ESD worksurfaces are typically an integral part of the ESD workstation, particularly in areas where hand assembly occurs. The purpose of the ESD worksurface is two-fold:
(1) To provide a surface with little to no charge on it.
(2) To provide a surface that will remove ElectroStatic charges from conductors (including ESD Susceptible devices and assemblies) that are placed on the surface.
For an electronics manufacturing environment, the worksurface resistance point-to-point (Rtt) and point-to-ground (Rtg) of 1.0 x 106 to less than 1.0 x 109 ohms is recommended(ANSI/ESD S4.1). Worksurfaces can be defined as any surface that un-packaged ESD susceptible devices or assemblies placed in direct contact with. Worksurfaces can include, mats, laminates, trays, carts, shelves, and other storage or handling devices that do not have an ESD shielding function. A ground wire from the mat should connect to the common point ground which is connected to ground, preferably equipment ground.
All ESD worksurfaces should be systematically tested for Rtg using a resistance megohmmeter with a 5 lb. electrodes. This should become part of a regular compliance verification plan. Surfaces should be confirmed at least two to four times per year and whenever a workstation or bench is moved or installed. ESD mats where the Rtg measures > 5 x 108 or greater should be considered for replacement before they are out of spec.
The worksurface must be maintained and should be cleaned with an worksurface ESD cleaner. Regular cleaners typically contain silicone. They should never be used on an ESD worksurface because the silicone will add an insulating layer. The ESD control plan should require testing the resistance to ground periodically.
For more information on best practices for establishing and ESD workstation, contact Desco.
Desco manufacturers products in several factories in the USA. In addition, Desco leases 450 acres of prime agricultural property in Chino Valley where we grow coil cords for wrist strap production. An on-site facility conducts research in pest control, soil testing, and bio-sustainability. Future plans include growing ground cords, vinyl berries, and breeding alpacas for improving their natural static-dissipating hair for use in woven wristbands. Desco is committed to using local-sourced materials that are eco-friendly and sustainable.
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ANSI/ESD S20.20 now requires the Operator Walking Test (ANSI/ESD STM97.1) and conformance to Operator Resistance Measurements (ANSI/ESD STM97.2) for product qualification of flooring / footwear systems for grounding personnel.
What is the importance of the Walking Test?
- It is necessary to qualify the Footwear / Flooring personnel grounding system for certification to ANSI/ESD S20.20.
- It can provide records to prove that Footwear / Flooring personnel grounding system used as a static control method is actually providing the performance expected.
- It is used when testing samples for qualification of a Footwear / Flooring personnel grounding system or on an existing installed floor when evaluating a change in footwear or flooring maintenance.
For Product Qualification, what Operator Resistance Measurements does ANSI/ESD S20.20-2014 state product should test to?
- < 100 volt peak voltage per ANSI/ESD STM97.2 Voltage in Combination with a Person.
- < 1 x 109 ohms Resistance per ANSI/ESD STM97.1 Resistance in Combination with a Person.
These are both to be performed in a walk-in environment chamber, including 12% RH.
For more information on the Walking Test and ANSI/ESD STM97.2, click HERE for the ESD Association’s PowerPoint.
Want more information? Sign up for a Free Flooring / Footwear Assessment. The Assessment will only take about 30 minutes of your time. We will demonstrate the Walking Test and provide more information on how you can be sure your Flooring / Footwear personnel grounding system meets the current S20.20 guidelines.
DESCO now manufactures a series of bags which are qualified to MIL-PRF-81705E Type III, Class 2. These products are on the Qualified Product Listing under the Defense Standardization Program. Check our listing HERE.
Standard stock bags are converted per MIL-DTL-117H. For standard sizes and further see the product pages HERE.
The DESCO 81705 bags are made and stocked in our Sanford, NC facility.
Please contact Service@Desco.com for more information.
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 continuous monitors are not used, a wrist strap should be tested while being worn at least daily. 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. Specially formulated ESD lotion can solve this problem. Failures may also be caused by dirty or loose wristbands which should be cleaned or tightened. When a wrist strap fails a test, the supervisor should be contacted, and the failure effectively addressed or the wrist strap replaced.
A Footwear / Flooring 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. Both ESD footwear and ESD flooring are required. Wearing ESD footwear on a regular, insulative floor is a waste of time and money.
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.
