Category Archives: How-To
There is a lot of confusion out there as to what the difference is between resistivity and resistance. We get asked questions on a regular basis so hopefully this post will put an end to any misunderstanding – we’ll explain the difference between the two and will point out the measurements you really need to worry about when it comes to your ESD Control Programme.
The difference between Resistivity and Resistance
“Resistance or resistivity measurements help define the material’s ability to provide electrostatic shielding or charge dissipation.“ [Source]
However, resistance and resistivity values are not interchangeable. Let’s get a bit technical here to illustrate the difference between the two:
1. The resistance expresses the ability of a material to conduct electricity. It is therefore related to current and voltage. In fact, the surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
With a pure resistive material,
– R is the resistance (expressed in Ohm Ω),
– U is the voltage (expressed in Volt) and
– I is the current (expressed in Amp).
The unit of measure for surface resistance is W. 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 Programme compliant to the ESD Standard EN 61340-5-1, it is therefore vital to carry out surface resistance measurements as described in the Standard itself.
2. The surface resistivity of a material describes a general physical property. It is not influenced by the shape of the electrodes used or the distance between them. “Surface resistivity, ρ, can be defined for electric current flowing across a surface as the ratio of DC voltage drop per unit length to the surface current per unit width.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
As Dr. Jaakko Paasi describes in his research note, surface resistivity can be expressed by using a concentric ring probe as
– k is the geometrical coefficient of the electrode assembly,
– rcentre is the outside radius of the centre electrode and
– router is the inside radius of the outer electrode.
For the electrodes recommended by EN 61340-5-1, the coefficient k = 10.
The unit of measure for surface resistivity is W but in practice you will often see W/square (which technically is not a physical unit).
As previously explained, the surface resistivity does not depend on shape or distance of the electrodes used when performing the test. You can compare results freely – no matter what type of electrode was used to get the measurements in the first place.
Converting from Resistivity to Resistance
“Values of surface resistance and surface resistivity become comparable if the measured surface resistance value is multiplied by the geometrical coefficient of the used electrode fixture.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
If you measure surface resistance according to EN 61340-5-1, then the corresponding surface resistivity can be calculated by multiplying the resistance value by the geometrical coefficient factor k = 10. Likewise, surface resistivities can be converted to surface resistances by dividing the surface resistivity value by 10.
Per User guide EN 61340-5-2:1999 Clause 4.1.1 “Point-to-point resistance has been discussed, rather than the surface and volume resistivity which was found in previous standards and reports. This change has been made to cater for non-homogenous materials, which are becoming increasingly common in these applications, as well as ease of measurement.”
Particular care is needed in interpreting results when measuring non-homogeneous materials such as multilayer mats or conductive-backed synthetic fibre carpeting containing a small amount of conductive fibre. Buried conductive layers can provide shunt paths. Be clear when stating what you have measured!
A few notes in regards to measuring surface resistance and resistivity:
- On large surfaces, such as bench-mats, readings will sometimes vary with increasing time of measurement. This is due to the ‘electrification’ of the mat beyond the area measured. It is therefore important to measure properly and to keep the duration of measurement constant. Fifteen seconds is an arbitrary but practical duration for measurement time.
- Moreover, the materials needing to be checked in an EPA are most of the time, non-conductive polymers that have been made conductive or antistatic by addition of conductive particles or by special treatments during manufacture. The resistivity of such materials may vary from one point to another or they may be direction dependent (anisotropic).
- EN 61340-5-1 goes some way to specifying the procedures to be followed and test probes to be used, so that the results can be compared, at least roughly.
- Also, the resistance of some materials may vary with humidity level and temperature. It is therefore good practice to take a note of these two parameters when measuring.
So now that we’ve identified what the difference is between surface resistance and resistivity, there is one more thing we want to cover in today’s post: the different types of surface resistances you will come across when dealing with ESD and how to measure them:
1. Resistance to Ground (Rg)
“Resistance to Ground is a measurement that indicates the capability of an item to conduct an electrical charge (current flow) to an attached ground connection. The higher the resistance in the path, the more slowly the charge will move though that defined path.” [Source]
The Resistance to Ground is measured to ensure that surfaces in an EPA are correctly grounded. This is certainly one of the most useful measurements in an EPA.
Performing a Resistance to Ground Test
To perform the test:
- One 2.3kg cylindrical probe is required for this measurement.
- Connect the probe to a megohm meter and place it on the surface to test.
- Connect the other ohmmeter lead to earth or to an ESD ground point.
- Measure the resistance at 10V for conductive items and 100V for dissipative items.
2. Resistance Point-To-Point (Rp-p)
“A point-to-point measurement used during the qualification process evaluates floor and worksurface materials, garments, chair elements, some packaging items, and many other static-control materials.“ [Source]
Resistance Point-To-Point is used to assess the performance of an item used in an EPA.
Performing a Resistance Point-To-Point Test
To perform the test:
- Two 2.3kg cylindrical probes are required for this measurement
- Connect the probes to a megohm meter.
- Place the material to be tested on an insulative surface such as clean glass and place the probes on the material.
- Measure the resistance at 10V for conductive items and 100V for dissipative items.
- Move the probes so as to measure in a cross direction and repeat the test.
“Point-to-point measurements are important during the qualification process for proper evaluation of flooring and worksurface materials. After installation, the resistance-to-ground measurement is more applicable since it emulates how the material really behaves in practice.” [Source]
3. Volume Resistance (RV)
Although this is one of the less common measurements when it comes to ESD, it’s still worth to mention the volume resistance here. You would measure the volume resistance when a non-grounded item such as a container is to be placed on a grounded item, such as a mat. The volume resistance will indicate whether the item can be used in the desired manner.
Performing a Volume Resistance Test
To perform the test:
- Two 2.3kg cylindrical probes are required for this measurement
- Connect the probes to a megohm meter.
