Category Archives: Articles

Managing your ESD Control Programme

It’s Thursday and time for a new blog post! Although, today’s post is a little different…

A few years ago, a great article was published on the Circuits Assembly website. The article was written by the ESD Association and is titled Managing Your ESD Program.
The article discusses the challenges manufactures face in designing and maintaining an ESD control programme for their operations. The article goes on to breakdown managing an ESD programme in the following areas:

  • Identify and establish ESD teams
  • Identify your losses
  • Identify ESD sensitive items
  • Evaluate your facility and processes
  • Establish and implement procedures
  • Train personnel
  • Evaluate, adjust and provide feedback

We wanted to share the article with you because it contains loads of information that we think you will find useful!

Here is a link to the article – happy reading!

5 (additional) common mistakes in ESD Control & how to avoid them

A little while ago we published a post listing some of the most common issues we see when visiting EPAs and how to fix them. We had a lot of positive feedback from this post so thought we’d create a follow-up post with another 5 mistakes that are creeping up on a regular basis.

Introduction
You may remember how we talked in the previous post about companies wasting a lot of money by misusing their ESD products? No? Catch-up here.
The bottom line is: the job doesn’t end with purchasing ESD control items. Operators need to be trained on how to use their ESD products and ESD products need to be checked on a regular basis. If this doesn’t happen, you might as well throw the money you invested in your ESD Control Programme out the window…
Remember: ElectroStatic Discharge (ESD) is silent, quick and potentially lethal to electronic parts. When electronic parts are not properly handled during manufacturing, assembly, storage or shipping, damage from ESD can reach into the millions of dollars each year.

5 (additional) common Mistakes in ESD Control

1. Using insulators at the workstation
Non-essential insulators at an ESD protective workstation might include regular packaging, document holders, binders and tape. In addition, workers like to personalise their work areas so they might have high charging plastics in the form of radios, picture frames, purses, drinking cups etc. on the bench.
None of these are essential to get your job done and all of them pose a risk to your sensitive components.

 Check Insulators can be controlled by doing the following within an EPA:
• Keep insulators a minimum of 30cm from ESDS items at all times or
• Replace regular insulative items with an ESD protective version or
• Periodically apply a coat of topical antistat.
 Standard All non-essential insulators and items (plastics and paper), such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS are handled.
The ESD threat associated with process essential insulators or electrostatic field sources shall be evaluated to ensure that:
• the electrostatic field at the position where the ESDS are handled shall not exceed 5 000 V/m;
or

• if the electrostatic potential measured at the surface of the process required insulator exceeds 2 000 V, the item shall be kept a minimum of 30 cm from the ESDS; and
• if the electrostatic potential measured at the surface of the process required insulator exceeds 125 V, the item shall be kept a minimum of 2,5 cm from the ESDS.
[IEC 61340-5-1:2016 clause 5.3.4.2 Insulators]

If you want to learn more about controlling insulators, have a look at this post.

2. Using open shielding bags or containers
So, you may have heard of a Faraday Cage but do you know what role it plays in ESD Protection? We see a lot of companies that have a state-of-the-art EPA but when it comes to shipping sensitive components, everything falls apart. They may use component shippers but without a lid or they use shielding bags that are stapled together. None of these practises will do your sensitive components any good.

 Check In ESD Protection, the Faraday Cage effect causes charges to be conducted around the outside surface of the conductor. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’. However, to complete the enclosure, make sure to place lids on boxes/containers and seal shielding bags using a label or tape.
This is the only way to ensure ESD sensitive devises placed inside the shielding bag are protected.
 Standard Transportation of sensitive products outside of an EPA shall require packaging that provides both:
– dissipative or conductive materials for intimate contact;
– a structure that provides electrostatic discharge shielding.”
[EN 61340-5-3 Clause 5.3 Outside an EPA]

3. Ungrounded ESD Work Surface
ESD mats and laminate work surfaces cost a lot more than their regular insulative counterparts. The ESD dissipative characteristics are added so when charged conductors (conductive or dissipative) items are placed on the surface, a controlled discharge occurs and electrostatic charges are removed go ground. However, this only occurs if the ESD work surface is actually connected to ground.

 Check Best industry practice is that ESD ground connections should be firm fitting connecting devices such as metallic crimps, snaps and banana plugs that shall be connected to designated ground points. The use of alligator clips is not recommended. The companies’ Compliance Verification Plan should include periodic checks of worksurfaces measuring Resistance-to-Ground from the work surface centre or the most worn area to ground.
Many companies also use a daily checklist, which requires the operator to verify that ground cords are firmly connected.
 Standard Periodic testing of work surfaces is necessary to ensure that they continue to meet specifications. Resistance to ground measurements are typically used to verify that the path to ground is intact. In cases where the resistance go ground measurements exceeds the established resistance limits, the following steps can be taken to identify the cause of the high resistance readings:

  • Verify visually that the work surface is connected to the ground reference.
  • Clean the work surface. Sometimes a dirty surface can cause the resistance to exceed acceptable limits. Once the surface has been cleaned (note: clean the bottom of the resistance measuring electrode as well) repeat the resistance to ground measurement. If the second measurement is within specification this might lead to a further investigation concerning the cleaning practices used by the organization.
  • Disconnect the grounding wire and measure the resistance from the top surface of the work surface to the work surface groundable point. This measurement shall show whether or not the work surface is functioning as designed and it will verify that there is a good connection between the groudable point and the work surface.
  • Using an ohmmeter, measure the resistance of the wire used to ground the work surface. The measurement is made from the point where the wire is connected to the work surface’s groundable point to the ground reference.

The frequency of periodic testing is normally specified in corporate operating procedures. However, a common guide would be to conduct these measurements at least quarterly.
[CLC TR 61340-5-2 User guide Work surface clause 4.7.1.4.3 Periodic tests]

The most important functional consideration for work surfaces is the resistance from the top of the surface to the groundable point. This establishes the resistance of the primary path to ground for items placed on the surface. IEC 61340-5-1 has set a resistance to ground range for work surfaces of less than 1,0 x 109.
[CLC TR 61340-5-2 User guide Work surface clause 4.7.1.2.5 Electrical considerations]

For more information how to ground and look after your ESD work surface, have a look at this post.

