Electrostatic Discharge

Have you ever walked across a car park on a bright cold winter’s day only to get zapped by your car’s door handle? It’s commonly known that these ‘zaps’ are much more common in cold dry weather. It begs the question: if there are less ‘zap, will using air humidifiers in a manufacturing environment prevent ESD damage of sensitive components? Let’s find out!

 

Humidity

Humidity describes the amount of water vapor in the air. There are 3 main measurements of humidity with the most common one being the relative humidity (RH). It is expressed in percent and describes “how much humidity there is in the air, compared to how much there could be. Meteorologists often use the relative humidity as a measurement to describe the weather at various places.” [Source]

At 0% the air is completely dry; at 100% it is so moist that mist or dew can form. The optimum relative humidity level is somewhere between 40% and 60%:

  • A lower relative humidity increases charge generation as the environment is drier.
  • If the humidity level is too high, condensation can form on surfaces.

 

Charge Generation and ElectroStatic Discharge (ESD)

The simple separation of two surfaces generates an ElectroStatic charge. Examples:

  • Unwinding a roll of tape
  • Gas or liquid moving through a hose or pipe
  • A person walking across a floor with heels and soles contacting and separating from the floor

 

Walking across a floor generates ElectroStatic charges.
Walking across a floor generates ElectroStatic charges.

 

The amount of static electricity generated varies and is affected by materials, friction, area of contact and the relative humidity of the environment. A higher charge is generated at low humidity or in a dry environment.

Once an item has generated a charge, it will want to come into balance. If it is in close enough proximity to a second item, there can be a rapid, spontaneous transfer of electrostatic charge. This is called discharge or ElectroStatic Discharge (ESD).

Going back to our earlier example of getting a zap from your car’s door handle:

  1. Charge generation: you walk across the car park with your soles contacting and separating from the floor. A charge is built-up on you.
  2. ElectroStatic Discharge (ESD): you touch the door handle. Charges move from your body to your car until both are balanced out.

 

Impact of relative humidity on ESD

Many people will notice a difference in the ability to generate static electricity when the air gets dryer (relative humidity decreases). Relative humidity (RH) directly affects the ability of a surface to store an electrostatic charge. “With a humidity level of 40% RH, surface resistance is lowered on floors, carpets, table mats and other areas. … the moisture in the air forms a thin protective “film” on surfaces that serves as a natural conductor to dissipate electric charges. When humidity drops below 40% RH, this protection disappears, and normal employee activities lead to objects being charged with static electricity.” [Source]

ESD Damage on an integrated circuit. No Magnification, 400x magnification, 5,000x magnification.

In an electronics manufacturing environment lower humidity may result in lower output from production due to an increase in ESD events during manufacturing processes.

 

Air Humidification and ESD

Air humidifiers are used to add moisture to the air and are commonly used in drier environments to keep humidity at a constant (optimum) level. Given that a lower humidity level increases the risk of ESD events, the obvious questions are:

  1. Can air humidifiers replace normal ESD Control measures?
  2. Are air humidifiers required for complete ESD protection?

Let’s address both questions:

  • Let’s be very clear about one thing here: air humidifiers cannot replace ESD Control measures.

As explained further above, ESD is caused by two items that are at a different electrostatic equipotential and want to equalize their charges. Adding moisture to the air using humidifiers will not stop this discharge from happening. The only thing you may achieve is a reduction in the number of ESD events. BUT: they will still happen; just walking across a carpet will generate a charge on an operator. If they then touch an ESD sensitive component, discharge will still occur and may damage the component. No humidifier will prevent this.

The only way to control electrostatic charges on a person or object is through ESD grounding – this will ensure any charges generated dissipate to earth:

For more information on how to create a ESD workstation and how to correctly ground all elements, have a look at this post.

Wrist-Strap.jpg
Grounding of an operator using a wrist strap
  • Low air humidity can increase the number of ESD events so it may make sense to keep a factory at a higher humidity level. However, there are many other factors that come into play when choosing the ‘right’ humidity for a manufacturing environment. The recommended humidity range is usually determined by the specifications of the devices and components being assembled. Increasing the humidity in an electronics manufacturing facility can help to reduce ESD events but increased humidity can lead to other unwanted quality issues in an electronics manufacturing environment such as corrosion, soldering defects and the popcorn effect on moisture sensitive devices.

A normal range for humidity in electronics manufacturing is between 30% RH and 70% RH. Some facilities try to maintain a constant moderate RH (~50%), whereas other environments may want lower % RH due to corrosion susceptibility to humidity sensitive parts.
And remember: you will not eliminate ESD by using humidifiers and keeping humidity levels at a higher level. You need an ESD Control Program in place to avoid ESD and associated damages.

 

Conclusion

Air humidification can help reduce the number of ESD events in an electronics manufacturing environment but at the same time there are other factors (e.g. moisture sensitivity of components) that need to be considered.

A lower relative humidity level increases charge generation as the environment is drier. This will result in more ESD events which can potentially damage sensitive components. The only way to protect sensitive components from ESD damage is by having proper ESD control measures in place and connecting operators, objects and surfaces to ground. This will ensure each element is kept at the same electrical potential and any electrostatic discharge is being removed to ground.

For more information on how to get your ESD control program off the ground or improve an existing program, request a free ESD/EOS Assessment at your facility by one of our knowledgeable local representatives to evaluate your ESD program and answer any ESD questions!

 

Introduction

Electronic devices and systems can be damaged by exposure to high electric fields as well as by direct electrostatic discharges. A good circuit layout and on-board protection may reduce the risk of damage by such events, but the only safe action at present is to ensure that devices are not exposed to levels of static electricity above the critical threshold.

This can only be achieved by introducing a static control program which usually involves setting up an ESD Protected Area (EPA) in which personnel are correctly grounded and all meet the ESD Standard. However, setting up an EPA does not of itself guarantee a low static environment. Production procedures may change, new materials may be introduced, the performance of older materials may degrade and so on.

Measuring Effectiveness of an ESD Control Program

To ensure the effectiveness of any static control program it is important that regular measurements are carried out:

  1. to determine the sensitivity to ESD of devices being produced or handled.
  2. to confirm that static levels are lower than the critical level, and that new or modified work practices have not introduced high static levels.
  3. to ensure that both new and existing materials in the EPA meet the necessary requirements.

Only after an ‘operational baseline’ has been established by regular auditing will it become possible to identify the origin of unexpected problems arising from the presence of static.

1. Determining the sensitivity of ESD sensitive Devices

It is important to understand the sensitivity of ESD sensitive devices before an action plan can be created. Once you know the sensitivity of the items you are handling, can you work towards ensuring you’re not exceeding those levels.

