Imagine this scenario: you come to work in the morning and test your wrist strap per your ESD program’s recommended test frequency procedure. The wrist strap passes and you start work on your ESD sensitive devices. 3 hours later, when you come back from your tea break, you test your wrist strap again before continuing work and the wrist strap fails.
What to do? It is unknown when exactly the wrist strap failed in those 3 hours after your first periodic test in the morning and it is possible the devices you worked on during that time frame have been damaged. You don’t know which products have been damaged – latent defects are not visible and failures may only occur at a later time, reducing the potential reliability of the products.

Periodic testing is commonly used in an ESD program, however using continuous monitoring while working on those sensitive devices will alert the operator as soon as their wrist strap and/or workstation path-to-ground connection fails. Today’s blog post will highlight various benefits of continuous monitoring.

Introduction
Wrist straps are considered the first line of ESD Control. They are used to link people to ground ensuring operators are kept at the same potential as surfaces, objects and ESD sensitive devices (ESDs). Before handling sensitive items, wrist straps need to be visually inspected and checked (while worn) which will alert the operator to potential faults.
Per ESD Handbook TR 20.20 paragraph 5.3.2.4.4 Test Frequency, “Because wrist straps have a finite life, it is important to develop a test frequency that will guarantee integrity of the system. Typical test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that knowledge of a continuous, reliable ground is needed, then continuous monitoring should be considered or even required.

Continuous monitors come in different styles and sizes but are intended to be kept on the workstation. Some units just ‘sit’ on the bench; others are attached to the working surface matting; some can even be attached underneath the workbench so they don’t take away valuable workspace. Operators connect their wrist strap to the unit to allow for real-time continuous monitoring. If the wrist strap fails, the unit will alarm. Many continuous monitors also feature a parking snap providing a means for the operator to disconnect when leaving their workstation.
There are two different types of continuous monitors available:

  • Single-wire continuous monitors allow the use of any standard, single-wire wrist strap and coiled cord. The monitor / wrist strap system life-cycle costs are significantly lower compared to dual-wire systems. While they would not be suitable for the most critical applications, single-wire continuous monitors are an economical way to monitor both the operator’s wrist strap and/or workstation surface.
  • Dual-wire continuous monitors provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire continuous monitors provide redundancy because even if one dual-wire wrist strap conductor is severed, the operator still has a reliable path-to-ground with the other conductor. Dual-wire technology requires the use of dual-wire wrist straps and coiled cords.

 1. Instant Feedback
Continuous monitors provide operators with instant feedback on the status and functionality of their wrist strap. The instant an operator’s wrist strap or cord fails, the monitor will issue audible and visual (LEDs) alarms alerting the user and supervisor of the problem. The faulty wrist strap can be replaced with a new one from stock.

The SCS 724 Workstation Monitor in Use
The SCS 724 Workstation Monitor in Use

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

*The resistance threshold limits can vary between brands and models (and can sometimes also be adjusted by the user) so make sure you do your homework before committing to a particular unit and check the limit meets your individual requirements.

724 Monitor Installation
Installing the SCS 724 Workstation Monitor to ground the worksurface

Some continuous monitors can monitor worksurface ground connections. A test signal is passed through the worksurface and ground connections. Discontinuity or over limit resistance changes cause the monitor to alarm.
Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. However, the monitor will not detect insulative contamination on the worksurface and test methods such as those outlined in ESD TR53 can be used to isolate this problem. ” [ESD TR20.20 Continuous Monitors Clause 18.4.2 Worksurface Ground Monitoring].

3. Detection of Initial Flex Fatigue
Unlike wrist strap testers, 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.

Using the SCS Iron Man® Plus Monitor in conjunction with Dual-Wire Smocks
Using the SCS Iron Man® Plus Monitor in conjunction with Dual-Wire Smocks

During operation, wrist straps might be stressed and flexed to their limits at a workstation. While a wrist strap is being checked it is typically not stressed, as it would be under working conditions. Openings in the wire at the coiled cord’s strain relief are sometimes only detected under stress. Even if the wrist strap is working properly, a bad or intermittent ground connection will render the wrist strap system less than 100% effective.” [ESD TR20.20 Continuous Monitors Clause 18.2 Wrist Strap Checkers]

4. Elimination of Periodic Testing
Many customers are eliminating periodic touch testing of wrist straps and are utilizing continuous monitoring to better ensure that their products were manufactured in an ESD protected environment. Continuous monitors also eliminate the need for users to test wrist straps and log the results.