- Put the first probe upside down and ‘sandwich’ the test sample between it and the second probe placed on top.
- Measure the resistance.
So hopefully we have put an end to any confusion in regards to surface resistivity and resistance and answered all your questions. If there is anything else you’d like to know, let us know in the comments.
- Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”
- David E. Swenson, Affinity Static Control Consulting: “Electrical Resistance and Resistivity”
- ESD Association, Inc.: ESD Fundamentals – Part 3: Basic ESD Control Procedures and Materials
We’ve previously learned that the simple separation of two surfaces can cause a transfer of electrons resulting in one surface being positively and the other negatively charged. A person walking across a floor and soles contracting & separating from the floor is such an example. The resulting static charges that generate are an annoying and costly occurrence for office and factory employees. The thing is, they can easily be controlled with existing carpets and tiled floors. Learn how in today’s post.
What is Static Electricity?
Static electricity is an electrical charge that is at rest – as opposed to electricity in motion or current electricity. Static charges can be generated by either friction or induction. Typical examples are the Wimshurst machine that uses friction and the Van de Graaff generator using electrostatic induction.
How is Static Electricity generated?
The most common generation of static charge is the triboelectric charge or the friction electricity developed when rubbing together and then separating two masses. For example, when two blocks are rubbed together and then separated, a triboelectric charge is developed on each block. The two blocks will have opposite polarities; one will be negatively charged and the other will be positively charged. Other examples include:
- Unwinding a roll of tape
- Gas or liquid moving through a hose or pipe
- A person walking across a floor and soles contacting & separating from the floor.
Charge Generation: Unwinding a Roll of Tape
Static Charge Generation from flooring
When a person walks across a carpeted or tiled floor, a triboelectric charge builds up in the body due to the friction between the shoes and floor material. The more you generate, the greater the voltage potential developing in the body – you are basically acting as a capacitor.
Everyone’s capacitance to hold charges is different. However, a sure sign of static presence is hair standing on end or static discharge sparks. Static discharges can be noticed when you touch an object of lower 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 discharged to the lower potential object.
This discharge that can be felt as well as seen, is commonly referred to as an electrostatic discharge, or “ESD”.
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 ESD event can be felt at the human sensation threshold of 3000 volts. If one feels or sees the static shock, it is a minimum of 3000 volts. The potential static charge that can develop from walking on tiled floors is greater than 15,000 volts, while carpeted floors can generate in excess of 30,000 volts.
The problem with ESD
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 electronic manufacturing, repair facilities or 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 programme. The costs involved with static damage not only include the immediate cost of the damaged component but the contributing cost of diagnostic and repair labour that is needed to replace or fix the component. In most cases, the labour involved will far exceed the component cost. If the damaged component performs enough to pass Quality Control (QC), it is called a soft failure as opposed to a hard failure when it does not pass the QC. It is far more expensive for a soft failure occurring at the manufacturer which then leads to a hard failure in the field which escalates product returns and field service cost.
As with any type of control, there are several levels of protection. The method for choosing the necessary degree of ESD protection starts with defining your static sensitivity for electronic components. The ESD Association defines different classes of sensitivity for the HBM (Human Body model) and CDM (Charged Device Model).
ESDS Component Sensitivity Classification
How can you determine the class of sensitivity of the devices within your facility? Look at your product flow through your facility, start at receiving and walk the components or products through until they are at dispatch ready to ship. Chances are, you have several different product flows through your facility. Each flow or loop will have specific components that enter or travel the loop. Make a list of all the sensitive components in each loop and determine the static voltage sensitivity or rating from each of the manufacturers. The lowest voltage sensitivity will dictate the sensitivity class of each loop. The philosophy here is “the chain is only as strong as the weakest link”. Each loop should have the required ESD protection for the most sensitive components that will travel this loop. This will define what class of protection is needed for each loop. You can have different class loops as long as the loops are closed, not allowing other components in. The objective here is to define a static control programme to safeguard your most sensitive component.
ESD control carpet and conventional carpet with antistatic treatments can still generate up to 1,500 volts, far exceeding the class 1 limits for the HBM. These carpets, however, when properly maintained, can provide safe grounding and electrostatic propensity below the class 2 and 3 sensitivity range.
Proper maintenance for ESD control carpets is rather simple but very important. For conventional carpets that are treated with a topical antistat or other treatment, it is required that the treatment is replenished on the carpet as it wears away due to foot traffic. The amount of treatment on the carpet can be determined by testing with a surface resistance meter. The higher the resistance readings of the floor, the lower the amount of static control treatment that is present on the carpet. The level of treatment should be monitored by resistance readings and kept between 1 x 106 and 1 x 1010 ohms. Some ESD floor finishes can be used as a carpet treatment. This requires a simple spray bottle filled with 50/50 mix of ESD control floor finishes and water. Always check with the floor finish manufacturer before use. Application of diluted floor finish usually requires a 1 to 2 spray coat on the carpet depending on the level of resistance you want.
Reztore® Topical Antistat – for more information click here
ESD control carpets are made with static dissipative yarn and only require that the yarn is kept clean and free of insulative dirt, dust and spray cleaners.
ESD control floor tiles can also generate triboelectric charges depending on the construction of the tile. The tile (dissipative or conductive) may have voids between the impregnated conductive sections which allows triboelectric charges to be generated and then drained. This cyclic voltage can be very harmful to sensitive components.
ESD control floor finishes alone can provide both non-triboelectric charging as well as a path to ground. Such floor finishes can be applied on many surfaces including sealed concrete, vinyl tile and especially ESD control tiles. If the ESD control tile is generating triboelectric charges, ESD control floor finish will complement these tiles with its non-triboelectric properties, as well as enhancing the surface’s electrical properties. The ease of maintenance for an ESD control floor finish is another benefit when used on top of any tile floor, especially carbon impregnated conductive tile that may form streaks of black carbon on the surface.
