Thin film switches, commonly known as membrane switches, have a specific layered structure that allows them to function. These components work together to create a durable, lightweight, and user-friendly interface. Here's a detailed breakdown of each layer in a standard thin film switch structure:
- Overlay or Graphic Layer This is the top-most layer of a membrane switch visible to the user. It can include graphical text and designs, which indicate where to press. Materials typically used for this layer are:
- Polyester (more durable and has better chemical resistance)
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Polycarbonate (more cost-effective but less durable than polyester)
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Graphic Adhesive Beneath the graphic overlay is a thin layer of adhesive used to stick the overlay to the next layer in the stack. This adhesive must be durable enough to withstand the flexing of the switch during use as well as environmental factors such as moisture.
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Dome Retainer Layer (Optional) This layer, usually made of polyester, holds any tactile domes in place. Tactile domes are typically made from stainless steel or polyester and give the user a tactile response ("click" feel) when the switch is pressed.
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Spacer Layer The spacer layer separates the top circuit from the bottom and ensures that they do not touch until a user applies pressure on the front side of the switch. This layer generally has holes where the switch contact is made.
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Top Circuit Layer This layer consists of the conductive traces printed onto a flexible substrate such as polyester. The circuit can be printed using conductive inks or etched copper on polyimide substrates for more rigorous applications.
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Circuit Spacer Another layer of spacing material may be inserted here if needed, depending on the design of the membrane switch. It could further prevent unwanted circuit contact and may carry additional circuitry or components.
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Lower Circuit Layer Mirroring the top circuit layer, this also contains conductive traces and is lined up with the top layer so that when pressure is applied, the circuits connect at specific touch points.
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Rear Adhesive Layer This adhesive bonds the entire membrane switch package to the product or service it is being used with. It can be designed to stick the switch to a variety of surface materials.
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Backing Material (Optional) A rigid backing material, like aluminum or rigid plastic, can be added for switches that require additional structural support.
The thin film switch works when a user presses a button on the graphic overlay, which then flexes down to close the circuit between the top and bottom layers. This action is detected by the electronic circuit to which the thin film switch is connected, resulting in the desired operation.
This construction allows for a compact, versatile, and visually appealing user interface that can be sealed against environmental conditions, making membrane switches suitable for a wide range of applications, from medical devices to industrial control panels.