ESD Protection

Electrostatic Discharge (ESD)
Have you ever defied gravity by rubbing a ballon on your hair, then making it stick to a wall or ceiling? This attraction results a small negative charge that rubbing created in the balloon. Your body can build a similar, but much greater, charge when it rubs against certain materials (e.g., carpet). Touching a grounded object causes a quick discharge of the energy because unlike charges attract. In other words, the free electrons you carry jump to the positively charged atoms of grounded objects.

This phenomenon, called electrostatic discharge (ESD), can become a problem with unprotected electronic devices, which 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. 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 it’s critical that membrane switches be designed to prevent the ESD arc from reaching their sensitive circuitry and components.

Destination: earth ground
ESD can reach switch circuitry in two ways. It can punch directly through the layers above the circuitry, or it can flash around and between the layers that protect the circuitry. To prevent ESD from damaging the switch, you must provide a “path of least resistance” for the charge to follow to an earth ground and away from the circuitry.

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 material’s 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 material’s 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 it’s 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 can’t reach circuitry or an earth ground, the user continues to carry the ESD potential, and the switch is protected.

ESD shielding
To prevent ESD from damaging your switches, you may need to shield them. Here are a guidelines to consider:

  • Make sure any shielding is connected to an earth ground. An ungrounded shield is merely a floating barrier that will distribute the charge to other potentially damaging locations in the switch assembly.
  • To prevent punch through, make sure the dielectric strength of the surface(s) over the circuitry are greater than a typical ESD event. Again, to calculate a substrate’s specific dielectric strength, multiply the dielectric strength of the material per unit thickness (a value typically provided by the substrate manufacturer) by the layer’s actual thickness. If the substrate’s dielectric strength is too low to prevent punch through, then you may need to use a full shield to protect the circuit.
  • Maintain as big a distance as possible between all edges of the keypad and the switch’s circuit traces. The greater the distance, the greater the flash around protection you have.
  • If edge distance is a problem, try printing an independent, earth-grounded loop on the edge of the circuit layer, but away from any circuit traces. Any ESD voltage present will hit the loop and be channeled away from sensitive circuitry.
  • If you use a full shield, make sure it is placed between the top layer and circuit layer and that it extends to all edges to prevent flash around. Shields can be printed as grids of conductive ink, but must also be continuous and extend to the assembly’s outer edges.
  • To protect against flash around ESD voltages in critical applications, you may need to manufacture the switches with a conductive, grounded perimeter bezel as well. A non-conductive bezel can lead to flash around.
  • Design circuit traces with round corners. Sharp corners are more prone to attract a charge than rounded ones.

Testing for ESD shielding effectiveness
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:

  • mounting conditions for the switch assembly (details about housing, connections, etc.)
  • discharge test method to employ (contact and/or air gap)
  • for air-gap test, distance between discharge tip of ESD gun and keypad
  • ESD voltage protection required (kV)
  • description of discharge tip on ESD gun
  • maximum voltage spike to measure

Note that in ESD punch through is extremely rare, especially in switch assemblies that feature a polyester graphic layer. Polyester has a dielectric strength of more than 3 kV per mil, which means it would take a pulse of over 21 kV to get through the graphic layer alone.

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. Don’t add a full shield layer when you don’t have to—that’s money in your pocket.

Oringally posted at by Alan Burke