PaperPile
No more paper logs!

When using continuous monitoring, operators:

  • Don’t have to waste time queuing at a wrist strap test station before each shift.
  • Don’t have to remember to complete their daily test logs.

Conclusion
If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products”? Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.
Advantages of Continuous Monitors are plentiful:

  • Immediate feedback should a wrist strap fail
  • Monitoring of operators and work stations
  • Detection of split-second failures
  • Elimination of periodic testing

All of the above advantages of Continuous Monitors will lead to a reduction in overall costs. Savings comes from:

  1. Eliminating time/labor required in verifying a wrist strap before handling ESDs
  2. Reducing damage to ESDs from broken wrist straps that may go unnoticed with standard wrist strap testers.

For more information and an overview of SCS Workstation Monitors, have a look at our Selection Guide.

There is a very common misconception that if a person is wearing a wrist strap, an ESD garment (ESD smock, ESD shirt, ESD coat etc.) is unnecessary. Operators falsely believe that any charge on a person and their clothes will find its way to ground via the wrist strap.
Today’s blog post will explain the importance of ESD smocks and why they should be considered for your ESD Protected Area (EPA).

Purpose of ESD smocks
The main reason people wear ESD smocks is to shield their insulative clothing and minimize the electric fields generated from their clothing. As we learned previously, all process essential insulators should be kept at a minimum distance of 12 inches from ESD susceptible items. Clothing fabric, particularly when made from synthetic fibres, is a significant charge generator. Non-ESD clothing fabric is an isolated charged insulator which cannot be grounded and the resulting charges can threaten ESD control.
An insulator (like clothing) will not let charges flow and holds the charge until either neutralized naturally over time (hours or days) or with an air ionizer (artificially under a few seconds). Until the charges are neutralized, your clothing may have several thousand volts that could suddenly discharge and damage nearby static-sensitive items.
The ESD Standard does not require ESD smocks, however they are a very practical solution for minimizing ESD events from a person’s clothing. ESD smocks can be an important step to demonstrating commitment to an ESD control program.

Example of an ESD Smock
Example of an ESD Smock

Garments are intended to attenuate electrostatic fields that may be present on personnel clothing. The need for ESD protective garments is generally determined based on the sensitivity of the ESD items being handled where ESD control is a requirement.
While a person may be grounded using a wrist strap or other grounding methods, that does not mean that insulative clothing fabrics can dissipate a charge to that person’s skin and then to ground. Personnel clothing usually is electrically separate or isolated from the body.” [ESD TR20.20 Clause 5.3.13 Garments]

ESD smock properties
Most smocks are constructed of a dissipative material which incorporates texturized 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 Smocks are an ESD protective product that should possess the following ESD control characteristics:

  • Antistatic low-charging so they minimize 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 Smock and
  • Groundable so the user can easily and reliably connect them to ground.

The majority of ESD smocks on the market are single-wire ESD smocks meaning they provide one electrical connection to ground.
The all-new SCS Dual-Wire Static Control Smocks are designed for use with dual-wire grounding systems. The smock’s dual-wire circuit loop closes when its two conductive cuffs are bridged by the skin on the wearer’s wrists.

  • One conductive cuff is electrically bonded to the dissipative garment and one snap stud at the hip.
  • The second conductive cuff uses an isolated conductive path to bond to a second snap stud at the hip.

When used in conjunction with a dual-wire continuous monitor, SCS Dual-Wire Static Control Smocks provide assurance of a proper dissipative path from operator to ground at all times.
For more information on the differences of single-wire and dual-wire systems, please review this post.

Installation and grounding of ESD smocks
Follow the directions below for proper installation and grounding of the ESD smock.

