ESD

How to Neutralize a Charge on an Object that Cannot be Grounded

Image scsstaticcontrol March 9, 2018 0

We have learned in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials.

But what do you do if an item in your EPA is essential to assembly and it cannot be grounded? Don’t sweat, not all hope is lost! Let us explain a couple of options which will allow you to use the non-groundable item in question.

Conductors and Insulators

In ESD Control, we differentiate items as conductors and insulators.

Materials that easily transfer electrons are called conductors. Examples of conductors are metals, carbon and the human body’s sweat layer.

Grounding cable snap with connection to a ground. — *A charged conductor can transfer electrons which allows it to be grounded*

Insulators are materials that do not easily transfer electrons are non-conductors by definition. Some well-known insulators are common plastics, polystyrene foam, and glass.

Plastic cup with charged electrons — *Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.*

Both, conductors and insulators, may become charged with static electricity and discharge.

Electrostatic charges can effectively be removed from conductive or dissipative conductors by grounding them. A non-conductive insulator will hold the electron charge and cannot be grounded and “conduct” the charge away.

Conductors and Insulators in an EPA

The first two fundamental principles of ESD Control are:

Ground all conductors (including people).
Remove all insulators.

To ground all conductors per the first ESD Control principal, all surfaces, products and people are electrically bonded to ground. Bonding means linking or connecting, usually through a resistance of between 1 and 10 megohms.

Wrist straps and worksurface mats are some of the most common devices used to remove static charges:

Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices.
Movable items (such as containers and tools) are bonded by standing on a bonded surface or being held by a bonded person.

If the static charge in question is on something that cannot be grounded, i.e. an insulator, then #2 of our ESD Control principles will kick in and insulators must be removed. Per the ESD Standard ANSI/ESD S20.20, “All nonessential insulators such as coffee cups, food wrappers and personal items shall be removed from the EPA.” [ANSI/ESD S20.20 clause 8.3.1 Insulators]

The ESD Standard differentiates between these two options:

If the field measured on the insulator is greater than 2000 volts/inch, keep it at a minimum distance of 12 inches from the ESDs or
If the field measured on the insulator is greater than 125 volts/inch, keep it at a minimum distance of 1 inch from the ESDs.

Moving an insulated keyboard away from ESD sensitive workspace — *Aim to keep insulators away from ESDs*

“Process-Essential” Insulators

Well, nothing in life is black and white. It would be easy if we were always able to follow the above ESD Control ‘rules’ but there are situations where said insulator is an item used at the workstation, e.g. hand tools. They are “process-essential” insulators – you cannot remove them from the EPA or the job won’t get done.

How do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA?

Here are four ways to reduce the ESD risk of these insulators:

Keep all insulators a minimum of 1 inch or 12 inches from ESDs at all times per recommendation of the ESD Standard.
This reduces the chance of insulators coming in contact with ESDs during workstation processes and assembly.
Replace regular insulative items with an ESD protective version.
There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers, soldering tools, brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench.
Periodically apply Topical Antistat on non-ESD surfaces.
After Topical Antistat has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts. It therefore gives non-ESD surfaces electrical properties until the hard coat is worn away.
Neutralization with Ionization
If these three options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralization using an ionizer becomes a necessary part of your ESD control program. This allows for control of charged particles that can cause ESD events which we will cover next.

Neutralization

Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionization.

Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.

Electronic enclosures are process-essential insulators (shown on ESD workstation) — *Electronic enclosures are process-essential insulators*

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

An ionizer creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionization can neutralize static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.

The charged ions created by an ionizer will:

neutralize charges on process required insulators,
neutralize charges on non- essential insulators,
neutralize isolated conductors and
minimize triboelectric charging.

SCS Benchtop ionizer on a workstation removing charges from isolated conductors on PCB Board — *Insulators and isolated conductors are common in ESDs – Ionizers can help*

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

Summary

The best way to keep electrostatic sensitive devices (ESDs) from damage is to ground all conductive objects and remove insulators. This is not always possible because some insulators are “process-essential” and are necessary to build or assemble the ESDs.

Insulators, by definition, are non-conductors and therefore cannot be grounded, but they can be controlled to minimize potential ESD damage.

