Welcome back to “A Minute with Miranda.” This week we will be covering how to launder your ESD Smocks.
SCS Static Control Smocks should be laundered periodically for proper operation. Smocks should be washed by hand or with a standard household washing machine using cold or warm water with a non-ionic liquid detergent. Avoid using bleach, fabric softeners or dry detergents as these can contaminate the conductive fibers and make the garment insulative .
After washing the smocks they should either be hung dry or tumbled dry at low heat. High heat can degrade the conductive fibers within the garment and degrade the ESD properties.
With normal wearing and washing conditions, SCS Static Control Smocks will maintain their ESD properties for a minimum of 100 washings.
SCS Smocks meet the requirement for Groundable Static Control Garment System per ANSI/ESD S20.20 required limit of less than 3.5 x 107 ohm Rtg tested per ANSI/ESD STM2.1 and ESD TR53.
Welcome back to “A Minute with Miranda.” This week we will be covering ESD Worksurface Mats.
SCS worksurface mats serve two purposes for an ESD worksurface in an EPA:
they provide a surface that does not generate a static charge and
they remove charges from all charged conductors, which include ESD susceptible devices and assemblies, that are placed on the surface.
SCS worksurface mats are constructed from either dissipative 2 layer rubber or 3 layer vinyl material and are available in both rolls and mat kits.
We can also quote custom sizes SCS worksurface mats meet ANSI/ESD STM4.1 ANSI/ESD S20.20 required limit of 1 x 106 to less than 1 x 109 ohms for Rtt and Rtg and the recommendations of ANSI/ESD S4.1
For more information on SCS Worksurface matting, check out our Selection Guide here.
When referring to an “ESD Protected Area” or “EPA”, a lot of people imagine rooms or even whole factory floors with numerous workstations. This very common misconception leads to nervousness and even fear when it comes to implementing an ESD Control Program. There is a concern regarding the cost and time implications when establishing an EPA. However, most often, a simple ESD workstation is completely sufficient to fulfill a company’s needs to protect their ESD sensitive products. Today’s post will provide a step-by-step guide on:
How to create an EPA at an existing workstation,
What ESD control products are required
How to correctly set up ESD control products
What is an “ESD Protected Area” or “EPA”?
An EPA is an area that has been established to effectively control Electrostatic Discharge (ESD) and its purpose is therefore to avoid all problems resulting from ESD damage, e.g. catastrophic failures or latent defects. It is a defined space within which all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. This is achieved by simply using only ‘groundable’ materials for covering of surfaces and for the manufacture of containers and tools. All surfaces, products and people are grounded to Ground.
What is Grounding?
Grounding means linking, usually through a resistance of between 1 and 10 megohms. Movable items (such as containers and tools) are grounded by virtue of lying on a grounded surface or being held by a grounded person. Everything that does not readily dissipate a charge must be excluded from the EPA.
How big does an EPA need to be?
An EPA can be just one workstation, or it could be a room containing several different workstations. “The definition of an EPA depends somewhat on the user environment. An EPA may be a permanent workstation within a room or an entire factory floor encompassing thousands of workstations. An EPA may also be portable as used in a field service situation.” [Handbook ESD TR20.20-2016 Clause 9.0 ESD Protected Areas]
What is needed to convert a Workstation into an EPA?
Creating an EPA at an existing workstation does not need to be complicated or expensive. There are just a few things that are required:
1. Wrist Strap
Wrist straps are the most common personnel grounding devices and are used to link people to ground. They are required if the operator is sitting. A wrist strap is made up of two components:
A wristband that is worn comfortably around the wrist and
A coiled cord that connects the band to Ground or a Wrist Strap Grounding System as explained in #4.
2. Wrist Strap Grounding System
These have been designed to be installed underneath bench tops where they are easily accessible to operators and where they are unlikely to be knocked and damaged or hinder the operator. The grounding cord of the Grounding System needs to be connected to a suitable Ground.
3.Worksurface Mat
ESD protective worksurfaces 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. The purpose of the ESD worksurface 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) that are placed on the surface.
4. Worksurface Mat Grounding Cord
An ESD worksurface needs to be grounded using a ground cord. A ground wire from the surface should connect to 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.
Where sitting personnel will be grounded via a wrist strap, this method is not feasible for operators moving around in an ESD Protected Area. In those situations, a flooring / footwear system is required.
5. Foot Grounders
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.
6. Floor Mat
Floor matting is an essential component in the flooring / footwear system when grounding moving or standing personnel. The path to Ground from operators via heel grounders to Ground is maintained by using dissipative or conductive flooring.
Floor mats don’t just ground personnel; they are also used to ground ESD control items (e.g. mobile carts or workstations).