Part of every ESD control plan is to identify items in your company that are sensitive to ESD. At the same time, you need to recognize the level of their sensitivity. As explained by the ESD Association, how susceptible to ESD a product is depends on the item’s ability to either:

  • dissipate the discharge energy or
  • withstand the levels of current.

2. Measurements to prove the effectiveness of an ESD Control Program

Measuring electrostatic quantities poses special problems because electrostatic systems are generally characterized by high resistances and small amounts of electrical charge. Consequently, conventional electronic instrumentation cannot normally be used.

Measuring Electrical Field

Wherever electrostatic charges accumulate, they can be detected by the presence of an associated electric field. The magnitude of this field is determined by many factors, e. g. the magnitude and distribution of the charge, the geometry and location of grounded surfaces and the medium in which the charge is located.

The current general view of experts is that the main source of ESD risk may occur where ESDS can reach high induced voltage due to external fields from the clothing, and subsequently experience a field induced CDM type discharge.” [CLC TR 61340-5-2 User guide Garments clause 4.7.7.1 Introductory remarks]

718_Use2.jpg
Using the 718 Static Sensor to test static fields

A static field meter is often used for ESD testing of static fields. It indicates surface voltage and polarity on objects and is therefore an effective problem-solving tool used to identify items that are able to be charged.

A field meter can be used to:

  • verify that automated processes (like auto insertion, tape and reel, etc.) are not generating charges above acceptable limits.
  • measure charges generated by causing contact and separation with other materials.
  • demonstrate shielding by measuring a charged object and then covering the charged item with an ESD lab coat or shielding bag. Being shielded the measured charge should be greatly reduced.

 

Measuring ESD Events

ESD events can damage ESD sensitive items and can cause tool lock-ups, erratic behavior and parametric errors. An ESD Event Detector like the EM Eye ESD Event Meter will help detect most ESD events. It detects the magnitude of events and using filters built into the unit, it can provide approximate values for some ESD events for models (CDM, MM, HBM) using proprietary algorithms.

Using the EM Eye ESD Event Meter to detect ESD Events

Solving ESD problems requires data. A tool counting ESD events will help carry out a before-and-after analysis and will prove the effectiveness of implementing ESD control measures.

 

3. Checking Materials in your EPA

When talking about material properties, the measurement you will most frequently come across is “Surface Resistance”. It expresses the ability of a material to conduct electricity and is related to current and voltage. The surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
It is important to remember that the surface resistance of a material is dependent on the electrodes used (shape as well as distance). If your company implements an ESD control program compliant to the ESD Standard ANSI/ESD S20.20, it is therefore vital to carry out surface resistance measurements as described in the Standard itself. For more information on the definition of resistance measurements used in ESD control, check out this post.

A company’s compliance verification plan should include periodic checks of surfaces measuring:

  • Resistance Point-to-Point (Rp-p) and
  • Resistance-to-ground (Rg).
SRMeter2_use.jpg
Measuring Surface Resistance of worksurface matting using the
SRMETER2 Surface Resistance Meter

Surface resistance testers can be used to perform these tests in accordance with ANSI/ESD S20.20 and its test method ANSI/ESD S4.1; if these measurements are within acceptable ranges, the surface and its connections are good. For more information on checking your ESD control products, catch-up with this. It goes into depth as to what products you should be checking in your EPA and how they should be checked.

 

Conclusion

Measurements form an integral part of any ESD control program. Measuring devices help identify the sensitivity of ESD devices that ESD programs are based on, and also are used to verify the effectiveness of ESD control programs set in place. High quality instruments are available commercially for measuring all the parameters necessary for quantifying the extent of a static problem.

We hope the list above has introduced the techniques most commonly used. For more information on how to get your ESD control program off the ground, Request a free ESD/EOS Assessment at your facility by one of our knowledgeable local representatives to evaluate your ESD program and answer any ESD questions!

 

 

The best-equipped service bench in your shop can be a real money-maker when set up properly. It can also be a source of frustration and lost revenue if the threat of ElectroStatic Discharge (ESD) is ignored.

A typical scenario might be where an electronic product is brought in for service, properly diagnosed and repaired, only to find a new symptom requiring additional repair. Unless the technician understands the ESD problem and has developed methods to keep it in check damage from static electricity cannot be ruled out as a potential source of the new problem.

Static electricity is nothing new; it’s all around us and always has been. What has changed is the spread of semiconductors in almost every consumer product we buy. As device complexity increases, often its static sensitivity increases as well. Some semiconductor devices may be damaged by as little as 20-30 volts!

It is important to note that this post is addressing the issue of ESD in terms of control, and not elimination. The potential for an ESD event to occur cannot be completely eliminated outside of a laboratory environment, but we can greatly reduce the risk with proper training and equipment. By implementing a good static control program and developing some simple habits, ESD can be effectively controlled.

The Source of the Problem

Static is all around us. We occasionally will see or feel it by walking on carpet, touching something or someone and feeling the “zap” of a static discharge. The perception level varies but static charge is typically 2000-3000 volts before we can feel it. ESD sensitivity of some parts is under 100 volts – well below the level that we would be able to detect.

Even though carpet may not be used around the service bench, there are many other static “generators” may not be obvious and frequently found around or on a service bench. The innocent-looking Styrofoam coffee cup can be a tremendous source of static. The simple act of pulling several inches of adhesive tape from a roll can generate several thousand volts of static! Many insulative materials will develop a charge by rubbing them or separating them from another material. This phenomenon is known as “tribocharging” and it occurs often where there are insulative materials present.

Tape.JPG
Sources of Charge Generation: Unwinding a Roll of Tape

People are often a major factor in generation of static charges. Studies have shown that personnel in a manufacturing environment frequently develop 5000 volts or more just by walking across the floor. Again, this is “tribocharging” produced by the separation of their shoes and the flooring as they walk.

A technician seated at a non-ESD workbench could easily have a 400-500 volt charge on his or her body caused not only by friction or tribocharging, but additionally by the constant change in body capacitance that occurs from natural movements. The simple act of lifting both feet off the floor can raise the measured voltage on a person as much as 500-1000 volts.

Setting up a “Static Safe” Program

Perhaps the most important factor in a successful static control program is developing an awareness of the “unseen” problem. One of the best ways to demonstrate the ESD hazard is by using a “static field meter”. The visual impact of locating and measuring static charges of more than 1000 volts will get the attention of skeptical individuals.

718.jpg
Static Field Meter – find more information here

Education of Personnel

ESD education and awareness are essential basic ingredients in any effective static control program. A high level of static awareness must be created and maintained in and around the protected area. Once personnel understand the potential problem, reinforce the understanding by hanging up static control posters in strategic locations. The technician doesn’t need an unaware and/or unprotected person wandering over and touching things on the service bench.

