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Resistive Touchscreen Integration

The following will guide you through AMT resistive touch screen design and LCD front bezel design, in order to ensure reliability in the end product.

Terms

Touch Screen Drawing

Touch Screen Drawing

  1. Outside Dimension: The overall external size of the touch screen, which is usually the size of the glass substrate of the touch screen. Overall size tolerance is specified in the drawing of the touch screen.
  2. View Area: This dimension is a reference for mechanical engineers when designing the size of the LCD front bezel opening.
  3. Active Area: Touch screen’s functional area.
  4. Tail: Touch screen’s circuit connection line, there are 4-, 5-, and 8-wire variations.
  5. Touch Screen Surface: The front side of touch screen is the touch function side. The material is a film, and the surface of this film can be an antiglare, clear, or Anti-Reflective hard-coating. The touch screen surface should be facing outward.

LCD Display Module Drawing

LCD Display Module Drawing

  1. Outside Dimension: The overall external size of the LCD module, i.e. size of the metal casing that houses the LCD panel and the PCB.
  2. Bezel Opening Area: Also known as LCD’s View Area. This is the exact cutout opening area of the LCD metal frame. The LCD’s metal frame must be fully covered by the system’s front bezel.
  3. Active Area: LCD’s functioning area.

Design Notes

  1. In order for the LCD Active Area (functioning area) to be completely covered by the touch screen, the LCD Active Area must at least equal the touch screen’s Active Area. Giving further consideration to production tolerance, it is recommended that the LCD Active Area be smaller than the touch screen’s Active Area by approximately 1mm on each side.
  2. Considerations regarding LCD front bezel design:
    • AMT suggests that LCD Bezel Opening Area (or View Area) should be smaller than or, at least equal to, the Touch Screen View Area, so that the area not intended for viewing is fully covered by the LCD’s bezel.
    • In order to maintain linearity along the boundary of the touch screen’s Active Area, be sure to note the following:
      • System’s or LCD’s front bezel must be designed with its opening cutout edge falling between the touch screen’s View Area and Active Area, preferably closer to the Active Area.
        VA and AA
      • With AMT’s resistive touch screens, there is an Isolation Ink Area between the Active Area and View Area, which may or may not be a fully transparent area. This area should also be covered by system’s or LCD’s front bezel.
  3. In case your LCD has narrow borders, AMT may need to enlarge the touch screen design in order to ensure lasting touch screen reliability.
  4. AMT recommends that the LCD tail should exit on the wide border side.
  5. Bezel/Surface Gap:
    A gap of approximate 0.5 mm is recommended between the bezel and the top ITO PET. This will prevent unwanted contact reception.
    BezelSurface Gap
  6. Front Gaskets:
    The front gasket provides a buffer for the touch screen when it is tightly fitted to the front bezel. A single-side adhesive front gasket is recommended. Do not use high bond double-sided tape (VHB type adhesive). It is recommended that the front gasket be bonded to the touch screen perimeter. Please ensure that the view area of the touch screen is not enclosed/encased by the front gasket.Front Gaskets
    The gasket expands when compressed. Thus, make sure it does not encroach into the viewing area of the touch screen. Compression force around the perimeter of the touch screen should be even. The front bezel itself must not come into contact with the view area of the touch screen. The thickness of the gasket is recommended to be less than 1.0 mm, except for special considerations or designs. See the following illustration.Front Gasket Alignment
  7. Tail:
    The prescribed bend radius of the tail must not be exceeded, as poor conduction or even breakage can result. Integral tail types can tolerate a bend radius of 5.0mm, HSC types up to 1.5mm, and 1.0mm for FPC tails.
    Tail
  8. Mounting:
    The touch screen must be secured from below through the application of double-sided adhesive to the display. If the screen is affixed to the bezel using very high bond double side adhesive, the top ITO PET may separate from the screen. Contact us for more information.
    Mounting
  9. Non-Input Area:
    A zone near the edge insulation is particularly susceptible to contact damage, especially from stylus use. This area must be covered by the bezel and protected from contact. If you have special application requirements, please contact us for more information.
    Non-Input Area
  10. Venting:
    Note: Venting is generally not recommended unless the touch screen is to be applied in extra high / low altitudes. AMT’s standard touch screens feature no air vent. A Few touch screen models will be design with an air vent to equalize internal and external atmospheric conditions. The vent must be clear, and contact with foreign matter, particularly liquids, must be avoided.
    Venting
  11. Attention for GFG Mounting
    When a Glass-Film-Glass (GFG) resistive touch screen is constructed, a thin layer of glass is laminated to the front of a resistive touch screen. Thus, the edges and 4 corners of the touch screen are marked red in the drawing below as a reminder to be extra careful when mounting.
    GFG Mounting
  12. Critical Area of GFG
    The zone within 4 mm of the adhesive opening is particularly susceptible to damage by high force impact from sharp objects, (see below) especially when high force is applied along the perimeter of the touch screen.
    Critical Area
    Since the top glass is fragile and can be easily broken due to the height of the adhesive around the perimeter of the touch screen, this area must be protected from high force.

Reiteration of Design Notes

  1. LCD Active Area should equal or be smaller than Touch Screen Active Area (by approximately 1mm on each side).
  2. LCD Bezel Opening Area should equal or be smaller than Touch Screen View Area.
  3. The System’s or LCD’s front bezel opening cutout edge should fall between the touch screen’s View Area and Active Area and be closer to the Active Area.
  4. The Isolation Ink Area should be covered by the LCD’s front bezel.
  5. The Tail should exit the wide border side of LCD.