Avoiding yield loss: removing ESD from your cleanroom

Published: 6-Sep-2023

Experts estimate that up to one third of printed circuit board (PCB) failures are from electrostatic discharge (ESD). To ensuring reliability in your manufacturing, ESD needs to minimised in any way possible. Words by Elizabeth Norwood

Experts estimate that up to one third of printed circuit board (PCB) failures are from electrostatic discharge (ESD). It poses one of the most significant and persistent challenges in cleanroom environments, particularly in industries such as medical electronics, aerospace components, and semiconductor production.

These sectors rely heavily on delicate electrical assemblies that are vulnerable to ESD-induced damages.

This damage can lead to costly warranty returns, time-consuming PCB rework, and even scrapped boards. Not to mention the negative impact to a company’s reputation. Therefore, it’s important to minimise ESD and its impact within cleanrooms.

Understanding 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”.

There are two common types of ESD damage: catastrophic and latent

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, MOSFETTs, transformers, transistors, integrated circuits, sensors, switches and relays.

Two types of ESD damage

There are two common types of ESD damage: catastrophic and latent. Catastrophic failure causes permanent damage that renders the PCB unusable. Although easier to detect during the inspection, catastrophic damage requires a complete PCB replacement.

Latent ESD damage may not be immediately apparent and often goes undetected during initial inspections. The PCB can be partially degraded and continue to temporarily operate. However, over its lifetime, the component may have compromised or intermittent operational behavior, 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.

Sources of ESD

ESD can occur at any stage of the manufacturing and assembly process, but human touch remains the most common route. The human body and certain clothing can accumulate static electricity levels ranging from 500 to 2,500 volts during a typical workday. Manufacturing areas involving "hands-on" processes, such as receiving, assembly, repair, cleaning, inspection, and packaging, are particularly susceptible to ESD.

Other sources of ESD include ungrounded electrical equipment, such as oscilloscopes used to troubleshoot electronic circuitry. Machinery and other production equipment such as conveyors or carts can also generate friction, and ultimately create ESD as their component materials rub together.

Each stroke should overlap the previous one by 10 to 25%

A rapid movement of air near an electronic assembly, such as using compressed air to clean or dry a PCB assembly can be another source of ESD.

Managing ESD in cleanroom environments

Although it is impossible to prevent all ESD charges completely, there are several ways ESD can be reduced during surface mount technology (SMT) production. Governing bodies such as the International Electrotechnical Commission (IEC) and American National Standards Institute (ANSI) recognise the threat ESD poses to PCBs and include guidelines for ESD control in their standards.

Here are some recommended methods for managing ESD during SMT production:

Environmental control

  • Use anti-static wrist straps or heel straps connected to a grounded line for workers handling sensitive PCBs. 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.
  • Install conductive floor mats where technicians walk and stand to discharge static build-up from shoes.
  • Ensure workers wear anti-static clothing and shoe coverings.
  • Ground all work tables, floor mats, and wrist straps.
  • Maintain air humidity between 40-70% and regulate the temperature to 18-22°C (64-70°F) to prevent static build-up. Cooler humid air helps to dissipate static charge.
  • Avoid using compressed air for drying PCBs. Fast moving air can cause charge separation, resulting in a build-up of static.
  • Remove unnecessary items from the work area that can generate ESD. For example, plastic or polystyrene trays or even notepads and pens can create an ESD charge.
  • Ship all assembly parts and PCBs in static-controlled containers and packing materials.

Wipe away static

Nearly everything in or around the PCB production line can be affected by static charges. Everything 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.

Static charges can be dissipated 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. Be sure to wipe in straight lines with overlapping strokes to cover all areas thoroughly. Each stroke should overlap the previous one by 10 to 25%.

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Presaturated ESD-dissipating cleaning wipes are excellent at removing contamination, but look for wipes with a low level of alcohol. Alcohol is an ideal non-conductive workplace cleaner, but it tends to dry out mats, cause fissures or leave surfaces brittle. Many good suppliers will provide free wipe samples to ensure the presaturated wipes work effectively for your facility and technicians.

Use static dissipating cleaning tools

Some flux removers and other cleaning fluids used within PCB production can generate as much as 12,000 volts of ESD, which is more than enough to damage or destroy electronic components. This build-up of ESD comes from the friction of the static molecules as they move down the plastic spray tube, which is commonly used to dispense these cleaning products. There are, however, alternative tools that can be used with cleaning aerosols to dissipate any static charge.

By removing the plastic straw and using a static zapping dispensing tool that attaches to the top of the aerosol can, ESD can be reduced. Made from durable aluminium and stainless steel, this static dissipating tool makes the user part of the grounding circuit by maintaining contact with it. This reduces static charges to just 50 volts to prevent ESD damage.

Use a static-safe flux remover dispenser

An ESD-safe controlled flux remover dispensing system is another tool that helps to eliminate static when cleaning PCBs.  It not only improves the outcome of benchtop cleaning but improves ESD safety.

The dispensing system usually includes brush and syringe attachments. These help to clean under low surface mounted components for thorough cleaning. The scrubbing action of the brushes increases the effectiveness of the fluid, which loosens and rinses away contaminants with little effort and minimal cleaner.

Alcohol is an ideal non-conductive workplace cleaner, but it tends to dry out mats, cause fissures or leave surfaces brittle

Using a controlled dispensing system keeps the flux remover clean for each use delivering the correct amount of fluid to wet the PCB completely but without overspray or waste. It reduces fluid use by up to 60% to help cut cleaning costs and ensure every drop inside the aerosol can is used. This in turn makes it easier to dispose as non-hazardous waste.

Cleaning with this tool also helps to control ESD by carrying down any charges to the ground, cutting the static going to the PCB, its microchips and other sensitive electronic components.

Reduce ESD: Enhance PCB reliability

Electrostatic discharge poses a significant threat to sensitive electronic components inside cleanrooms. Small, complex PCB assemblies with their highly sensitive components are vulnerable to ESD spikes that can cause either immediate catastrophic failure or latent in-the-field malfunction. So, it is important to ensure ESD is removed from the cleanroom during PCB production and packaging.

The careful selection and use of static-dissipating cleaning tools and fluids, along with other environmental controls helps protect electronic assemblies from ESD spikes, ensuring their reliability, longevity and overall quality.

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