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» TEST & QUALITY ASSURANCE Managing electrostatic discharge (ESD) Defending against the invisible enemy in PCB reliability One of the most common causes of PCB failure is electrostatic discharge (ESD). This invisible threat can have catastrophic effects for manufacturers. In this article, MicroCare’s Elizabeth Norwood explains how manufacturers can control ESD to increase both production efficiency and product quality. Source: MicroCare As PCBs get more sophisticated, their sensitivity to ESD spikes increases ESD can damage electronic components and result in significant losses for electronics manufacturing companies in the form of warranty returns, rework, scrapped boards, wasted time and missed deadlines. This is not only costly, but may also negatively affect the company’s reputation. Experts estimate that up to one third of PCB failures are from ESD. Today’s electronic assemblies are smaller, more complex and incorporate increasingly more expensive parts, so it makes sense to reduce and control ESD and protect PCBs during assembly. What is ESD? Electrostatic discharge is caused by two surfaces or objects at different levels of electrostatic charge coming into close contact with each other. When one is positively charged, and the other negatively charged, the protons and electrons that carry these charges attempt to balance each other out by rapidly exchanging at the point of contact. The sudden release or discharge of the built-up charge causes an ESD ‘spike’. Although ESD is invisible, it is a severe threat in many electronics manufacturing facilities. Static charges of all proportions can cause irreversible damage to delicate electronic components including resistors, capacitors, inductors, diodes, integrated circuits, MOSFETs, transformers, transistors, integrated circuits, sensors, switches and relays. There are two common types of ESD damage: catastrophic and latent. Catastrophic failure causes permanent damage so that the PCB will never function again. Although easier to detect during the inspection, catastrophic damage requires a complete PCB replacement. Latent ESD damage may not be immediately apparent. The PCB can be partially degraded and continue to function. However, the component may lose overall function or have intermittent faults during its lifetime, resulting in an unreliable electronic device. Although latent damage accounts for the biggest cause of ESD failure, it is also harder to detect. A PCB with latent damage can still be undetected during rigorous testing. This makes failure from latent damage extremely costly to the manufacturer. Where does ESD come from? There are several ways in which ESD occurs, but the most common is through human touch. During everyday activities, the human body and some clothing store anywhere from just a few volts of static electricity to many thousands during a typical working day. When hand contact is made with the PCB, it often results in ESD. An electronic manufacturing facility has many other circumstances where ESD could damage PCB assembly. For example, using ungrounded electrical equipment such as an oscilloscope when troubleshooting electronic circuitry is a major cause. However, it’s not just electronic equipment that causes ESD. Trolleys and conveyors can also build up friction or machinery or materials rubbing against each other to generate a charge. Even simple items like plastic cups placed near electronic circuitry can generate a charge. Creating a rapid movement of air near an electronic assembly can be another source of ESD. For 46 EPP Europe » 04 | 2023
example, using compressed air to clean a PCB assembly, or placing a fan nearby on a hot day can be problematic. Preventing ESD damage In an ideal world, PCBs would be designed using components that are not sensitive to ESD. However, as electronic assemblies become smaller incorporating delicate components on compact, densely-packed boards, this is not an easy task. The more complex and sensitive a PCB becomes, the more challenging it is to prevent ESD. Although it is impossible to prevent all ESD charges completely, there are several ways ESD can be reduced during SMT (surface mount technology) production. Eliminating ESD from work area The area where the PCB is made can significantly affect ESD. Some work spaces are more vulnerable to electrostatic discharge than others. For example, receiving, assembly, repair, cleaning, inspection and packaging are the most susceptible production areas. fortunately, practical procedures and measures can be implemented to reduce ESD in the work environment. • Wear anti-static wrist straps or heel straps. ESD anti-static straps are a simple and effective way of dissipating static from workers. Wrist straps can be connected to a grounded line, keeping the person continuously grounded while they work on sensitive PCBs. • Place conductive floor mats where technicians stand to help discharge static build-up from shoes. • Establish protocols to ensure all workers wear anti-static clothing and shoe coverings. • Ensure all work tables, equipment racks, floor mats and wrist straps are grounded. • Prevent static build-up by maintaining the temperature to 22–18 °C. Also, keep air humidity between 40–70% in the production area. Humid air helps to dissipate static charge. • Remove any unnecessary items from the work area. For example, plastic or polystyrene cups or even notepads and pens that can create an ESD charge. • Ship all assembly parts and PCBs in staticcontrolled containers and packing materials. Wipe away static Static can easily build up within electronic manufacturing facilities. Work surfaces, where devices are assembled, to the tools used to clean and test PCBs can hold a static charge. Nearly everything in or around the production line can be affected, from wooden workbenches and vinyl chair cushions to computer monitors and plastic trays. Even electric soldering irons and reflow and wave soldering ovens are conductive. When manufacturing PCBs, it is vital to remove any built-up static by dissipating the charge. This can be done by regularly wiping all items, including work surfaces and tools, with pre-saturated, ESD-reducing cleaning wipes. By using a high-quality ESD cleaning wipe, contamination like fingerprints, grease and oil are cleaned off without leaving behind lint, debris or static charge. ESD cleaning wipes are excellent at removing contamination, but look for one with a low level of alcohol. Alcohol is an ideal non-conductive workplace cleaner, although it tends to dry out mats, cause fissures or leave surfaces brittle. Source: MicroCare Zusammenfassung Eine häufige Ursache für das Versagen von Baugruppen bzw. Elektronik ist die elektrostatische Entladung (ESD). Im Artikel wird ein sachgemäßer ESD-Schutz diskutiert, der die Beschädigung oder gar Zerstörung elektronischer Bauteile und Geräte verhindert. Résumé Les décharges électrostatiques (ESD) sont une cause fréquente de défaillance des modules ou de l’électronique. L’article traite d’une protection ESD appropriée, laquelle empêche la détérioration ou même la destruction des composants et appareils électroniques. Резюме A static dissipating tool prevents ESD damage by reducing static charges Частой причиной отказа модулей и электроники является электростатический разряд. В статье рассматривается надлежащая защита от электростатического разряда, позволяющая избежать повреждения или даже разрушения электронных компонентов и устройств. EPP Europe » 04 | 2023 47
