Electrostatic discharge (ESD) can pose a serious threat to sensitive electronic equipment. It can damage circuitry, disable components and generally pose a threat to the working life of electrical equipment. In the long run, if a business in the electronics industry is unable to clamp down on this threat, ESDs can have a significant impact on productivity, product reliability, rework costs and ultimately profitability. In fact, ESD Association estimates that as much as 25 per cent of all electronics that get damaged for unknown reasons can be attributed to ESD, and that in total ESDs cost the electronics industry US$ 5 billion a year.
An ESD event can permanently damage a device’s circuitry so that it stops functioning entirely or partially by causing a variety of problems such as metal melt, junction breakdown or oxide failure. If the damage happens after the item’s final test in a plant, the product could be sold or used despite this fault. When the flaw leads to a problem further down the line, it can have repercussions on reputation of the manufacturer.
An ESD event could take place at any point in the product’s lifecycle, from its production to being put into operation, and normally it results from a device being handled in an uncontrolled environment or with poor ESD-control practices. This makes it imperative for electronics manufacturers to ensure that they have effective static-control measures in place; otherwise they will struggle to routinely produce high-quality devices.
Certain materials coming into contact with each other and then separating generate an electrostatic charge. This can be caused by a long list of things such as when a device slides into a bag. However, one of the most common causes is people walking across the floor; with each step the contact and separation of the sole of the shoe with the floor’s surface creates a small charge.
This charge can quickly build up when it is subjected to regular traffic from people and wheeled equipment. Factors such as the material of the floor, type of footwear and even humidity can have an effect on the amount of charge generated.
One of the most common materials that generate ESD is human skin itself, as it is very conductive in electrostatic terms. If a person is electrically-isolated by synthetic soles of shoes, the charge can accumulate on the body like in a capacitor, with voltage potentially rising to several kilovolts.
You are unlikely to know that you are carrying around this charge until you touch a grounded object and receive a shock, as the body releases all of its accumulated charge at once and a spark jumps from the body to the object. If it is an electrical device that you touched, it could inadvertently get damaged.
Antistatic flooring solutions can reduce this risk. However, there are several things to consider when selecting an ESD-control floor. The first and foremost is whether to opt for a conductive or static dissipative system.
A conductive material, because of its low electrical resistance, allows electrons to flow easily through it or across its surface. When a conductive material becomes charged, the charge gets uniformly distributed across the surface of the material.
If the charged conductive material makes contact with another conductive material, electrons move between the materials easily. If the second conductor is attached to an AC equipment’s ground or any other grounding point, electrons flow to ground and excess charge on the conductor gets neutralised.
Static dissipative materials have electrical resistance between insulative and conductive materials. There can be electron flow across or through the dissipative material, but surface resistance or volume resistance of the material controls it.
Charge can be generated triboelectrically on a static dissipative material. However, like the conductive material, a static dissipative surface allows the transfer of charge to ground or other conductive objects. Transfer of charge from a static dissipative material generally takes longer than from a conductive material of equivalent size. Charge transfers from static dissipative materials are significantly faster than from insulators, and slower than from conductive substances.
Which control technique to use out of the two largely depends on the application. In a general office environment, a coating with antistatic characteristics that reduces triboelectric charging on contact surfaces will normally be sufficient.
However, in an area where any significant charge accumulation may damage sensitive components or create a spark risk, emphasis is on dissipating electrostatic charge before it can build up to dangerous levels. For example, conductive flooring (25,000-ohm to 100,000-ohm resistance) is commonly used in facilities such as ammunition manufacturing, while electronics manufacturing will most often specify flooring in the dissipative range of 106 to 109 ohms.
The amount of resistance in a static-control floor determines the amount of electrical current that can pass through the floor. As resistance decreases, electrical current increases. In practice, ESD control is usually achieved by a combination of static dissipative footwear and flooring, so that there is an adequately low-resistance path from the person via footwear and flooring to ground (zero/earth potential). In many cases, dissipative flooring is the most cost-effective option.
The types of antistatic flooring are categorised into several standards by ESD/EOS Association Inc. The following points identify the main categories and key criteria that a floor has to fulfil in order to meet these:
1. ANSI/ESD S20.20 (S20.20) requires a system resistance of a person through the floor and to ground of less than 3.5×107 ohms (35 megaohms) when the floor is to be used as the primary ground. Many dissipative floors have resistance ranges above 35 megaohms before considering operator resistance.
2. ANSI/ESD S20.20 revised in 2014 for ESD workplace compliance mentions three standards within the specification relating to flooring materials; all three standards are required to be met for compliance.
3. ANSI/ESD STM 7.1 Floor Materials ‘Resistive Characterisation of Materials’ states that any flooring material must measure below 1.0×10E9 ohms to ground.
4. ANSI/ESD STM 97.1 Floor Materials and Footwear ‘Resistance in Combination with a Person’ states that the recommended maximum system resistance is 3.5×10E7.
5. ANSI/ESD STM 97.2 Floor Materials and Footwear ‘Voltage Measurement in Combination with a Person’ suggests that the recommended maximum voltage allowed is 100 volts.
Selection of an effective ESD floor-covering system should always begin with a thorough evaluation of the intended use and possible future use. Intended use should include evaluation of devices or processes in the environment that are most sensitive to ESD events, chemical resistance and aesthetic requirements.
Understanding the need for and importance of ESD-compliant footwear is also critical in the performance of the ESD flooring chosen. Grounding personnel require effective ESD footwear that is appropriate to the application, properly worn and in good repair.
A variety of antistatic flooring systems are available for static-sensitive environments that allow architects and consultants to meet the specific needs of the work area in question. This comprehensive product range includes floor finishes for production and assembly rooms, storage areas, office space, R&D facilities or clean rooms, to name a few.
These surfaces do not require periodic treatment to maintain antistatic control capability that complies with the latest ANSI/ESD standards.
Ensuring that the floor area is tailored to the level of electrostatic charge that is likely to be generated within an environment with sensitive electronic equipment is key to avoiding failures and resultant business issues that can stem from the charges sparking into devices and ruining components within.
When specifying an antistatic floor finish, it is important to work with both a supplier and applicator who are well versed with the facts around electrostatic charge buildup in the floor and how this risk can be effectively reduced.
V. Srinivasan is sales director at Flowcrete India Pvt Ltd