Preventing ESD In The Cleanroom
Electrostatic Discharge (ESD) is a long standing and major concern within cleanrooms and the electronics, aerospace, and semiconductor industries who utilize them. The components in these industries are susceptible to damages caused by ESD, and as a result a lot of attention must be focused on ensuring a static-free environment. In this blog post, we’ll discuss what ESD is, some causes, and some ways to mitigate it in the cleanroom.
Electrostatic Discharge is a sudden flow of electricity between two electrically charged objects. It can cause significantly more damage than particulate contamination to small electronics. It occurs through contact, an electrical short, or dielectric breakdown. One of the biggest causes of ESD is static electricity, which is generated through tribocharging, or the idea that the when two materials are brought into contact then separated then there could be a release of electric energy. Tribocharging is what most people are familiar with in terms of ESD – it occurs through friction, like when someone walks on a rug, or rubs a plastic comb against dry hair. An example of this can be demonstrated through rubbing a balloon against a sweater or other material to generate a small spark of electricity. This little spark can carry upwards of a few thousand volts, but it only takes a small charge (30V +) to completely obliterate sensitive electrical circuits. In many instances, a person may not even feel the discharge of static electricity, though it is often accompanied by a visible spark. This can be devastating to work within a cleanroom, with electronic components that are extremely sensitive. As such, special precautions and materials are used to mitigate the potential damage that ESD can cause.
What are ESD Protective Materials?
ESD Protective Material attracts less particulate contamination to its’ surface than insulate material. This is because fewer charges are generated, or accumulated, on the surface and particles are attracted to charged surfaces. ESD Protective Materials may also be labeled in a variety of different ways, but most equipment falls into 3 categories; Conductive, Anti-Static, or Static Dissipative. One similarity of these materials is that they are all measured by surface resistivity units of ohms per square, with the exception of Anti-Static, which has no official resistance or resistivity.
Let’s take a deeper look at the 3 main categories; Anti-Static, Conductive, and Dissipative.
Anti-Static materials refer to materials which prevent triboelectric charging. This type of material is commonly found in the packaging of electronics and other sensitive products. Anti-Static cleanroom items are usually products like clothing, anti-fatigue mats, cleanroom curtains, gloves, and a variety of different wipes and pre-moistened towelettes. In these instances, Anti-Static products are used to eliminate or control static electricity. It is important to note that Anti-Static is not defined by a resistance or resistivity of a material, whereas Conductive and Dissipative materials are. Conductive materials carry a low electrical resistance, which allows for electrons to easily flow across the surface or through the material itself. For these types of materials, the electric charge either goes to the ground or to another conductive object through direct contact. All conductive materials have a surface resistivity equal to or greater than 1 x 10^5 ohms/square foot, or a volume resistivity of less than 1 x 10^4 ohm-cm. This allows a charge to be easily grounded or transferred in close proximity. Conductive cleanroom equipment can include products like shoe covers, cleanroom chairs, and even gloves and tape. Static Dissipative materials allow for an electric charge to be grounded more slowly, in a more controlled manner, than conductive materials and may be better suited for situations that require greater control over electric current. Static Dissipative materials have a surface resistivity equal to or greater than 10^5 ohms/square foot, but less than 1 x 10^12 ohms/square. The volume resistivity is equal to or greater than 1 x 10^e ohm-cm, but less than 1 x 10^11. In the cleanroom, Static Dissipative materials are generally items such as; curtains, nitrile gloves, nylon gloves, Mini & Micro CleanFoam, surface cleaners, and wipes.
Readying The Work Area:
While ESD needs vary room to room and industry to industry, it is best, when building an appropriate ESD work station, to start from the ground and move up. A floor literally has to be grounded, or connected to a sink with the potential to absorb the current, and only conductive or dissipative level of resistance materials can be grounded. When choosing your cleanroom floor, it is imperative to choose flooring that meets these standards, or else there is no static protection provided – even from ground strips or conductive adhesive. Flooring can also be tested by using an ohmmeter to determine the electrical resistance, and it is recommended to test and find exactly what your cleanroom demands are. Removing a floor due to inefficiency can be much more expensive than getting it right in the first place. To do this, make sure to match the properties of the floor with that of the work environment where it will be used.
The next step is to ensure that all individuals within a cleanroom are properly grounded, and this can be accomplished through special footwear or foot grounders. It is critical to properly match footwear with the floor where it will be used and the total resistance of the floor plus the person plus the static control footwear must measure below 35 megohms (35,000,000 ohms). Generally it is best to wear two foot grounders to prevent triboelectric generation through walking, but ESD shoes are a suitable replacement.
An ESD protected workstation is the next focus. This is where an individual will physically work, and they are constructed and equipped with materials and equipment designed to limit potential damage to any sensitive items. ESD protective workstations don’t necessarily have to be located in a cleanroom, but the same rules still apply. It is best to keep all materials and staff at the same electrical potential, and the workstation usually aids in this through construction with materials that are static dissipative, as well as a common grounding connection, and a way of grounding personnel, generally through a wrist strap. Through these means of grounding, the workbench reaches equilibrium with the electrical potential between the device being worked on and the personnel. However, depending on how sensitive the device is, this may not supply enough protection. For this, it is best to use additional ESD control materials, like ESD table mats, ESD floor mats, as well as ESD shelves and even ESD ready chairs if appropriate. To further reduce potential triboelectric build up, we encourage the use of ESD protective clothing and Anti-Static gloves. In Conclusion Electrostatic Discharge represents a serious threat to sensitive components in cleanrooms. It must be dealt with in a controlled and calculated manner that varies from room to room and the nature of the work. Make sure to do your homework before purchasing, otherwise it can be an expensive mistake.