If an operator leaves the ESD Protected Area 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 ESD Protected Area.
Contact Desco for more information on Personnel Grounding.
To shop personnel grounding products, click HERE.
How to Measure Resistance Point-to-Point (Rtt) or Surface-to-Ground (Rtg) in Accordance with ESD Association Documents
The Desco Digital Surface Resistance Meter Kit measures resistance point-to-point (Rtt) or surface-to-ground (Rtg) of worksurfaces, flooring systems, garments, and other materials in accordance with ESD Association documents: ESD TR53, ANSI/ESD S4.1, ANSI/ESD S7.1, ANSI/ESD STM97.1 and other documents.
Learn more about Desco’s Digital Surface Resistance Meter Kit Here
Think Of Static Electricity as Germs and Contamination!
Daily life has other examples of hidden enemies where careful procedures must be followed to regularly obtain positive results. One example is sterilization, which combats germs and contamination in hospitals.
Damage caused by invisible and undetectable events can be understood by comparing ESD damage to medical contamination of the human body by viruses or bacteria. Although invisible, they can cause severe damage. In hospitals, the defense against this invisible threat is extensive contamination control procedures including sterilization.
We are aware of the benefits of sterilization in medicine. We must develop the same attitude towards ESD control and “sterilize” against its contamination. Just as you would never consider having surgery in a contaminated operating room, you should never handle, assemble, or repair electronic assemblies without taking adequate measures against ESD. For the hospital to sterilize most of the instruments is not acceptable; actually it may waste money. Each and every instrument needs to be sterilized. Likewise, it is not acceptable to protect the ESD sensitive items most of the time. Effective ESD control must occur at each and every step where ESDS items are manufactured, processed, assembled, installed, packaged, labeled, serviced, tested, inspected, transported, or otherwise handled.
The Operator’s Role in Controlling ESD
In order for the ESD control program to be effective, operators must be aware of the threat of ESD, and understand and adhere to the rules of controlling static electricity, and how to properly use ESD Protected Area (EPA) ESD control items.
ESD control items are ESD protective products that have been specially formulated to possess at least one of the following ESD control properties:
1) low charging (antistatic) – refers to the property of a material that inhibits triboelectric charging.
2) resistance (conductive or dissipative) – able to be grounded
3) shielding – based on a the Faraday Cage concept and creates an enclosure that attenuates a stationary electrostatic field.
These products should be identified by the ESD Protective Symbol. Note: the ESD Protective Symbol has an arc while the ESD Susceptibility Symbol does not.
The ESD Protective Symbol identifies products designed to provide ESD control protection.
For more information on ESD Control and ESD Control products, visit Desco.com
This is the fifth in a series of blogs concerning the Basics of ESD. Be sure to sign up for our blog to follow the series.
At this point in our blog series concerning ESD Awareness, we should consider some basic physics:
Ohm’s law is an extremely useful equation in the field of electrical/electronic engineering because it describes how voltage, current, and resistance are interrelated.
Ohm’s law states that, in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference (.i.e. voltage drop or voltage) across two points, and inversely proportional to the resistance between them.
Resistance determines how much current will flow through a component. A very high resistance allows a small
amount of current to flow. A very low resistance allows a large amount of current to flow. Resistance is measured in ohms.
An analogy to help understand these terms better is a water hose. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the hose size. For example, you can spray the water further away by increasing the water pressure and consequently the flow rate of the water. There is a direct relationship between pressure, flow and the hose diameter.
In a similar way, voltage, current and resistance are related to each other by the electrical engineering formula Ohm’s law.
Resistance to ground (Rtg) is a measurement that indicates the capability of an item to conduct an electrical charge (current flow) to an attached ground connection. For ESD control purposes, devices with a Rtg of less 1.0 x 106 are known as conductors (foam, floor mats) and must be connected to ground. ESD Control devices with an Rtg of 1.0 x 106 to < 1.0 x 109 are a special class of conductor, called dissipative (worksurface mats, gloves, smocks) and must also be grounded. The dissipative range of conductors is especially important for ESD control because it “slows down” a static discharge event. Devices with an Rtg of 1.0 x 1011 or greater are known as insulators. Insulators cannot be grounded and must be removed from the ESD Protected Area or neutralized with ionization.