Workstation Grounding

To minimize the threat of an ESD event, we need to bring all components of the system to the same relative potential and maintain that potential. Workstations can be grounded with the following options:

  1. Establish an ESD Common Grounding Point, an electrical junction where all ESD grounds are connected to. Usually, a common ground point is connected to ground, preferably equipment ground.
  2. The Service Bench Surface should be covered with a dissipative material. This can be either an ESD-type high-pressure laminate formed as the benchtop surface, or it may be one of the many types of dissipative mats placed upon the benchtop surface. The mats are available in different colors, with different surface textures, and with various cushioning effects. Whichever type is chosen, look for a material with surface resistivity of 1 x 109 or less, as these materials are sufficiently conductive to discharge objects in less than one second. The ESD laminate or mat must be grounded to the ESD common grounding point to work properly. Frequently, a one Megohm current limiting safety resistor is used in series with the work surface ground. This blog post will provide more information on how to choose and install your ESD working surface.
ESD-Worksurface-Matting.jpg
Types of Worksurface Matting – click here for more information
  1. A Dissipative Floor Mat may also be used, especially if the technician intends to wear foot-grounding devices. The selection of the floor mat should take into consideration several factors. If anything is to roll on the mat, then a soft, cushion-type mat will probably not work well. If the tech does a lot of standing, then the soft, anti-fatigue type will be much appreciated. Again, the mat should be grounded to the common ground point, with or without the safety resistor as desired.
  2. Workstation Tools and Supplies should be selected with ESD in mind. Avoid insulators and plastics where possible on and around the bench. Poly bags and normal adhesive tapes can generate substantial charges, as can plastic cups and glasses. If charge-generating plastics and the like cannot be eliminated, consider using one of the small, low cost air ionizers It can usually be mounted off the bench to conserve work area, and then aimed at the area where most of the work is being done. The ionizer does not eliminate the need for grounding the working surface or the operator, but it does drain static charges from insulators, which do not lend themselves to grounding.

Personnel Grounding

People are great static generators. Simple movements at the bench can easily build up charges as high as 500-1000 volts. Therefore, controlling this charge build-up on the technician is essential. The two best known methods for draining the charge on a person are wrist straps with ground cords and foot or heel grounders. Personnel can be grounded through:

  1. Wrist Straps are probably the most common item used for personnel grounding. They are comprised of a conductive band or strap that fits snugly on the wrist. The wrist strap is frequently made of an elastic material with a conductive inner surface, or it may be a metallic expandable band similar to that found on a watch. For more information on wrist straps, check out this post.
  2. Ground Cords are typically made of a highly flexible wire and often are made retractable for additional freedom of movement. There are two safety features that are usually built into the cord, and the user should not attempt to bypass them. The first, and most important, is a current limiting resistor (typically 1 Megohm) which prevents hazardous current from flowing through the cord in the event the wearer inadvertently contacts line voltage. The line voltage may find another path to ground, but the cord is designed to neither increase or reduce shock hazard for voltages under 250 volts. The second safety feature built into most cords is a breakaway connection to allow the user to exit rapidly in an emergency. This is usually accomplished by using a snap connector at the wrist strap end.
    Wrist-Strap.png
  3. Foot or Heel Grounders are frequently used where the technician needs more freedom of movement than the wrist strap and cord allow. The heel grounder is often made of a conductive rubber or vinyl and is worn over a standard shoe. It usually has a strap that passes under the heel for good contact and a strap of some type that is laid inside the shoe for contact to the wearer. Heel grounders must be used with some type of conductive or dissipative floor surface to be effective and should be worn on both feet to insure continuous contact with the floor. Obviously, lifting both feet from the floor while sitting will cause protection to be lost.Don’t forget to regularly check and verify your personnel grounding items:
PersonnelGroundingTesters.png
The Personnel Grounding Checklist

 

Summary

An effective static control program doesn’t have to be expensive or complex. The main concept is to minimize generation of static and to drain it away when it does occur, thereby lessening the chance for an ESD event to happen. The ingredients for an effective ESD program are:

  1. Education: to ensure that everyone understands the problem and the proper handling of sensitive devices.
  2. Workstation Grounding: use a dissipative working surface material and dissipative flooring materials as required.
  3. Personnel Grounding: using wrist straps with ground cords and/or foot-grounding devices.
  4. Follow-up to ensure Compliance: all elements of the program should be checked frequently to determine that they are working effectively.

The ESD “threat” is not likely to go away soon, and it is very likely to become an even greater hazard, as electronic devices continue to increase in complexity and decrease in size. By implementing a static control program now, you will be prepared for the more sensitive products that will be coming.

Stop by the Desco Industries Inc booth to see a demo of the SMP Static Management Program.

SMP-Screenshot.JPG

SCS Static Management Program (SMP) continuously monitors your ESD process control system throughout all stages of manufacturing. SMP captures data from SCS workstation, equipment and ESD event continuous monitors and allows you to pinpoint areas of concern and prevent ESD events. Quantifiable data allows you to see trends, become more proactive and prove the efficiency of your ESD process control system.

We will also be featuring a selection of our ESD monitoring equipment:

The exhibition area is open Monday, September 24 – Wednesday, September 26.

Peppermill Resort & Casino
2707 South Virginia Street
Reno, NV 89502

View the 2018 Program Guide HERE.

Static discharges can be noticed when you touch an object of different electrical potential such as a door knob, and a bolt of electricity flows from your charged body to the door knob. This flow of electricity is actually a result of the stored static charge that is being rapidly transferred to the knob. This discharge that can be felt as well as seen, is commonly referred to as an electrostatic discharge, or “ESD”.

The generated static charges are a potentially costly occurrence for office and factory employers. You will learn in today’s post how they can easily be controlled with different types of floor material.

 

Static Charge Generation from Flooring

When a person walks across a floor, a triboelectric charge builds up in the body due to the friction between the shoes and floor material. The simple separation of two surfaces (such as a person walking across a floor with soles contracting and separating from the floor) can cause a transfer of electrons resulting in one surface being positively and the other one negatively charged, resulting in static charges.

walkingacrosscarpet.png
Generating Charges by walking across carpet

It is not necessarily the static charge generated in the body that does the damage as much as it is the difference in potential that creates an electrostatic discharge.

 

The Problem with ESD (Electrostatic Discharge)

The generation of a static charge can pose quite a problem for environments that contain sensitive equipment or components that are vulnerable to static damage, such as electronics manufacturing, repair facilities and medical facilities – including computer rooms and clean rooms.

Controlling the damage and costs caused by ESD is usually the main concern that drives a company to implement a static control program. The costs involved with static damage not only include the immediate cost of the damaged component, but the contributing cost of diagnostic, repair and labor that is needed to replace or fix the component. In many cases the labor involved can far exceed the component cost.

