Electrostatic discharge (ESD) can wreck the electronic components on a PCB (printed circuit board) or even the board itself. When insulators or unconnected conductors accumulate positive or negative charges, with the charge amount depending on the insulator or conductor’s stored capacitance as it relates to objects in its proximity that are oppositely charged, the static formulated cannot be transferred by electromotive force, rendering it unable to equalize itself.
This will happen if the value of the capacitance between the board’s components is not decreased. Effectively, this static has no path by which to discharge. If a conducting path is present, the charge either discharges quickly or bleeds slowly away, depending on how much resistance is offered in it. However, when the discharge happens rapidly, it generates the phenomenon called ESD, and it can cause significant damage to the PCB and electronic equipment.
The Source Of Static Electricity
While static electricity can source from all materials, some are more susceptible to negative charges, while others are to positive ones. Materials like styrofoam and plastic, for instance, generate negative charges, while human skin and animal fur produce positive ones. How much charge is produced can vary depending on how much static the material can store. For example, the capacity abled by a human body is about 250 picofarads which roughly equates to a static storage charge of 25 kV of electromotive force.
Electronic Circuitry And ESD
Imagine the lightning you see during thunderstorms on a far more miniature scale. This tiny lightning amount is referred to as electrostatic discharge (ESD). The laws of physics dictate that a path of least resistance will cause a large amount of electromotive force to flow in its current form across it. Generally, the current being discharged travels via the metal frame or chassis to the ground.
When an operator is assembling a PCB, for example, the electric charge stored by his or her body can discharge through the circuitry on the board through a metal chassis or the grounded holder. The current will attempt to travel to reach the ground via several available paths. If one of those paths happens to go through the PN junction of the integrated circuit, the current will burn through it, and leave visible holes in its wake. This is a result of the I2R loss, with R being the resistance and the flow of current represented by the I. If the resistance is low, a high current flow of electromotive force is permitted. Because the material cannot dissipate the extensive volume of generated heat, the result is a burn.
How much damage takes place is governed by the amount of heat that the electromotive force carries via the current flow. Therefore, not every single charge will irreparably damage the device. However, when the device is later stressed during typical operations, the seemingly minimal damage caused by that discharge can totally disrupt the device’s operations. The same reasoning applies to a situation where smaller redundant charges flow without dissipation. Over time, they degrade the internal structure and integrity of the device’s performance.
The Nature Of Electrostatic Discharge
ESD can manifest in a variety of ways. The most common way is through devices that are sensitive to human touch. The human body, as well as particular clothing materials, can store anywhere from 500 to 2,500 Volts of static electricity every workday. This is a level too low for human perception, but it is more than enough to damage electronic components.
But human touch is not the only method by which electric currents can damage printed circuit board assembly. The use of ungrounded electrical equipment like an oscilloscope to troubleshoot circuitry, the near proximity of plastic or styrofoam, and even the generation of rapid air movement near the electric assembly can produce currents that induce electrical damage. The latter can happen when, for instance, the electronic equipment is near air handling systems, blowing air on electronics with fans, and even use of compressed air to clean PCB.
In the case of humans collecting static charge, the carriers of it are unaware, and it can take place without physical contact between the PCB and the charged object.
ESD-Induced Damage To PCBs
The possibility of ESD is always present when any object that has a charge is near a PCB. The greatest chance of an ESD is when the objects come in contact, but the distance separating the points of charge, as well as the electromotive force between the two sides, are also factors in how great the chances of an ESD taking place.
Even with the protection mechanisms within ICs built in by manufacturers, ESD may not be avoidable. The damage it causes can be one of two types: severe or latent. Latent damage suffered by a device because of an ESD is unlikely to cease its operation, but it will partially degrade it. With time, especially after repeated latent ESD damage, the device will function more poorly, reducing its lifetime of operation.
Bare PCBs can suffer latent damage as well, such as in the case of an electrical charge from one track to its grounded neighbor. The discharge can reduce the path’s conductivity to only partial form by charring the top layer of the insulation on the PCB. This potentially results in degraded performance of the circuit in the post-assembly phase.
Severe, or catastrophic ESD-induced damage renders a device permanently damaged. This can typically be detected with a performance test. The sole benefit of this occurrence is that the damage being so severe is open to early detection in the manufacturing stage of the PCB assembly process, so it is less costly than the damage incurred over a period of time through latent damage because products with such can pass inspection and testing but fail field operations.
Designing To Mitigate ESD
There are standard practices and methods that designers can follow to help mitigate the chances of ESD, protecting PCBs from its disastrous effects.
- Rather than placing components on metallic connectors, they should be laid on pads made from a material less severely impacted by ESD.
- Since connection and disconnection of a cable from the system could cause ESDs, using transient voltage suppressors or surge protection diodes at both the input and output ports of the circuit can reduce ESD damage.
- Using ground planes can help to bypass ESD to the ground, avoiding the probability that it passes through the PCB’s active components.
- Parasitic inductance hinders the protection circuits from working quickly enough to prevent ESD-induced damage. A way to reduce the effect is to reduce line lengths.
- Traces carrying different voltages should be adequately spaced, especially when the voltage is high. Since the PCB insulation material may not have sufficient insulation resistance, the designer may need to include an air gap or a slot on the PCB.
- Avoid running tracks that lead to IC pins near the PCB’s extremities. Because operators handle PCBs almost entirely by their edges, those areas are more susceptible to ESD. When possible, the board ground track should run around the PCB’s periphery.
The Process Of Preventing ESD
The prevention of ESD by process requires awareness by all who interact with it and the training of all personnel that handle PCBs and the included components and stores them. To prevent any buildup of static electricity, proper grounding of equipment and personnel is pivotal. This requires personnel to wear anti-static attire (shoes, overalls, caps, etc.). Wristbands that connect to the ground should always be worn before the PCB assembly is handled.
Extra care needs to be taken regarding the installation of metal workbenches. They should be grounded to electrical earthing systems and have anti-static mats on top. The materials for anti-static grounding have high-value resistors in series which allow the static charge to be drawn away, protecting personnel from experiencing electrical shocks inadvertently. More information and best practices in this regard can be found in the NFPA-77.
To ensure that the grounding systems are working accordingly, periodic checks should be conducted. Wristbands and ground wires have a limited life period, so they need to be examined and replaced as needed. The anti-static mats tend to have their surface wear off over time, affecting their resistivity, meaning that they should be periodically replaced. For floors and carpets, treatment compounds that help to subvert the static charge build-up are also available.
It is particularly important to curb the use of synthetic materials in production, especially concerning the area of material inspection, storage facilities and their containers, the PCB assembly and testing areas, as well as during packaging and shipping. Another important consideration is the humidity level. Dry air is more prone to ion generation, allowing for greater accumulation of static charges, while humid air helps them dissipate more readily. Therefore, those areas that have air conditioning must also make use of humidifiers to keep the air from being overly dry.
Summary on ESD for Printed Circuit Boards
Sometimes the above-described conditions and needs are not possible for the entire floor of the PCB assembly shop. In those cases, it is vital to have an ESD-protected zone that is clearly identifiable and demarcated. When surfaces are all linked to the ground, objects, people, and any equipment are sensitive to ESD at a similar potential. The area should be set apart and only accessed by trained staff, as well as those responsible for conducting the above-noted effectivity checks.
Vinatronic‘s expert staff has extensive knowledge of the affects of ESD on PCBs and takes extreme measures to mitigate it in our manufacturing facility. If you have an idea or design, we’d be happy to assist in turning that dream into a reality. Explore our website for our offerings.