How can you protect exposed ground pins from ESD?

Question

If I have a DC powered PCB that is floating (not tied to earth ground), and a human touches the ground plane causing an ESD event to occur, what can be done in terms of design to prevent damage to the ground pins of IC's and other sensitive components?

As an example I've drawn a circuit where there are two exposed pins which power an LED. Consider C1, IC, and TVS to be part of the PCB, and the DC power supply to be connected externally. The boxed in portion is the exposed area of the circuit which a human can touch.

If the system is powered and the PSU (or battery) is connected to the circuit, then I would assume that the ESD current would flow back into the power supply negative. But what if the system is unplugged? Are the GND pins at risk of damage here?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

You're supposed to have a separate chassis ground that maximizes free space capacitance such as a large area conductive plate (can be very thin) or enclosure. You then ESD shunt everything to that, including your circuit ground. The large free space capacitance reduces the accumulated voltage on the device when your body capacitance equalizes charge with it.

Obviously signals must be shunted to the chassis ground through devices like TVS diodes, but I recall that shunting circuit ground to chassis ground can be done directly or through TVS diodes. I am unclear which is more effective. It would seem that direct shunting is the lower impedance more expedient path but relies solely on the equalization of charge to reduce the voltage spike on the circuit ground, whereas the TVS diode is less ideal path but has an energy dissipation mechanism to hard limit the accumulated voltage on the circuit ground. I suppose the higher the free space capacitance of the chassis ground, the more you would prefer direct shunting.

Care must be taken when shunting to this chassis ground so that the impedance is low enough that the fast ESD transients at the entry points you are designing for favour going to the chassis ground over your circuit board. This can sometimes involve a mini split-plane on your PCB that nearby connectors shunts to which then connects to your chassis ground. This mini-plane will need to be connected via a bridge to the real ground plane and signals entering the connectors need to be routed over the bridge accordingly to prevent EMC problems regarding discontinuities or unecessarily large loops for the return currents.

You can't effectively shunt ESD that can enter everywhere and anywhere on the board. You need to identify the ESD entry points you want to protect against and prevent contact everywhere else.

Taken from Electromagnetic Compatibility, 2009, Henry Ott R.I.P.

Answer 2

Simple: make the ground connection solid enough, and wide enough (and I mean geometrically, as a ground plane, or enclosing the circuit even), that ESD voltage is not dropped along the path between components.

ESD current flows as a traveling wave, through space, along wires, and over (metallic) enclosures; a minuscule EMP blast, from the spark, expanding outward at light speed. The easiest mitigation is to direct that current around the equipment -- hence, a metallic shielding enclosure. Any wires/connectors/cables can potentially bring ESD back into the circuit (within the enclosure, or inside the perimeter of the PCB ground plane), and filtering/protecting those is another matter.

Answer 3

If you touch the PCB GND and there is an ESD event, the PCB GND voltage may jump up or down by few to few hundred volts.

But since PCB ground is (hopefully) good solid ground, also everything related to it such as signals and supplies jump by an equal amount.

Depends on what your power supply is, but any modern 5V DC output mains power supply will have an EMI Y capacitor between mains side and low voltage side, so when charge jumps to PCB ground, there is current on 5V DC wires to charge that Y capacitor, and the capacitance prevents sudden jumps because charge is small so voltage change over the capacitance.

Answer 4

Electrostatic discharge only occurs when the current has somewhere to go. The current is coming from the ambient capacitance between human and surrounding environment, which is usually at ground potential.

Example situations:

1. Grounded appliance, user touches signal wire: ESD current flows through signal wire to component with lowest breakdown voltage, and then continues to mains ground.
2. Grounded appliance, user touches ground wire: ESD current flows through ground wire to mains ground.
3. Ungrounded appliance, user touches any wire: ESD current flows towards any circuit nodes that have large ambient capacitance. If you have large copper planes or metal enclosure, towards them.

Thus in general touching ground on a portable device wouldn't cause damage, as the path of the current doesn't go through any components.

The situation will change if for some reason you have large ambient capacitance on some other part of the circuit, or if there are any cables connected. For example antennas often have large surfaces and consequently large ambient capacitance.

Similarly if the electrical connection between the point of discharge and the plane with large capacitance is narrow and causes a voltage drop during the discharge, the current may take a shortcut through some component.