 

ESD Flooring Materials

There are several options available on the market ranging from coatings (floor finish or paint) to coverings (vinyl or rubber). The choice of material depends on the mechanical and optical properties required as well as the available budget.

In general, floor coverings will last longer (10 years or more) than a floor coating. They are more durable and have a specific resistance to ground that remains constant over time.

esd-floor-coverings.jpg
Types of Floor Coverings – click here for more information

Coatings are easier to apply and repair and their initial cost is considerably lower. Coatings are usually applied to existing floors and often serve to convert a conventional floor into an ESD floor. However, regular maintenance is required as coatings will lose their ESD properties over time.

 

ESD Floor Coatings

  • Topical Antistat:
    Conventional carpets can be treated with a Topical Antistat or other treatment. It is required that the treatment be replenished on the carpet as it wears away due to foot traffic.
    ESD carpet is available but proper maintenance is very important.
  • ESD Floor Finish:
    Existing hard surfaces (e.g. concrete, sealed or painted wood, linoleum, asphalt) can be treated with ESD Floor Finish to eliminate the need for ESD control flooring. Repeat applications are required periodically to keep ESD properties within specification.
  • ESD Paint:
    Paint is ideal for providing a cost effective static-free environment and is very effective as a

static control floor coating for electronics manufacturing, assembly and storage. It controls dissipation of static electricity and provides path to ground.

ESD Floor Coverings:

Floor coverings will have either “conductive” or “dissipative” electrical properties.

  • Conductive materials have a resistance to ground (RG) of greater than 1 x 103 ohms but less than 1 x 105
  • Dissipative materials have a resistance to ground (RG) of greater than 1 x 105 ohms but less than 1 x 1012

It is recommended to use conductive flooring material; S20.20 requires ESD flooring to be less than 1 x 109 ohms (RG). The same standard requires a person/footwear/flooring to be less than 3.5 x 107 ohms (resistance in series of operator plus footwear plus floor). Remember that floors get dirty which can raise floor resistance. Therefore, it is good to start off with a floor that is conductive (less than 1 x 106 ohms). So even if the resistance increases, you’re within the required limits of the ESD Standard.

  • ESD Carpet:
    ESD control carpets are made with static dissipative yarn and only require that the yarn be kept clean and free of insulative dirt, dust and spray cleaners.
  • ESD Matting:
    Types of matting range from vinyl to rubber and anti-fatigue matting.
    Vinyl (e.g. SCS 8200 Series) is generally cheaper and provides high resistance to many chemicals. Rubber (e.g. SCS CONDFM Series) on the other hand is more durable and can withstand extreme hot and cold temperatures. Anti-fatigue matting (AFM Series) is designed to provide comfort for personnel that must stand or walk for long periods.

 

Considerations when Using Flooring Materials

1. Grounding

ANSI/ESD S20.20 requires that all conductors in an ESD protected area, including personnel, must be grounded. This includes ESD flooring. The ESD ground must be tied directly to and at the same potential as the building or “green wire” equipment ground. The SCS floor mat ground cord FGC151M is just one option for grounding floor matting.

2. Periodic Verification

All ESD control items (including ESD flooring) have to be tested:

  • Prior to installation to qualify product for listing in user’s ESD control plan.
  • During initial installation.
  • For periodic checks of installed products as part of ANSI/ESD S20.20 clause 7.4 Compliance verification plan.
SRMeter2_use.jpg
Measuring Surface Resistance of ESD Floor Matting – click here for more information

A surface resistance meter (e.g. SCS SRMETER2) can be used to verify compliance of the ESD floor with the ESD standard.

3. Person/Footwear/Flooring System

ESD flooring does not ensure protection from ESD damage unless operators walking across the ESD floor wear ESD footwear, either ESD shoes or ESD foot grounders.

ESD foot grounders are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic charges from personnel. They are easy to install and can be used on standard shoes by placing the grounding tab in the shoe under the foot.
Foot grounders must be worn on both feet to maintain the integrity of the body-to-ground connection Wearing a foot grounder on each foot ensures contact with ground via the ESD floor even when one foot is lifted off the floor. This will more reliably remove static charges generated by human movement.

SCS offers a number of different foot grounder types for your requirements.

 

Conclusion

Static charges can easily be controlled with different types of floor material which vary in their properties, cost and durability. The best static control systems are not only the ones that protect sensitive components and equipment but are: A) at hand and readily available, B) easily maintained. Floor coverings are long lasting and maintain their ESD properties over time, while existing floors can be economically converted for use in an ESD control program using various types of coatings.

Remember that all ESD control items such as flooring, personnel grounding and specialty equipment should be grounded and tested periodically to verify all components are within specification.

Not sure which ESD flooring is right for you? Request a free ESD/EOS Assessment at your facility by one of our knowledgeable local representatives to evaluate your ESD program and answer any ESD questions!

Most ESD Protected Areas (EPAs) will contain a bench or a series of benches. It is important that each bench, or worksurface, is covered with the correct ElectroStatic Discharge (ESD) protective material. They also have to be properly connected to earth using a system of cords and common point grounds. Today’s post will explain in more detail how these ESD protective worksurface work and what you need to look out for.

Introduction

The purpose of an ESD protective worksurface is to aid in the prevention of damage to ESD sensitive items (ESDS) and assemblies from electrostatic discharge.
ESD worksurfaces, such as mats, are typically an integral part of the ESD workstation, particularly in areas where hand assembly occurs. An ESD protective worksurface provides protection in two ways:

  1. Providing a low charging (antistatic) worksurface area that will limit static electricity to be generated below potentially damaging levels.
  2. Removing the electrostatic charge from conductive objects placed on the worksurface.

 

Types of ESD protective worksurfaces

When deciding to invest in ESD protective worksurfaces, you have the choice of ESD matting (laid-out on a standard non-ESD bench) or ESD benches. Performance-wise there is no difference.

Generally speaking, ESD matting offers a lower initial investment and is easier to replace. On the other hand, some people prefer the robust and consistent approach of ESD benches.

scs-esd-workstation-mats.jpg
Types of ESD protective worksurface matting – more information

An ESD protective worksurface is usually dissipative. Although conductive materials are the quickest to ground a charge, they can also cause damage by discharging too rapidly. Dissipative worksurfaces have a surface resistance of at least 1 x 104, but less than 1 x 109 ohms. Dissipative materials will dissipate a charge slower and are recommended for handling electronic components. Dissipative materials are usually the preferred choice for bench top worksurfaces.