Statguard® Floor Finish – for more information click here
The best static controls are not only the ones that protect sensitive components and equipment but are: A) at hand and readily available, B) easily maintained. For these reasons, carpets and tile floors should not be overlooked as sources for static control. Existing carpet or tile floors can be easily included into an ESD control programme.
Each component in an ESD protected area (EPA) plays a vital part in the fight against electrostatic discharge (ESD). If just one component is not performing correctly, you could harm your ESD sensitive devices potentially costing your company a lot of money. The problem with many ESD protection products is that you can’t always see the damage – think wrist straps! By just looking at a coiled cord, you can’t confirm it’s working correctly; even without any visible damage to the insulation, the conductor on the inside could be broken. This is where periodic verification comes into play.
ESD protected area (EPA) products should be tested:
- Prior to installation to qualify product for listing in user’s ESD control plan.
- During the initial installation.
- For periodic checks of installed products as part of IEC 61340-5-1 Edition 2 2016 clause 5.2.3 Compliance verification plan.
“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.” [IEC 61340-5-1:2016 clause 5.2.4 Compliance verification plan]
Components of a Verification Plan
As outlined in the User Guide 61340-5-2:2008, each company’s verification plan needs to include:
1. A list of items that are used in the EPA and need to be checked on a regular basis
This would include ESD working surfaces, personnel grounding devices like wrist straps or foot grounders, ionisers etc. It is recommended to create a checklist comprising all ESD control products: this will ensure EPAs are checked consistently at every audit.
2. A schedule specifying what intervals and how each item is checked
The test frequency will depend on a number of things, e.g. how long the item will last, how often it is used or how important it is to the overall ESD control programme.
As an example: wrist straps are chosen by most companies to ground their operators; they are the first line of defence against ESD damage. They are in constant use and are subjected to relentless bending and stretching. Therefore, they are generally checked at the beginning of each shift to ensure they are still working correctly and ESD sensitive items are protected. Ionisers on the other hand are recommended to be checked every 6 months: whilst they are in constant use, they are designed to be; the only actual ‘interaction’ with the user is turning the unit on/off. If however, the ioniser is used in a critical clean room, the test frequency may need to be increased.
It is recommended that Wrist Straps are checked before each shift
The user guide offers a solution: “Some organizations may want to increase the time between verifications of an ESD control item after it has been in use for a period of time. This is typically done by monitoring the failures of the ESD control item. Once the organization has evidence that there is an acceptable period of time where no failures were found, the time between verifications can be increased. The new verification interval is then monitored. If an unacceptable level of failures is identified, then the verification frequency should revert back to the previous level.” [User Guide 61340-5-2:2008 clause 4.3.3 Verification frequency]
The industry typically uses 2 types of verification to achieve maximum success: visual and measurement verification. As the name suggests, visual verification is used to ensure ESD working surfaces and operators are grounded, ESD flooring is in good shape or wrist straps are checked before handling ESD sensitive items.
Actual measurements are taken by trained personnel using specially designed equipment to verify proper performance of an ESD control item.
3. The suitable limits for every item used to control ESD damage
IEC 61340-5-1:2016 contains recommendations of acceptable limits for every ESD control item. Following these references reduces the likelihood of 100V (HBM) sensitive devices being damaged by an ESD event.
Please bear in mind that there may be situations where the limits need to be adjusted to meet the company’s requirements.
4. The test methods used to ensure each ESD product meets the set limits
Tables 1 to 3 of IEC 61340-5-1:2016 list the different test methods a company has to follow. If a company uses other test methods or have developed their own test methods, the ESD control programme plan needs to include a statement explaining why referenced standards are not used. The company also needs to show their chosen test methods are suitable and reliable.
It is recommended that written procedures are created for the different test methods. It is the company’s responsibility to ensure anybody performing the tests understands the procedures and follows them accordingly.
5. The equipment used to take measurements specified in the test methods
Every company needs to acquire proper test equipment that complies with the individual test methods specified in Tables 1 to 3 of IEC 61340-5-1:2016. Personnel performing measurements need to be trained on how equipment is used.
6. A list of employees who will be performing the audits
Part of the verification plan is the choice of internal auditors. A few suggestions for the selection process:
- Each induvial is required to know the ESD Standard IEC 61340-5-1 AND the company’s individual ESD programme.
- It is essential that the selected team member recognises the role of the ESD control programme in the company’s overall quality management system.
- It is recommended that each nominated worker has been trained on performing audits.
- The designated employee should be familiar with the manufacturing process they are inspecting.
7. How to deal with non-compliance situations
Once an audit has been completed, it is important to keep everyone in the loop and report the findings to the management team. This is particularly vital if “out-of-compliance” issues were uncovered during the audit. It is the responsibility of the ESD coordinator to categorise how severe each non-conformance is; key problems should be dealt with first and management should be notified immediately of significant non-compliance matters.
Results of audits (especially non-compliance findings) are generally presented using charts. Each chart should classify:
- The total findings of the audit
- The type of each finding
- The area that was audited
It is important to note that each company should set targets for a given area and include a trend report. This data can assist in determining if employees follow the outlined ESD control programme and if improvements can be seen over time.
Here is an example of a Verification Plan using ESD flooring for demonstration purposes. A few notes:
- Our sample company has 2 different areas where ESD floor matting is used: the packaging area and the main EPA.
- Flooring is not used for grounding personnel handling ESD sensitive items
- Our sample company has established that the limits outlined in the standard are suitable for their internal requirements.
Bear in mind that ALL your ESD control items need to be included in your verification plan. So if your company uses wrist straps, smocks, chairs, gloves etc. then ALL of them have to be listed as part your ESD control programme.
So we’ve talked about various components in an EPA before (e.g. wrist straps, bags, tape and so on). But there is one important part we have not covered yet: ESD protective working surfaces. So let’s change that right now!