4. Dissipative ESD Floor is measuring high
Electrostatic dissipative materials have a resistance to ground of greater than 1 x 105 ohm but less than 1 x 1012 ohm. EN 61340-5-1 requires the Resistance-to-Ground of ESD flooring to be less than 1 x 109 ohms. So, if you install new dissipative flooring and it measures 1 x 106 ohms, you’re all good. The problem with flooring is that when it gets dirty (and trust us, it will get dirty!), the resistance increases which potentially results in out-of-spec flooring.

 Check A regular maintenance schedule needs to be followed and floor resistance measurements needs to be taken as outlined in the companies’ Compliance Verification Plan. A dissipative floor finish can be used to reduce floor resistance. Periodic verification will identify how often the floor finish needs to be applied. As the layer(s) of dissipative floor finish wear, the resistance measurements will increase. So, after some amount of data collection, a cost-effective maintenance schedule can be established.
 Standard For standing operations, personnel can be grounded via a wrist strap system or by a footwear-flooring system. When a footwear-flooring system is used, personnel shall wear ESD footwear on both feet and the two following conditions shall be met:

  • the total resistance of the system (from the person, through the footwear and
    flooring to ground) shall be less than 1,0 × 109 ;
  • the maximum body voltage generation shall be less than 100 V.

[IEC 61340-5-1 Clause 5.3.3 Personnel grounding]

5. Poorly fitting Wrist Straps
As discharges from people handling sensitive items cause significant ESD damage, the wrist strap is considered the first line of ESD control. However, there are number of issues we see repeatedly when it comes to wrist straps:

  • Operators feel restricted by the wrist strap and stop wearing it altogether.
  • Operators leave their workstation and forget to re-connect their wrist strap when returning to their workstation.
  • Operators don’t pay attention when fitting their wrist straps resulting in an incorrect fit.
  • Operators use ripped wristbands or patched-up coiled cords.

Remember: if your wrist strap is worn incorrectly (or not at all), charges on your body will not dissipate to ground resulting in dangerous ESD exposure to sensitive ESD components.

 Check The wrist strap should be effectively tested while worn on the person and records should be kept. Wiggling the resistor strain relief portion of the coiled cord during the test will help identify failures sooner. Analysis and corrective action should take place when a wrist strap tester indicates a failure.

An even better solution is the use of continuous monitors that will alarm if the person is not properly grounded. Some monitors will beep if a discharge occurs or when a certain voltage level of electrostatic charge is on the person.

 Standard “Because wrist straps do not last forever, they should be tested periodically. A good testing program not only tests the wrist strap itself, but also indicates the quality of the skin contact when performing a system test. Wrist strap bands that are soiled, incorrectly sized or improperly worn will show resistance higher than acceptable.”
[CLC TR 61340-5-2 User guide Wrist strap clause 4.7.2.4 Wrist strap testing]Proper testing of the wrist strap includes the resistance of the groundable point on the end of the cord, the cord itself, the current-limiting resistor, the cord-to-band snap connector, the resistance of the interface of the cuff, the cuff/wrist interface and the resistance of the person between the wrist and the hand that contacts the test electrode. The maximum acceptable resistance for wrist strap grounding is less than 3,5 x107 .
[CLC TR 61340-5-2 User guide Wrist strap clause 4.7.2.4.2 Additional user wrist strap testing]

If you want to learn more about wrist straps, how to use and test them, we recommend having a look at this post.

Do you have anything to add? Let us know in the comments.

How do Foot Grounders work?

A question we repeatedly get from customers is in regards to foot grounders. Wrist straps are generally straight forward and people understand 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.

Purpose of Foot Grounders
A flooring / footwear system is an alternative for grounding standing or mobile workers. Where sitting personnel will be grounded via a wrist strap, this method is not feasible for operators moving around in an ESD Protected Area.
ESD foot grounders are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic charges from personnel. They are easy to install and can be used on standard shoes by placing the grounding tab in the shoe under the foot.

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 current should accidental exposure to electricity occur. The other end of the resistor is joined to a conductive sole. This sole contacts a grounded ESD floor mat or ESD flooring system.

Structure of a Foot GrounderStructure 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 ESDS.
EN 61340-5-1 recommends a minimum of 1 Megohm resistance to ground (Rg) in order to limit inadvertent electrical current exposure to a maximum of 0.00025 amperes.

For standing operations, personnel can be grounded via a wrist strap system or by a footwear-flooring system. When a footwear-flooring system is used, personnel shall wear ESD footwear on both feet and the two following conditions shall be met:
• the total resistance of the system (from the person, through the footwear and flooring to ground) shall be less than
1,0 × 109 Ω;
• the maximum body voltage generation shall be less than 100 V.
[IEC 61340-5-1 Clause 5.3.3 Personnel grounding]

Installation of Foot Grounders
1. Foot Grounders with Hook-and-Loop Straps
These heel grounders are 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 making contact with the shoe.
  • Insert the contact strip inside of the shoe and under the foot. Make sure that a solid contact is made between the sock and body. Cut contact strip to desired length.
  • Fasten hook and loop straps together, securing foot grounder firmly on shoe.
  • Test each foot grounder to confirm proper installation.

Installation of Foot Grounders with Hook-and-Loop StrapsInstallation of Foot Grounders with Hook-and-Loop Straps – more information

2. Foot Grounders with elastic Straps and Clip Fasteners
These heel grounders are equipped with a clip fasteners, a quick release fastening system.

  • Insert the contact strip inside of the shoe and under the foot. Make sure that a solid contact is made between the sock and body. Cut contact strip to desired length.
  • Fit the heel cup snugly to shoe and connect the Snap-Loc fastener together. Adjust elastic strap for comfortable fit. Tuck excess elastic strap behind itself.
  • Test each heel grounder to confirm proper installation.