  • Put on the smock, and close the garment by fastening all of the snaps on the front. Verify that no clothing is exposed outside of the smock.
    Installation of Smocks - Part 1
  • Fit the conductive knitted cuffs over the wrists. Ensure that the cuffs make contact with the skin. They should never be worn over the shirt sleeves.
    Installation of Smocks - Part 2
  • Ground the ESD smock. A popular way to ground an ESD smock is with a coiled cord either attached to a snap on the waist area of the smock or via a wrist strap snapped to the inside cuff of an ESD smock. If none of these methods are suitable, the smock should be grounded via the person’s wrist removing charges via ESD footwear to ESD protected flooring.
    Installation of Smocks - Part 3

 

Using Hip-to-Cuff Grounding with the SCS Dual-Wire Static Control Smock and SCS Iron Man® Plus Workstation Monitor
Using Hip-to-Cuff Grounding with the SCS Dual-Wire Static Control Smock and SCS Iron Man® Plus Workstation Monitor

After verifying that the garment has electrical conductivity through all panels, the garment should be electrically bonded to the grounding system of the wearer so as not to act as a floating conductor. This can be accomplished by several means:

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

[ESD TR20.20 Clause 5.3.13.2.6 Proper Use]

ESD Garments are a conductor and therefore should be grounded. If not grounded, the ESD smock can be a potentially threatening isolated charged conductor. If an operator is wearing a smock but is not electrically connecting the smock to either their body’s skin or ground, then charges on the smock may have nowhere to go or discharge to.

Testing of ESD smocks
Panel-to-panel conductivity is essential to ensure portions of the smock 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.
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 smock. Being shielded, the measured charge should be greatly reduced.

Static control garments that electrically bond to the test subject and provide a path to ground for the test subject (Category 3) shall be evaluated by all three methods:

  • the resistance point-to-point test method (Fig Sa, 5b and 5c);
  • the resistance point to groundable point test method (Figures 4, 6a, 6b and 7); and
  • the system test to determine the resistance from the person, through the garment

groundable point of the garment to the groundable point, including the ground cord (Figures 8a and 8b).” [ANSI/ESD STM2.1 Clause 1.3.2]

Using the SCS 701 Analog Surface Resistance Megohmmeter to test panel-to-panel conductivity
Using the SCS 701 Analog Surface Resistance Megohmmeter to test panel-to-panel conductivity

Cleaning of ESD smocks
Smocks must be laundered periodically for proper operation. The proper method to clean a smock is to:

  • Wash the garment by hand or with a standard household washing machine in cold or warm water.
  • Only use non-ionic liquid softeners and detergents when laundering. Do not bleach your ESD smocks!
  • Tumble dry with low heat or hang dry.

Do not use:

  • Hot water
  • Dry detergent
  • Bleach
  • Fabric softener
  • Industrial laundry machines

Please also note that smocks should not be altered in any way. The smocks effectiveness is in fully covering the human body and street clothes – especially at the wrists and front of the body. Altering the smock in any way will nullify its effectiveness.
The typical useful and effective life of an ESD smock under normal wearing and recommended washing conditions is a minimum of 75 washings. Under the same conditions, SCS smocks will maintain their usefulness and effectiveness for a minimum of 100 washings

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
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
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.
Installation of Cup Style Foot Grounders – more information
Installation of Cup Style Foot Grounders – more information

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
Installation of D-Ring Toe Grounders – more information
Installation of D-Ring Toe Grounders – more information

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.

Installation of Disposable Foot Grounders – more information
Installation of Disposable Foot Grounders – more information

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
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

  1. 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).
  2. 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.
  3. 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.
  4. A current limiting of one or two megohm resistor in series with the contact strip is recommended but not required.
  5. ESD foot grounders should be tested independently at least daily while being worn to periodically test for proper grounding.

SCS-NC1-holiday2017From all of us here at SCS, we would like to wish you a safe and Happy Holiday season!

We will be closed on Monday, December 25, 2017 and Monday, January 1, 2018.

As always, thank you for your business and the opportunity to provide you with a Static Control Solution.

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
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.

770075-UseExample of a single-wire capacitance monitor – more information

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.

724useExample of a dual-wire pulsed monitor – more information

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.

 

We get a lot of inquiries regarding wrist straps: what they do, why there are different types, how they are used, etc. So, the purpose of today’s blog post is to answer all those questions for you. If there is something we did not cover in the blog post make sure you ask us in the comments!
Let’s get started!

Introduction
The ESD Standard S20.20 requires “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]

The most common personnel grounding device is a wrist strap which is used to connect people to ground.
A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and, in turn, 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. Wrist straps are required if the operator is sitting. They 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.
A wrist strap is made up of two components:

  • a wristband that is worn comfortably around your wrist and
  • a coil cord that connects the band to ground.