Definition of an ESD protective working surface
ESD protective working surfaces aid in the prevention of damage to ESD sensitive items (ESDS) and assemblies from electrostatic discharge.
ESD working surfaces, such as mats, are typically an integral part of the ESD workstation, particularly in areas where hand assembly occurs. The purpose of the ESD working surface is two-fold.
- To provide a surface with little to no charge on it.
- To provide a surface that will remove ElectroStatic charges from conductors (including ESDS devices and assemblies) that are placed on the surface.
Types of ESD protective working surface
When deciding to invest in ESD protective working surface, you have the choice of ESD matting (laid-out on a standard non-ESD bench) or ESD benches. Performance-wise there is no difference so what option you go for depends solely on your personal preference.
Examples of ESD protective working surface matting – for more details click here
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.
Grounding your ESD protective working surface
Whatever your choice, your ESD protective working surface needs 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 working surface 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.
Example of an ESD Protective Working Surface – 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]
Working at an ESD protective working surface
Operators should ensure that the working surface is organised 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.
An operator installing an ESD protective working surface mat
Insulators can be a considerable threat to your products. Remember that an insulator cannot be grounded so it will retain its charge for a long time. Removing all non-essential insulators from the ESD protective workstation is an important rule. If not, your company’s investment in the grounded ESD working surface may be wasted.
“The biggest threat is Field Induced Discharges, which can occur even at a properly grounded ESD working surface. If an ESDS is grounded in the presence of an ElectroStatic charge, instead of the ESDS having charges removed from it, the ESDS may become charged with a voltage induced on it. Then, when placed on the grounded ESD work surface, a discharge occurs. If the ESDS is removed from the presence of the ElectroStatic charge and grounded again, a second discharge may occur.“ (Ref. ESD Handbook, ESD TR20.20, section 2.7.5).
Maintaining your ESD protective working surface
The ESD working surface must be maintained and should be cleaned with an ESD cleaner. Regular cleaners typically contain silicone and should never be used on an ESD working surface.
Example of an ESD cleaner – click here for more information
The ESD control plan should require testing of the resistance to ground periodically. For more information on testing your ESD working surface, check this post. However, the operator should be on guard every day and check visually that the ground wire is attached.
So you’ve identified ESD sensitive items in your factory and you realise that you need to implement ESD Control measures. But where do you start? There is so much information out there and it can be completely overwhelming. Don’t panic – today’s blog post will provide you with a step-by-step guide on how to set-up a suitable ESD Control Plan.
“The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
• compliance verification
• grounding / equipotential bonding systems
• personnel grounding
• EPA requirements
• packaging systems
• marking” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.1 ESD control program plan]
“Each company has different processes, and so will require a different blend of ESD prevention measures for an optimum ESD control program. It is vital that these measures are selected, based on technical necessity and carefully documented in an ESD control program plan, so that all concerned can be sure of the program requirements.” [EN 61340-5-1 Edition 1.0 2007-08 Introduction]
1. Define what you are trying to protect
A prerequisite of ESD control is the accurate and consistent identification of ESD susceptible items. Some companies assume that all electronic components are ESD susceptible. However, others write their ESD control plan based on the device and item susceptibility or withstand voltage of the most sensitive components used in the facility. A general rule is to treat any device or component that is received in ESD packaging as an ESD susceptible item.
An operator handling an ESD susceptible item
2. Become familiar with the industry standards for ESD control
A copy of EN 61340-5-1:2007 can be purchased from British Standards: “BS EN 61340-5-1:2007 applies to activities that: manufacture, process, assemble, install, package, label, service, test, inspect, transport or otherwise handle electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 V human body model (HBM). BS EN 61340-5-1 provides the requirements for an ESD control program. The user should refer to IEC 61340-5-2 for guidance on the implementation of this standard.
3. Select a grounding / equipotential bonding system
The elimination of differences in potential “can be achieved in three different ways:
- grounding using protective earth:
the first and preferred ESD ground is protective earth if available. In this case, the ESD control elements and grounded personnel are connected to protective earth;
- grounding using functional ground:
the second acceptable ESD ground is achieved through the use of a functional ground. This conductor can be a ground rod or stake that is used for grounding the ESD control items in use at a facility. In order to eliminate differences in potential between protective earth and the functional ground system it is highly recommended that the two systems be electrically bonded together;
- equipotential bonding:
in the event that a ground facility is not available, ESD protection can be achieved by connecting all of the ESD control items together at a common connection point.” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.3.1 Grounding/equipotential bonding systems]
Example of a grounding/equipotential bonding system
4. Determine the grounding method for operators (Personnel Grounding)
The two options for grounding an operator are:
- a wrist strap or
- foot grounders/footwear.
In some cases, both (wrist strap and foot grounders) will be used.
5. Establish and identify your ESD Protected Area (EPA)
ESD Control Plans must evolve to keep pace with costs, device sensitivities and the way devices are manufactured. Define the departments and areas to be considered part of the ESD Protected Area. Consider if customers and/or subcontractors should be included. Implement access control devices, signs and floor marking tape to identify and control access to the ESD Protected Area.
Example of an ESD Protected Area including signs and floor marking tape
6. Select ESD control items to be used in the EPA based on your manufacturing process
Elements that should be considered include: working surfaces, flooring, seating, ionisation, shelving, mobile equipment (carts) and garments. Check-out this post for more information.
7. Develop a Packaging (Materials Handling & Storage) Plan
When moving ESD susceptible devices outside an ESD protected area, it is necessary for the product to be packaged in an enclosed ESD Shielding Packaging.
8. Use proper markings for ESD susceptible items, system or packaging
From EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.1: “The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include: …marking”.
If you are handling ESD sensitive devices, there are 3 symbols you need to know.