Installation of Foot Grounders with elastic Straps and Clip FastenersInstallation of Foot Grounders with elastic Straps and Clip Fasteners – more information

3. D-Ring Toe Grounders
Toe grounders with the elastic D-ring fastening system are designed for use on heeled shoes.

  • Insert the grounding tab inside of the shoe and under the foot. Make sure that a 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 making contact with 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

D-RingInstallation 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 is 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 to 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 well 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

DisposableHeelGroundersInstallation of Disposable Foot Grounders – more information

Testing of Foot Grounders
Proper testing of your foot grounders involve testing the individual foot grounder, the contact strip and the interface between the contact strip and the wearer’s perspiration layer.
There are a number of personnel grounding testers on the market designed to properly test foot grounders. For more detailed information on how personnel grounding testers work and how to operate them, have a look at this post.
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.

Personnel Grounding ChecklistEnsure your Foot Grounders are working before handling ESDs

Cleaning of Foot Grounders
Foot grounders are to ground static charges, while dirt generally 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, e.g Desco Europe’s Reztore™ Antistatic Surface & Mat 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 and warm water are recommended. However, care must be taken to ensure that these detergents are silicone free.

 Conclusion

  • It is recommended that ESD foot grounders are worn on both feet in order to ensure that a continuous path to ground is maintained at all times (even when lifting one foot).
  • Contact strips should be tucked inside the shoe with as much contact area as possible to the bottom of the stockinged foot. ESD foot grounders rely upon the perspiration layer inside of the shoe to make contact through the stocking.
  • Foot grounders must be used with an ESD protected floor (such as correctly grounded ESD floor finish, carpet tiles or floor mats) to provide a continuous electrical path from the user directly to the ESD ground. 
  • A current limiting of one or two megohm resistor in series with the contact strip is recommended but not required.
  • ESD foot grounders should be tested independently at least daily while being worn.

 

How to neutralise a charge on an object that cannot be grounded

We have learnt in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials. But what do you do if you absolutely need an item in your EPA and it cannot be grounded? Don’t sweat, not all hope is lost! There are a couple of options which will allow you to use the item in question. Let us explain…

Conductors and Insulators
In ESD Control, we differentiate conductors and insulators.
Materials that easily transfer electrons are called conductors. Some examples of conductors are metals, carbon and the human body’s sweat layer.

ConductorA charged conductor can transfer electrons which allows it to be grounded

Materials that do not easily transfer electrons are called insulators and are by definition non-conductors. Some well-known insulators are common plastics and glass.

InsulatorInsulators will hold the charge and cannot be grounded and “conduct” the charge away

Both, conductors and insulators, may become charged with static electricity and discharge.
Electrostatic charges can effectively be removed from conductors by grounding them. However, the item grounded must be conductive or dissipative. An insulator on the other hand, will hold the charge and cannot be grounded and “conduct” the charge away.

Conductors and Insulators in an EPA
The first two fundamental principles of ESD Control are:

  1. Ground all conductors including people.
  2. Remove all insulators.

To achieve #1, all surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Wrist straps and work surface mats are some of the most common devices used to remove static charges. Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices. Movable items (such as containers and tools) are bonded by virtue of standing on a bonded surface or being held by a bonded person.

However, what if the static charge in question is on something that cannot be grounded, i.e. an insulator? Then #2 of our ESD Control principles will kick in. Per the ESD Standard, “All non-essential insulators and items (plastics and paper), such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS are handled.
The ESD threat associated with process essential insulators or electrostatic field sources shall be evaluated to ensure that:

  • the electrostatic field at the position where the ESDS are handled shall not exceed 5 000 V/m;

or

  • if the electrostatic potential measured at the surface of the process required insulator exceeds 2 000 V, the item shall be kept a minimum of 30 cm from the ESDS; and
  • if the electrostatic potential measured at the surface of the process required insulator exceeds 125 V, the item shall be kept a minimum of 2,5 cm from the ESDS.”

[IEC 61340-5-1:2016 clause 5.3.4.2 Insulators]

Always keep insulators a minimum of 31cm from ESDS itemsAlways keep insulators a minimum of 31cm from ESDS items

“Process-essential” Insulators
Well, we all know that nothing in life is black and white. It would be easy to just follow the above ‘rules’ and Bob’s your uncle – but unfortunately that’s not always possible. There are situations where said insulator is an item used at the workstation such as a hand tools. They are essential – you cannot just throw them out of the EPA. If you do, the job won’t get done.
So, the question is – how do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA? There are 2 options you should try first:

1. Replace regular insulative items with an ESD protective version
There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers and brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench. For more information on using ESD safe tools and accessories, check this post.

2. Periodically apply a coat of Topical Antistat
The Reztore® Topical Antistat (or similar solution) is for use on non-ESD surfaces. After it has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts. It therefore gives non-ESD surfaces electrical properties until the hard coat is worn away.

If these two options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.

Neutralisation
Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionisation.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.

Electronic enclosures are process-essential insulators
Electronic enclosures are process-essential insulators

An example of isolated conductors can be conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.

An ioniser creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionisation can neutralise static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
The charged ions created by an ioniser will:

  • neutralise charges on process required insulators,
  • neutralise charges on non- essential insulators,
  • neutralise isolated conductors and
  • minimise triboelectric charging.

Ioniser ExampleInsulators and isolated conductors are common in ESD Sensitive (ESDS) Devices – Ionisers can help

For more information on ionisers and how to choose the right type of ioniser for your application, read this post.

Summary
Insulators, by definition, are non-conductors and therefore cannot be grounded. Insulators can be controlled by doing the following within an EPA:

  • Keep insulators a minimum of 31cm from ESDS items at all times or
  • Replace regular insulative items with an ESD protective version or
  • Periodically apply a coat of Topical Antistat

When none of the above is possible, the insulator is termed “process-essential” and therefore neutralisation using an ioniser should become a necessary part of your ESD control programme.

