In our last post, we talked about ESD: what it is, what types of ESD damage there are and what costly effects ESD can have. Missed our very first post? Catch-up here.
All caught up? Right, moving on. Today you will learn how to avoid ESD damage and protect your ESD sensitive items. So, let’s jump right in.
The fundamental ESD Control Principles
We’ve established that ESD is the hidden enemy in the electronics industry. Therefore, the BIG question is: how exactly do you control ElectroStatic Discharge (ESD) in your workplace? Easy – just follow these ESD fundamentals:
- Ground all conductors including people
- Remove all unnecessary non-conductors (also known as insulators)
- Place ESD sensitive devices inside of shielding packaging when transported outside of an ESD Protected Area (EPA)
Per ESD Handbook ESD TR20.20-2008 section 2.4 “It should be understood that any object, item, material or person could be a source of static electricity in the work environment. Removal of unnecessary nonconductors, replacing nonconductive materials with dissipative or conductive materials and grounding all conductors are the principle methods of controlling static electricity in the workplace, regardless of the activity.”
These are the essential principles of ESD Control. If you implement all three points above, you will be in control of ESD and your sensitive items will be protected. Well, that wasn’t hard, was it? Don’t be terrified – we’ll go through everything in detail. We’ll cover #2 and #3 in future points – today’s focus is #1.
Definition of an ESD Protected Area (EPA)
An ESD Protected Area (EPA) is a designated zone – all surfaces, objects, people and ESD Sensitive Devices (ESDs) within are kept at the same electrical potential. This is achieved by simply using ‘groundable’ materials for covering of surfaces and for the manufacture of containers and tools. This applies to all items with an electrical resistance of less than 109 ohms.
An EPA could be just one workstation or it could be a room containing several different workstations. It can be portable as used in a field service situation or permanent.
The user guide CLC/TR 61340-5-2:2008 defines an EPA as follows:
“An ESD protected area (EPA) is an area that is equipped with the ESD control items required to minimize the chance of damaging ESD sensitive devices. In the broad sense, a protected area is capable of controlling static electricity on all items that enter that work area. Personnel and other conductive or dissipative items shall be electrically bonded together and connected to ground (or a common connection point when a ground is not available) to equalize electrical potential among the items. The size of an EPA can vary greatly. A protected area may be a permanent workstation within a room or an entire factory floor encompassing thousands of workstations. A protected area may also be a portable worksurface or mat used in a field service situation.” [CLC/TR 61340-5-2:2008 Use guide clause 4.6 Protected areas (EPA)]
You’re probably wondering now, how exactly you can get all surfaces, objects and operators to the same electrical potential. Fear not – we’ve got you covered!
- Personnel Grounding
As previously stated, a fundamental principle of ESD control is to ground conductors including people at ESD protected workstations.Wrist straps are the first line of defense against ESD, the most common personnel grounding device used, and are required to be used if the operator is sitting. The wristband should be worn snug to the skin with its coil cord connected to a common point ground which is connected to ground, preferably equipment ground.
If you are not using a continuous or a constant monitor, a wrist strap should be tested while being worn at least daily. This quick check can determine that no break in the path-to-ground has occurred. Part of the path-to-ground is the perspiration layer on the person; an operator with dry skin may inhibit the removal of static charges and may cause a test failure.
“The wrist strap system should be tested daily to ensure proper electrical value. Nominally, the upper resistance reading should be ” [ANSI/ESD S1.1 Annex A, 3 Frequency of Functional Testing]
A Flooring / Footwear system is an alternative for personnel grounding for standing or mobile workers. Foot grounders or other types of ESD footwear are worn while standing or walking on an ESD floor. ESD footwear is to be worn on both feet and should be tested independently at least daily while being worn. Unless the tester has a split footplate, each foot should be tested independently, typically with the other foot raised in the air.
“Compliance verification should be performed prior to each use (daily, shift change, etc.). The accumulation of insulative materials may increase the foot grounder system resistance. If foot grounders are worn outside the ESD protected area testing for functionality before reentry to the ESD protected area should be considered.” [ESD SP9.2 APPENDIX B – Foot Grounder Usage Guidance]
Both ESD footwear and ESD floor are required. Wearing ESD footwear on a regular, insulative floor is a waste of time and money.
Part of the path-to-ground is the perspiration in the person’s shoes. The conductive tab or ribbon of foot grounders should be placed inside the shoe under the foot with the excess length tucked into the shoe. Thanks to the perspiration in the shoe, direct contact with the skin is normally not necessary.