The measurement may be shown in various ways. Most commonly:
- 1 kilohm
- 1 x 103 ohm
- 10^3 ohm
For more information on ESD Control and ESD Control products, visit Desco.com
This is the fourth in a series of blogs concerning the Basics of ESD. Be sure to sign up for our blog to follow the series.
Many firms consider all components ESD sensitive. However, it is critical to be aware of the most sensitive item being handled.
Any ESD sensitive item should be identified with the ESD susceptibility symbol, either on itself or its container. The ESD Susceptibility Symbol (also called Sensitivity or Warning Symbol) identifies items that can be damaged by ESD and must be un-packaged and handled by a properly grounded operator at an ESD protected workstation.
Most firms use the ANSI/ESD S20.20 document to construct their ESD control plan which is based on handling ESD sensitive items having a Human Body Model withstand voltage of 100 volts or greater. The Human Body Model simulates discharges from a person and increasingly tests an electronic device at higher and higher discharges until it fails, thus establishing the device’s withstand voltage.
There are three ESD Awareness Symbols defined in ANSI/ESD S8.1:
To view information about each symbol please click the link or image above.
ESD Susceptibility Symbol
The ESD susceptibility symbol incorporates a reaching hand in a triangle with a slash through it and is used to indicate that an electrical or electronic device or assembly is susceptible to damage from an ESD event. Used to identify ESDS [ESD sensitive items] and that personnel should be grounding when unpackaging or handling that item. It is also referred to as the ESD sensitivity symbol or ESD warning symbol.
Application: The ESD susceptibility symbol should be used on assemblies and devices that have a sensitivity to ESD events. The symbol may be incorporated on a sticker used to close or seal ESD protective packaging to indicate that materials inside the package are ESD susceptible.
Format: The symbol is a reaching hand with defined fingers and fingernail, in a contrasting triangle with a slash in front of the hand.
Color: The choice of color for this symbol is arbitrary. The color red shall not be used because it suggests a hazard to personnel. The preferred color is a yellow hand and slash on a black background.
Note: Three Arrows In A Circle Symbol, per ESD Handbook ESD TR20.20 “Military organizations sometimes use MIL-STD-1285 for hardware marking and that document once required the use of the “three arrows in a circle” symbol. That symbol has been changed for consistency with industry marking to the “hand in triangle” ESD Susceptibility symbol”
ESD Protective Symbol
The ESD protective symbol differs from the ESD susceptibility symbol, by the addition of an arc around the outside of the triangle and the omission of the slash across the hand and the triangle.
Application: The ESD protective symbol should be used to identify items that are specifically designed to provide ESD protection for ESDS items. Examples of these are packaging, ESD protective clothing and personnel grounding equipment. The ESD protective symbol should also be used on items designed to replace static generative materials. Examples of these items are ESD protective work station equipment, trash can liners, and chairs. The item is to be ESD protective or non-static generative by design.
Color: The choice of color for this symbol is arbitrary. The color red shall not be used because it suggests a hazard to personnel. The preferred color is a yellow hand on a black background.
Note: In Europe per Packaging standard EN 61340-5-3 there is a requirement to place a letter under the triangle denoting the product’s primary ESD control function:
S electrostatic discharge shielding
F electrostatic field shielding
C electrostatic conductive
D electrostatic dissipative
If the letter is “L” the primary ESD control property is low charging (antistatic); if “EPA” is meant to indicate the product is designed to be used in an ESD protected area.
ESD Common Point Ground
This symbol is established to indicate an ESD common point ground, which is defined by ANSI/ESD-S6.1 as “a grounded device where two or more conductors are bonded.”
Application: The ESD common point ground symbol should be used to indicate the location of an acceptable common point ground as defined by ANSI/ESD-S6.1.
Format: This symbol consists of a bold outer circle inside of which are the words, ESD COMMON POINT GROUND, in bold type. Inside that are two thick contrasting circles and one thick circle that may fill the center or extend to the center where a snap, plug or other fastener may be connected.
Color: The choice of color for this symbol is arbitrary but black or white on green is suggested. The color red shall not be used because it suggests a hazard to personnel.
Note: In Europe, for the same purpose the Earth Bonding Point symbol is used:
For more information on ESD Control and ESD Control products, visit Desco.com
This is the third in a series of blogs concerning the Basics of ESD. Be sure to sign up for our blog to follow the series.