 

Grounding of ESD protective worksurfaces

ESD protective worksurfaces need to be grounded. A ground wire from the surface should connect to the common point ground which is connected to ground, preferably equipment ground. For electronics manufacturing a worksurface resistance to ground (Rg) of 1 x 104 to less than 1 x 109 ohms is recommended. 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.

WorkstationGraphic.png
Grounding an ESD Protective Worksurface – click here for more grounding products

Using a current limiting resistor in the ground cord is the user’s choice. However, the resistor is not for ESD control purposes. The ESD Association standard for grounding is ANSI/ESD S6.1 which recommends a hard ground (no resistor) but allows the use of a current limiting resistor in the mat’s ground cord. “The grounding conductors (wires) from wrist straps, working surfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common point ground or common connection point is acceptable and recommended.

Note: Manufacturers may add resistance to the grounding conductors for purposes other than ESD (e.g. current limiting). Added resistance is acceptable for the purposes of controlling ESD provided electrostatic accumulation does not exceed specific EPA requirements. The typical added resistance in grounding conductors is 1 megohm, although other values may be specified.” [ANSI/ESD S6.1 section 5.3.3 ESD Technical Element Conductors]

 

Using ESD protective worksurfaces

Operators need to ensure that the ESD workstation is organized to perform work and that all unnecessary insulators and personal items are removed. Regular plastics, polystyrene foam drink cups and packaging materials etc. are typically high charging and have no place at an ESD protective workstation.

When working at an ESD workstation, users have to be grounded, too. A wrist strap is arguably the best way to provide a safe ground connection to the operator. While it does not prevent the generation of charges, its purpose is to dissipate these charges to ground as quickly as possible.

When working on high-end sensitive components, the use of Continuous Monitors is recommended. Operators connect their wrist strap to the unit to allow for real-time continuous monitoring. If the wrist strap fails, the unit will alarm.

724use.jpg
Using a Continuous Monitor at an ESD Workstation – more information

An option available with most Continuous Monitors is the ability to monitor worksurface ground connections. “Some continuous monitors can monitor worksurface ground connections. A test signal is passed through the worksurface and ground connections. Discontinuity or over limit resistance changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. The monitor however, will not detect insulative contamination on the worksurface.” [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]

When the monitor is connected to an ESD worksurface mat, the amount of current that flows is a function of the total resistance between the monitor and through the working surface to ground. When the resistance of the worksurface is below a pre-set threshold, the monitor will indicate good. Conversely, if the resistance level is high when compared to the monitor’s reference, the unit will alarm. This is an integrating resistance measuring circuit, therefore it is relatively insensitive to externally induced electromagnetic fields.

 

Maintaining your ESD protective worksurface

An ESD worksurface 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.

Operators need to be on guard every day and check visually that ground wires are attached correctly. The company’s compliance verification plan should also include periodic checks of worksurfaces measuring:

  • Resistance Point-to-Point (Rp-p) and
  • Resistance-to-ground (Rg).

Surface resistance testers can be used to perform these tests in accordance with S20.20 and its test method ESD TR53. If these measurements are within acceptable ranges, the worksurface and its connections are good.

770007_UseMatt

Verifying Surface Resistance using the SCS 701 Analog Surface Resistance Megohmmeter

Conclusion

Most people in the industry consider worksurfaces to be the second most important part of an ESD Control Program, with personnel grounding being the most important.

It is therefore important to install, use and maintain ESD protective worksurfaces correctly. Following all steps outlined above will ensure your ESD sensitive components are protected.

Not sure which ESD worksurface is right for you? Request a free ESD/EOS Assessment for your facility by one of our knowledgeable local representatives to evaluate your ESD Program and answer any ESD questions!

 

Many companies implement an ElectroStatic Discharge (ESD) Control Program with the aim of improving their operations. Effective ESD control can be a key to improving:

  • Productivity
  • Quality
  • Customer Satisfaction

Problems arise when an organization invests in ESD protective products and/or equipment and then misuses them. Misuse of ESD protective products and/or equipment wastes invested money and can also be causing more harm than good. Today’s blog post will highlight some of the major issues we have come across and how you can avoid or fix them.

About ESD Control and ESD Protection

Remember that for a successful ESD control program, ESD protection is required throughout the manufacturing process: from goods-in to assembly all the way through to inspection. Anybody who handles electrical or electronic parts, assemblies or equipment that are susceptible to damage by electrostatic discharges should take necessary precautions.

Just like viruses or bacteria that can infect the human body, ESD can be a hidden threat unable to be detected by human eyes. Hidden viral/bacterial threats in hospitals are controlled by extensive contamination control procedures and protective measures such as sterilization. The same principles apply to ESD control: you should never handle, assemble or repair electronic assemblies without taking adequate protective measures against ESD.

Common Mistakes in ESD Control

1. Ionizers are poorly maintained or out-of-balance

If an ionizer is out of balance, instead of neutralizing charges, it will produce primarily positive or negative ions. This results in placing an electrostatic charge on items that are not grounded, potentially discharging and causing ESD damage to nearby sensitive items.

Step3 Remember to clean emitter pins and filters using appropriate tools. Create a regular maintenance schedule which will extend the lifespan of your ionizers tremendously.

Consider using ionizers with “Clean Me” and//or “Balance” alarms. These will alert you when maintenance is required.

Step2.png All ionization devices will require periodic maintenance for proper operation. Maintenance intervals for ionizers vary widely depending on the type of ionization equipment and use environment. Critical clean room use will generally require more frequent attention. It is important to set up a routine schedule for ionizer service.”

[ESD TR20.20 Handbook Ionization clause 15.8 Maintenance / Cleaning]

If you would like to learn more about how ionizers work and what type of ionizer will work best for your application, check out this post for detailed coverage.

2. ESD Garments are Ungrounded

We’ve seen it so many times: operators wearing an ESD coat (without appropriate wrist straps and/or footwear/flooring) thinking they are properly grounded. However, without proper electrical bonds to a grounding system they are not grounded!

Step3 Every ESD garment needs to be electrically bonded to the grounding system of the wearer. Otherwise it just acts as a floating conductor. There are a few options to choose from:

  • Wrist Straps
  • ESD footwear/flooring
  • Hip-to-Cuff grounding
Step2 After verifying that the garment has electrical conductivity through all panels, the garment should be electrically bonded to the grounding system of the wearer so as not to act as a floating conductor.

This can be accomplished by several means:

  1. Ground the garment to the body through a wrist strap-direct connection with an adapter.
  2. Ground the garment through conductive wrist or heel cuffs in direct contact with the skin of a grounded operator.
  3. Ground the garment through a typical separate ground cord, directly attached to an identified groundable point on the garment.
  4. Garments should be worn with the front properly snapped or buttoned to avoid exposure of possible charge on personal clothing worn under the garment.