Insulators can be controlled by doing the following within an EPA:

Keep insulators a minimum distance from ESDS at all times (1 or 12 inch minimum distance depending on field voltage measurements of the insulator per ESD Standard recommendation)
Replace regular insulative items with ESD protective versions
Periodically apply a coat of Topical Antistat
Neutralize charges for “process-essential” insulators with ionization

With these steps added to your ESD control process, all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential in an ESD Protected Area (EPA) to reduce the risk of ESD events and ESD damage.

How to Reduce the Risk of Damaging ESD Sensitive Devices in Critical Applications

Image scsstaticcontrol February 22, 2018 0

Do your employees handle ESD-sensitive high-end components that are expensive to replace if they failed? If so, reducing the possibility of ESD damage is an important part of an ESD control program. Today’s blog post will look at one option of protecting your critical applications: Dual-Wire Wrist Straps.

Introduction

In an ESD Protected Area (EPA), all surfaces, objects, people and ESD sensitive devices (ESDs) are kept at the same electric potential. This is achieved by using only ‘groundable’ materials that are then linked to ground.

This is in line with the requirements of ANSI/ESD S20.20: “The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
– Training
– Product Qualification
– Compliance Verification
– Grounding / Equipotential Bonding Systems
– Personnel Grounding
– ESD Protected Area (EPA) Requirements
– Packaging Systems
– Marking”
[ANSI/ESD S20.20 clause 7.1 ESD Control Program Plan]

Wrist Straps

Wrist straps are the most common personnel grounding device and are used to link people to ground. They are required if the operator is sitting.

A wrist strap is made up of two components:

A wrist band that is worn comfortably around your wrist and
A coiled cord that connects the band to a Common Grounding Point.

Components of a Wrist Strap

Dual-Wire Wrist Straps

Dual-Wire Wrist Straps have two conductors (compared to single-wire monitors which have only one conductor inside the insulation of the coiled cord). They offer a reduced risk of damaging ESD sensitive devices because if one conductor is severed or damaged, the operator still has a reliable path-to-ground with the second conductor. For that reason, they dual-wire wrist straps are generally used in critical applications.

Advantages of using Dual-Wire Wrist Straps:

Elimination of intermittent failures
Extension of wrist strap lifespan
Compatible with high performance continuous monitors

The MagSnap 360™ Dual-Wire Wrist Strap and Coil Cord – more information

Dual-Wire Continuous Monitors

For maximum benefit, dual-wire wrist straps should be used together with dual-wire continuous monitors. Instead of connecting a coil cord directly to a common grounding point, the operator connects to a continuous monitor. The operator is grounded through the continuous monitor and the operator-to-ground connection is monitored.

The monitors provide operators with instant feedback on the status and functionality of their wrist strap and/or workstation. Continuous monitors detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” which could result in damage to ESD sensitive components. The “intermittent” stage is characterized by sporadic failures as the cord is not completely severed. Once the cord is fully split, the “open” stage is reached.

The WS Aware Dual-Wire Workstation Monitor – more information

“Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task.” “While effective at the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge. If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift’s work could be suspect.” “Wrist strap checkers are usually placed in a central location for all to use. Wrist straps are stressed and flexed to their limits at a workstation. While a wrist strap is being checked, it is not stressed, as it would be under working conditions. Opens in the wire at the coiled cord’s strain relief are sometimes only detected under stress.“ [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]

Resistance (or dual-wire) constant monitors are “… used with a two wire (dual) wrist strap. When a person is wearing a wrist strap, the monitor observes the resistance of the loop, consisting of a wire, a person, a wristband, and a second wire. If any part of the loop should open (become disconnected or have out of limit resistance), the circuit will go into the alarm state.” “While the continuity of the loop is monitored, the connection of the wrist strap to ground is not monitored.” “There are two types of signals used by resistance based constant monitors; steady state DC and pulsed DC. Pulsed DC signals were developed because of concerns about skin irritation. However, pulse DC units introduce periods of off time (seconds) when the system is not being monitored.“ [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]

Conclusion

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

And there you have it: dual-wire wrist straps together with dual-wire continuous monitors offer better protection than intermittent monitoring or testing if you have a critical application.

Check-out the SCS Wrist Strap Selection Guide and Workstation Monitor Selection Guide to find the correct products for your application.

Visit SCS at the 2018 IPC Apex Expo in San Diego, CA – February 27 – March 1 Booth#3749

scsstaticcontrol January 26, 2018 0

Visit SCS at the 2018 IPC Apex Expo in San Diego, CA (Feb. 27 – March 1) Booth#3749. See a demo of next gen ESD control: SCS Static Management Program (SMP).

