type integrates the touch sensor physically inside the LCD or OLED display module. The in-cell type is employed mostly for the small-to-medium-sized mobile touch applications similarly to the on-cell type. In the manufacturing process of the LCD panel, the electrodes are fabricated by connecting the segmented VCOMs through the metal in the TFT glass. However, in the manufacturing process of the OLED panel, the electrodes are fabricated by connecting the segmented cathode electrodes through the metal, but the cathode patterning process can degrade the electrical characteristics of the OLED. Therefore, in-cell type is predominantly adopted in the LCD panels. Since the touch sensor is integrated with the TFT glass in the display panel, it has less weight, thinner thickness, and lower manufacturing cost than the add-on and on-cell types. The segmented VCOMs are used for the electrodes of the touch operation and for the display operation, separately. Therefore, the frame time is divided into touch sensing and display times, and thereby the touch sensing time of the in-cell type can be reduced compared with that of the add-on and on-cell types. In addition, the in-cell type touch sensor is located further away from the finger and closer to the display than the on-cell and add-on types. Therefore, the in-cell type has a lower SNR than the add-on and on-cell types.
Key takeaways from this section:
Add-On type of Touch Screen Sensors is gradually being replaced by the newer applications of On-Cell and In-Cell, which provide lightness and thinness, while being cheaper to manufacture at the same time.
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