Workspace ESD Protection
Electro-Static Discharge (ESD) is essentially the sudden flow of current between two objects with a different electrical charge when there’s a conducting path between them. This could be lightning flowing from a charged cloud down to the ground via the dielectric breakdown of the air between them or that short sharp electric shock you feel when the static electricity that’s built up in your clothing flows to a metal door handle when you touch it. A visible spark often characterizes ESD; the intensity and duration depend on the size of the potential difference between objects.
Electronic components, particularly integrated circuits, can be damaged by ESD, which is why it is such an essential consideration in any workplace where such parts are routinely handled. Various causes can cause the build-up of charge; most commonly in the workplace, it will come from triboelectric charging. Here, different materials are rubbed together, or make and break contact, to create a charge. This effect is most commonly seen when walking across a carpet or removing nylon clothes. It’s the effect seen if an inflated party balloon is rubbed on a woolen jumper, allowing the balloon to stick to a wall or make a person’s hair hilariously stand on end.
The other common cause is electrostatic induction, where a charged object induces a charge on anything, or anyone, that comes into proximity with it. The charged object creates an electrostatic field that causes electrical charges to be redistributed on nearby objects. While it does not change the object’s net electrostatic charge, it will develop regions with positive and negative charges. As commonly used in disposable cups, Styrofoam material can induce an electrostatic charge on nearby ESD-sensitive components and cause an ESD discharge if the component is then touched with an earthed object.
ESD can also be caused by exposure to high levels of energetically charged particles such as radiation or EM emissions, though this is unlikely in most normal workplace environments.
To give an idea of the size of charges we are discussing, the electric spark that can pass from a person’s hand to a door handle requires a field strength of more than 40 kV/cm to occur. With potentials of that magnitude being routinely present, we can see why touching an electrostatic-sensitive component can potentially be so harmful.
ESD and sensitive components
The effect of ESD on electrostatic-sensitive devices will depend on many factors; the size of the charge, the device’s nature, how the charge enters and exits the device. One failure mechanism is when the static charge creates a high potential difference that causes a high peak current to flow through the device. Such a current can cause the burnout of those parts of the device through which the current flows. This will be influenced by how the charge is applied to the device and how it is grounded. Typical damage seen includes fused or melted interconnecting wires on the die and damaged substrate for integrated circuits. An alternative failure effect that may be seen in integrated circuits is where the insulating oxide layer breaks down within the component. Both types of damage may wreck the component, resulting in a latent failure that only materializes in-service or simply results in poor or degraded performance of one or more functions.
ESD storage precautions
Electrostatic-sensitive devices will require protection during shipping and storage. They pass through uncontrolled environments that may include charge objects that may generate electrostatic induction, EM emitters generating highly-energetically charged particles, or natural radiation sources. A typical journey from the manufacturer where components are shipped via air freight flying at high altitude will typically expose the cargo to all these sources.
Any build-up and discharge of static electricity can be minimized by using protective packaging with appropriate surface resistance and volume resistivity properties and resistance to triboelectric charging effects from movement during transit. Electrostatic sensitive components are usually supplied in black conductive foam contained within an anti-static bag. A typical anti-static bag used for integrated circuits uses partially conductive plastic, which acts as a Faraday cage to provide electromagnetic shielding and protect the contents against an external ESD.
ESD handling precautions
The dielectric nature of electronic components means that electrostatic charging cannot be prevented entirely during handling. This risk is typically greatest at the time when the electrostatic-sensitive devices are removed from the protective packaging, used while in storage, and installed onto a PCB as part of the manufacturing process. Most of these components are of a size that necessitates the use of automated equipment for placement on the PCB, focusing the time of most significant ESD risk at the point where components are loaded into the assembly equipment.
The most effective ESD precautions in these circumstances are the grounding of all equipment parts that can contact the components using static dissipative materials that allow the continuous slow leakage of charge, preventing the build-up of static to the ground connection. This will eliminate the risk of any sudden discharge if a statically charged component comes into contact with the equipment.