The ESD Susceptibility (left) and ESD Protective Symbol (right)
9. Implement a Compliance Verification Plan
Developing and implementing an ESD control programme is only the first step. The second step is to continually review, verify, analyse, evaluate and improve your ESD programme.
“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 must occur. The compliance verification plan must document the test methods used for process monitoring and measurements… 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.” [EN 61340-5-1 Edition 1 2007-08 clause 5.2.3 Compliance verification plan]
Regular programme compliance verification and auditing is a key part of a successful ESD control programme.
10. Develop a Training Plan
“The training plan shall define all personnel that are required to have ESD awareness and prevention training. At a minimum, initial and recurrent ESD awareness and prevention training shall be provided to all personnel who handle or otherwise come into contact with any ESDS [ESD sensitive] items. Initial training shall be provided before personnel handle ESD sensitive devices. The type and frequency of ESD training for personnel shall be defined in the training plan. The training plan shall include a requirement for maintaining employee training records and shall document where the records are stored. Training methods and the use of specific techniques are at the organization’s discretion. The training plan shall include methods used by the organization to ensure trainee comprehension and training adequacy.” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.2 Training Plan]
11. Make the ESD Control Plan part of your internal quality system requirements
A written ESD Control Plan provides the “rules and regulations”, the technical requirements for your ESD Control Programme. This should be a controlled document, approved by upper management initially and over time when revisions are made. The written plan should include following:
- Qualified Products List (QPL): a list of EPA ESD control items is used in the ESD control Plan
- Compliance Verification Plan: includes periodic checking of EPA ESD control items and calibration of test equipment per manufacturer and industry recommendations.
- Training Plan: an ESD Programme is only as good as the use of the products by personnel. When personnel understand the concepts of ESD, the importance to the company of the ESD control programme and the proper use of ESD products, they will implement a better ESD control programme improving quality, productivity and reliability.
A video making the rounds on Social Media this week highlights a very common problem in ESD Protection: dry air.
The report shows energy minister Simon Bridges walking around his office spraying the floor with water, applying tape to door handles and using an antistatic mat at his desk. The reason? Simple: a new carpet which was installed at the Beehive and the “little tickles” ministers are getting as a result.
Simon Bridges spraying the floor in his office with water (Source)
Jim Robb who is in charge of maintaining the parliament building provides the following explanation: “We’re in the middle of winter, we’ve got cold polar air, there’s no moisture in it and it’s a very common problem, I’ve been dealing with this for thirty years.”
While the ministers at the Beehive seem to have figured out solutions to avoid the “zap”, we want to show you how you can solve the issue of dry air in your EPA.
Impact of relative humidity on ESD
Just like the ministers in the above report, many people will notice a difference in the ability to generate static electricity when the air gets dryer (the % RH decreases).
Relative humidity (RH) directly affects the ability of a surface to store an electrostatic charge. As RH increases, the time a surface will hold a charge will decrease and the dissipation rate will increase.
An example: walking across a carpet can yield a charge of 35kV at 10% RH (very dry air) but will drop significantly to 7.5kV at 55% RH. In an electronics manufacturing environment lower humidity may result in lower output from production due to an increase in ESD events during manufacturing processes.
A normal range for humidity in an electronics manufacturing environment is be 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.
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 sn electronics manufacturing environment such as corrosion, soldering defects and the popcorn effect on moisture sensitive devices.
The popcorn effect (Source)
Dealing with dry air in an EPA
While not a replacement for grounding, shielding or ESD working surfaces, ionisation can mitigate ESD events in areas where dry air is normal. For more information on ionisation and the different types of ionisers, we recommend reading these two posts:
In summary, worksurface ionisers (Bench Top and Overhead Ionisers) produce positively and negatively charged ions that are moved to the controlled area with fan driven airflow. Point-of-use air ionisers use compressed gas to combat electrostatic attraction neutralising charges on particles causing contamination or visual defects on products.
Using a point-of-use air ioniser
The ESD Standard on Relative Humidity
The ESDA’s TR20.20-2008 discusses RH in a few different places. Some significant statements are listed below.
ESD Handbook ESD TR20.20-2008 section 2.3 Nature of Static Electricity:
“The moisture level in the air, or relative humidity in the environment, are important considerations in the liberation and accumulation of static electricity. It is well known that static electricity in the form of static cling and static shocks are more prevalent when the air is dry. Heating interior air in the winter months dries out the already dry air in the higher latitudes. Static charge accumulation is easier on dry materials since moisture on surfaces tends to allow charges to slowly dissipate or recombine.
However, it is impractical to use humidity control alone to provide static control since static charges are developed even at relative humidity levels of 90% and greater. For most situations, 30 to 70% RH is considered the appropriate range. Special areas, such as wafer fabrication, may require lower humidity control for processes that are affected by moisture (e.g., photoresist application). Soldering is known to be affected by high relative humidity conditions (>70%). For areas that have low ambient humidity, ionization is an important consideration to aid in reducing charge accumulation levels and provide neutralization of charges after they are developed but before they can cause difficulties.”
ESD Handbook ESD TR20.20-2008 section 5.3.16 Humidity:
“Humidity is beneficial in all ESD control program plans. Contact and separation of dry materials generates greater electrostatic charges than moist materials because moisture provides conductivity that helps to dissipate charge. For this reason, ESD effects are most noticeable in the winter since heating systems reduce building environment moisture. Geographic location (desert vs. coastland) is also a major contributor to ambient conditions inside buildings. Any circumstance that results in a low relative humidity will permit a greater accumulation of electrostatic charges. Relative humidity above 30% in ESD protective areas is desirable as long as other adverse conditions are not created as a result of humidity levels. Generally speaking an upper limit of 70% is desirable to prevent corrosive effects on the metal portions of electronic devices and assemblies.