[ESD TR20.20 Handbook Garments clause 19.4 Proper Use]

ESD clothing loses their ESD properties over time. It is therefore an important part of the ESD Control Program to incorporate periodic checks (see #3 below) of ESD garments.

If you need more information on ESD garments, we recommend having a look at this post.

3. No Compliance Verification Plan / Not Checking ESD Control Products

Companies can invest thousands of dollars in purchasing and installing ESD control products but then waste their investment by never checking their ESD items. This results in ESD equipment that is out of specification. Without the tools in place to check their ESD items, companies may have no idea if they are actually working correctly. Remember: ESD products (like any other product) are subject to wear and tear, and other errors when workstations get moved, ground cords get disconnected…etc. The list goes on.

Step3 When investing in ESD control products, make sure you also establish a Compliance Verification Plan. This ensures that:

  • ESD equipment is checked periodically
  • Necessary test equipment is available
Step2 A compliance verification plan shall be established to ensure the organization’s fulfilment of the requirements of the plan. Process monitoring (measurements) shall be conducted in accordance with a compliance verification plan that identifies the technical requirements to be verified, the measurement limits and the frequency at which those verifications shall occur. The compliance verification plan shall document the test methods used for process monitoring and measurements. If the organization uses different test methods to replace those of this standard, the organization shall be able to show that the results achieved correlate with the referenced standards. Where test methods are devised for testing items not covered in this standard, these shall be adequately documented including corresponding test limits. Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements.
The test equipment selected shall be capable of making the measurements defined in the compliance verification plan.
”[ANSI/ESD clause 7.4 Compliance verification plan]

We provide detailed instructions on how to create a Compliance Verification Plan in this post.

4. Improperly Re-Using Shielding Bags / Using Shielding Bags with Holes or Scratches

ESD Shielding Bags are used to store and transport ESD sensitive items. When used properly, they create a Faraday Cage effect which causes charges to be conducted around the outside surface. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’. However, if the shielding layer of an ESD Shielding Bag is damaged, ESD sensitive items on the inside will not be protected anymore.

Step3 Re-using shielding bags is acceptable as long as there is no damage to the shielding layer. Shielding bags with holes, tears or excessive wrinkles should be discarded.

Use a system of labels to identify when the bag has gone through five (5) handling cycles. When there are five broken labels, the bag is discarded.

Step2 ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area.

Transportation of ESDS items outside an ESD Protected Area (hereafter referred to as “EPA”) requires enclosure in static protective materials, although the type of material depends on the situation and destination. Inside an EPA, low charging and static dissipative materials may provide adequate protection. Outside an EPA, low charging and static discharge shielding materials are recommended. While these materials are not discussed in

the document, it is important to recognize the differences in their application. For more clarification see ANSI/ESD S541.

[ANSI/ESD Foreword]

This post provides further “dos and don’ts” when using ESD Shielding Bags.

5. Using Household Cleaners on ESD Matting

The use of standard household cleaners on ESD matting can put an ESD Control Program at risk and damage the ESD properties of items. Many household cleaners contain silicone or other insulative contaminants which create that lovely shine you get when wiping surfaces in your home. The problem is that silicone and other chemical contaminates can create an insulative layer which reduces the grounding performance of the mat.

Step3 Don’t spend all this extra money on ESD matting and then coat it with an insulative layer by using household cleaners. There are many specially formulated ESD surface and mat cleaners available on the market. Only clean your ESD working surfaces using those cleaners.
Step2 “Periodic cleaning, following the manufacturer’s recommendations, is required to maintain proper electrical function of all work surfaces. Ensure that the cleaning products used to not leave an electrically insulative residue which is common with some household cleaners that contain silicone.”

[ESD TR20.20 Handbook Worksurfaces clause 10.5 Maintenance]

Conclusion

There are many more issues we see when setting foot into EPAs and the above list is by no means complete. These are the most common issues we’ve found when assessing EPAs.

It is important to train all personnel using ESD products and/or equipment to follow proper ESD control programs, and maintenance procedures to avoid common ESD control mistakes. Basic ESD control principles should be followed for an ESD control program to be successful:

  • Ground conductors.
  • Remove, convert or neutralize insulators with ionizers.
  • Shield ESD sensitive items when stored or transported outside the EPA.

What mistakes do you commonly see when walking through an EPA? Let us know what you commonly see in the comments and your solutions for fixing them!

For more information on how to get your ESD control program off the ground and create an EPA, check this post.

ElectroStatic Discharge (ESD) can pose danger to a Printed Circuit Board (PCB). A standard bare PCB (meaning that it has no semiconductor components installed) should not be susceptible to ESD damage, however as soon as you add electronic (semiconductor) devices, it becomes susceptible according to each of the individual’s susceptibility.

While ESD damage can post a danger, there is another risk factor many operators forget: moisture.

Today’s blog post is going to address both risks and will explain how you can protect your PCBs from both when storing them.

The problem with moisture

If you have been following along with our blogs, you will be well aware of the problems ESD damage can cause.

Moisture, on the other hand, may be a new issue to you. Surface Mounted Devices (SMDs), for example, absorb moisture and then during solder re-flow operations, the rapid rise in temperature causes the moisture to expand and the delaminating of internal package interfaces, also known as “pop corning.” The result is either a circuit board assembly that will fail testing or can prematurely fail in the field.

Moisture.png
Moisture from air diffuses inside the plastic body & collects in spaces between body & circuit, lead frame and wires. Expanding vapor can crack (popcorn) the plastic body or cause delamination.

Storing PCBs

All PCBs should be stored in a moisture barrier bag (MBB) that is vacuum sealed. In addition to the bags, Desiccant Packs and Humidity Indicator Cards must be used for proper moisture protection. This ‘package’ is also known as a dry package.

Most manufacturers of the Moisture Sensitive Devices (MSD) will dictate how their product should be stored, shipped, etc. However, the IPC/JEDEC J-STD-033B standard describes the standardized levels of floor life exposure for moisture/reflow-sensitive SMD packages along with the handling, packing and shipping requirements necessary to avoid moisture/reflow-related failures.

The ESD Handbook ESD TR20.20 mentions the importance of moisture barrier bags in section 5.4.3.2.2 Temperature: “While only specialized materials and structures can control the interior temperature of a package, it is important to take possible temperature exposure into account when shipping electronic parts. It is particularly important to consider what happens to the interior of a package if the environment has high humidity. If the temperature varies across the dew point of the established interior environment of the package, condensation may occur. The interior of a package should either contain desiccant or the air should be evacuated from the package during the sealing process. The package itself should have a low WVTR.