Where electrostatic-sensitive devices are manually handled, then the best approach for preventing ESD is to create an Electrostatic Protected Area (EPA) where all electrostatic sensitive devices can be managed safely. The EPA can be anything from a small area on a desk to a large industrial facility. The EPA’s main principle is eliminating any highly electrically charged objects where possible, grounding to eliminate any electric charge that may be present on objects that enter the area, and preventing charge building up on objects within the area. These can include personnel as well as goods, equipment, and other items needed within the EPA. Strict controls will be necessary to ensure that any static charge is dissipated and the environment provides protection for all components and assemblies. The EPA’s ESD protective measures will only be effective if the personnel is correctly trained and effective procedures have been put in place.
The philosophy driving the ESD protection strategy is to ground or bring all EPA elements to the same potential, eliminating any potential difference between any items that might cause a current flow. This strategy is applied to absolutely everything within the EPA-protected area, including components, assemblies, surfaces, tools, and personnel. Grounding must use a common grounding point and be wired suitably and safely by electrical safety rules and regulations. Mains earth is generally used for grounding but requires that care must be taken to ensure there is no possibility of the workstation becoming live electrically. The use of special earth connection plugs with no live or neutral connection is recommended.
Typical methods of preventing and dissipating charge build-up in areas accessed by personnel include grounding using anti-static mats or conductive materials for flooring and work surfaces. The use of earthed metal furniture, such as workbenches, chairs, and storage cabinets, can provide additional protection. It’s important to note that metal furniture and fixtures are only practical if coated in a suitable static-dissipative paint to prevent any sudden discharges that would potentially occur with uncoated metal. Even worse, using a standard paint coating with insulating properties would defeat the object of using a conducting base material. The critical protection comes from personnel wearing specialized workwear containing conductive filaments, footwear that provides a conductive path to the floor, and the use of conducting wrist straps and foot-straps.
Another useful method in the workplace where conditions allow is humidity control. The moisture present in humid conditions can help prevent electrostatic charge generation by dissipating any electric charge before it reaches harmful levels. The downside of this technique is it can’t be used where any parts are sensitive to humidity, or their performance could be adversely affected.
Suppose insulative or dielectric materials are utilized within the EPA, or any other materials cannot for any reason be grounded. In that case, they must be kept sufficiently physically separated from electrostatic sensitive devices. If such materials are present and this separation is not possible, then ionization systems can be employed to neutralize any charged surfaces on these materials.
ESD in-service protection
Any device containing electrostatic-sensitive devices will be able to service being used in the real world; the circuit will need to incorporate protection against ESD threats. Protection circuitry is well understood, and components readily available. Commonly such protection is applied at any point where ESD from the outside world can affect the parts inside the device’s casing.
This has the added benefit that no special workplace protective measures are required for the device, inside or outside the EPA, once assembled. Of course, once you open up the device to troubleshoot, repair, or upgrade the components, this protection is compromised, and the same protection required during assembly will now be needed.
A device is made from subassemblies that may be separately handled, such as multiple PCBs within an enclosure. These subassemblies will either require their protection circuitry or rely on handlers’ compliance with special handling requirements.
Creating the EPA
Having established the need to create an EPA, the next step is to decide what proportional measures to take. Tempting as it might be to try and make the EPA as large and possible and look to buy every conceivable product on the market, this can get very expensive very quickly. By careful consideration, a well-designed EPA can save money by reducing wastage from damaged components and improving device reliability.
Your EPA’s critical design criterion is to provide an environment that will minimize ESD build-up and quickly dissipate any static charge safety and away from any sensitive electronic components. This starts with the flooring, which should be static dissipative. A wide range of tile and carpet products are available, though take note that some flooring types may require regular treatment to maintain their dissipative properties.
If feasible, the environment should include air conditioning or specific humidifier equipment to control humidity levels. Very dry atmospheres promote the generation of static charges, so maintaining a comfortable and practical humidity level that minimizes static will make a significant contribution to the EPA’s effectiveness. Consider adding ionizers as part of any air conditioning solution if additional protection across the entire EPA is necessary. Otherwise, localized ionizers can be used if a specific location contains insulative or dielectric materials that require extra protection.