Why Wave Distortion Technology is superior

We talked in the past about the benefits of continuous monitors and also introduced the different types (single-wire vs. dual-wire) to you. The focus of today’s post is the technology behind continuous monitors – how they work and how they compare to each other. So, let’s jump right in.

Introduction to Continuous Monitors
While wrist straps are the first and best line of defence against ElectroStatic Discharge (ESD), they must be tested to ensure that they are installed and working properly. On-demand or “touch” testers have become the most common testing method; they complete a circuit when the wrist strap wearer touches a contact plate.
One drawback with on-demand type testers is that they require a dedicated action by the wearer of the wrist strap to make the test. Also, 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 time that the wrist strap was not working properly.
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.

Technologies used for Continuous Monitors
There are three types of wrist strap monitoring on the market today:
1. Basic Capacitance / Impedance Monitoring,
2. Resistance Monitoring and
3. Wave Distortion Capacitance / Impedance Monitoring.

So, let’s look at all 3 types in a bit more detail:

1. Basic Capacitance / Impedance Monitoring
This single-wire technology makes 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.

AC capacitance monitors have a few drawbacks:

  1. 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.
  2. 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.
  3. Since the capacitance and therefore the impedance of the circuit will also vary with such things as the persons 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.

2. Resistance Monitoring
Dual-wire resistance monitors were developed to overcome some of the problems with the AC capacitance types. Here again the concept is simple. By providing a second path to ground (without relying on the capacitor above) we can apply a tiny DC current. It is then simple to measure the DC resistance of the circuit and alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted). Thus, a two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement. The shortcomings with the AC capacitance monitor are eliminated.

Two-wire monitors require two wires to work. 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.
There have been some reports that a constant DC voltage applied to the wristband causes skin irritations.

3. Wave Distortion Capacitance / Impedance Monitoring
Wave Distortion Technology continuous monitors feature:

  • low test voltage,
  • a low monitor range for 1 megohm of resistance in the operator’s wrist strap and
  • instantaneous detection of an intermittent or failure of the path-to-ground of the operator or work surface that other monitors / technologies miss.

Continuous monitors using wave distortion technology apply a continuous test voltage (1.2 volts peak- “Wave Distortion” or vector impedance works by applying a continuous test voltage of 1.2 volts peak-to-peak at 1 to 2 microamperes (0.000002 amperes) to the wrist strap that is connected to the continuous or constant monitor. The test voltage creates a sine wave that the monitor circuit compares to established patterns. By monitoring the “distortions”, or shape of the sine wave, Wave Distortion Technology determines if the monitored circuit is complete – the operator is in the circuit and the total equivalent DC resistance is within specifications. Wave Distortion Technology produces a very fast alarm time (<50 milliseconds) and minimal false alarms.

Comparing Continuous Monitor Technologies
We’ll compare the three different technologies using the following parameters:
1. Safety Resistor Monitoring
2. Test Voltage
3. Banana Jack & 10mm Socket Monitoring
4. Response Time
5. In-Use Verification

1. Safety Resistor Monitoring
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. “Wrist straps have a current limiting resistor moulded into the ground cord head on the end that connects to the band. The resistor most commonly used is a 1 x 106W, 1/4 watt with a working voltage rating of 250 V.” [IEC TR 61340-5-2 User Guide, Clause 4.7.2.5 Current limiting]

Neutral Basic Capacitance / Impedance Monitoring
Happy Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

2. Test Voltage
We’ve mentioned further above that some people have reported skin irritations when using resistance monitors which apply a constant DC voltage to the wristband. The problem is that the test voltages of resistance monitors is quite high (up to 16V). You have a similar issue with basic capacitance/impedance monitors (3.5V). Another thing to remember is that higher test voltages increase the risk of damage when handling ESD susceptible devices. Luckily for you, wave distortion monitors only use a test voltage of 1.2 – way below the other two technologies.

Neutral Basic Capacitance / Impedance Monitoring
Happy Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

3. Banana Jack & 10mm Socket Monitoring
Coiled cords with banana jack and 10mm sockets are commonly used in the electronics industry. Unfortunately, these cannot be used with dual-wire resistance monitors. As mentioned further above, special dual-conductor wrist straps need to be purchased.

Happy Basic Capacitance / Impedance Monitoring
Sad Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

4. Response Time
Detecting intermittent or complete failures in the path-to-ground of the operator or working surface is the job of a continuous monitor – but, it’s also important to look at how long it takes the monitor to report the issue. What’s the point of using a continuous monitor, if it takes the monitor 5 minutes to tell you there is an issue? All the sensitive devices you handled in the last 5 minutes may have been damaged. An instantaneous detection/alarm is therefore crucial. The slower the response time, the higher the potential impact on sensitive items. Response times for basic capacitance/impedance and resistance monitors is ~1s and ≤ 2s respectively. Wave distortion monitors on the other hand have a response time of <50ms.

Neutral Basic Capacitance / Impedance Monitoring
Neutral Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

5. In-Use Verification
So, imagine this scenario: you received a new constant monitor, you found a nice new home for it, you install it and use it. 12 months down the line, it’s time to verify/calibrate the monitor. You have to remove the monitor from its cosy place, complete the calibration and put it back. What a pain, right? The good news is: the test limits of wave distortion monitors can be verified without removing them from the workstation. Sound like a dream, right?

Sad Basic Capacitance / Impedance Monitoring
Neutral Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

We’ve created the below table for you to easier compare the different technologies:

Comparison of Continuous Monitors Technologies

As you can see, the latest Wave Distortion Technology provides the most reliable and stable confirmation of an operator’s continuous path-to-ground to ensure ESD sensitive product is protected at all times.
Shop our range of Wave Distortion Monitors here.