Besides the increasing propensity to generate electrostatic charges on dry materials in general, performance of many ESD protective materials degrade. In fact, when exposed to low humidity conditions, some ESD protective materials become totally ineffective or become sources of electrostatic charges. Therefore, evaluation of ESD control materials should include performance testing in controlled environments at the lowest expected operating relative humidity level. Manufacturers of ESD protective materials should be able to provide performance data in regards to relative humidity. Likewise, materials should be tested in moderate humidity conditions as well to ensure they do not become “too conductive” and present a potential safety hazard to personnel working with substantial voltages. See the Personnel Safety section of this handbook for further guidance in this area.
Humidity control in factories or physically large areas or buildings can be difficult and expensive. In smaller rooms or areas, it may be possible to use portable humidifiers to raise the immediate area humidity. However, in large facilities and factories the environmental systems many need to include steam generation and monitoring equipment to control humidity. This type of equipment is expensive to install and purchase especially in pre-existing facilities. To reduce the total cost impact, companies should consider the need for humidification equipment when planning new facility construction.”
ESD TR20.20-2008 can be purchased directly from the ESD Association.
Most people tend to believe that if a person is wearing a wrist strap, an ESD lab coat (also known as smocks) is redundant. This is due to the belief that any charge on the person or their clothes would find its way to ground via the wrist strap. This is a very common misconception and today’s blog post will explain in more detail why you should be considering the use of ESD lab coats in your ESD Protected Area (EPA).
Purpose of ESD lab coats
Although the ESD Standard does not require ESD lab coats, they are a very practical. Some even believe, ESD lab coats represent the single most important step to demonstrate commitment to an ESD control programme.
As we have learnt previously, all process essential insulators should be kept at a minimum distance of 31cm from ESD susceptible items. Clothing, particularly when made from synthetic fibres, are significant charge generators. Worse for ESD control, the fabric is an insulator so the result can be very threatening: an isolated charged insulator which cannot be grounded.
An insulator will not let charges flow and will therefore hold the charge until either neutralised over time (naturally over hours or days) or with an air ioniser (artificially under a few seconds).
In the meantime, your sleeves, waist, etc. may have several thousand volts (a very significant electric field to expose nearby conductors) that may induce charges on nearby isolated conductors. This is the main reason people wear ESD lab coats: so they can shield the insulative clothing and minimise the electric fields generated from their clothing.
Examples of lab coats – for more details click here
“The ESD risk provided by everyday clothing cannot be easily assessed. The current general view of experts is that the main source of ESD risk may occur where ESDS [ESD sensitive items] can reach high induced voltage due to external fields from the clothing, and subsequently experience a field induced CDM [Charged Device Model] type discharge. So ESD control garments may be of particular benefit where larger ESDS having low CDM withstand voltage are handled, and operators habitually wear everyday clothing that could generate electrostatic high fields.” [CLC TR 61340-5-2 2008 User guide Garments clause 22.214.171.124 Introductory remarks]
ESD lab coat properties
Most lab coats are constructed of a dissipative material which incorporates texturised polyester and carbon nylon fibres. The conductive nylon fibres are woven in a chain-link design throughout the material, providing continuous and consistent charge dissipation.
ESD lab coats are an ESD protective product that should possess the following ESD control characteristics:
- Antistatic low-charging so they minimise the generation of electrostatic charges;
- Dissipative so when grounded they will remove charges to ground;
- Shielding creating a “Faraday Cage” effect so they will restrict charges generated on the user’s clothing to the inside of the ESD lab coat and
- Groundable so the user can easily and reliably connect them to ground.
Installation and grounding of ESD lab coats
Follow the directions below for proper installation and grounding of the ESD lab coat:
- Put on the lab coat and fasten all of the snaps on the front of the lab coat, making sure that clothing is not exposed outside of the lab coat.
- Throughout use, it is essential that the conductive cuff is in intimate contact with the wrist skin. The conductive cuff should never be allowed to be pulled up and over the shirt sleeve.
- Ground the ESD lab coat. A popular way to ground an ESD lab coat is with a coiled cord either attached to a snap on the waist area of the lab coator via a wrist strap snapped to the inside cuff of an ESD lab coat. If none of these methods are suitable, the lab coat should be grounded via the person’s wrist removing charges via ESD footwear to ESD protected flooring.
Wearing your ESD lab coat correctly
“Garments on which high levels of static electricity can be generated are one of the causes of ESD damage. It is important that such charged garments do not come into contact with ESDS. The covering garments need to be grounded, either through direct contact with the wearer’s skin, or by alternative means such as being electrically connected to a wrist strap. It is important that the ESD protective garment sleeves cover the end of the inner garment sleeves.” [EN 61340-5-2 paragraph 5.2.5.]
Grounding a lab coat using the snap at the waist
ESD lab coats are a conductor and therefore should be grounded. If not grounded, the ESD garment can be a potentially threatening isolated charged conductor. If an operator is wearing a lab coat but is not electrically connecting the lab coat to either their body’s skin or ground, then charges on the lab coat may have nowhere to go or discharge to.
Testing of ESD lab coats
Panel-to-panel conductivity is essential to ensure portions of the lab coat are not left as isolated charged conductors. A Resistance Test Kit can quickly measure resistance of the fabric and ensure panel-to-panel conductivity by placing five pound electrodes on different fabric panels.
Testing panel-to-panel conductivity using 222635
To ensure that the fabric is low tribocharging, a Static Field Meter can be used to measure charges generated by causing contact and separation with other materials. In addition, the Static Field Meter can demonstrate shielding by measuring a charged object and then covering the charged item with the ESD lab coat. Being shielded the measured charge should be greatly reduced.
Cleaning of ESD lab coats
The proper method to clean a lab coat is to wash the garment in cool or warm water, tumble dry with low heat or hang dry. Do not bleach your ESD lab coats! Make sure you only use non-ionic softeners and detergents when laundering.
