Setting up a new ESD Protected Area (EPA) can be a little daunting, especially if you are unfamiliar or relatively new to the concept of ESD Control. It would be nice to have a blue print for establishing an EPA, but in truth there is no exact formula as everyone’s situation is different. However, there are general guidelines that make for a good starting point.
The best place to start for a good static control program is with a list of ESD Control items by function and Technical Requirements for each. Once it’s decided how many Technical Requirements to employ, then an ESD control provider can recommend specific part numbers based on performance and budget. Ultimately, it important to implement all the Technical Requirements of ANSI/ESD S20.20.
Personnel Grounding
Grounding the human body is the single most important Technical Requirement for ESD control. This is not an optional requirement, unless there is a safety hazard like the presence of high voltage. What is optional is how you ground the human body.
You can wear a wrist strap and or ESD footwear to ground the human body (ESD footwear must be worn on an ESD protective floor). However, the rule is that when you are seated you shall wear a wrist strap. When wearing a wrist strap the human body Rtg (resistance to ground) required limit is <3.5 x 10^7 ohms. This Rtg guarantees the body will not retain more than 100 volts. When grounding through the footwear, required limit through the human body is <1 X 10^9 ohms but also cannot generate more than 100 volts.
Workstation Grounding
The second most important Technical Requirement for most companies would be a grounded dissipative mat.or Worksurface. Anywhere an exposed ESD susceptible (ESDS) device is placed or has intimate contact with would be considered an ESD Worksruface. For instance, a work station, shelving unit, mobile cart, inspection stations in-between machines on an SMT line, or even an ESDS item sitting on a static shielding bag. This would all be optional if the ESDS device is in a shielded container, although this would not be practical for most applications.
ESD Flooring
While not mandatory, ESD Flooring is one of the best static control Technical Requirements to consider. Not only does it minimize charge generation but it allows for mobile grounding of personnel, carts, and ESD chairs. This make the material handling process much safer and easier within an EPA. ESD flooring can be any type from permanent materials, such as ESD tile, ESD matting, or ESD epoxy and paints, to protective materials such as dissipative floor finishes, This should be given a priority consideration when setting up a new unpopulated EPA or facility. It’s much easier and less expensive to do it upfront than after the area is populated.
ESD Garments or Smocks
ESD garments and smocks control static generation on clothing. Clothing can be considered an insulator and cannot be grounded. They are also a great way to establish and identify personnel that belong in the EPA.
ESD Packaging Materials
These would be material handling such as static shielding and moisture barrier bags, totes, bins, foams, tapes, and other forms of packaging. Note that when an ESD susceptible (ESDS) device is transported outside of the EPA, it shall be in a discharge shielding package of some type. If the EPA does not have mobile grounding, then the same rule applies when transporting that device from point A to point B within the EPA.
Ionization – Ionization is used to neutralize static generation on isolated conductors and process required insulators.
Insulators cannot be grounded and therefore require ionization to minimize and neutralize static charges. Isolated conductors that are not grounded may also retain or hold charges and must be neutralized via ionization. These might include: ungrounded components, totes, hand tools, metal fixtures, machinery, etc. The ANSI/ESD S20.20 standard requires that all conductors be at < +/- 35 volts.
Process Required or essential insulators include items such as the enclosure of product, such as cell phone cases, plastic connector bodies, in addition to packaging materials, tapes and cleaning processes. So, while ionization is often thought as a last resort, it is one of the best methods to control static.
Shelving and Carts – it is necessary to ground shelves and carts if an “exposed” ESDS device is placed on them. If the device is enclosed in shielded packaging, then this Technical Requirement is optional.
ESD Chairs – ESD chairs would provide a non-static generating seating surface and possibly redundant grounding for the body. They also eliminate a potential Process Required insulator from inducing charges onto ESDS devices.
Compliance Verification
Specific types of test equipment should be used periodically to verify that each Technical Requirement is working and in spec (i.e. wrist straps, ESD mats, ionizers, etc)., For personnel grounding items, compliance verification is required daily and a written log shall be maintained.
Determining how much static control is needed depends a lot upon what the HBM Class rating is of your most sensitive device, or how much a customer may require that you have. Safely handling a 3,000 volt Class 3 device is a whole lot easier than handling a 250 volt Class 1 device. ANSI/ESD S20.20-2014 now requires that you document your most sensitive device and then you would use that as your gauge. Once it is determined how much static control you need, recommended best solutions can be given.
Want more information and guidance on setting up and EPA? Contact Desco.