Wristband and coil cord of a wrist strapWristband and coil cord of a wrist strap

The key to the wrist strap is the intimate contact of the conductive band to the skin and of course the coil cord connecting to ground. It doesn’t matter if the contact point to your body is on your wrist, finger, forearm, ankle, etc., as long as it is in direct contact with your skin. The skin is electrically continuous over your whole body. The wrist is just a convenient place to couple the band to.

Styles of Wrist Straps
Operators can choose between elastic and metal wristbands:

  • Elastic wristbands are the most popular wristband as they are comfortable to wear and easy to adjust. Compared to metal wristbands they are also less expensive.
  • Some people prefer metal wristbands as they are generally longer lasting and easier to clean.

The key to personnel 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 wristbands.

Both elastic and metal wristbands are (to a certain degree) adjustable. Metal wristbands 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 wristbands if you need a tighter/looser fit.
To adjust your wristband, follow the below steps:
1. Elastic wristbands:

  • Open the clasp by pulling upward on the “tail” of material that extends out from the clasp.
  • Tighten or loosen the elastic material through the clasp until the wristband fits snug but comfortably.
  • We recommend that you close the clasp and wear the band with the excess tail extended for a day to be sure the adjustment is snug, comfortable, and has the proper electrical contact with the skin before cutting.
  • Test the wrist strap system to be sure of proper electrical resistance and skin contact.
  • When you are ready to cut off excess material, mark with a pencil where excess material is to be trimmed.
  • Remove band from wrist. Open clasp. Cut off strip excess material about 1/4″ short of pencil mark so that the end of material is concealed by cap. This will eliminate the possibility of frayed ends.
  • Close clasp and use as a fixed elastic wristband.

Adjusting an elastic wristbandAdjusting an elastic wristband

2. Metal wristbands:

  • Insert the link end of the wristband 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 size, 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.
  • Test the wrist strap system to be sure of proper electrical resistance and skin contact.

Adjusting a metal wristbandAdjusting a metal wristband

1 megohm Safety Resistors
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.
Such safety resistors are built into the wrist straps themselves and also in such wrist strap monitors as WS Aware, Iron Man® Plus and Ground Man Plus manufactured by SCS. 

Typical Problems with Wrist Straps
Some of the typical problems with proper grounding of an operator using a wrist strap are:

  • worn out wrist strap which no longer has good electrical properties
  • stretched out wrist strap which doesn’t make good electrical contact with the skin
  • loosely-worn wrist strap which doesn’t make good electrical contact with the skin either
  • dry skin of an operator increasing electric resistance of a contact beyond specification
  • improper placement of a wrist strap, such as over the cuff of the garment

Also, another issue we often see is that wrist strap users connect their 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 ANSI/ESD S20.20 table 2.

Testing of Wrist Straps
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 wristband, 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.

Verifying a wrist strap using a wrist strap/footwear testerVerifying a wrist strap using a wrist strap/footwear tester

If the wrist strap tester outputs a FAIL test result, stop working. Test the wristband 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 coiled cord with a new one and repeat the test.

If the operator and wrist strap system fails high:

  • Make sure the coiled 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 coiled 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-moisturize the skin thereby increasing its conductivity. Retest. Operators with dryer skin should wear metal banded wrist straps to minimize 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.

Now that we’ve covered the basics of wrist straps, we will dive into the different types of wrist straps – but that will have to wait until next time as this post is already very long. Stay tuned!

People pose the biggest threat to ESD sensitive components. However, when properly trained, operators can become the key weapon in the fight against ESD. Every person coming into contact with ESD sensitive items should be able to prevent ESD related problems before they occur or provide immediate action when they do occur. Today’s blog post will explain in detail the role operators play in ESD Protection and how your company can support them in the fight against ESD.