An Introduction to Wrist Straps

Image scsstaticcontrol November 30, 2017 3

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 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 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 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 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!

The importance of employees in the fight against ESD

Image scsstaticcontrol November 16, 2017 0

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]

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.

EOS and ESD

Image scsstaticcontrol June 15, 2017 1

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 recognized 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]

*ESD 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]

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 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 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 Program will provide protection against ESD but not EOS.

Now that you have learned 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].

*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]

References:

Industry Council on ESD Targets: White Paper 4: Understanding Electrical Overstress – EOS
In Compliance: Rethinking Electrical Overstress
EEWeb: Electric Overstress (EOS) and Its Effects on Today’s Manufacturing
Dangelmayer Associates/ESD Association: Electrical Overstress – Many Sources; Any Solutions?

Introduction to ESD

Image scsstaticcontrol May 4, 2017 4

Welcome to this little corner of the interwebs! Today marks the beginning of something truly amazing: our very own blog! You’re currently reading through our first blog post and we appreciate you taking time out of your busy life and spending it here.

The intention of this blog is to provide you with resources, information and tools – all focused around ESD! So, if you have any ideas for future blog posts or suggestions on what we could do better, don’t be a stranger and leave a comment! We look forward to hearing from you.

Now, for this first post we thought we’d start right at the beginning: what is ESD? It’s the core of our business but a lot of people don’t understand what it’s all about so let’s clear that up – right here and right now!

ElectroStatic Charge
Everything you see around you is made from atoms – your mouse, keyboard, screen, cup of coffee etc. Every atom is constructed of a nucleus, which includes positively charged protons, and one or more negatively charged electrons bound to the nucleus. As atoms have an equal number of electrons and protons, it balances out having no charge. No problems so far!

Structure of an Atom (Source)

Unfortunately, all materials can tribocharge and generate ElectroStatic charges. Most of the time this happens through contact and separation; some everyday life examples are:

Opening a plastic bag
Combing hair
Walking across a floor

Walking across a floor.png Walking across a floor can generate an ElectoStatic Charge

“For most people, static electricity is represented by the noise or crackle heard on a radio that interferes with good reception or the shock experienced when touching a metal object after walking across a carpeted room or sliding across a car seat. Static electricity is also observed as static cling when clothes are stuck together after coming out of a clothes dryer. Most of the time, people observe static electricity when the weather is cold and dry.” “While many people tend to think of static electricity as being at rest or not moving, static electricity causes the most concern when it ceases to be stationary.” [ESD Handbook ESD TR20.20 section 2.1 Basics of Static Electricity, Introduction]

When two materials make contact and are then separated, a transfer of electrons from one surface to the other may take place. The amount of static electricity generated depends upon the materials subjected to contact or separation, friction, the area of contact or separation, and the relative humidity of the environment. At lower relative humidity (as the environment is drier) charge generation will increase significantly. Common plastics generally will create the greatest static charges.
“Electrostatic charge is most commonly formed by the contact and separation of two materials. The materials may be similar or dissimilar although dissimilar materials tend to liberate higher levels of static charge. An example is a person walking across the floor. Static electricity is produced when the person’s shoe soles make contact, then separate from the floor surface. Another example is an electronic device sliding into or out of a bag, magazine or tube.” [ESD Handbook ESD TR20.20 section 2.3 Nature of Static Electricity]

ElectroStatic Discharge (ESD)
If two items are at different ElectroStatic charge levels (i.e. one is positively and the other negatively charged) and approach one another, a spark or ElectroStatic Discharge (ESD) can occur. This rapid, spontaneous transfer of an ElectroStatic charge can generate heat and melt circuitry in electronic components.

ElectroStatic Discharge (ESD)

ESD events are happening around us all the time – yet, most of these cannot be seen or felt. For a person to sense ElectroStatic Discharge (ESD) (the dreaded ‘zap’), a discharge of about 2,000V is needed. To actually see ESD (in form of an arc, e.g. lightning) even greater voltages are required.

While ESD in your home can be annoying, it’s generally harmless. However, in the electronics industry ESD is the hidden enemy. Damages caused by invisible and undetectable ESD 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. In the electronics industry, it’s ESD Protection; we will get into more detail on that in a later post.

