Protecting Membrane Switches from ESD
When your body rubs against certain materials it builds a negative charge and when touching grounded objects these free electrons that we carry jump to the positively charged atoms of the grounded objects, causing a quick discharge of energy know as electrostatic discharge (ESD).
Normally ESD is not much cause for concern; however unprotected electronic devices can be damaged by the discharge. It’s not uncommon for a human body and earth-grounded object to have a potential difference of 15,000 volts or more.
The point of human contact in many electronic devices is the Membrane Switch keypad, and so it is crucial that the Membrane Switch is designed to prevent ESD from reaching their sensitive circuitry and components.
There are two ways in which ESD can reach the sensitive circuitry of the Membrane Switch; ESD can punch directly through all the layers above the circuitry, or it can flash around the layers that protect the circuitry. If the ESD cannot reach the circuitry or an earth ground, the user continues to carry the ESD leaving the Membrane Switch protected.
Ways in which to protect Membrane Switches from ESD include:
- A thin plastic layer can be used to insulate the graphic overlay that is touched by the user. For ESD to punch through the plastic layer, it would have to have a voltage higher than the material’s dielectric strength (the maximum electric field that the material can withstand without breaking down). For example if the insulating plastic is 5 mils thick and has a dielectric strength of 2kV per mil, then the point at which ESD could punch through the insulating layer and reach the circuitry would be equal to 2x5 = 10kV.
- Ensuring a big as distance as possible between all edges of the keypad and the switches circuit traces is also important, as the greater the distance, the greater the flash around protection there is.
- Shields that can be printed as grids of conductive ink extending to all edges of the keypad can prevent flash around of the ESD. A conductive grounded perimeter bezel can also be used, to prevent flash around.
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