Single-Wire vs. Dual-Wire Monitors

A wrist strap is arguably the best way to provide a safe ground connection to the operator in order 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. One drawback with on-demand type testers is that they require a dedicated action by the wearer of the wrist strap to make the test. Also, 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.

Wrist StrapA Wrist Strap

Both types of wrist straps – when in good condition and properly worn – provide equally good connection of operator to ground. A single-wire wrist strap is undoubtedly less expensive than its dual counterpart. However, for applications where sensitive components are being handled, the share of dual-wire wrist straps is growing rapidly. The reason for this is its ability to guarantee that the wrist strap indeed provides proper dissipation of charges on the operator. The way to ensure that the wrist strap is worn properly at all times is to utilise a continuous wrist strap monitor. These units monitor proper connection of the operator to ground and alarm should this connection fail. If you want to learn more about the benefits of continuous monitoring, we recommend you read this post.

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 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. AC capacitance monitors have a few drawbacks:

  • 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.

This technology is still around today and is purchased by some because of its low cost and a lack of knowledge by the End-User. A big plus of this technology is the ability to use any standard single-wire wrist strap.

2. Wave Distortion Monitors
Many of the short comings of the capacitance and other earlier monitors have been overcome with the development of AC monitors that use the concept of the wrist strap wearer as a capacitor, but in a different way. The concept of the wrist strap and wearer as an RC circuit is not wrong but it is an over simplification. The total circuit actually contains resistance, capacitance and inductance (RCL). Each component value will vary with the environment, size of wearer, and the other factors that affect the accuracy of the AC capacitance monitor. 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 WDM 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.

Using ESD shielding bagsExample of a single-wire wave distortion monitor

Wave distortion monitors solves many of the problems of the other types:

  • It allows the use of any brand of single-wire wrist strap
  • It cannot be tricked like the AC capacitance units
  • It provides a warning if the lower (safety) resistance limits are compromised
  • The tiny amount of current required to generate the waveform has never caused reported skin irritation.

As an added bonus, wave distortion monitors will also detect an open circuit or bad ground all the way back to the building ground point. This is a fundamental advantage of this kind of monitor. Other monitors may insure that the operator is connected to the monitor. No other monitor automatically ensures that the user is actually grounded.

Dual-Wire Wrist Strap Monitoring
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. It is then simple to measure the DC resistance of the circuit and alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted). Thus, a two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement. The shortcomings with the AC capacitance monitor are eliminated.
Two-wire monitors require two wires to work. 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.

Example of a Dual-Wire Continuous MonitorExample of a dual-wire monitor

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
Dual polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire systems are used to create redundancy. In critical applications, you build-in redundancy to have a backup if your primary option fails. With dual-wire wrist straps the redundancy is there as a protection rather than an alternative. If you are monitoring your dual-wire wrist strap and one wire fails, then the unit will alarm. You will still be grounded by the other wire, so there will be a significantly reduced risk of damaging ESD sensitive components if you happen to be handling them when the wrist strap fails. The wrist strap would still need to be replaced immediately. So, 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.

6 Tips for handling “Class 0” Items

When talking about ESD Classifications a little while ago, we identified a “class 0” item as withstanding discharges of less than 250 volts.
The introduction of IEC 64340-5-1 states “This part of IEC 61340 covers the requirements necessary to design, establish, implement and maintain an electrostatic discharge (ESD) control program for activities that: manufacture, process, assemble, install, package, label, service, test, inspect, 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), 200 V charged device model (CDM) and 35 V on isolated conductors.

So, the obvious question is: how do you handle items that are susceptible to voltages of less than 100V? That’s what we’re going to answer in today’s blog post.

Introduction
Years ago, it was common for devices to be vulnerable to voltages greater than 100V. As the need for smaller and faster devices increased, so did their sensitivity to ElectroStatic Discharges as circuit-protection schemes were removed to stay ahead of the market. These new extremely sensitive components are now susceptible to discharges nearing 0V. Obviously, this causes problems for companies handling these devices: while their ESD programme may be in compliance with the ESD Standard, extremely sensitive devices require tighter ESD Control to protect them from ESD failures.

Definition of “Class 0”
Before moving any further, we need to qualify the term “class 0” as there is a lot of confusion out there as to what this term actually means. As stated above, the HBM Model refers to any item with a failure voltage of less than 250V as a “class 0” component. However, in recent times, the term has been used more and more to describe ultra-sensitive devices with failure voltages of less than 100V. Whilst the following tips and tricks really work on any “class 0” item, they are specifically designed to protect extremely sensitive items that withstand discharges of less than 100V.

People are often a major factor in the generation of static chargesUltra-sensitive devices are extremely common these days

Do your homework
Imagine someone (a customer, your boss etc.) is approaching you and demands you to update all internal procedures so your company can handle “class 0” components. Do you know how to handle this request? Or would you be pulling out your hair trying to figure out what needs to be done? As explained further above, “class 0” refers to a wide range of items and there are a few things you should remember before making any changes to your existing ESD programme:

  1. Verify what ESD Model your company/engineers/customers etc. are referring to. As we have learnt in the past, there are different ESD models (HBM, CDM, MM) as well as individual classifications for each model. A lot of people get confused when it comes to citing ESD classifications. In reality, there is only one “class 0” which refers to the human body model (HBM) but it’s always best to check.
  2. Check the specific withstand voltage an individual part is susceptible to. “Class 0” refers to all items that withstand discharges of less than 250V. However, there is a big difference between a failure voltage of 240V or 50V. You need to have detailed ESD sensitivity information available before being able to make decisions on how to improve your existing ESD control programme. This step is actually part of creating a compliance verification plan.
  3. A part’s ESD classification is only of importance until it is ‘merged’ into an assembly. So, the ESD classification of a device only refers to the stand-alone component. Once it goes into another construction, the classification of the whole assembly is likely to change.

Below are 6 tips that will help your company to upgrade your ESD control programme so you can effectively and efficiently handle ultra-sensitive items without risking ESD damage.