Introduction
As an employee, the invisible threat of ESD should be of great concern to you. ESD damage can significantly reduce your company’s profitability. This may affect your company’s ability to compete in the marketplace, your profit sharing and even your employment. Everyone likes to take pride in their work, but without proper ESD controls, your best efforts may be destroyed by ElectroStatic discharges that you can neither feel nor see.

motherboard doctors
People are often a major factor in the generation of static charges

Perhaps the most important factor in a successful static control program is developing an awareness of the “unseen” problem. People are often a major factor in the generation of static charges. Studies have shown that personnel in a manufacturing environment frequently develop 5000 volts or more by just walking across the floor. This is “tribocharging” produced by the separation of their shoes and the flooring as they walk.
A technician seated at a non-ESD workbench could easily have a 400-500 volt charge on his or her body caused not only by friction or tribocharging but additionally by the constant change in body capacitance that occurs from natural movements. The simple act of lifting both feet off the floor can raise the measured voltage on a person by as much as 500-1000 volts.
Educating your personnel is therefore an essential basic ingredient in any effective static control program. A high level of static awareness must be created and maintained in and around the ESD protected area. Once personnel understand the potential problem, it might help to reinforce this understanding by hanging up a few static control posters in strategic locations. No technician needs an unprotected person wandering over and touching things on the service bench.

The invisible enemy
The biggest issue with ElectroStatic discharges is that you can neither see nor feel the threat. Daily life has other examples of hidden enemies where careful procedures must be followed to regularly obtain positive results. One example is sterilization which combats germs and contamination in hospitals.
Damage caused by invisible and undetectable events can be understood by comparing ESD damage to medical contamination of the human body by viruses or bacteria. Although invisible, they can cause severe damage. In hospitals, the defense against this invisible threat is extensive contamination control procedures including sterilization.

A medical team performing an operation
Would you consider having surgery in a contaminated operating room?

We are aware of the benefits of sterilization in medicine. We must develop the same attitude towards ESD control and “sterilize” against its contamination. Just as you would never consider having surgery in a contaminated operating room, you should never handle, assemble or repair electronic assemblies without taking adequate measures against ESD. For the hospital to sterilize most of the instruments is not acceptable; actually, it may waste money. Every single instrument needs to be sterilized. Likewise, it is not acceptable to protect the ESD sensitive items most of the time. Effective ESD control must occur at each and every step where ESDS items are manufactured, processed, assembled, installed, packaged, labelled, serviced, tested, inspected, transported or otherwise handled.
Everyone handling sensitive components should:

  • recognize ESD threat
  • know what equipment to use, and how to use it
  • know the correct ESD procedures, and work to them
  • know how to check equipment
  • know which packaging to use
  • take corrective actions when required.” [Source]

It is obvious that ESD training of personnel is a prerequisite for a functioning ESD control program.

Training
ESD training needs to be provided to everyone who handles ESD sensitive devices – that includes managers, supervisors, subcontractors, cleaners and even temporary personnel. Training must be given at the beginning of employment (BEFORE getting anywhere near an ESDS) and in regular intervals thereafter.
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 ESDS items. 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 the methods used by the Organization to verify trainee comprehension and training adequacy.” [ANSI/ESD S20.20-2007 section 7.2]

Training
Training is an essential part of an ESD Control Program

ESD training should include:

  • an introduction to ESD – what it is, what it’s caused by and how to control it
  • how to handle sensitive devices and what precautions to take when coming into contact with them
  • how to identify and mark ESD sensitive items
  • an overview of the ESD Standard.

For operators working in assembly, repair or field service, job specific training will be required, too.
If visitors are entering an EPA, they must possess basic ESD awareness and understand how to use their wrist straps and footwear.

Operator’s safety comes first
One final word of warning: while ESD control is important, it is of secondary importance to employee safety. ElectroStatic charges or static electricity can be everywhere; however, conductors can be effectively grounded and charges removed to ground. A fundamental rule in ESD control is to ground all conductors, including people. BUT: Personnel should not be grounded in situations where they could come into contact with voltage over 250 volts AC.

Last time, we discussed the storage requirements of ESD sensitive items. Today we want to focus on the overall selection process for ESD bags: from choosing the correct type of material all the way through to determining the right size for your application. Sounds complicated? Honestly – it’s not and once you have the right tools (you’re welcome!), you’ll be an expert in no time. So, let’s go!