Please also note that lab coats should not be altered in any way. The lab coats effectiveness is in fully covering the human body and street clothes – especially at the wrists and front of the body. Altering the lab coat in any way will nullify its effectiveness.
The typical useful and effective life of a lab coat under normal wearing and recommended washing conditions is a minimum of 75 washings.
Questions for you: Do you use lab coats? If so, what’s the reason you started using them?
We’ve mentioned the term “ESD Protected Area (EPA)” many times in previous blog posts but what exactly is it?
- Why do you need an EPA?
- How can you identify one?
- And most importantly what do you need to create an EPA?
This posts will help shed some light on these very common questions so let’s go!
Definition of an ESD Protected Area (EPA)
An ESD Protected Area (EPA) 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 (i.e. materials with an electrical resistance typically of less than 109 ohms) for covering of surfaces and for the manufacture of containers and tools. All surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Movable items (such as containers and tools) are bonded by virtue of standing on a bonded surface or being held by a bonded person. Everything that does not readily dissipate charge must be excluded from the EPA.
Example of an EPA
The user guide CLC/TR 61340-5-2:2008 defines an EPA as follows:
“An ESD protected area (EPA) is an area that is equipped with the ESD control items required to minimize the chance of damaging ESD sensitive devices. In the broad sense, a protected area is capable of controlling static electricity on all items that enter that work area. Personnel and other conductive or dissipative items shall be electrically bonded together and connected to ground (or a common connection point when a ground is not available) to equalize electrical potential among the items. The size of an EPA can vary greatly. A protected area may be a permanent workstation within a room or an entire factory floor encompassing thousands of workstations. A protected area may also be a portable worksurface or mat used in a field service situation.” [CLC/TR 61340-5-2:2008 Use guide clause 4.6 Protected areas (EPA)]
An EPA could be just one workstation or it could be a room containing a number of different workstations.
So to sum-up, in an EPA you:
- ground all conductors (including people),
- remove all insulators (or substituting with ESD protective versions) or
- neutralise process essential insulators with an ioniser.
Purpose of an ESD Protected Area (EPA)
Electrostatic Discharge [ESD] can damage components and products containing electronics. It is the hidden enemy in many high-tech factories. Often this damage cannot be detected by quality control inspections which can be very frustrating. ESD damage can adversely impact productivity, quality, product reliability and thus a company’s reputation and profitability.
An EPA is an area that has been established to effectively control ESD and its purpose is therefore to avoid all problems resulting from ESD damage. Operators need to understand and follow the basics of ESD control to limit the generation of electrostatic charges, limit and slow discharges in the EPA.
Identifying an ESD Protected Area (EPA)
An EPA needs to be identified as such. You can use products such as floor marking tape and/or signs which are designed to attract attention and deliver a clear message to personnel and visitors, i.e. “You are entering an ESD Protected Area” or “You are leaving an ESD Protected Area”.
Example of an EPA Warning sign
Creating an ESD Protected Area (EPA)
In its simplest form, an EPA area is a basic workstation and consists of the following components:
- an ESD working surface mat,
- a grounding cord,
- a wrist strap,
- a coiled cord and
- an Earth Bonding Point Plug (EBP Plug).
Set-up of a basic EPA – watch the video
To create an EPA:
- Bond the operator to the EBP Plug using the wrist strap and coiled cord.
- Connect the ESD working surface mat to the EBP Plug using the grounding cord.
By following the above two steps, each element connected to the EBP Plug (the surface and the operator) are kept at the same electrical potential and any electrostatic discharge (ESD) is being removed to ground via the EBP Plug. The EBP Plug provides a common ground point for grounding using protective earth. The plug fits into the mains supply socket, making a connection with the earth conductor only. In place of the live and neutral pins are moulded insulating plastic pins to allow positive location in the socket.
Do you have any questions for us? Let us know in the comments!
Today’s blog post is a continuation of last week when we covered the basics of wrist straps – what they are, what different styles there are etc. Missed the first post? Catch-up here!
Today we’ll be taking a closer look at queries we receive in connection with using wrist straps. So without further ado, let’s jump right in!
What is the 1 megohm Resistor for?
The purpose of the 1 megohm resistor found in series with wrist straps is solely to provide safety to the human body by limiting the amount of current that could be conducted through the body. The 1 megohm resistor is designed to limit the current to 250 microamps at 250 Volts rms AC. This is just below the perception level (and a bit before the nervous system goes awry) of most people. Physical perception of current traveling in/on the body varies depending on size, weight, water content, skin conditions, etc. Remember that the termination of the coil cord with the 1 megohm resistor must always be connected to the operator.
Can I connect to ground via a bench mat?
Many wrist strap users connect the wrist cord to a stud on their ESD protective mat. This process is not recommended as it can increase the total system resistance to ground to over the 35 megohm limit required by EN 61340-5-1 table 1.
It is recommended to connect a wrist strap to an Earth Bonding Point (EBP)
Can I buy a wireless wrist strap?
Passive “wireless” or “cordless” wrist straps have severe limitations. Assuming you were tribocharged to 10 KV and wearing the “wireless” wrist strap, it would take many hours (days even depending on the ambient relative humidity) to get you below 5 KV, nevertheless 10 Volts. Most (if not all) of the charge reduction would be due to natural recombination of the charges on your skin with the air molecules and the natural conductance of the air through water vapour content.
At this point there is no “cordless” wrist strap system on the market that works at all. In fact “cordless” wrist straps will damage ESDS devices because you will not be grounding your body.
Do I need to wear a wrist strap on both hands?
Good question, since the concept of grounding an individual is not very intuitive. The skin is one of the largest organs of the human body. The resistance of the human body is averaged to 1,500,000 ohms (or 1.5 x 103 ohms) from the ESD-STM5.1 [ESDA Standard for Human Body Model]. The resistance of an individual may vary from 1 kilohm to over 1 megohm. In either case, the skin is conductive in the sense it can conduct electrical current. Therefore, since the skin is (for the most part) continuous, i.e., the ball of your foot is electrically connected to your index finger, then grounding the skin at any point will in fact ground all of the skin. So you can in good conscious say that if you properly wear one grounded wrist strap, then both hands are grounded as well as other exposed skin areas.