7. Floor Mat Grounding Cord
Just like worksurface matting, floor matting needs to be connected to Ground. This ensures that any charges on the operator are dissipated through their heel grounders and the floor matting to Ground. A floor mat grounding cord is used to link the floor mat to Ground.
Alternatively, matting can be grounded via a strip of copper foil.
Installing an ESD Workstation
To install the ESD workstation, it is necessary to ground the worksurface and operator with the following steps:
1.
Lay the worksurface mat flat on the workbench with the stud(s) facing upwards.
2.
Connect the worksurface mat grounding cord to the worksurface mat.
3.
Connect the other end of the worksurface mat grounding cord to Ground.
4.
Place the wristband on the wrist.
5.
Connect the coiled cord to the wristband.
6.
Attach the Wrist Strap Grounding System to the bench. Remember that it needs to be connected to a suitable Ground.
7.
Connect the other end of the coiled cord to the Wrist Strap Grounding System and verify personnel is properly grounded.
If your operators are standing or mobile and grounding via a wrist strap is not feasible, ground the worksurface, and the ESD flooring:
1.
Ground the worksurface mat by following steps #1 to #4 above
2.
Lay the floor mat flat on the floor with the stud(s) facing upwards.
3.
Connect the floor mat grounding cord to the floor mat.
4.
Connect the other end of the floor mat grounding cord to Ground.
5.
Place the foot grounders on the feet and verify personnel is properly grounded.
Conclusion
An EPA can be created at an existing workstation in a facility. To establish an EPA it is important to:
Ground all conductors (including people),
Remove all insulators (or substituting with ESD protective versions) or
Neutralize process essential insulators with an ionizer.
With a few simple steps, you can convert your existing workstation into an ESD workstation. You will need:
Worksurface Mat
Worksurface Mat Grounding Cord
Wrist Strap
Wrist Strap Grounding System
Optional:
Foot Grounders
Floor Mat
Floor Mat Grounding Cord
We hope this article has introduced the basics of an ESD Protected Area (EPA), and the steps needed to create an ESD Workstation.
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!
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.
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.
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.
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.
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!
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.
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.
“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.”
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!
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
“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:
Ground the garment to the body through a wrist strap-direct connection with an adapter.
Ground the garment through conductive wrist or heel cuffs in direct contact with the skin of a grounded operator.
Ground the garment through a typical separate ground cord, directly attached to an identified groundable point on the garment.
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 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.
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
“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.
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.
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.
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.
“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.”
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.
When talking about ESD Classifications a little while ago, we identified a “class 0” item as withstanding discharges of less than 250 volts.
The introduction of ANSI/ESD S20.20 states: “This standard covers the requirements necessary to design, establish, implement and maintain an Electrostatic Discharge (ESD) Control Program for activities that manufacture, process, assemble, install, package, label, service, test, inspect or otherwise handle electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts Human Body Model (HBM) and 200 volts Charged Device Model (CDM).”
So how do you handle items that are susceptible to voltages of less than 100V? That’s what we’re going to answer in today’s blog post.
Introduction
Years ago, it was common for devices to be vulnerable to voltages greater than 100 V. As the need for smaller and faster devices increased, so did their sensitivity to ElectroStatic Discharges as circuit-protection schemes were removed to stay ahead of the market. These new extremely sensitive components are now susceptible to discharges nearing 0 V. This causes problems for companies handling these devices: while their ESD program may be in compliance with the ESD Standard, extremely sensitive devices require tighter ESD Control to protect them from ESD failures.
What is a “Class 0” device?
Before moving any further, we need to qualify the term “class 0”. As stated above, the HBM Model refers to any item with a failure voltage of less than 250 V as a “class 0” component. However, in recent times, the term has been used more and more to describe ultra-sensitive devices with failure voltages of less than 100 V. Whilst the following tips and tricks work on any “class 0” item, they are specifically designed to protect extremely sensitive items that withstand discharges of less than 100 V.
Ultra-sensitive devices are extremely common
Before Updating Your ESD Program
“Class 0” refers to a wide range of items and there are a few things you should remember before making any changes to your existing ESD program:
Verify what ESD Model your company/engineers/customers etc. are referring to. As we have learnt in the past, there are different ESD models (HBM, CDM, MM) as well as individual classifications for each model. A lot of people get confused when it comes to citing ESD classifications. There is only one “class 0” which refers to the human body model (HBM) but it’s always best to check.
Check the specific withstand voltage an individual part is susceptible to. “Class 0” refers to all items that withstand discharges of less than 250 V. However, there is a big difference between a failure voltage of 240 V or 50 V. You need to have detailed ESD sensitivity information available before being able to make decisions on how to improve your existing ESD control program. This step is part of creating a compliance verification plan.
