ESD FAQs

Static is a naturally occurring phenomenon and is generated from the friction of two separating materials. This is referred to as Tribo-Electric generation.

Materials are made from atoms that are normally electrically neutral – they contain equal amounts of positive (protons) and negative (electrons) charges. As the contacting surfaces of the materials are pulled apart, electrons are transferred from one material to the other. This leaves both materials charged. The transfer invariably leaves one surface with more electrons (negatively charged) and one surface with less electrons (positively charged).

The spark is a neutralization of the charge and is created when the excess charge is either bought near a conductive surface and the excess charge “earths” itself. Or when the charge comes into contact with a region with excess charge of the opposite polarity (positive or negative).

Everyday materials are susceptible to this. Some of these include:

• Air (Always Positive)
• Human skin and hair
• Nylon, wool, cotton and silk
• Paper, wood and some rubbers
• Rayon, Teflon (always Negative) and Silicon
• Polyester, Polyethylene and PVC

Actions that generate charge include:

• Walking across a carpet generates between 1,500 to 35,000 volts
• Walking over untreated Vinyl floors generates between 250 to 12,000 volts
• Operator working at bench can generate between 700 to 6,000 volts

As you can see, it is easy to generate static and easy to create that spark. Humidity will also effect how much charge is generated.

Some materials are said to have “free” electrons. In physics these are referred to as a Charge Carrier. In terms of ESD, we refer to them as Conductive. A common material that has “free” electrons is Metal. In metals, one or two valence electrons (an electron from the outer shell of the atom) are able to move freely between the crystal structure of the metal – making them conduction electrons (hence Conductive material). Other conductive materials include water, human sweat and carbon.

Some materials do not allow for the easy transfer of electrons. This is caused when a material has a large gap between the valence electrons on the outer shell of the atom and the next one. If this gap is to big for the electron to cross then the charge is stopped there. We refer to this as an insulator. It should be noted that all insulator materials can be made conductive with a high enough charge. The electron can become so charged it can “jump” the gap.

Both insulators and conductors can be “charged” positively or negatively. Conductors allow for this charge to be rapidly discharged across the surface – allowing for easier flowing electrons and protons and increasing the risk of the spark.

ESD stands for Electrostatic Discharge and is often the term used for products designed to protect against the damage static can cause. It is a widely used acronym in the Electronics Industries as they often have to protect sensitive electronic equipment from static discharge. It is often incorrectly used as a term for something that is “Static Safe”.

Conductive, Insulative, Dissipative and Anti-Static are all terms that are used to subdivide ESD in more detail although some thing that is Insulative is not considered ESD safe.

In terms of ESD we class products and items into these categories based on their surface resistance

Surface Resistance is a measurement of how easily an electric charge can travel across a medium i.e. the surface of a work bench, coat, floor etc.

Conductive materials generally have a surface resistance of between 1 x 10³ ohms/square and 1 x 10⁶ ohms/square, have no initial charges, provide a path for the charge to bleed off and usually have carbon-particle or carbon-fibre throughout. Conductive materials, in terms of ESD, allow the charge to move too fast across the surface which can damage or “overload” components.

Insulative materials have a surface resistance of at least 1 x 10¹² ohms/square. Insulative materials hold onto charges so they don’t dissipate quickly.

Anti-Static materials are generally any material that inhibits tribo-electric charging (generation of static). These materials are not really Anti-Static more Non Static. Anti-Static materials have a surface resistance of between 1 x 10⁹ ohms/square and 1 x 10¹² ohms/square and may be made from surface resistive, surface coated or filled materials.

Static Dissipative materials have a surface resistance of between 1 x 10⁶ ohms/square and 1 x 10⁹ ohms/square and have low or no initial charges. Dissipative materials may be either surface coated or filled materials.

When you feel a static shock, you are experiencing a minimum of 3,000 volts of electricity. You can hear a static discharge of between 2,000 to 3,000 volts and you can see a static discharge of over 5,000 volts. By comparison, a ground to earth bolt of lightning typically has around 1 Billion Volts.

A person walking across a carpet in an environment with relative humidity of between 10% and 20% can generate up to 35,000 volts. Enough for it to cause you a brief snap of pain when it discharges. A vinyl floor could generate up to 12,000 volts.

In the grand scheme of things, given a bolt of lightning can generate a billion volts, a few thousand doesn’t seem to be that much of a concern. We have all stroked the cat whilst wearing a woolen jumper and felt and heard the click of a 15,000-volt shock with no harm done

But as electronic devices advance and become smaller and smaller, they become much more susceptible to Electrostatic Discharge. Modern Complementary Metal-Oxide Semi-Conductors (CMOS) that are in a mind-boggling range of electronic devices from cameras to satellites can be damaged by charges LESS than 1000 volts and some by charges as little as 5 volts. That is a major issue for electronics manufacturing

