In Part 1, we looked at charge generation and dissipation and how this leads to specifying a maximum resistance to ground Rg to control electrostatic charge buildup. Charge is stored in the capacitance C and, at the same time, dissipates away through Rg.
In Part 2, we look at the discharge path when electrostatic discharge (ESD) occurs and applying our understanding in ESD controls.
In our simple circuit, the components to the left of the vertical dotted line represent the ESD source capacitance C and its internal resistance Rs. Stored charge in C represents stored energy ready to dump into ESD. Most ESD sources are charged isolated conductors. IEC 61340-5-1 and ANSI/ESD S20.20 consider these to be conductors with resistance Rs less than 10 kΩ and resistance to ground Rg greater than 1000 MΩ. These might be metal or other low resistance items or a charged person.
Now, we will consider what happens when the stored charge can discharge as ESD into an external circuit (to the right of the dashed line) containing a victim ESDS. We have a charged capacitance C, which will discharge through the source series resistance Rs into the external circuit through the device resistance Rd and resistance of the external discharge path (Rdp).
For ESD to occur, two criteria must be fulfilled:
- The source capacitance C must have sufficient charge and voltage difference with the ESDS to cause ESD.
- The source must either contact or be close enough to the ESDS for discharge to occur.
If either of these cannot happen, we cannot get potentially damaging ESD. Eliminating either makes a good contribution to preventing ESD risk!
If we connect a low resistance Rg so that little or no voltage is produced on C under normal conditions, we can eliminate the risk of ESD from this source. This is “grounding”. The resistance from the item to ground, Rg, can be surprisingly high and still give effective grounding because the charging current is very low. Resistance to ground Rg up to 1000 MΩ is often used. The ground connection is always taken to a common connection ground conductor so that the voltage on all grounded items is the same – this is equipotential bonding. No voltage difference means no possibility of ESD. It is not necessary (but is often desirable) to also connect to physical earth. The equipotential bonding principle would equally work on the space station where no physical ground connection would be possible.
When ESD occurs, current flows through the external circuit and device is limited by the combined resistances of the source, ESDS, and discharge path Rs + Rd + Rdp. It’s worth noting that the energy released into the ESD is dissipated in each resistance according to its resistance value, the largest resistance dissipating the greatest portion of the ESD energy.
If Rs and Rdp are very low resistance (e.g., metal item ESD source and ESDS on a metal tray), the peak ESD current in the discharge can be high, more than tens of amps for a source voltage of even 100 V. ESDS are often susceptible to damage from even short duration high current ESD.
If the ESD source is a person, the source resistance Rs is body resistance and might be of the order 1500 Ω, which would limit the ESD current to around 67 mA for a 100 V source voltage. If we add additional resistance in the discharge path Rdp, say 1 MΩ, in a bench mat surface, the peak ESD current could be reduced to about 100 µA, which would be unlikely to give ESD damage.
If Rs and Rdp are low resistance compared to the device, all the energy stored in C is dissipated in the device. If they are larger than the resistance of the device, the discharge current is limited, and much of the stored energy is dissipated in these resistances rather than in the device. Resistance in the discharge path has a protective effect. This is particularly true for charged device ESD, where the device itself forms the charged isolated conductor ESD source. A minimum resistance is often specified for a surface which will contact ESDS.
ESD Control in the EPA
The principles of ESD control that come out of this discussion are surprisingly simple. Looking around the EPA, we can see many ESD control items designed according to these principles:
- Replace insulators with conductors or static dissipative materials and connect them to common point ground.
- Where possible, ground conductors that might contact the ESDS.
- Always ground personnel who handle ESDS.
- Where necessary, limit voltage differences between isolated conductors and any ESDS that they might contact
- Prevent discharges between ESDS and metal items – Provide resistive contact materials to limit ESD current.
Anything which stands on an ESD control floor may be grounded through it, if designed to do so. Chairs, trolleys, carts, and racks can be grounded through conducting feet or wheels in contact with the floor. Beware that good electrical contact is often prevented by incompatible contacting materials or by dirt buildup.
In the case of personnel, grounding might be done with a wire (wrist strap) connecting the body to ESD earth. Alternatively, make a connection through the ESD control footwear and floor.
Static dissipative bench surfaces provide an intermediate resistance surface that limits the ESD current when a charged ESDS is placed upon the surface. It will also dissipate charge from ESD control tools, tote boxes, or other portable items that are placed upon them, bringing them safely down to zero volts.
ESD control items often act as a system, such as a person grounded through their ESD control footwear and floor. Another case is a hand-held tool. An ordinary tool might have insulating handles, making the metal parts of the tool into isolated conductors that are likely to become charged and a source of ESD. For an ESD control tool, the insulating parts are replaced by static dissipative material, allowing charge to dissipate from the tool to the user’s grounded hand. If the user must wear gloves, these must be made of static dissipative material to maintain the ground path from the tool through to the hand.
Understanding how ESD risks arise and can be controlled allows us to focus our resources on developing and implementing an effective ESD control program.
References and Further Reading
- International Electrotechnical Commission, Electrostatics – Part 5-1: Protection of electronic devices from electrostatic phenomena – General requirements, IEC 61340-5-1
- ESD Association, ESD Association Standard for the Development of an Electrostatic Discharge Control Program for – Protection of Electrical and Electronic Parts, Assemblies and Equipment (excluding Electrically Initiated Explosive Devices), ANSI/ESD S20.20-2021.
- Smallwood J. M., The ESD Control Program Handbook, Wiley ISBN 978 1 118 31103-5, 2020.