One thing to note: proactive actions are critical. There is no point in figuring out how to protect your components from ESD damage AFTER you have received them. Trust us: it’s gonna go wrong! Instead, focus on getting things sorted BEFOREHAND. That’s the best approach to stay ahead of the game.

1. Improve Grounding
So, you will already know that inside an EPA, all conductors (including people) are grounded. Now you’re probably thinking: “But I’ve already grounded my operators and worksurfaces. What else is there left to do?”. Firstly, well done for properly grounding the ‘objects’ in your EPA – trust us, that’s not a given! The next step is to tweak things a bit to allow for even better protection. Here are some suggestions:

Personnel:

  • Decrease the wrist strap and ESD footwear upper limit. The ESD Association has test data showing charge on a person is less as the path-to-ground resistance is less.
  • Use continuous monitors and ESD smocks
  • Introduce/increase use of ESD flooring
  • Use sole or full coverage foot grounders (rather than heel grounders)

WorkstationFull coverage foot grounders are recommended when handling ultra-sensitive devices

Worksurfaces:

  • Reduce the required limit for Point-to-Point resistance of 1 x 109 per the ESD Standard to 106 to 108 ohms (see #5). The reason for this reduction is simple: 1 x 109 is too high as it still produces thousands of volts of in electrostatic charges. However, the resistance cannot be too small either as this can lead to a sudden ‘hard discharge’ potentially damaging ESD sensitive components.

Other:

  • Improve grounding of carts, shelves and equipment to Ground
  • Minimise isolated conductors like devices on PCBs

2. Minimise Charge Generation
The best form of control is to minimise charge generation. First of all, you should always use shielding packing products like bags or containers (especially when outside an EPA) as these protect from generating charges in the first place. For more information on choosing the correct type of ESD Packaging, we recommend reading this post.

The next step is to eliminate charges once they are generated – this can be achieved through grounding and ionisation. We’ll cover ionisation in #3 and #4. We’ve already talked about improved grounding in #1. However, for ultra-sensitive components, we also recommend the following:

Both types of ESD products create a low tribocharging coating which allows charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts.
For more tips on managing charge generation from flooring, check this post.

3. Remove Insulators
When talking about conductors and insulators, we explained that insulators cannot be grounded and can damage nearby sensitive devices with a sudden uncontrolled discharge. It is therefore critical to eliminate ALL insulators that are not required in your EPA: plastic cups, non-ESD brushes, tapes etc. How? Here are a couple of options:

EBP-Bar-for-FlyerUse ESD safe accessories whenever possible

If an insulator is absolutely necessary for production and cannot be removed from the EPA, you could consider a topical treatment which will reduce triboelectric charges.
Is this not an option, then move on to tip #4.

4. Use Ionisation
First of all, ionisation is not a cure-all. We’ve learnt that ionisers neutralise charges on an insulator.
However, that does not mean that you can just have any insulator in your EPA because the ioniser will “just fix it”. No, in this instance, prevention is generally a better option than the cure. So, your priority should ALWAYS be to remove non-process essential insulators from your EPA – see tip #3. If this is not possible – then ionisation becomes essential:

  • Ionisers can be critical to reduce induction charging caused by process necessary insulators
  • Ionisers can be critical in eliminating charges on isolated conductors like devices on PCBs
  • Offset voltage (balance) and discharge times are critical considerations depending on the actual application
  • Ionisation can reduce ElectroStatic Attraction (ESA) and charged particles clinging and contaminating products.

It is recommended to use ionisers with feedback mechanisms so you’re notified if the offset voltage is out of balance.

5. Increase ESD Training and Awareness
ESD Training is a requirement of every ESD Programme. When handling ultra-sensitive devices, it is even more important to remind everyone what pre-cautions are necessary to avoid damage. Regular ‘refreshers’ are a must and it is recommended to verify the effectiveness of the training programme, e.g. through tests. So, who, when and what should be taught? Easy!

Training is an essential part of an ESD Control ProgrammeESD Training is a vital part of every successful ESD Control Programme

  • ESD training needs to be provided to everyone who handles ESD sensitive devices – that includes managers, supervisors, subcontractors, visitors, cleaners and even temporary personnel.
  • Training must be given at the beginning of employment (BEFORE getting anywhere near a sensitive products) and in regular intervals thereafter.
  • Training should be conducted on proper compliance verification procedures and on the proper use of equipment used for verification.

6. Create an enhanced Compliance Verification Plan
We talked in a previous post about compliance verification, what it is and how to create a plan that complies with the ESD standard. So, if you already followed our steps and have a plan in place, you’re probably wondering how you can possibly improve on that? Here are a few tips:

  • Use a computer data collection system for wrist straps and foot grounders testing, e.g. SmartLog Pro™
  • Increase the testing frequency of personnel grounding devices from once per day to every time the operator enters the EPA
  • Use continuous monitors where operators are grounded via wrist straps. Consider computer based monitor data collection system, e.g. SMP. This should include continuous monitoring of the mat Ground.
  • Use Ground continuous monitors, e.g. SCS Ground Master. At a large facility, the most frequent reoccurring violation is the ESD mat ground cord either becoming disconnected from the mat or grounding point. As Ground continuous monitors will only test the fact that the mat is grounded, it is still imperative that the Resistance to Ground of the mat is regularly tested. Remember that the use of improper mat cleaners can raise the mat surface resistance above the upper recommended level of <109
  • Test ionisers more frequently or consider self-monitoring ionisers. Computer based data collection systems are a good alternative, too.
  • Increase the use of a static field meter and nano coulomb testing to verify that automated processes (like auto insertion, tape and reel, etc) are not generating charges above acceptable limits.