A part’s ESD classification is only of importance until it is ‘merged’ into an assembly. So, the ESD classification of a device only refers to the stand-alone component. Once it goes into another construction, the classification of the whole assembly is likely to change.
Tips for handling “Class 0” Items
Below are 6 tips that will help your company to upgrade your ESD control program so you can effectively and efficiently handle ultra-sensitive items without risking ESD damage.
One thing to note: The best approach to stay ahead of the game is taking proactive actions. It is critical to figure out how to protect your components from ESD damage before you receive them. If actions are taken after components are received, the components are susceptible to receiving ESD damage.
1. Improve Grounding
Inside an EPA, all conductors (including people) are grounded. Now you’re probably thinking: “But I’ve already grounded my operators and worksurfaces. What else is there left to do?”. Firstly, well done for properly grounding the ‘objects’ in your EPA. The next step is to adjust and improve your current program to allow for even better protection. Here are some suggestions:
Personnel:
Decrease the wrist strap and ESD footwear upper limit. The ESD Association has test data showing charge on a person is less as the path-to-ground resistance is less.
Full coverage foot grounders are recommended when handling ultra-sensitive devices
Worksurfaces:
Reduce the required limit for Point-to-Point resistance of 1 x 109 per the ESD Standard to 106 to 108 ohms (see #5). The reason for this reduction is simple: 1 x 109 is too high as it still produces thousands of volts of in electrostatic charges. However, the resistance cannot be too small either as this can lead to a sudden ‘hard discharge’ potentially damaging ESD sensitive components.
Other:
Improve grounding of carts, shelves and equipment to Ground
Minimize isolated conductors like devices on PCBs
2. Minimize Charge Generation
The best form of control is to minimize charge generation. First, you should always use shielding packing products like bags or containers (especially when outside an EPA) as these protect from generating charges in the first place. For more information on choosing the correct type of ESD Packaging, we recommend reading this post.
The next step is to eliminate charges once they are generated – this can be achieved through grounding and ionization. We’ll cover ionization in #3 and #4. We’ve already talked about improved grounding in #1. However, for ultra-sensitive components, we also recommend the following:
Personnel: Use low-charging floor finish
Surfaces: Use low-charging topical antistatic treatments
Both types of ESD products create a low tribocharging coating which allows charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts.
3. Remove Insulators
When talking about conductors and insulators, we explained that insulators cannot be grounded and can damage nearby sensitive devices with a sudden uncontrolled discharge. It is therefore critical to eliminate ALL insulators that are not required in your EPA: plastic cups, non-ESD brushes, tapes etc. How? Here are a couple of options:
Replace regular production supplies and fixtures with dissipative, low charging versions, e.g. ESD dissipative brushes, ESD dispensers, ESD tape, ESD Chairs etc.
If an insulator is absolutely necessary for production and cannot be removed from the EPA, you could consider a topical treatment which will reduce triboelectric charges.
Is this not an option, then move on to tip #4.
4. Use Ionization
First, ionization is not a cure-all. We’ve learnt that ionizers neutralize charges on an insulator.
However, that does not mean that you can just have any insulator in your EPA because the ionizer will “just fix it”. No, in this instance, prevention is generally a better option than the cure. So, your priority should ALWAYS be to remove non-process essential insulators from your EPA – see tip #3. If this is not possible – then ionization becomes essential.
Ionization:
Ionizers can be critical to reduce induction charging caused by process necessary insulators
Ionizers can be critical in eliminating charges on isolated conductors like devices on PCBs
Offset voltage (balance) and discharge times are critical considerations depending on the actual application
Ionization can reduce ElectroStatic Attraction (ESA) and charged particles clinging and contaminating products.
It is recommended to use ionizers with feedback mechanisms, so you’re notified if the offset voltage is out of balance.
5. Increase ESD Training and Awareness
ESD Training is a requirement of every ESD Program. When handling ultra-sensitive devices, it is even more important to remind everyone what pre-cautions are necessary to avoid damage. Regular ‘refreshers’ are a must and it is recommended to verify the effectiveness of the training program, e.g. through tests. So, who, when and what should be taught?
ESD Training is a vital part of every successful ESD Control Program
ESD training needs to be provided to everyone who handles ESD sensitive devices – that includes managers, supervisors, subcontractors, visitors, cleaners and even temporary personnel.
Training must be given at the beginning of employment (BEFORE getting anywhere near a sensitive products) and in regular intervals thereafter.
Training should be conducted on proper compliance verification procedures and on the proper use of equipment used for verification.