This damage can occur at any time to the electronic component from receiving and incoming goods, stores, inspection, kitting, assembly, auto insertion, soldering (including wave and re-flow), testing, reworks and shipping of the goods. The damage is more often than not to small to see with the human eye and won’t be discovered until the component is used. It can then either not work at all (direct catastrophic failure) or fail whilst in use (latent failure) or won’t work as intended (upset failure)- usually causing serious problems to the user

Direct catastrophic failures occur when a component is damaged to the point where it is now dead and will never work again. This type of damage can usually be spotted during quality control. Latent failures occur when ESD weakens or wounds the component to the point where it will still function and may pass testing, but over time the component will continue to deteriorate faster and can cause poor system performance and failure before its intended life span. As latent failures happen after quality testing and once the system has been deployed to the customer, the costs of repair can be high either from warranty claims or repair bills. It can also severely damage the providers’ reputation

Upset failures occur when the ESD has damaged the component just enough to cause damage so the component won’t work correctly but not enough to cause total failure. This can be extremely frustrating as it can be very hard to identify the faulty component and can lead to long periods of downtime and investigation to find the fault.

Proper ESD Control needs careful planning and implementation. Much like a risk assessment or a COSHH assessment, the risks need to be identified, worked out how they occur and decide on what needs to be changed/ added to the process to either mitigate or remove the risk. Also exactly like a Risk or COSHH assessment an ESD control program comprises of:

• Identify the appropriate and effective ESD control measures – Tailored for your application, reduce the number of ESD opportunities, Reduce the magnitude of the ESD to an insignificant level.
• Implement an effective training program – Tells operators why it is needed (dangers and costs), how they should do it (equipment procedures), ESD Awareness
• Implement effective Testing, Auditing and maintenance – Are we doing it right and consistently? Is the equipment OK and working properly? Is there a maintenance procedure in place?

All three stages must be recorded and documented properly. If they are not checked, recorded and documented properly then they probably aren’t being implemented properly

The international standard for ESD Control is EN61340-5-1. In order to meet compliance, you will need to have all three of these stages in place.

The above can seem a little open to interpretation. But when identifying the danger points ask your self several key questions:

• How is the charge generated?
• How will it reach the component?
• Can it be taken away before reaching the component (earthed via a wrist band, ESD work mat, ESD flooring etc)? or can the component be protected against the charge (such as in ESD packaging or containers)?

There is a “minimum” level of ESD protection which comprises of an ESD operator worn wrist strap, connected to an ESD work mat via an ESD cord, connected to a common earth grounding point. A common earth grounding point is usually a plug the mat/ wrist strap connects to. They are normally yellow and have several stud connectors on (or similar) and a special earth bonding plug into the buildings mains electricity via a normal socket.

Whilst this is the minimum level it should not be considered the be-all and end-all of your ESD Control Program – more the starting point.

An electrostatic field is a type of electric field (a vector field surrounding an electric charge) that exerts forces on other charges, attracting or repelling them. Static electricity is an electric charge and, at a sufficient-enough charge, will generate an electric field that can extend beyond the source. The below image is an example of an electrostatic field (arrows) being emitted from a charged source.

Static fields can be measured with an electric field monitor.

When a charge becomes sufficiently powerful enough it can generate an electric field. When this field comes close enough to a conductive material it causes electrical charges to be redistributed around the material, causing one side to become negatively charged and the other positively charged. Because of the electric field, this can be done without the source physically touching the objects around it.

The positively charged source attracts the opposite charge in the objects around it – in this case, it attracts the negative charges (blue). The positive charges in the object (red) are repelled. The negative charges then attract positive charges (protons) from the source to balance out the negatively charged electric field.

These charges can be 1,000’s of volts. A sudden influx of electrons on a sensitive conductor like circuitry on a PCB or Complementary Metal-Oxide Semi-Conductor (CMOS) can damage the very delicate circuitry. Some CMOS components can be damaged by as little as 30 volts, so a sudden influx of 1,000’s of volts can cause critical failure in the component.

We can protect ESDS devices from ESD damage (including induction from an electrostatic field) by implementing proper ESD systems and controls such as identifying and grounding potential sources and protecting ESDS devices in conductive packaging and on ESD safe furniture.

A constant monitor is a device used to monitor the connections between the operators ESD wrist strap, bench mat/ bench or the earth whilst checking the earth to ground system is working properly.

If one of the ESD items is disconnected or the earth to ground system is interrupted then an alarm will sound. Many also have a green and red light display.

ESDS stands for Electrostatic Discharge Sensitive. An ESDS Device is something that is susceptible to damage from a static charge. Many electronic components, such as a Printed Circuit Board (PCB) or a Complementary Metal-Oxide Semi-Conductor (CMOS) are ESDS devices.

To ensure your ESD controls are still effective you will need to test them periodically. Testing ESD surfaces is the best way to test an ESD controlled flat surface, something like a floor or bench mat.

ESD surface testers use a point to point systems the tests the surface resistance between two points. They also test point to ground resistance and Earth Bonding Points (EBP) to ground.

Wrist band and foot tester systems check the resistance through the wrist band, cord and wearer.