Do wrist straps need to be tested?
Yes, wrist straps need to be checked regularly to ensure they are faultless and ground the operator properly. 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) and the interface with the operator’s skin.
Wrist straps need to be checked before each use. Periodic testing is not required if continuous monitors are used. They provide instant feedback should the wrist strap fail while handling ESD sensitive devices.
Example of a Wrist Strap Tester
What should I do if I fail my wrist strap test?
If the wrist strap tester outputs a FAIL test result, stop working. Test the wrist band and cord individually to find out which item is damaged. There are some methods to troubleshoot your wrist straps. First make sure your tester is properly adjusted and calibrated.
If the operator and wrist strap system fails low:
- Make sure that the person is not directly connected to ground via another path, i.e., touching a grounded metal structure.
- The most common cause of a fail low is a shorted resistor in the wrist strap coil cord. Replace the coil cord with a new one and repeat the test.
If the operator and wrist strap system fails high:
- Make sure the coil cord has a secure connection both the banana jack/socket to tester and the stud snap to wrist strap buckle.
- Ensure there is continuity in the coil cord (you can test with an ohmmeter).
- Remove the wrist strap and hold the bottom part of the band tightly between the operator’s thumb and index finger and test. If the test fails high, the band may be soiled and needs cleaning or the buckle to band connection may be suspect. Either replace the band or clean and then retest.
- If the above test is okay, then the skin of the operator’s wrist may be too dry. Apply ESD lotion to the wrist to re-moisturise the skin thereby increasing its conductivity. Retest. Operators with dryer skin should wear metal banded wrist straps to minimise the contact resistance. If their skin is very dry, application of an ESD lotion may be required as part of their donning process.
You need to obtain a PASS test result before beginning work.
Did we miss any question(s)? Let us know in the comments!
We get a lot of inquiries in regards to wrist straps: what they do, why the different types, how they are used, etc. So we thought it might be helpful for you to collect the most common queries in a Q&A style blog post. There is quite a bit of ground to cover in regards to wrist straps so to make it easier on your eyes and head, we spread the information over 2 blog posts. The first post will cover the basics from what a wrist strap is to introducing the different styles. The second post will focus on the correct use of wrist straps. Let’s get started!
What is a wrist strap?
A wrist strap is made up of two components:
- a wrist band that is worn comfortably around your wrist and
- a coil cord that connects the band to an Earth Bonding Point (EBP).
Wrist band and coil cord of a wrist strap
Why do I need a wrist strap?
In an ESD Protected Area (EPA) all surfaces, objects, people and ESD sensitive devices (ESDs) are kept at the same potential. This is achieved by simply using only ‘groundable’ materials that are then linked to ground. 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 (see question below).
Are wrist straps necessary?
Wrist straps are not necessary if an operator is wearing two foot grounders on a conductive grounded floor and doesn’t lift both heels/toes at the same time. As some people lift both feet off the ground while seated, wrist straps are essential for sitting personnel.
Does a wrist strap work if it’s near your forearm?
Yes. The key to the wrist strap is the intimate contact of the conductive band to your skin and of course the coil cord connecting it to ground. It doesn’t matter if the contact point to your body is on you wrist, finger, forearm, ankle, etc., as long as it is in direct contact with your skin. The skin is electrically continuous over you whole body. The wrist is just a convenient place to couple the band to.
What styles of wrist bands are available and how do they differ in their effectiveness?
Operators can choose between elastic and metal wrist bands:
- Elastic wrist bands are the most popular wrist band as they are comfortable to wear and easy to adjust. Compared to metal wrist bands they are also less expensive.
- Some people prefer metal wrist bands as they are generally longer lasting and easier to clean.
The key to personal grounding is to have an adequate path to ground so that there is never a potential difference with respect to ground on the human body for longer than 150 milliseconds (ms) body movement time. Such rapid grounding is accomplished well by elastic or metal wrist straps. So in terms of their effectiveness to protect against ESD, there is no difference between elastic and metal wrist bands.
What size wrist band do I need?
Both elastic and metal wrist bands are (to a certain degree) adjustable. Metal wrist bands offer less adjustment, so you will find those are generally available in different sizes depending on the circumference of your wrist. However, you are still able to adjust metal wrist bands if you needed a tighter/looser fit.
To adjust your wrist band, follow the below steps:
1. Elastic wrist bands:
- Place the wrist band on the wrist.
- Pull the “tail” of material that extends out from the clasp to tighten the elastic material until the wrist band fits snugly but comfortably.
Adjusting an elastic wrist band
2. Metal wrist bands:
- Insert the link end of the wrist band into the slotted opening on the cap. Insert it at a downward angle to allow the links to slide inside the channel in the backplate.
- Change the size of the band by sliding the links in or out of the stainless steel backplate. For extra small you can cut off excess links with cutters.
- Lock the links into place by pulling down on the band, seating the band securely over the lip on the edge of the backplate.
Adjusting a metal wrist band
What is the difference between single-wire and dual-wire wrist straps?
Single-wire wrist straps have one conductor inside the insulation of the coil cord. They offer significantly lower life-cycle costs compared to dual-wire wrist straps. While they would not be suitable for the most critical applications, single-wire wrist straps are an economical way to ground an operator.
Dual-wire wrist straps have two conductors. They offer a reduced risk of damaging ESD sensitive devices because even if one conductor is severed, the operator still has a reliable path-to-ground with the other conductor. For that reason they are generally used in critical applications. For maximum benefits dual-wire wrist straps should be used together with dual-wire continuous monitors.
Find part 2 of this series here.