If an operator leaves the EPA and walks outside wearing ESD footwear, care should be taken not to get the ESD footwear soiled. Dirt is typically insulative, and the best practice is to re-test the ESD footwear while being worn each time when re-entering the EPA.
- Working Surfaces
ESD working surfaces, such as mats, are typically an integral part of the ESD workstation, particularly in areas where hand assembly occurs. The purpose of the ESD working surface is two-fold:
- To provide a surface with little to no charge on it.
- To provide a surface that will remove ElectroStatic charges from conductors including ESDS devices and assemblies) that are placed on the surface.
ESD mats need to be grounded. A ground wire from the mat should connect to the common point ground which is connected to ground, preferably equipment ground. For electronics manufacturing a working surface resistance to ground (RG) of 1 x 106 to less than 1 x 109 ohms is recommended.
“The single most important concept in the field of static control is grounding. Attaching all electrically conductive and dissipative items in the workplace to ground allows built-up electrostatic charges to equalize with ground potential. A grounded conductor cannot hold a static charge.” [Grounding ANSI/ESD S6.1 Foreword]
Per ANSI/ESD S20.20 section 188.8.131.52 Grounding / Bonding Systems Guidance, “In most cases, the third wire (green) AC equipment ground is the preferred choice for ground.”
Best practice is that ground connections use firm fitting connecting devices such as metallic crimps, snaps and banana plugs to connect to designated ground points. The use of alligator clips is not recommended.
The working surface must be maintained and should be cleaned with an ESD cleaner. Regular cleaners typically contain silicone, and should never be used on an ESD working surface. ESD Handbook ESD TR20.20-2008 section 184.108.40.206 Maintenance “Periodic cleaning, following the manufacturerís recommendations, is required to maintain proper electrical function of all worksurfaces. Ensure that cleaners that are used do not leave an electrically insulative residue common with some household cleaners that contain silicone.“
- Other moveable objects
Moveable items (such as containers and tools) are grounded when placed on a grounded surface or being held by a grounded operator. Everything that does not readily dissipate charge must be excluded from the EPA (refer to #2 of our ESD Control Principles above). Regular plastics, polystyrene foam drink cups and packaging materials, etc. are typically high charging and have no place at an ESD protective workstation. Intention of an ESD Protected Area (EPA)We’ve learnt in our previous blog post that ElectroStatic discharge (ESD) can damage components and products that contain electronics. A lot of the time, this damage is not detected during quality inspection and can cause significant problems further down the line.An ESD Protected Area (EPA) is an area that has specifically been created to control ESD; its purpose is therefore to avoid ALL problems resulting from ESD damage. Workers need to understand AND follow the basics of ESD control to limit the generation of electrostatic charges as well as limit and slow discharges in the EPA.Recognizing an ESD Protected Area (EPA)
An ESD Protected Area must be clearly identified using signs and/or aisle tape. This ensures operators and visitors are alerted when entering (or leaving) an ESD Protected Area which require special precautions (grounding via wrist straps and/or foot grounders etc.). It also indicates that they are entering (or exiting) areas where exposed ESDS items can be handled safely.Remember to be consistent throughout your shop floor, i.e. use the same signs. This will avoid confusion for your operators.
“While signs are one way of indicating the boundaries of an EPA, it is not the only way. Any alternate method that alert the personnel that an EPA begins is acceptable to ANSI/ESD S20.20. Some of the alternate ways to mark the boundaries of an EPA are:
- tape on the floor
- different color floor tiles
- different color carpet
- any other way to establish boundary conditions
Anyway to distinguish the boundaries of an EPA would be acceptable as long as the personnel are aware of the indications and take the proper precautions while inside the EPA.” [ESD TR20.20-2016 section 9.1.2 EPA Boundary Indicators]
Building an ESD Protected Area (EPA)
A basic form of an ESD Protected Area is a workstation consisting of the following components:
- An ESD working surface mat
- A grounding cord
- A wristband
- A coiled cord
- A common point ground
To set-up an EPA:
- Connect the ESD working surface mat to the common point ground using the grounding cord.
- Link the operator to the common point ground using the wristband and coiled cord.
Congratulations – you’ve just created an ESD Protected Area!
By following the above steps, each component (the ESD mat and the operator) is kept at the same electrical potential (ground). Any ElectroStatic charge (ESD) is removed to ground via the common point ground.