6. Create an enhanced Compliance Verification Plan
We talked in a previous post about compliance verification, what it is and how to create a plan that complies with the ESD standard. So, if you already followed our steps and have a plan in place, here are a few tips to improve your compliance verification plan:
Use a computer data collection system for wrist straps and foot grounders testing
Increase the test frequency of personnel grounding devices from once per day to every time the operator enters the EPA
Use continuous monitors where operators are grounded via wrist straps. Consider computer based monitor data collection system, e.g. SMP. This should include continuous monitoring of the mat Ground.
Use Ground continuous monitors, e.g. Ground Master. At a large facility, the most frequent reoccurring violation is the ESD mat ground cord either becoming disconnected from the mat or grounding point. As Ground continuous monitors will only test the fact that the mat is grounded, it is still imperative that the Resistance to Ground of the mat is regularly tested. Remember that the use of improper mat cleaners can raise the mat surface resistance above the upper recommended level of <109
Test ionizers more frequently or consider self-monitoring ionizers. Computer based data collection systems are a good alternative, too.
Increase the use of a static field meter and nano coulomb testing to verify that automated processes (like auto insertion, tape and reel, etc.) are not generating charges above acceptable limits.
Conclusion
“Class 0” items require additional measures of ESD protection due to their sensitivity to ESD damage. The best way to protect these ultra-sensitive components is to increase ESD protective redundancies and periodic verifications to all ESD Control technical elements.
To decrease the probability of ESD damage while handling ultra-sensitive items, additional precautions are required. This includes additional and/or more stringent technical requirements for ESD control products, increasing redundancies, and more frequent periodic verifications or audits.
Additionally, ESD control process systems should be evaluated as to their performance as a system. It is important to understand how the technical elements in use perform relative to the sensitivity of the devices being handled. Thus, tailoring the process to handle the more sensitive parts. For example: If the footwear/flooring allows a person’s body voltage to reach 80 volts and a 50 withstand voltage item gets introduced into the process, you must either allow only handling via wrist straps or would have to find a way to modify the footwear/flooring performance to get peak voltages below the 50 volt threshold.
Remember: The ESD Standard gives recommendations that will always be behind current/future developments. As soon as a Standard is published, technology will have progressed. In order to protect your devices and company reputation for reliable devices – it is recommended your company take responsibility to implement methods/procedures that exceed the recommendations of the ESD Standard to fit your sensitive component requirements.
We have learned in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials.
But what do you do if an item in your EPA is essential to assembly and it cannot be grounded? Don’t sweat, not all hope is lost! Let us explain a couple of options which will allow you to use the non-groundable item in question.
Conductors and Insulators
In ESD Control, we differentiate items as conductors and insulators.
Materials that easily transfer electrons are called conductors. Examples of conductors are metals, carbon and the human body’s sweat layer.
A charged conductor can transfer electrons which allows it to be grounded
Insulators are materials that do not easily transfer electrons are non-conductors by definition. Some well-known insulators are common plastics, polystyrene foam, and glass.
Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.
Both, conductors and insulators, may become charged with static electricity and discharge.
Electrostatic charges can effectively be removed from conductive or dissipative conductors by grounding them. A non-conductive insulator will hold the electron charge and cannot be grounded and “conduct” the charge away.
Conductors and Insulators in an EPA
The first two fundamental principles of ESD Control are:
Ground all conductors (including people).
Remove all insulators.
To ground all conductors per the first ESD Control principal, all surfaces, products and people are electrically bonded to ground. Bonding means linking or connecting, usually through a resistance of between 1 and 10 megohms.
Wrist straps and worksurface mats are some of the most common devices used to remove static charges:
Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices.
Movable items (such as containers and tools) are bonded by standing on a bonded surface or being held by a bonded person.
If the static charge in question is on something that cannot be grounded, i.e. an insulator, then #2 of our ESD Control principles will kick in and insulators must be removed. Per the ESD Standard ANSI/ESD S20.20, “All nonessential insulators such as coffee cups, food wrappers and personal items shall be removed from the EPA.” [ANSI/ESD S20.20 clause 8.3.1 Insulators]
The ESD Standard differentiates between these two options:
If the field measured on the insulator is greater than 2000 volts/inch, keep it at a minimum distance of 12 inches from the ESDs or
If the field measured on the insulator is greater than 125 volts/inch, keep it at a minimum distance of 1 inch from the ESDs.
Aim to keep insulators away from ESDs
“Process-Essential” Insulators
Well, nothing in life is black and white. It would be easy if we were always able to follow the above ESD Control ‘rules’ but there are situations where said insulator is an item used at the workstation, e.g. hand tools. They are “process-essential” insulators – you cannot remove them from the EPA or the job won’t get done.
How do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA?
Here are four ways to reduce the ESD risk of these insulators:
Keep all insulators a minimum of 1 inch or 12 inches from ESDs at all times per recommendation of the ESD Standard.