The workbench is the crucial area for ESD protection; it provides the core elements for protecting electrostatic sensitive devices during high-risk manual handling. Not only does the workbench provide a static dissipative working surface, but it also enabled the connection of any other grounded elements of the static protection equipment. Any workbench within the EPA should be fitted with ESD wrist and ankle straps to ensure the user is correctly grounded using a high resistance dissipative rather than a conductive path for safety reasons. All associate furniture, including seats and storage cabinets, should be similarly protected.
Surfaces that provide a conductive path offer a low impedance discharge path that will result in the short circuit of any devices placed on the surface during prototyping and testing. Any ESD event or an inadvertent application of a power source to the wrong part of the device will result in a large current flowing through it. Using a surface with a high resistance dissipative path will limit current flow while still fulfilling the function of preventing a static charge build-up and allowing its dissipation. More critically, a conductive surface connected to a wrist strap could present a safety hazard should the ground become live. In the improbable event of the ground connection of the bench becoming live, the high impedance becomes a much safer option. A further reason for using a high resistance dissipative surface rather than a conductive surface is one of safety. If any high voltages are present in a device, any inadvertent contact between the conductive surface and the high voltage source could be hazardous to personnel touching that surface.
One thing to watch out for is that ESD dissipative surfaces can be prone to damage from sharp tools. This can create areas of the surface left with compromised dissipative properties that may not be readily detectable. Another issue is the contamination of the surface by chemicals, oils, and general dirt that may also affect performance. Keeping surfaces clean using an appropriate cleaning solution following the manufacturer’s recommendations will be required. Never use unapproved solvents or abrasive products as these can remove or damage the ESD dissipative surface.
Personnel working within the EPA will require specialist ESD tools where necessary. A good example is soldering irons that can effortlessly transfer static directly to components. Because the tip in contact with the component is metallic, it provides the perfect transfer path. It maximizes the current passed to the component during an ESD event ensuring maximum damage. Most soldering irons are pretty suitable for work with static-sensitive devices. The main requirement being that the tip should be earthed, and the thermostatic control should use a zero voltage switching system to prevent significant current spikes with the soldering iron caused by the switching of the thermostat from inducing an electrostatic charge on nearby components.
The use of ESD protective clothing within the EPA is critically important because the items of regular clothing worn by personnel are great at generating and holding static charges. Clothes made using man-made fibers, especially nylon, can create highly high charge levels simply by the wearer undertaking normal activities such as walking. Having a protective outer layer in an ESD lab coat over regular clothes can provide sufficient protection in most normal circumstances. The problem can also be significantly reduced using conductive straps around the shoes to provide a dissipative path to the flooring. The majority of footwear includes high insulative rubber or leather soles.
Users should bear in mind that any parts subject to regular movement, particularly the ESD wrist strap, will be prone to failure. Straps especially will fail as an open circuit fault, which will not be noticeable unless it’s a complete separation of parts but will result in a total loss of ESD protection. The protection mechanisms must be subject to regular tests to ensure they stay within spec. Ideally, items like wrist straps should be tested each time they are used. If budgets allow, continuous monitoring systems are available as an integral part of the ESD workbench.
So far, we’ve talked about what you need to purchase. An EPA’s effectiveness is also affected by what items and materials are allowed to enter the area. Where possible, all static generating material should be kept out of the EPA. The EPA’s outer borders should be managed so that any such materials outside but close to the EPA’s boundary cannot cause an adverse effect within the EPA. Such materials include plastic cups, expanded polystyrene used in packaging and disposable cups, and some types of bubble wrap and other packaging materials. Food and drink should be kept away from electronic manufacturing facilities anyway. However, there is a temptation to use plastic or Styrofoam cups to store small loose items like screws or spare components. Training and periodic walk-around checks can help eliminate this issue.
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