Conclusion
The bottom line is: the only way to protect ultra-sensitive components is to increase ESD protective redundancies and periodic verifications to all ESD Control technical elements.
If you handle ultra-sensitive items, to decrease the probability of ESD damage, additional precautions are required including additional and/or more stringent technical requirements for EPA ESD control products, increasing redundancies, and more frequent periodic verifications or audits. Additionally, ESD control process systems should be evaluated as to their performance as a system. You will need to understand how the technical elements in use perform relative to the sensitivity of the devices being handled. Thus, tailoring the process to handle the more sensitive
parts. For example: If the footwear/flooring allows a person’s body voltage to reach say 80 volts and a 50 withstand voltage item gets introduced into the process, you will either have to allow only handling via wrist straps or would have to find a way to modify the footwear/flooring performance to get peak voltages below the 50 volt threshold.

Remember: it is YOUR responsibility to do protect YOUR devices and YOUR reputation. The ESD Standard can only give recommendations and it’ll always be behind current/future developments. As soon as a Standard is published, technology will have progressed. So, if – in order to protect your devices – your company needs to implement methods/procedures that exceed the recommendations of the ESD Standard, so be it.

References:

Best Storage Conditions for PCBs

Most people are aware of the dangers ElectroStatic Discharge (ESD) can pose on a Printed Circuit Board (PCB). A standard bare PCB (meaning that it has no semiconductor components installed) should not be susceptible to ESD damage. However, as soon as you stuff it with electronic (semiconductor) devices, it becomes susceptible according to each of the individual’s susceptibility.
However, there is another risk factor many operators forget: moisture.
So, today’s blog post is going to address both issues and will explain how you can protect your PCBs from both when storing them.

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

The problems moisture causes in SMDsMoisture from air diffuses inside the plastic body & collects in spaces between body & circuit, lead frame and wires. Expanding vapour can crack (popcorn) the plastic body or cause delamination.

Storing PCBs
All PCBs should be stored in a moisture barrier bag (MBB) that’s vacuum sealed. In addition to the bags, Desiccant Packs and Humidity Indicator Cards must be used for proper moisture protection. This ‘package’ is also known as a dry package.
Most manufacturers of the Moisture Sensitive Devices (MSD) will dictate how their product should be stored, shipped, etc. However, the IPC/JEDEC J-STD-033B standard describes the standardised levels of floor life exposure for moisture/reflow-sensitive SMD packages along with the handling, packing and shipping requirements necessary to avoid moisture/reflow-related failures. The ESD Handbook ESD TR20.20 mentions the importance of moisture barrier bags in section 5.4.3.2.2 Temperature: “While only specialized materials and structures can control the interior temperature of a package, it is important to take possible temperature exposure into account when shipping electronic parts. It is particularly important to consider what happens to the interior of a package if the environment has high humidity. If the temperature varies across the dew point of the established interior environment of the package, condensation may occur. The interior of a package should either contain desiccant or the air should be evacuated from the package during the sealing process. The package itself should have a low WVTR.

Components of a dry package
A dry package has four parts:

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

Moisture Barrier Bag (MBB)

Moisture Barrier Bags (MBB) work by enclosing a device with a metal or plastic shield that keep moisture vapour from getting inside the bag. They have specialised layers of film that control the Moisture Vapour Transfer Rate (MVTR). The bag also provides static shielding protection.

204519
Desiccant is a drying agent which is packaged inside a porous pouch so that the moisture can get through the pouch and be absorb by the desiccant. Desiccant absorbs moisture vapour (humidity) from the air left inside the barrier bag after it has been sealed. Moisture that penetrates the bag will also be absorbed. Desiccant remains dry to the touch even when it is fully saturated with moisture vapour. The recommended amount of desiccant is dependent on the interior surface area of the bag to be used. Use this desiccant calculator to determine the minimum amounts of desiccant to be used with Moisture Barrier Bags.

Humidity Indicator Cards (HICs)


Humidity Indicator Cards (HICs) are printed with moisture sensitive spots which respond to various levels of humidity with a visible colour change from blue to pink. The humidity inside barrier bags can be monitored by the HIC inside. Examining the card when you open the bag will indicate the humidity level the components are experiencing so the user can determine if baking the devices is required.

The Moisture Sensitive Level (MSL) label


The Moisture Sensitive Level (MSL) label tells you how long the devices can stay outside the bag before they should be soldered onto the board. This label is applied to the outside of the bag. If the “level” box is blank, look on the barcode label nearby.

 

Creating a dry package
Now that you know the components of a dry package, you’re probably wondering: but how do I put it all together? Not to worry – we’ve got you covered! If you follow these steps, you will create a secure dry package and your PCBs will be protected – against ElectroStatic Discharge and moisture.

Place the desiccant and HIC onto the tray stack. Trays carry the devices. Remember to store desiccant in an air tight container until it used.
Dry Packaging - Step 1

Place the MSL label on the bag and note the proper level on the label.
Dry Packaging - Step 2

Place the tray stack (with desiccant and HIC) into the moisture barrier bag.
Dry Packaging - Step 3

Using a vacuum sealer, remove some of the air from the bag, and heat seal the bag closed. It is not good to take all the air out of the bag. Only slight evaluation is needed to allow the bag to fit inside a box.
Dry Packaging - Step 4

Now your devices are safe from moisture and ESD.
Dry Packaging - Step 5

Do you use moisture barrier bags in your facility? What are your experiences? We’d love to hear from you in the comments!

And that’s a wrap! Just to let everyone know that we will be taking a little summer break over the next few weeks so there won’t be any new posts going up until the end of September. Don’t miss us too much…

 

 

The role of employees in ESD Protection

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 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.

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

Perhaps the most important factor in a successful static control programme 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 just by walking across the floor. Again, this is “tribocharging” produced by the separation of their shoes and the flooring as they walk.
A technician seated at a non-ESD workbench could easily have a 400-500 volt charge on his or her body caused not only by friction or tribocharging but additionally by the constant change in body capacitance that occurs from natural movements. The simple act of lifting both feet off the floor can raise the measured voltage on a person as much as 500-1000 volts.
Educating your personnel is therefore an essential basic ingredient in any effective static control programme. A high level of static awareness must be created and maintained in and around the 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. The technician doesn’t need 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 sterilisation 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 defence against this invisible threat is extensive contamination control procedures including sterilisation.