This reduces the chance of insulators coming in contact with ESDs during workstation processes and assembly.
Replace regular insulative items with an ESD protective version.
There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers, soldering tools, brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench.
Periodically apply Topical Antistat on non-ESD surfaces. After Topical Antistat has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts. It therefore gives non-ESD surfaces electrical properties until the hard coat is worn away.
Neutralization with Ionization If these three options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralization using an ionizer becomes a necessary part of your ESD control program. This allows for control of charged particles that can cause ESD events which we will cover next.
Neutralization
Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionization.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.
Electronic enclosures are process-essential insulators
An example of isolated conductors are conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.
An ionizer creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionization can neutralize static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
The charged ions created by an ionizer will:
neutralize charges on process required insulators,
neutralize charges on non- essential insulators,
neutralize isolated conductors and
minimize triboelectric charging.
Insulators and isolated conductors are common in ESDs – Ionizers can help
For more information on ionizers and how to choose the right type of ionizer for your application, read this post.
Summary
The best way to keep electrostatic sensitive devices (ESDs) from damage is to ground all conductive objects and remove insulators. This is not always possible because some insulators are “process-essential” and are necessary to build or assemble the ESDs.
Insulators, by definition, are non-conductors and therefore cannot be grounded, but they can be controlled to minimize potential ESD damage.
Insulators can be controlled by doing the following within an EPA:
Keep insulators a minimum distance from ESDS at all times (1 or 12 inch minimum distance depending on field voltage measurements of the insulator per ESD Standard recommendation)
Replace regular insulative items with ESD protective versions
Periodically apply a coat of Topical Antistat
Neutralize charges for “process-essential” insulators with ionization
With these steps added to your ESD control process, all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential in an ESD Protected Area (EPA) to reduce the risk of ESD events and ESD damage.
Do your employees handle ESD-sensitive high-end components that are expensive to replace if they failed? If so, reducing the possibility of ESD damage is an important part of an ESD control program. Today’s blog post will look at one option of protecting your critical applications: Dual-Wire Wrist Straps.
Introduction
In an ESD Protected Area (EPA), all surfaces, objects, people and ESD sensitive devices (ESDs) are kept at the same electric potential. This is achieved by using only ‘groundable’ materials that are then linked to ground.
This is in line with the requirements of ANSI/ESD S20.20: “The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
– Training
– Product Qualification
– Compliance Verification
– Grounding / Equipotential Bonding Systems
– Personnel Grounding
– ESD Protected Area (EPA) Requirements
– Packaging Systems
– Marking”
[ANSI/ESD S20.20 clause 7.1 ESD Control Program Plan]
Wrist Straps
Wrist straps are the most common personnel grounding device and are used to link people to ground. They are required if the operator is sitting.
A wrist strap is made up of two components:
A wrist band that is worn comfortably around your wrist and
A coiled cord that connects the band to a Common Grounding Point.
Components of a Wrist Strap
Dual-Wire Wrist Straps
Dual-Wire Wrist Straps have two conductors (compared to single-wire monitors which have only one conductor inside the insulation of the coiled cord). They offer a reduced risk of damaging ESD sensitive devices because if one conductor is severed or damaged, the operator still has a reliable path-to-ground with the second conductor. For that reason, they dual-wire wrist straps are generally used in critical applications.
Compatible with high performance continuous monitors
The MagSnap 360™ Dual-Wire Wrist Strap and Coil Cord – more information
Dual-Wire Continuous Monitors
For maximum benefit, dual-wire wrist straps should be used together with dual-wire continuous monitors. Instead of connecting a coil cord directly to a common grounding point, the operator connects to a continuous monitor. The operator is grounded through the continuous monitor and the operator-to-ground connection is monitored.
The monitors provide operators with instant feedback on the status and functionality of their wrist strap and/or workstation. Continuous monitors detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” which could result in damage to ESD sensitive components. The “intermittent” stage is characterized by sporadic failures as the cord is not completely severed. Once the cord is fully split, the “open” stage is reached.
“Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task.” “While effective at the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge. If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift’s work could be suspect.” “Wrist strap checkers are usually placed in a central location for all to use. Wrist straps are stressed and flexed to their limits at a workstation. While a wrist strap is being checked, it is not stressed, as it would be under working conditions. Opens in the wire at the coiled cord’s strain relief are sometimes only detected under stress.“ [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]
Resistance (or dual-wire) constant monitors are “… used with a two wire (dual) wrist strap. When a person is wearing a wrist strap, the monitor observes the resistance of the loop, consisting of a wire, a person, a wristband, and a second wire. If any part of the loop should open (become disconnected or have out of limit resistance), the circuit will go into the alarm state.” “While the continuity of the loop is monitored, the connection of the wrist strap to ground is not monitored.” “There are two types of signals used by resistance based constant monitors; steady state DC and pulsed DC. Pulsed DC signals were developed because of concerns about skin irritation. However, pulse DC units introduce periods of off time (seconds) when the system is not being monitored.“ [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]
Conclusion
Dual Polarity Technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire systems are used to create redundancy. In critical applications redundancy is built-in to have a backup if the primary source fails. With dual-wire wrist straps the redundancy is there as a protection rather than an alternative. If you are monitoring your dual-wire wrist strap and one wire fails, then the unit will alarm. You will still be grounded by the other wire, so there will be a significantly reduced risk of damaging ESD sensitive components if you happen to be handling them when the wrist strap fails. The wrist strap still needs to be replaced immediately.
And there you have it: dual-wire wrist straps together with dual-wire continuous monitors offer better protection than intermittent monitoring or testing if you have a critical application.
Wrist straps are generally straight forward with what they do and how they work, but when it comes to foot grounders there is still a lot of confusion out there – something we want to address in today’s post. So, let’s get started.
Introduction An Electrostatic Discharge (ESD) flooring / footwear system is an alternative for grounding standing or mobile workers. Sitting personnel are usually grounded via a wrist strap, but this method is not feasible for operators moving around in an ESD Protected Area (EPA).
ESD foot grounders are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic charges from personnel. ESD foot grounders are easy to install and can be used on standard shoes by placing the grounding tab in the shoe under the foot.
Per ANSI/ESD S20.20 Clause 8.2 Personnel Grounding: “For standing operations, personnel shall be grounded via a wrist strap or by a footwear/flooring system meeting the requirements of”:
the total resistance of the Footwear / Flooring system shall be less than 1.0 × 109 ohms
the maximum body voltage generation shall be less than 100 V.
Structure of Foot Grounders Foot grounders discharge static from a person to ground by connecting the person to a grounded walking surface. A conductive ribbon placed inside the wearer’s shoe or sock makes electrical contact with the skin through perspiration. The ribbon is joined to a resistor which limits electrical current should accidental exposure to electricity occur. The other end of the resistor is joined to a conductive sole. The sole contacts a grounded ESD floor mat or ESD flooring system.
Structure of a Foot Grounder
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 and more reliably protect ESD sensitive devices (ESDS).
Installation of Foot Grounders 1. Standard Style Foot Grounders Standard D-ring heel grounders are equipped with an elastic D-ring fastening system which provides adjustable cinching of an ankle strap and allows “flex” during walking. They are designed for use on most types of shoes and boots.
Place the grounding tab in the shoe so that it will lay under the heel. Once heel is repositioned inside tied shoe, tuck excess ribbon material into side of shoe.
Place heel cup onto the shoe. For models with a non-marking interior, install so that the lined cup surface is making contact with the shoe.
Pull the strap through the D-ring and cinch down for snug, comfortable fit.
Test each heel grounder to confirm proper installation.
Installation of Standard Style Foot Grounders – more information
2. Cup Style Foot Grounders Cup Style Foot Grounders are heel grounders designed for use on standard shoes and can be easily adjusted to fit the individual wearer.
Place the foot grounder on the shoe so that the lining is contacting the shoe.
Insert the grounding tab inside of the shoe and under the foot. Make sure that solid contact is made between the sock and body. Cut contact strip to desired length.
Fasten hook and loop straps together, securing the foot grounder firmly on shoe.
Test each foot grounder to confirm proper installation.
3. D-Ring Toe Grounders Toe Grounders with the elastic D-ring fastening system are designed for use with a variety of men’s and women’s shoes including high heels, cowboy boots, flat shoes, loafers and safety shoes.
Insert the grounding tab inside of the shoe and under the foot. Make sure that solid contact is made between the sock and body. Cut grounding tab to desired length.
Place rubber toe material under toe area of shoe sole. Pull hook-and-loop strap over top of shoe and cinch down until snug. Install so that the lined surface is contacting the shoe.
Pull elastic strap around the back of the heel. Adjust D-ring plastic loop for a comfortable fit.
Test each toe grounder to confirm proper installation
4. Disposable Foot Grounders Disposable Foot Grounders are designed for applications where the use of permanent foot grounders are not economical or practical. They are constructed so that it may be used once and then discarded.
Remove shoe. Wipe any excess dirt from underside of heel. Remove release paper from heel grounder.
Apply the adhesive end of the heel grounder to the underside of heel of the shoe. Wrap the tape snugly around the outside of the shoe.
Insert the non-adhesive end of the heel grounder inside the shoe so that the black dot is over the middle of the heel area facing upwards.