Components of a dry package

A dry package has four parts:

  1. Moisture Barrier Bag (MBB)
  2. Desiccant
  3. Humidity Indicator Card (HIC)
  4. Moisture Sensitive Label (MSL)

 

 3371014.jpg Moisture Barrier Bags (MBB) work by enclosing a device with a metal or plastic shield that keep moisture vapor from getting inside the bag. They have specialized layers of film that control the Moisture Vapor Transfer Rate (MVTR). The bag also provides static shielding protection.
Desiccant is a drying agent which is packaged inside a porous pouch so that the moisture can get through the pouch and be absorb by the desiccant. Desiccant absorbs moisture vapor (humidity) from the air left inside the barrier bag after it has been sealed. Moisture that penetrates the bag will also be absorbed. Desiccant remains dry to the touch even when it is fully saturated with moisture vapor.

The recommended amount of desiccant  depends on the interior surface area of the bag to be used. Use this desiccant calculator to determine the minimum amounts of desiccant to be used with Moisture Barrier Bags.

1-6PLDES1200.jpg
3HIC125.jpg Humidity Indicator Cards (HICs) are printed with moisture sensitive spots which respond to various levels of humidity with a visible color change from blue to pink. The humidity inside barrier bags can be monitored by the HIC inside. Examining the card when you open the bag will indicate the humidity level the components are experiencing so the user can determine if baking the devices is required.
The Moisture Sensitive Level (MSL) label tells you how long the devices can stay outside the bag before they should be soldered onto the board. This label is applied to the outside of the bag. If the “level” box is blank, look on the barcode label nearby. 113LABEL.jpg

5 Steps to Create a Dry Package

Now that we know the risks moisture poses to ESD components, follow these 5 steps to create a secure, dry package which will protect your PCBs against ElectroStatic Discharge and moisture:

  1. Place the desiccant and HIC onto the tray stack. Trays carry the devices. Remember to store desiccant in an air tight container until it used.
    Dry-Packaging-Step1.png
  1. Place the MSL label on the bag and note the proper level on the label.
    Dry-Packaging-Step2.png
  2. Place the tray stack (with desiccant and HIC) into the moisture barrier bag.
    Dry-Packaging-Step3.png
  3. Using a vacuum sealer, remove some of the air from the bag, and heat seal the bag closed. It is not good to take all the air out of the bag. Only slight evaluation is needed to allow the bag to fit inside a box.
    Dry-Packaging-Step4.png
  4. Now your devices are safe from moisture and static.
    Dry-Packaging-Step5.png

With the steps taken above, your package should now be properly sealed from moisture and protected from ElectroStatic discharge.

Looking for a moisture barrier bag for your application? See the SCS Moisture Barrier Bag Selection Guide to find the packaging that fits your specifications!

In a previous post we learnt how to select the correct ESD bag for your application, we want to focus on the next step: how to correctly use your ESD bag. We’ll use shielding bags as an example as they are the most commonly used ESD bags. However, the below can be applied to all types of ESD bags.

There are a few “dos and do-nots” you should keep in mind to ensure you get the most from your ESD bags. Nothing is worse than investing in all the right equipment and then using it incorrectly rendering all your efforts void. So, on that note, we have comprised a list of 5 tips for you on how to most efficiently use your shielding bags.

5 Tips On Efficient Use of Shielding Bags With ESD Sensitive Items:

1. Enclose Your ESD Sensitive Item with a Shielding Bag

Shielding bags should be large enough to enclose the entire product within. The shielding bag should be closed with a label or tape. Alternatively, you can use a zipper-style shielding bag. Following this advice ensures a continuous Faraday Cage is created which provides electrostatic shielding. This is the only way to ensure ESD sensitive devices placed inside the shielding bag are protected. If you are unfamiliar with the term “Faraday Cage”, scroll to the bottom of this page – we’ve included a more detailed explanation at the end of the post.

 

Enclose_Shielding_Bags
Enclose your ESD sensitive item

 

Please do not staple your shielding bag. The staple punctures the shielding layers and will provide a conductive path from the outside of the shielding bag to the inside. Charges outside the shielding bag could potentially charge or discharge to ESD sensitive components inside the shielding bag.

If you’re unsure as to what the correct size is for your application, catch-up on this post which will provide all the required information.

2. Remove Charges from Shielding Bags

When receiving an ESD sensitive device enclosed in a shielding bag, make sure you place the closed shielding bag on an ESD worksurface before removing the product. This will eliminate any charge that might have accumulated on the surface of the shielding bag.

 

Remove_Static_Charges.jpg
Remove charges

 

 3. Do Not Overuse Shielding Bags

Re-using shielding bags is acceptable as long as there is no damage to the shielding layer. Shielding bags with holes, tears or excessive wrinkles should be discarded.

 

Dont_overuse_shielding_bags
Don’t overuse shielding bags


 4. Shielding Bags Are Not A Working Surface

Do not use a shielding bag as an ESD worksurface. Although a shielding bag is safe to use around ESD susceptible products, it is not intended to be a worksurface for product. When working on ESD sensitive devices, do so using ESD worksurfaces that are grounded correctly.

Shielding_Bags_are_no_ESD-Worksurface.jpg
Don’t use shielding bags as your ESD worksurface

 5. A Shielding Bag Is Not A “Potholder” Or “Glove”

Do not use a shielding bag as an “ESD potholder” or “ESD glove”. This type of use offers no ESD protection to the product.

If you need to handle ESD sensitive devices, make sure you are properly grounded using wrist straps or heel grounders.

Shielding_Bags_are_no-Gloves
Shielding bags are no “ESD glove” or “ESD potholder”

Some of you may have read through this post and have stumbled across the term “Faraday Cage” as you have not come across it before. We’ve also mentioned it before when talking about storing and transporting ESD sensitive items. However, we’ve never actually explained what a Faraday Cage is – so let’s rectify that!

What Is A “Faraday Cage” Or “Faraday Shield”?

A Faraday Cage or Faraday shield is an enclosure formed by conducting material or by a mesh of conductive material. Such an enclosure blocks external static and non-static electric fields. Faraday Cages are named after the English scientist Michael Faraday, who invented them in 1836.

What Is An Example of Faraday Cage Effect?

An impressive demonstration of the Faraday Cage effect is that of an aircraft being struck by lightning. This happens frequently but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, a car may be a safe place during a thunderstorm.

 

Lightning.jpg
Lightning striking an airplane

 

How Is A Faraday Cage Effect Used In ESD Protection?

In ESD Protection, the Faraday Cage effect causes charges to be conducted around the outside surface of the conductor. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’.

Examples of ESD control products that provide a Faraday Cage or shielding include Metal-In and Metal-Out Shielding bags.

When Is ESD Shielding Packaging Used?

ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area.