Many of the common activities you perform daily may generate charges on your body that are potentially harmful to electronic components. Some of these activities include:

Walking across a carpet: 1,500V to 35,000V
Walking over untreated vinyl floor: 250V to 12,000V
Worker at a bench: 700V to 6,000V
Picking up a common plastic bag from a bench: 1,200V to 20,000V

Many of the CMOS technology components can be damaged by discharges of less than 1,000 volts. Some of the very sophisticated components can be damaged by charges as low as 10 volts.

Types of ESD Device Damage
So, we’ve established what ESD is and learned that ESD can damage electronics components. But what exactly does this damage look like? We’re so glad you asked!

The industry differentiates between catastrophic failures and latent defects. Per ESD Handbook ESD TR20.20 section 2.7 Device Damage – Types and Causes “Electrostatic damage to electronic devices can occur at any point, from the manufacture of the device to field service of systems. Damage results from handling the devices in uncontrolled surroundings or when poor ESD control practices are used. Generally damage can manifest itself as a catastrophic failure, parametric change or undetected parametric change (latent defect).”

Catastrophic failures occur when a component is damaged to the point where it is DEAD NOW and will never again function. In these cases, the ESD event may have caused a metal melt, junction breakdown or oxide failure. This is the easiest type of ESD damage to find since it can be detected during inspection and testing.
“When an electronic device is exposed to an ESD event it may no longer function. The ESD event may have caused a metal melt, junction breakdown, or oxide failure. The device’s circuitry is permanently damaged, resulting in a catastrophic failure.” [ESD Handbook ESD TR20.20 section 2.7.1 Catastrophic Failures]

Catastrophic Failures

Catastrophic failures will lead to completely failed or dead components.

Latent defects occur when ESD weakens or wounds the component to the point where it will still function properly during testing, but over time the wounded component may cause poor system performance. Later, after final inspection, perhaps in the hands of your customer, a latent defect may become a catastrophic failure.
“A device that is exposed to an ESD event may be partially degraded, yet continue to perform its intended function. However, the operating life of the device may be reduced dramatically. A product or system incorporating devices with latent defects may experience a premature failure after the user places them in service. Such failures are usually costly to repair and in some applications may create personnel hazards.” It is easy with the proper equipment to confirm that a device has experienced catastrophic failure or that a part is degraded or fails test parameters. Basic performance tests will substantiate device damage. However, latent defects are virtually impossible to prove or detect using current technology, especially after the device is assembled into a finished product. Some studies claim that the number of devices shipped to users with latent defects exceeds the number that fail catastrophically due to ESD in manufacturing.” [ESD Handbook ESD TR20.20 section 2.7.2 Latent Defects]

Latent Defects

Latent defects lead to degraded or wounded components

Costly Effects of ESD
Catastrophic failures are straight forward: they can be detected and repaired at an early manufacturing stage. This is the least costly type of ESD damage.

Latent defects on the other hand are not only hard to find, but they can also severely affect the reputation of your company’s product. Latent defects can cause upset or intermittent failures and can be very frustrating: customers return a product with a problem which the factory fail to detect so it ends up at the customer’s again with the problem unresolved.

ESD Damage on an Integrated Circuit

The cost for repairing latent defects increases as detection of the failure moves through the system. One study indicated the repair cost to be:

$10 Device
$10 Device in board: $100
$10 Device in board and in system: $1,000
$10 Device and system fails: $10,000

Industry experts estimate that product losses in the electronics industry due to static discharge range from 8 to 33%. Others believe the actual cost of ESD damage amount to billions of dollars annually.

Conclusion
ESD is the hidden enemy in the electronics industry:

It cannot be felt
It cannot be seen
It cannot necessarily be detected through normal inspection procedures.

Therefore, it is absolutely crucial to be aware of the most sensitive items in your factory. Technology advances all the time: electronic circuitry gets progressively smaller which leads to a reduction of microscopic spacing of insulators and circuits within components. “Electronic items continued to become smaller, faster and their susceptibility to static damage increased…all electronic devices required some form of electrostatic control to assure continued operation and product reliability.” [ESD Handbook ESD TR20.20 section 2.2]. While this is great news for the consumer with better, faster and stronger computers, tablets, phones etc., it’s bad news for the manufacturers. The evolution of technology leads to devices being even more sensitive to ESD. As a result, the need for appropriate ESD Protection is now more important than ever.

Over the next few weeks, we will provide you with all the tips, tools and techniques to create an effective ESD Control Program so that your sensitive components are protected against damages from ESD.