Part 1: Charge Buildup and Resistance to Ground

The normal world is full of electrostatic discharge (ESD) risks to unprotected ESD susceptible devices (ESDS). To handle these, we must set up an ESD Protected Area (EPA) in which we have reduced the ESD risks to a low level. In Parts 1 this article will look at how static electricity works. Part 2 we will look at how we control it in an EPA.

Where does static electricity come from? Science tells us that every material is made of atoms. Atoms are made of electrical charges – positive protons in the atomic nucleus and negative electrons around them. When these are present in equal numbers, their electrical effects cancel.

When two materials touch, some charges move from one material to the other. When the materials separate, each takes a small surplus of one type of charge. Repetition can build the charge imbalance. The charges will try to move to restore balance if they can, but if prevented – static electricity starts to accumulate!

These processes happen very easily. Several thousand volts can be measured on the surface of a plastic sheet lifted from a pile of materials. Packing tape pulled from the reel can give readings of tens of thousands of volts. The repeated shoe-floor contact of a person walking can raise their body to several thousand volts – when they discharge by touching something, they get a shock, as most of us have experienced!

The buildup of charge is rather like the buildup of water in a wash basin. A basin has a tap that allows water in and a drain that lets water out. If there is no plug in the drain, and only a small amount of water flows in, it can run out as fast as it enters with no buildup. Similarly, if charge can flow away (dissipate) as quickly as it is separated, there is no buildup of static electricity. If, however, the basin has a partly blocked or small drain and the tap is full-on, the water level in the basin can build up. If a basin has a blocked drain or the plug is in the plughole, it only needs a dripping tap to cause water level buildup. Similarly, if charge cannot dissipate, only a small amount of charge separation can lead to a buildup of charge and high voltage levels.

Insulators and Conductors

The materials that act like the plug in the plughole are insulators. An insulator is a material that does not allow charge to move away quickly enough to avoid charge buildup. These are materials like ordinary plastics and rubber. Insulators promote charge buildup and ESD risks, so we prefer not to have them near ESDS in an EPA.

Any item which is not an insulator allows charge to move around quickly enough to avoid charge buildup, providing the charge has somewhere to go. These are the conductors, static dissipative, and conductive materials. The IEC 61340-5-1 and ANSI/ESD 20.20 standards have specific definitions, classifying an insulator as a material or item having resistance over 100 GΩ (10¹¹ Ω) and a conductor as an item or material with resistance less than 10 kΩ. “Static dissipative” items and materials are those with intermediate resistance. Be careful – not all industries and standards use the same definitions! Examples of conductors and static dissipative materials can be metals, ESD control materials and water. The human body is mainly water and has resistance between about 1 kΩ and 100 kΩ depending on many factors.

A little theory….

Simple circuits can be used to help understand how static electricity works and how ESD control can be established. Many situations can be simply modeled as a current generator I (charge separation), feeding into some charge storage (capacitance C, the potential ESD source) and leakage (resistance R_g). One side of C and R_g are often earth – in ESD control this can be the common point ground. Charge is partly stored in C and, at the same time partly leaks away through R_g.

If the voltage V is constant, from Ohm’s Law

V = IR_g

If the charging current I = 10 nA and R_g = 1 GΩ, then V is only 10 V, which would not usually be noticed as static electricity. However, if R_g is 1000 GΩ, V would be 10 kV, which would likely be noticed, with a strong possibility of ESD occurring! So, the maximum resistance (usually resistance to ground) effectively sets the maximum voltage generated by a given charging current.

The parallel resistance R_g and capacitance C define a time constant, the product R_g C. If charge generation stops, the charge stored in C flows through R_g and an exponential decay of voltage V occurs. V drops to 5 % of its initial value in 3 x R_g C. If, for example, C = 100 pF (10^-10 F, e.g., a charged person) and R_g is 1 GΩ (10⁹ Ω), the product R_g C is 0.1 seconds – the voltage drops within a second and would not be noticed. If R_g were 100 GΩ in the same circumstance, the voltage decay time would be 10 seconds – long enough to potentially cause a problem. So, a second important effect of limiting the maximum resistance can be to keep the charge dissipation time short.

Another important factor is atmospheric humidity. We’re all familiar with the condensation that often forms on our cold drink glass – this, of course, comes from moisture in the air. Similarly, many materials have a very thin surface moisture layer that can provide a path for charge to dissipate. It tends to be thicker and has lower electrical resistance when the air humidity is higher. Below about 30% RH, the moisture layer breaks up and charge dissipation is inhibited. So, materials and items tend to charge more easily below 30 % RH, but at high RH, it can be difficult to generate electrostatic charge.

So, in ESD control, we replace insulators with conductive or static dissipative materials where possible and provide a path for the charge to dissipate. Any charged conductor can act as an ESD source, so we usually set a maximum resistance to ground R_g to limit voltage buildup in expected situations and reduce the time taken to dissipate charge. If R_g is too high, the voltages generated by charging currents can be too high, and the charge dissipation time can be too long. Providing these controlled dissipation paths largely removes any reliance on moisture from atmospheric humidity in controlling charge buildup. If we specify our materials to give low enough resistance at low humidity, humidity becomes irrelevant.

In Part 2, we’ll look at how resistance in the discharge path when ESD occurs can have a protective effect and how these ideas are applied in practical ESD control.

References and further reading

1. International Electrotechnical Commission, Electrostatics – Part 5-1: Protection of electronic devices from electrostatic phenomena – General requirements, IEC 61340-5-1.
2. 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.
3. Smallwood J. M., The ESD Control Program Handbook, Wiley, 2020, ISBN 978 1 118 31103-5.