The contact screen, simply put, is the combination of input and output, no longer need mechanical buttons or sliders, the display screen is a human-machine interface.
The whole contact screen module is composed of LCD, contact screen, contact screen controller, main CPU, and LCD controller. The touch screen and the touch screen controller are the core locations of the whole module, so we will focus on these two parts.
The structure of the contact screen is generally from top to bottom: 1. Exterior shield; 2. a masking layer; 3. mask layer & marking layer; 4. optical glue; 5. a first layer of induction unit and substrate; 6. Optical glue; 7. a second layer of induction unit and substrate; 8. Air layer or optical glue; 9LCD display.
The outer shield is usually less than 100um thick. All plastic masking layers need hard shields, this is because finger contact will scratch the plastic surface, assuming that the masking layer is glass can not need the outer shield, but the glass needs to be chemically strengthened or quenched, and the outer shield needs to be optically matched with the masking layer to avoid excessive light loss.
The masking layer can be 0-3mm thick, not all contact screens need a masking layer, the thinner the masking layer, the higher the signal-to-noise ratio and better sensing sensitivity. Commonly used materials are: polycarbonate, plexiglass and glass.
The third layer is the mask layer and the marking layer, which is roughly 100mm thick. The mask layer sits underneath the mask and hides wiring, the edges of the LCD, etc. It is permissible to add descriptive text or icons in the plan, but it is necessary that the marker is pressed fairly flat on the substrate of the ITO, and the marker information should be conductive or wrong.
The fourth layer is optical glue, with a thickness of about 25-200mm. The thinner the adhesive, the better the signal-to-noise ratio, and the higher the dielectric constant (ER) optical adhesive can have better sensing finger capacitance, and then a higher signal-to-noise ratio can be achieved. PSA pressure sensitive adhesive is usually applied.
The fifth layer is the induction unit and the substrate, the thickness of the ITO coating is less than 100nm, and the ITO coating substrate can be 100um-1mm glass (IR-1.52) or 25mm-300mm PET film (IR-1.65). The thicker the ITO, the lower the resistance per unit area and the better the signal-to-noise ratio. The thinner the ITO, the better the light transmittance. The substrate can be either film or glass. Assuming that ITO is made on the underside of the glass substrate, the glass substrate can be used as an exterior mask.
The sixth layer is another layer of optical adhesive, compared with the previous layer of optical adhesive, the thicker the optical adhesive, the better the signal-to-noise ratio, this layer of optical adhesive is usually used in combination with ACA-anisotropic conductive adhesive.
The seventh layer is also the sensing unit and substrate, which is the same as the first layer of substrate. Be careful not to mix film and glass. Assuming that ITO is on the substrate, a thick substrate can achieve a higher signal-to-noise ratio; Assuming that ITO is on the underside of the substrate, a thin substrate results in a higher signal-to-noise ratio. Similarly, in the marginal area, it is required to use heterogeneous conductive adhesives. Single-substrate processes are now available to simplify production and reduce costs.
The eighth layer is the air or optical adhesive layer, and we know that the dielectric constant of air is equal to 1, which can reduce the parasitic capacitance from the surface of the LCD. Assuming the use of optical glue, the device can be more solidified. It is necessary to match the optical parameters to make the light loss smaller, to choose the optical adhesive with the lowest possible dielectric constant, and to ensure that the separation between the ITO sensing unit and the surface of the LCD is at least 250mm.
Finally, it is the LCD screen, for the contact screen planning, it is a noise source, the noise comes from the backlight, the LCD pixel drives the control signal, usually do not choose the forced dot matrix screen, which will generate a high-voltage signal on the front of the LCD, try to use the active dot matrix screen with Vcom, which can constitute a virtual ground or shielding function; Assuming that it is indeed necessary to choose a forced dot matrix screen, it is necessary to add an ITO shield layer to the contact screen, and the shield layer must be grounded to remove the influence of parasitic capacitance CP.
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