Would you consider having surgery in a contaminated operating room?Would you consider having surgery in a contaminated operating room?

We are aware of the benefits of sterilisation in medicine. We must develop the same attitude towards ESD control and “sterilise” 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 sterilise most of the instruments is not acceptable; actually, it may waste money. Each and every instrument needs to be sterilised. 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:

  • recognise 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 prerequisite for a functioning ESD control programme.

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.

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]

Training
Training is an essential part of an ESD Control Programme

ESD training should include:

  • theory and causes of electrostatic charging, and basic ESD understanding
  • handling procedures
  • knowledge of, use, and limitations of protective equipment
  • identification of ESDS, and understanding of ESDS sensitivity
  • Safety aspects and high voltage precautions
  • New techniques, processes, facilities and equipment before they are implemented
  • Awareness of the 61340-5-1 standard. [Source]

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.

The Difference between EOS and ESD

Electrical Overstress, or EOS, has become a widely-used term over the past few years. However, a lot of people are still unsure as to what exactly it is and how it differs from ElectroStatic Discharge (ESD). Today’s blog post is intended to put an end to the confusion.

What is Electrical Overstress?

One huge problem with Electrical Overstress, or EOS, is the fact that people use the phrase in different ways. Up until now there has been no widely recognised definition. A White Paper on EOS published by the Industry Council on ESD Target Levels in 2016 uses the following definition: “An electrical device suffers an electrical overstress event when a maximum limit for either the voltage across, the current through, or power dissipated in the device is exceeded and causes immediate damage or malfunction, or latent damage resulting in an unpredictable reduction of its lifetime.
Simplified, EOS is the exposure of a component or PCB board to a current or voltage beyond its maximum ratings.  This exposure may or may not result in a catastrophic failure.

ElectoStatic Discharge (ESD) versus Electrical Overstress (EOS)

You can compare an ESD event with a knocked-over glass of water on a floor: you’ll get a small puddle but once all the water has spilt from the cup, it’s gone. There is no more water left and the damage is fairly limited. [Source]

Knocked-over glass of waterESD can be compared to a knocked-over glass of water

However, an EOS event can be compared to an open tap; there may be just a little drip in comparison but there is an unlimited amount of water available. After a while, the entire floor may be flooded and could cause some serious damage. As you can see, EOS events last several magnitudes longer than most ESD events. [Source]

Dripping tabEOS can be compared to a dripping tab

By many, ESD is seen as just one type of electrical stress. EOS on the other hand, describes a wide number of outcomes resulting from multiple stresses or root causes.
ESD does not require a “victim” or damaged product. There will be an ESD event if two objects are at different charge levels and a rapid, spontaneous transfer of an ElectroStatic charge between them occurs. An electrical stress can only become an overstress (as in EOS) if we’re aware of how much stress the “victim” (i.e. sensitive device) can withstand. One specification used to document these limits is the “Absolute Maximum Rating” (AMR). More on that in a little while. Back to EOS and ESD for now. The below image highlights the relationship and contrast between EOS and ESD:

Relationship-EOS-ESDRelationship between EOS and ESD [Source]

Generally speaking, EOS describes extreme signals other than ESD. The following table lists the main differences:

 ESD Event  EOS Event
 Cause

Rapid discharge of accumulated charge

Voltage and/or currents associated with operation of equipment or with power generating equipment
 Duration Once accumulated charge is consumed, ESD event can no longer manifest itself Lasts as long as originating signals; no inherent limitation
 Characteristics Have specific waveform which includes rapid rising edge and asymptotic read edge Can have any physically possible waveform as sources of EOS are often unpredictable
 Occurrence Non-periodic and non-repeatable (accumulation of charge cannot be guaranteed) Mostly (but not always) periodic and repeatable

Differences between EOS and ESD [Source]

The importance of Electrical Overstress (EOS)

Many failures in the electronics industry can be contributed to EOS. Yes, ESD has received a lot of attention over the past years. However, ESD represents only a small percentage of total EOS damages.

Typical causes of device failuresTypical causes of device failures [Source]

As explained further above, EOS and ESD are NOT the same thing. This is extremely important because:

  • EOS damages are much more common compared to failures caused by ESD.
  • A comprehensive ESD Control Programme will provide protection against ESD but not EOS.

Now that you have learnt what EOS is, how it’s different from ESD and that ESD protection is not effective for EOS damage, the obvious question will be “How can I protect my sensitive devices from EOS failures?”. That’s where we go back to our “Absolute Maximum Rating” (AMR) mentioned earlier.

Absolute Maximum Rating (AMR) and Electrical Overstress (EOS)

We’ve established earlier that EOS is caused by exceeding specific limits of a device, the so called Absolute Maximum Rating or AMR.
AMR represents “the point beyond which a device may be damaged by a particular stress” [Source].

Interpretation of AMR* [Source]

Interpretation of AMR* [Source]

*the yellow line represents the number of components suffering catastrophic damage

  • Region A is the safe operating area in which devices are to operate as anticipated.
  • Region B does not guarantee for the device to function as it should. No physical damage is expected in this area; however, if a device is operated in this region for extended periods of time, it may cause reliability problems.
  • The upper limit of region B represents the AMR. Issues will arise if a device is operated beyond this point.
  • Region C is the first area of electrical overstress causing latent failures.
  • Region D is the second area of electrical overstress causing immediate damages.

Protecting your sensitive devices from Electrical Overstress (EOS)

As already stated, ESD Protection measures are useless when it comes to protecting your sensitive devices from EOS. “Rather, improvement and mitigation of EOS failure causes will only advance through better communication between the supplier and the customer. This includes proper understanding of AMR, realistic specifications for it, finding the root cause of EOS damage incidents, and identifying the field and system application issues.” [Source]

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