Put the shoe on.
Test each foot grounder to confirm proper installation.
NOTE: This product is not recommended for use on equipment with operating voltage.
Advantages of ESD Foot Grounders ESD foot grounders are often preferred over ESD shoes for several reasons:
One size fits many foot sizes, reducing stock holdings and simplifying operations.
ESD foot grounders usually pass the mandatory resistance test as soon as worn, whereas some ESD shoes require a ‘warm-up period’ in order for the operator’s Resistance to Ground (RG) to drop below 35 megohms.
The operator is allowed to wear their own footwear, increasing their comfort in the workplace and not limiting footwear selection to available ESD shoe styles.
Less initial investment cost in comparison to ESD shoes if outfitting all operators in an EPA.
Disadvantages of ESD Foot Grounders ESD foot grounders have a useful life that is dependent on the floor and it’s surface roughness, which can make them seem like they have shorter useful life in comparison to ESD shoes. However, there are a few simple tricks to avoid a quick ‘burn-out’:
We recommend ESD foot grounders only to be used indoors where floors are usually smoother (and where the ESD foot grounder is less likely to become wet, thereby short circuiting the resistor). The rougher the floor the greater the wear.
The manner in which the wearer walks can also affect the life span of the grounder.
In summary, with reasonable care and if used only indoors, ESD heel and toe grounders can last several weeks.
Testing of Foot Grounders Proper testing of your foot grounders involve testing:
the individual foot grounder
the contact strip
the interface between the contact strip
the wearer’s perspiration layer
There are personnel grounding testers on the market designed to properly test foot grounders. For more information, check out our selection chart.
If you obtain a fail reading from the tester you should stop working and test the foot ground and contact strip individually to find out which item has failed. Replace the foot grounder or replace the bad component if possible. Retest the system before beginning work.
Ensure your Foot Grounders are working before handling ESDs
Cleaning of Foot Grounders Foot grounders are used to ground static charges, however dirt provides an insulative layer adversely effecting reliability. For proper operation, the foot grounder and its conductive strip must be kept clean.
The rubber portion of the foot grounder should be cleaned using an ESD cleaner. Ensure that your ESD cleaner is silicone free. This is critical as silicone is an insulator. An alternative would be to clean using isopropyl alcohol. ESD cleaners should not be used to clean the nylon polyester grounding tab. Foot Grounders can be safely hand or machine washed on gentle cycle. Mild detergents, such as Woolite® or a liquid dish washing product used with warm water are recommended for cleaning, however care must be taken to ensure that these detergents are silicone free.
Conclusion
It is recommended that ESD foot grounders are worn on both feet to ensure that a continuous path to ground is maintained at all times (even when lifting one foot).
Contact strips should be tucked inside the shoe with as much contact area as possible to the bottom of the stockinged foot. ESD foot grounders rely upon the perspiration layer inside of the shoe to make contact through the stocking.
Foot grounders must be used with an ESD protected floor system (such as properly grounded ESD floor finish, carpet tiles or floor mats) to provide a continuous electrical path from the user directly to the ESD ground.
A current limiting of one or two megohm resistor in series with the contact strip is recommended but not required.
ESD foot grounders should be tested independently at least daily while being worn to periodically test for proper grounding.
As we have learnt in our last blog post, wrist straps are the most common personnel grounding device to ground operators. In today’s blog post, we will talk about the different types of wrist straps and also explain how continuous monitors can support you in your fight against ESD.
What is a Wrist Strap? A wrist strap is arguably the best way to provide a safe ground connection for the operator to dissipate accumulated static charges with the purpose to prevent dangerous ESD exposure to sensitive ESD components.
Wrist straps must be tested to ensure that they are installed and working properly.
On-demand or “touch” testers have become the most common testing method.
On-demand testers complete a circuit when the wrist strap wearer touches a contact plate.
On-demand testers require a dedicated action by the wearer of the wrist strap to make the test.
Knowing that the wrist strap has failed after the fact may possibly have exposed a highly sensitive or valuable assembly to risk.
Continuous monitors eliminate the possibility of a component being exposed to ESD during the period that the wrist strap was not working properly.
Types of Wrist Straps A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and, in turn, by itself is connected to a known ground point at a workbench or a tool. While a wrist strap does not prevent generation of charges, its purpose is to dissipate these charges to ground as quickly as possible.
A single-wire wrist strap is comprised of one conductive surface contacting the wrist of an operator and providing one electrical connection to ground.
A dual-wire wrist strap has two electrically-separate parts and two separate electrical connections to ground combined in one cord.
For more information on wrist straps, we recommend checking out our last blog post here.
