Normally, the damage caused by ESD is not apparent when the installation is started up or the equipment tested. However, it continues to degrade the unit over time and such accumulated deterioration eventually leads to a total failure of the component in a matter of days, months or years.
That is why there is great interest in the world of electronic components and product manufacturing to minimise and control electrostatic discharge.
This article attempts to answer the basic questions about the problem of ESD and discusses the most important ways of avoiding it.
What does ESD stand for?
ESD stands for Electro-Static Discharge.
What is ESD?
It is the transfer of a charge due to a difference in electric potential. It should be remembered when dealing with this question that a charge or discharge is less important than the rate or speed at which it occurs.
How is static electricity generated?
Static electricity is always caused by an imbalance between electrons or negative charges (-) and protons or positive charges (+) in the matter.
What processes typically generate static electricity?
Static electricity is generated by the following processes:
- Friction (or triboelectricity): when two materials are rubbed against each other or separated from each other. For example, when you walk across a carpet, your shoes rub against the carpet and produce static electricity.
- Contact: when two contacts come together and one is more highly charged than the other, the most charged substance releases its charge until the electric potential matches at a value that may not be zero. For example: you will probably have sometimes noticed a spark or even a shock when closing the car door – that is because the car has become charged due to friction and when you touch it, your electric potential levels out with the car’s.
- Corona discharge: this occurs when a very small surface (e.g. testing prongs) is highly energised and generates a corona discharge.
- Induction: an object (mainly conductors) produces an electrostatic field (e.g. a computer) that acts over a distance on nearby objects by polarising the energy charge. When the polarised object comes into contact with the other one, further charges are released or captured and the result is a charged object.
How perceptive are we to ESD?
The voltage in an electrostatic discharge and our sensitivity to it can generally be defined as follows:
Electrostatic discharge and our sensitivity
90% of all ESD events occur below the 3000 V threshold so we do not perceive them.
What about electronic components?
Some electronic components are very sensitive to ESD and even fail at voltage levels of less than 100 volts.
This information can be provided by component manufacturers and must be known when designing a system to protect against ESD.
Examples of the threshold voltage at which an electronic component may be damaged are detailed below:
Threshold voltage that may damage an electronic component
So what factors should be taken into account to protect electronic components against electrostatic discharge build-up?
There are two main objectives to bear in mind when protecting electronic components:
- Avoid generating or building up electrical charges
- Prevent sudden electrostatic discharges by restricting or limiting the discharge current. This can be achieved by using materials with a moderate surface resistance of around 106 ohms.
How are materials classified to help you know whether they are suitable?
With regard to surface resistance, materials are classified in three types:
- Static conducting materials: which have a surface resistance of below 104 ohms
- Static dissipative materials: which have a surface resistance of between 104 and 1011 ohms
- Static insulating materials: which have a surface resistance of over 1011 ohms
Static dissipative materials are most widely recommended for treating ESD, although a combination of static conducting and dissipative materials can also be used for certain applications.
Insulating materials should be avoided when handling electronic products given the ease with which they can be electrostatically charged.
Do any standards exist that set out criteria for protecting electronic components against ESD?
Yes, a number of standards do exist, of which the most widely used are:
- EN 61340-5-1:2007 Electrostatics. Part 5-1: Protection of electronic components against electrostatics. General requirements
- ANSI/ESD S20.20-2007 for the Development of an Electrostatic Discharge Control Program
The maximum electrostatic field accepted by standard EN 61340-5-1 for discharge to humans from electronic devices is 100 volt/cm.
Furthermore, the above standards refer to others that set out technical requirements for all materials, clothing and facilities to protect against ESD.
What steps can we take to minimise the effects of ESD?
The following actions should be taken to minimise the negative impact of ESD:
1. Identify and mark EPA (Electrostatic Protected Areas) in working environments:
- Entry into an EPA
Those areas considered to entail a risk of electrostatic discharge for electronic equipment must be clearly marked as restricted areas where ESD protective means and measures are needed, and therefore access to them is forbidden without such means.
Entry into an EPA
- Exit from an EPA:
This indicates that you are about to leave an ESD protected area and enter a UPA (Un-Protected Area), i.e. an area with no ESD protection.
Exit from an EPA
2. Static dissipative flooring
Flooring in an EPA must be a transmitter of electric current by means of dissipative floor paint, mats or carpets.
Static dissipative flooring
3. Human protection
All individuals should have permanent and secure connection to earth, as well as avoiding the generation of electrostatic charges. This is achieved by connecting people to earth by means of ESD-dissipating clothes and equipment, such as specific ESD-proof footwear, ankle bands, wrist bands, anti-static clothing (uniform or overalls).
Example of human protection
4. Protective safety in the workplace
The way to guarantee safety in the workplace is to use static dissipative or conducting materials connected to earth via a grounding lead with an Rg resistance rating of below 109 ohms. This heading includes the following items:
- Office furniture: tables, chairs, foot rests. Anti-ESD mats can be used on these surfaces
- Warehousing, storage and transportation equipment: racks, component lockers and trays, sticks, trolleys, shelving, cabinets, pallet carriers …
- Machinery
- Hand tools
Protective safety in the workplace
How are ESD-sensitive products marked?
- Markings referring to the contents: Indicate that the product is ESD sensitive and must therefore be handled within EPA areas.
Contents identification
- Markings referring to the packaging: Indicate that the packaging can be used with ESD-sensitive devices.
The arc denotes ESD protection. The (*) indicates the features of the container or packaging, namely:
S: static shielded
C: static conducting
D: static dissipative
L: low static generating
EPA: to be used in an EPA
Packaging identification
Pedro Granados, Technician at Quality and Environment Dept.