Choose the correct material for your ESD bag
Before you get started, you need to be clear about the purpose of your ESD bag and the environment it’s being used in. Make sure you have the answers to the following questions:

  1. What do you intend to put inside the ESD bag? ESD sensitive items? Non-ESD sensitive items?
  2. Is moisture an issue you need to consider?
  3. Do you need your ESD bags to be self-sealable? Or will you be using ESD tape/labels to close your bags?
  4. Are the items inside your ESD bag sensitive to physical damage?
  5. Are the items you’re storing in your ESD bag particularly sharp which could potentially damage the material?
  6. Will the ESD bags (and obviously the items inside) be stored on a grounded shelf inside an EPA or are they being transported outside of an EPA, as well?

Once you have answers to ALL of the above questions, you can move on to the below selection chart and choose the right material for your application.

ESD sensitive items Moisture protection Self-sealable Physical protection Inside/
Outside EPA
Metal-In x x Both
Metal-Out x x Both
MBB – High Barrier x x x Both
MBB – Low Barrier x x x Both
Bubble Shielding x x Both
Conductive Black Only if used on a grounded surface Inside only if used with ESD sensitive items

A few more details on the different types of materials listed in the above chart:

  1. Metal-In Shielding Bags
    ESD bags which protect ESD sensitive items. The ESD shielding limits energy penetration from electrostatic charges and discharge. The dissipative outer layer dampens any discharge and therefore reduces damaging electrostatic events. They offer good see-through clarity. Available with and without dissipative zipper.

    Metal-In Shielding Bag – more information
  2. Metal-Out Shielding Bags
    Integral antistatic and low tribocharging bags which will not electrostatically charge contents during movement. Bags have a protective coated aluminum metal outer layer of laminated film; this promotes a more rapid discharge of static fields creating the event which the metal layer then attenuates. If RF field sensitivity is an issue, metal-out bags may be unsuitable. The rapid discharge to the highly conductive outer layer can create a higher radiated field which in return can cause issues for objects inside or near the bag. Available with and without dissipative zipper.

    Metal-Out Shielding Bag – more information
  3. Moisture Barrier Bags (MBB) – Low Barrier
    Offers ESD and moisture protection and can be used to pack SMD reels or trays. Available with and without dissipative zipper (except Dri-Shield® 2700).

    Low Barrier Moisture Barrier (MBB) Bag – more information
  4. Moisture Barrier Bags (MBB) – High Barrier
    Offer ESD and moisture protection and are ideal for applications where high moisture protection or conformance to IPC/JEDEC J-STD-033 is required. Available with and without dissipative zipper (except Dri-Shield® 3000).

    High Barrier Moisture Barrier (MBB) Bag – more information
  5. Cushioned Bags
    These bags combine the “Faraday Cage” and mechanical protection. They shield about twice as well as normal shielding bags of equivalent size. Bubble cushion layer provides heavy-duty protection that absorbs impact and prevents product damage.

    Cushioned Bag – more information
  6. Conductive Black Bags
    Black conductive film is made of virgin low density materials with black conductive compound to achieve high toughness and strength. This is commonly used for material handling, shipping and storage.

    Conductive Black Bag – more information

Calculating the correct size for your ESD bag
Once you have selected the correct type of material, it’s time to choose the right size for your ESD bag. There are different ways to determine this based on the type of material you use:

Shielding and Black Conductive Bags:
A. Bag Width = Item’s Thickness + Item’s Width + 25mm
B. Bag Length = Item’s Thickness + Item’s Length + 50mm

Moisture Barrier Bags (MBB):
A. Bag Width = Item’s Thickness + Item’s Width + 25mm
B. Bag Length = Item’s Thickness + Item’s Length + 76mm

Cushioned Bags:
A. Bag Width = Item’s Thickness + Item’s Width + 76mm
B. Bag Length = Item’s Thickness + Item’s Length + 76mm

Bonus Tip: Measuring a bag
It might seem obvious to some of you but given that we do get these types of queries on a regular basis, we thought this would be a good opportunity to include. Imagine you already have ESD bags that you use in your company. Someone has just taken the last one off the shelf and you need to order some more. How do you know what size ESD bag you have in front of you so you can place a new purchase order? No worries – we have the answer:

A. The width is measured from inside seam to inside seam. This is also your opening.
B. The length is measured from the top of the opening to the bottom of the bag.