Example of a Wrist Strap
Both types of wrist straps – when in good condition and properly worn – provide equally good connection of operator to ground.
Wrist Strap Monitors Monitoring of single-wire and dual-wire wrist straps is fundamentally different:
Single-wire wrist strap monitors do not have a return signal path; the only physical parameter they can rely on is parasitic capacitance of the operator’s body to ground.
Dual-wire wrist strap monitors measure the resistance of the operator’s wrist between the two halves of the wrist strap.
Single-Wire Wrist Strap Monitoring 1. AC Capacitance Monitors
The first constant monitors developed made use of the fact that a person can be thought of as one plate of a capacitor with the other plate being ground. The ground and the person are both conductors and they are separated (sometimes) by an insulator (shoes, mats, carpet, etc.) thus forming a capacitor. The combined resistance of the wrist strap and person forms a resistor so that the total circuit is a simple RC circuit. A tiny AC current applied to this circuit will cause a displacement current in the capacitance to flow to ground providing a simple way to make sure the person (capacitor), resistor (wrist strap) and coil cord are all hooked up. Any break in this circuit results in a higher impedance that can be used to trigger an alarm.
This technology is still around today and is purchased by some because of its low cost. A big plus of this technology is the ability to use any standard single-wire wrist strap.
2. Wave Distortion Monitors
What the wave form distortion monitor looks at is not the impedance level, but at the waveform generated by the circuit. Current will lead voltage at various points due to the combinations of resistance and capacitive reactance. (There is a negligible amount of inductive reactance from the coil cord.) By monitoring these “distortions” or phase shifts the monitor will determine if the circuit is complete i.e. the wearer is in the circuit and the total equivalent DC resistance is within specifications given a range of installations. Essentially, the unit will monitor the operator by sending a “signature” signal down the coil cord to the operator’s wrist. The operator acts as a load and will reflect that signal back to the monitor with a different signature. The monitor will then compare the reflected signature to its factory pre-set signatures. If the signal is within the “good” range, the operator passes and the monitor will continue its work. If the signature is “not” good, the monitor will go into an alarm-state to warn the operator to stop working and fix the problem.
Dual-Wire Wrist Strap Monitoring A number of issues can come up when using single-wire monitors, such as:
They do not provide a reliable way to know if the total resistance of the circuit is too low, i.e., if the current limiting safety resistor is shorted.
Simple AC capacitance monitors can be tricked into thinking the person is wearing the wrist strap when they are not. For example, laying a wrist strap and cord on a grounded mat will increase the shunt capacitance, which allows the monitor to show a good circuit even with the person out of the circuit. Forming the cord into a tight bundle or stretching it can also provide false readings.
Since the capacitance and therefore the impedance of the circuit will also vary with such things as the person’s size, clothing, shoe soles, conductance of the floor, chair, table mat, the person’s positions (standing or sitting), etc., these monitors often have to be “tuned” to a specific installation and operator.
Dual-wire resistance monitors were developed to overcome some of the problems with the AC capacitance types. By providing a second path to ground (without relying on the capacitor above), we can apply a tiny DC current to measure the DC resistance of the circuit. The monitor will alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted).
If you are monitoring your dual-wire wrist strap and one wire fails, then the unit will alarm.
You will still be grounded by the other wire, so there will be a significantly reduced risk of damaging ESD sensitive components if you happen to be handling them when the wrist strap fails.
The wrist strap would still need to be replaced immediately if a wrist strap fails.
There have been some reports that a constant DC voltage applied to the wristband causes skin irritations. This has been addressed in some models by pulsing the test current and in others by lowering the test voltage.
Conclusion While both single-wire and dual-wire wrist strap monitors help to dissipate accumulated charges on an operator, only dual-wire wrist strap solutions provide assurance of a proper dissipative path from operator to ground. Dual polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards.
Dual-wire continuous wrist strap monitors ensure that the wrist strap is worn properly at all times. These units monitor proper connection of the operator to ground and alarm should this connection fail.
In critical applications, dual-wire systems have redundancy built-in to have a backup if the primary option fails. Two-wire monitors require two wires to create redundancy – this means that the wearer must wear a dual-wire two-conductor wrist strap / coil cord which are more expensive than standard single-wire wrist straps.
A two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement while eliminating the shortcomings of the AC capacitance monitors
For applications where sensitive components are being handled, the ability to guarantee that the wrist strap provides proper dissipation of charges on the operator is critical.
The share of dual-wire wrist straps in sensitive component handling is growing rapidly. Click here to view our range of dual-wire monitors.
Components of a Wrist Strap 
The WS Aware Dual-Wire Workstation Monitor – 
Example of a single-wire capacitance monitor – 
Example of a dual-wire pulsed monitor –