Electrostatic Discharge (ESD)
This phenomenon, called electrostatic discharge (ESD), can become a problem with unprotected electronic devices, which can be damaged by the discharge. Itís not uncommon for a human body and earth-grounded object to have a potential difference of 15,000 volts or more. Currents generated during ESD can be as high as 30 amperes, but last only for fraction of a nanosecond. Only a small portion of this brief discharge, however, is sufficient to cause a circuit malfunction or more serious damage.
In many electronic instruments, the point of human contact is a membrane-switch keypad. So itís critical that membrane switches be designed to prevent the ESD arc from reaching their sensitive circuitry and components.
Destination: earth ground
Thin plastic is most often for the insulating graphic layer that is touched by the a membrane-switch user. In order for ESD to punch through this layer, it would have to have a voltage higher than the materialís dielectric strength, measured in volts (V) per unit thickness. If the insulating material is 7 mils thick and has a dielectric strength of 2 kV per mil, then the point at which ESD breakdown could occur would be equal to 2◊7 = 14 kV.
If the discharge does not exceed the materialís dielectric strength, it can migrate across the surface of the overlay. And if it reaches a cutout or outer edge, it can potentially flash around the edge and work itís way between the switch layers as it searches for an earth ground. But it may find sensitive circuit traces first and damage the switch. If it canít reach circuitry or an earth ground, the user continues to carry the ESD potential, and the switch is protected.
You can test the effectiveness of ESD shielding using two methods proscribed by ASTM test method F1812: one for measuring contact discharge, and the other for measureing air-gap discharge.
Both methods require some form of shielding that is connected to an earth ground during testing. The contact discharge method measures shielding when the discharge tip of a special ESD gun is in direct contact with the keypad. The air-gap method measures shielding when the gun is discharged at a specified distance above the keypad.
In order to test the effectiveness of the shield, you monitor the I/O traces of the membrane switch during the discharge of the ESD gun (Figure 3). If you note a voltage spike on any of the I/O traces, you know that the shield is not intercepting the discharge properly.
In some cases, it is also usefule to perform ESD testing after the switch is mounted in the final equipment housing. This can be especially useful to determine if the edge of the housing or switch helps insulate agains ESD.
Ask your customer to provide construction and performance criteria for the switch so that you can accurately evaluate it under the same conditions they will. Information to request should include the following:
Flash around is a much more common occurance, so you must look at factors to protect against it. Get exact perforance specifications from customers, and consider how grounding loops, bezels, shields, and equipment housings may eliminate the problem. Look for the simplest solution. Donít add a full shield layer when you donít have toóthatís money in your pocket.
Oringally posted at Membraneswitchnews.com by Alan Burke