Bonus Tip 2: Remember your ESDS items
Outside an ESD protected area, the objective of ESD protective packaging is to prevent a direct electrostatic discharge to the ESD sensitive item contained within and allow for dissipation of charge from the exterior surface. In addition, the packaging should minimize charging of the ESD sensitive item in response to an external electrostatic field and triboelectrification. If the user does not know the sensitivity of the items being used, we would always recommend static shielding packaging to be on the safe side.

In our last post, we talked about the ESD protective packaging requirements for ESD sensitive items and provided you with 6 steps to choose the correct type of packaging. We thought today we could go in a little bit more detail and introduce you to some types of packaging and how to use them. If you read our recent post on Tips to Fight ESD, you will remember how important it is to protect your ESD sensitive items when leaving an EPA. Yet, too often we see customers who have the perfect EPA, but when it comes to transporting and storing their precious components, it’s all falling apart.

Packaging required for transporting and storing ESD sensitive items
During storage and transportation outside of an EPA, it is recommended that ESD sensitive components and assemblies are enclosed in packaging that possesses the ESD control property of shielding. See our last post for more details.

Remember:

  • In ‘shielding’ we utilize the fact that electrostatic charges and discharges take the path of least resistance.
  • The charge will be either positive or negative; otherwise the charge will balance out and there will be no charge.
  • Charges repel so electrostatic charges will reside on the outer surface.

The Faraday Cage effect
A Faraday Cage effect can protect ESD sensitive items in a shielding bag or other container with a shielding layer. To complete the enclosure, make sure to place lids on boxes or containers and close shielding bags.

Cover must be in place to create Faraday Cage and shield contents.

Types of shielding packaging
The below list gives a few examples of what types of shielding packaging is available on the market. This list is by no means complete; there are many different options out there – just make sure the specifications state “shielding” properties.

  • Metal-In Shielding Bags
    ESD bags which protect ESD sensitive items. The ESD shielding limits energy penetration from electrostatic charges and discharge. They offer good see-through clarity. Available with and without zipper.

    Example of a Metal-In Shielding Bag – Click here for more information
  • Metal-Out Shielding Bags
    Integral antistatic and low tribocharging bags which will not electrostatically charge contents during movement. Bags have an aluminium metal outer layer of laminated film. Available with and without zipper.

    Example of a Metal-Out Shielding Bag – Click here for more information
  • Moisture Barrier Bags
    Offer ESD and moisture protection and can be used to pack SMD reels or trays.

    Example of a Moisture Barrier Bag – Click here for more information
  • Cushioned Shielding Bags
    These bags combine the “Faraday Cage” and mechanical protection. They shield about twice as well as normal shielding bags of equivalent size.

    Example of a Cushioned Shielding Bag – Click here for more information

Additional options for storing ESD sensitive items
Do you have the following in place?

  • ESD flooring
  • Grounded personnel (using foot grounders)
  • Grounded racking

IF (and this is a BIG IF) the above requirements are fulfilled, you can use conductive bags or containers to store your ESD sensitive items. Conductive materials have a low electrical resistance so electrons flow easily across the surface. Charges will go to ground if bags or containers are handled by a grounded operator or are stored on a grounded surface.

Conductive materials come in many different shapes and forms:

Conductive Black Bags
Tough and puncture resistant bags which are made of linear polyethylene with carbon added. The bags are heat sealable.

Example of a Conductive Black Film – Click here for more information
  • Rigid Conductive Boxes
    Provide good ESD and mechanical protection. Boxes are supplied with or without high density foam for insertion of component leads or low density foam which acts as a cushioning material.
  • PCB Containers
    Are flat based and can be stacked. They are made of injection moulded conductive polypropylene.

Again, there are many more options available on the market so make sure you do your research.

Note: we do not recommend using conductive packaging to transport ESD sensitive devices. Also, pink antistatic and pink antistatic bubble bags are not suited for storing or transporting ESD sensitive components.

Final thoughts
Packaging with holes, tears or gaps should not be used as the contents may be able to extend outside the enclosure and lose their shielding as well as mechanical protection.

Also, do not staple ESD bags shut. The metal staple provides a conductive path from the outside of the ESD bag to the inside. The use of a metal staple would undermine the effectiveness of the ESD bag making a conductive path for charges outside the bag to charge or discharge to ESD sensitive components inside the bag. To close an ESD bag, it is recommended to heat seal or use ESD tape or labels after the opening of the bag has been folded over. Alternatively, you can use ESD bags with a zipper.