ESD Packaging Standards For Outside An EPA

Per Packaging Standard ANSI/ESD S541 clause 6.2 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides:

  • Low charge generation.
  • Dissipative or conductive materials for intimate contact.
  • A structure that provides electrostatic discharge shielding.

Additional ESD Definitions

Other helpful ESD related definitions from the ESD Association Glossary ESD ADV1.0 include:

Faraday Cage“A conductive enclosure that attenuates a stationary electrostatic field.
Electrostatic discharge (ESD) shield: “A barrier or enclosure that limits the passage of current and attenuates an electromagnetic field resulting from an electrostatic discharge.
Electrostatic shield: “A barrier or enclosure that limits the penetration of an electrostatic field.

So, hopefully we’ve clarified a few things today when it comes to the “shielding” property by explaining the phenomenon of the “Faraday Cage”. Don’t forget to implement our tips when it comes to using your ESD bags!

 

We already know that in an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same potential which is achieved by using ‘groundable’ materials that are then linked to ground. We have also learnt that the most common personnel grounding device to link people to ground are wrist straps. People who are moving around should instead wear ESD footwear.

So how do you know if your wrist straps and ESD footwear are working properly? Excellent question! And one we’ll answer in today’s post so let’s jump right in!

Purpose of Personnel Grounding Testers

Wrist straps and ESD footwear should be part of your Verification Plan. Each component in an EPA plays a vital part in the fight against electrostatic discharge (ESD). If just one component is not performing correctly, ESD sensitive devices can be damaged, potentially costing your company thousands of dollars.

Wrist straps and ESD footwear can fail and damage cannot always be detected by visual inspection. Just by looking at the items you would not know if they still provide sufficient protection. Personnel grounding testers should be used to provide feedback to verify the functionality of an operator’s wrist strap and/or footwear.

Your Personnel Grounding Checklist - Wear, Verify, Log, Handle
Your Personnel Grounding Checklist

Your Personnel Grounding Checklist:

  1. Wear your personnel grounding equipment such a wrist strap and/or footwear
  2. Verify your personnel grounding system using a wrist strap and/or footwear tester. Wrist straps and footwear, need to be tested at least daily before handling ESD sensitive devices and should be worn while checking.
  3. Log a record of each test. Records should be kept for quality control purposes.
  4. Handle ESD sensitive components ONLY if your wrist strap and/or footwear pass(es) the test.

Types of Personnel Grounding Testers

Personnel grounding testers can be purchased in two configurations:

  • Wrist strap tester
  • Wrist strap and footwear tester

As wrist straps are the most commonly used personnel grounding device to ground operators, you will find a lot of testers on the market that check wrist straps only. Combined wrist strap and footwear testers will verify your wrist straps AND footwear.

In addition to WHAT the testers verify, you will also be faced with a wide range of devices differing in HOW they test. Below you will see a (by no means complete) list of options:

  • Continuous and split footplate: You will find testers with a continuous footplate which require each foot to be tested separately one after the other. Dual-footplate or independent footwear testers feature a split footplate which allows the unit to verify both feet independently at the same time. This can be an efficient time-saver if you have a number of operators in your company who are required to check their personnel grounding devices.
  • Portable, wall-mountable and fitted testers: Portable battery-powered (predominantly) wrist strap testers are suitable for small labs or for supervisors to spot-check workers and ensure compliance. Wall-mountable units are generally supplied with a wall plate which attaches to a wall; the tester is then mounted on to the wall plate. Some personal grounding devices are accompanied by a stand (and built-in footplate) which allow for a more freely positioning of the unit within a room.
  • Relay terminal: A few testers on the market are fitted with a relay terminal (electrically operated switch) that can be integrated with electronic door locks, turnstiles, lights, buzzers, etc. This can be of advantage if companies only want to allow personnel in an EPA that have passed their wrist strap and/or footwear test.
  • Data acquisition: A growing number of personnel grounding devices allow for test activity data to be logged in a database. The units link to a computer which records operator identification, test results, resistance measurements, time and more. Paperless data can enhance operator accountability, immediately identifying problems while reducing manual logging and auditing costs.

Operation of Personnel Grounding Testers

Wrist strap testing:

If you are not using a continuous or a constant monitor, a wrist strap should be tested at least daily. This quick check can determine that no break in the path-to-ground has occurred. Wrist straps should be worn while they are tested. This provides the best way to test all three components:

  • the wrist band
  • the ground cord (including the resistor)
  • the interface (contact) with the operator’s skin
The SCS Combo Wrist Strap/ Footwear Tester
The SCS Combo Tester can be used to test wrist straps – more information

To ensure that the resistance to ground of personnel is within specification it is important to measure the entire system (i.e., wrist strap, person, and ground connection). The wrist strap system test method is described in ANIS/ESD S1.1. In general, the test method measures the resistance of the ground cord, wristband or cuff, and the interface of the band or cuff of the wearer.” [Handbook ESD TR20.20 Clause 8.2 Wrist Strap System]

The wrist strap system should be tested daily to ensure proper electrical resistance. Nominally, the upper resistance reading should be < 35 megohms or a user-defined resistance. Daily test records can provide evidence of conformity. Daily testing may be omitted if continuous monitors are used.” [ANSI/ESD S1.1 Clause A3. Frequency of System Testing]

If the wrist strap tester outputs a FAIL test result, stop working and test the wrist band and cord individually to find out which item is damaged. Replace the bad component and repeat the test. Obtain a PASS test result before beginning work. For more information on troubleshooting failed wrist straps, check this post.

Footwear testing:

If you are using a flooring / footwear system as an alternative for standing or mobile workers, ESD footwear should be tested independently at least daily while being worn. Proper testing of foot grounders involves the verification of:

  • the individual foot grounder
  • the contact strip
  • the interface between the contact strip and the operator’s perspiration layer

a) Place the foot grounders on the user’s shoes per the manufacturer’s instructions.
b) Place the left foot on the floor plate and touch the body contact area on the tester with one hand. Activate the tester per the manufacturer’s instructions.
c) Remove the left foot from the floor plate.
d) Repeat steps b and c with the right foot.
[ANSI/ESD SP9.2 Clause 6.2.2 Procedure (Integrated Tester)]

The SCS Dual Combination Tester is used to test wrist straps and footwear
The SCS Dual Combination Tester is used to test wrist straps and footwear – more information

If the footwear tester outputs a FAIL test result, stop working, and test the foot grounder and contact strip individually to find out which item is damaged. Replace the foot grounder. Obtain a PASS test result before beginning work.

Conclusion

Wrist straps and footwear need to be tested at least daily before handling any ESD sensitive devices. Personnel grounding devices need to be worn for verification using a wrist strap and/or footwear tester.

A record of each test has to be kept for quality control purposes.

Only handle ESD sensitive components if your wrist strap and/or footwear pass(es) the test.