HBM vs. CDM

As previously explained, an Electrostatic Discharge is a rapid, spontaneous transfer of an electrostatic charge induced by a high electrostatic field through a spark between two bodies at different electrostatic potentials as they approach or are separated from one another.
The ESD Association characterises three models of discharge, Human Body Model (HBM), Charged Device Model (CDM) and Machine Model (MM). Each model is intended to follow specific discharge properties such as the rise and fall times of the discharge current waveform.

Today, we will discuss HBM and CDM.

Human Body Model (HBM) simulates a person becoming charged and discharging from a bare finger to ground through the circuit under test. Humans are considered a primary source of ESD and HBM can be used to describe an ESD event due to the combination of the capacitance of a human body and resistance of skin touching a sensitive component. Typically, you need to pay better attention to personnel grounding to eliminate HBM.

Per ESD Handbook ESD TR20.20-2016 section 3.4.1 Human Body Model (HBM): “HBM has been in use for over 100 years. It was first defined to allow measurement and evaluation of explosion hazards for underground mining operations. There are a few different test standards describing the HBM for military and commercial applications, but the differences are in the application of the test, calibration of the system, and other ancillary items. The waveform, as defined by the human body resistance and capacitance, is virtually identical among all the test standards. The most widely used standard is ANSI/ESDA/JEDEC JS-001. The HBM test standard uses a stressing circuit which charges a 100 pF capacitor to a known voltage and discharges through a 1500-ohm resistor as shown in Figure 3. The simulators are verified by measuring various features of the current waveform, some of which are shown in Figure 4. Full details for tester qualification and waveform verification are described in ANSI/ESDA/JEDEC JS-001.

50528inuseAn operator handling an ESD sensitive device

Charged Device Model (CDM) simulates an integrated circuit becoming charged and discharging to a grounded metal surface. CDM can be used to describe an ESD event due to an integrated circuit that is suspended on a vacuum pick and then placed on a metal surface during assembly.
Manual operation and handling is much less likely these days as operations have become more automated. CDM is the most pragmatic discharge model in automated production today. Anytime a sensitive device is lifted from a tray and transported it is most likely generating a charge.

Per ESD Handbook ESD TR20.20-2016 section 3.4.2 Charged Device Model (CDM): “In principle, there are two variations of CDM. The first considers the situation of a device that is charged (through tribocharging) on its package, lead frame, or other conductive paths followed by a rapid discharge to ground through one pin or connector. The second considers the situation of a device which is placed in an electric field due to the presence of a charged object near the device. The device’s electrostatic potential is increased by this field. This process is sometimes referred to as static induction. The device will discharge if it is grounded while still in the electric field. In both cases, the device will discharge, the failure mode will be the same, and the failure type and location will be the same. The most widely used CDM standards use the static induction approach. In CDM simulators, the device is grounded by a pogo pin contacting one pin or lead of the device. The current through the pogo pin can be measured and recorded which is particularly important as the discharge current determines the ESD threshold, a schematic of this is shown in Figure 5.
Experimental results show that the CDM discharge current is very fast, with rise-times measured often below 100 ps with a “pulse width” (full width half-maximum [FWHM]) of less than 500 ps to1 ns, an example waveform with the key parameters is shown in Figure 6. By comparison, the HBM discharge has a typical rise-time of 2 to 10 ns and durations of hundreds of ns. Until 2014, the most commonly used CDM standards were JEDEC JESD22-C101 or ANSI/ESD STM5.3.1. These have now been superseded by ANSI/ESDA/JEDEC JS-002.

So, why does it matter?
Different types of discharge can affect devices in different ways. HBM is a somewhat slow discharge and ranges from 10 to 30 nanoseconds. CDM is a very fast discharge which in turn means the energy has no time to dissipate. The CDM-type damage threshold is often 10 to 20 times lower than the one for an HBM-type discharge. If an HBM-type discharge causes damage at 2000V, it is not uncommon to have the same component damaged by a 100 to 150V CDM event.

Per ESD Handbook ESD TR20.20-2016 section 3.2.1: “ESD threats in Electronic Production Lines ESD threats in electronics manufacturing can be classified into three major categories: 

  • Charged personnel – When one walks across a floor a static charge accumulates on the body. Simple contact of a finger to a device lead of a sensitive device or assembly which is on a different potential, e.g., grounded, allows the rapid transfer of charge to the device.
  • Charged (floating) conductor – If conductive elements of production equipment are not reliably connected to ground, these elements may be charged due to triboelectric charging or induction. Then these conductive elements may transfer charge to a device or assembly which is at a different potential.
  • Charged device/boards – During handling, devices or boards can acquire a static charge through triboelectric charging or can acquire an elevated electrostatic potential in the field of nearby charged objects. In these conditions, contact with ground or another conducting object at a different electrostatic potential will produce a very fast ESD transient.

This categorization is useful in that each category implies a set of ESD controls to be applied in the workplace. ESD threats from personnel are minimized by grounding personnel through the use of wrist straps and/or footwear/flooring systems. Discharges from conductive objects are avoided by assuring that all conductive parts that might contact devices are adequately and reliably grounded. The occurrence of ESD involving charged devices or boards is minimized by a) preventing charge generation (low-charging materials, ionization) or b) by providing low-current “soft landings” using dissipative materials.
Since these preventive measures are seldom perfectly deployed, the overall threat of ESD failure remains and the risk ultimately depends on how well the controls are maintained and the relative sensitivities of the devices being handled.”

Taking Action
Desco Europe recommends reviewing your manufacturing process and determining what model is the most relevant for your facility. Are your components handled directly by hand or by a hand tool such as tweezers or a vacuum pick?
Finding the root cause of ESD events is crucial to solving the problem. Desco Europe technology can identify events in areas like SMT line, soldering, printer and repair stations. Desco Europe has instrumentation to identify component sensitivity and measure ESD events as well as ensure compliance within your facility:

  • The SCS CTM082 ESD Pro Event Indicator has a special CDM filter switch to filter and reject EMI signals that are not caused by CDM discharges. Make sure to set requirements for static voltage and discharge strength within your production environment based on the most sensitive component in production.
  • The SCS CTM048-21 EM Eye ESD Event Meter will calculate the event magnitude for HBM and CDM. It also logs the events to a microSD card so they can be downloaded to a PC. Solving ESD problems requires data; a before-and-after analysis of data may now be measured and used to tailor your ESD control program.
  • The SCS 770063 EM Aware Monitor is ideal for automated equipment and will provide an approximate voltage for the ESD event based on HBM and CDM models. The EM Aware Monitor has Ethernet network connectivity and communicates with our Static Management Program (SMP). All activity is stored into a database for on-going quality control purposes. SMP allows you to pinpoint areas of concern and prevent ESD events. Quantifiable data allows you to see trends, become more proactive and prove the efficiency of your ESD process control system.
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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.

Resistivity and Resistance in ESD Control

There is a lot of confusion out there as to what the difference is between resistivity and resistance. We get asked questions on a regular basis so hopefully this post will put an end to any misunderstanding – we’ll explain the difference between the two and will point out the measurements you really need to worry about when it comes to your ESD Control Programme.

The difference between Resistivity and Resistance
Resistance or resistivity measurements help define the material’s ability to provide electrostatic shielding or charge dissipation.“ [Source]
However, resistance and resistivity values are not interchangeable. Let’s get a bit technical here to illustrate the difference between the two:

1. The resistance expresses the ability of a material to conduct electricity. It is therefore related to current and voltage. In fact, the surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
With a pure resistive material,

Resistance Calculation

where:

– R is the resistance (expressed in Ohm Ω),
U is the voltage (expressed in Volt) and
I is the current (expressed in Amp).

The unit of measure for surface resistance is W. It is important to remember that the surface resistance of a material is dependent on the electrodes used (shape as well as distance). If your company implements an ESD Control Programme compliant to the ESD Standard EN 61340-5-1, it is therefore vital to carry out surface resistance measurements as described in the Standard itself.

2. The surface resistivity of a material describes a general physical property. It is not influenced by the shape of the electrodes used or the distance between them. “Surface resistivity, ρ,  can  be  defined  for electric current flowing across a surface as the ratio of DC voltage drop per unit length to the surface current per unit width.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
As Dr. Jaakko Paasi describes in his research note, surface resistivity can be expressed by using a concentric ring probe as  

Resistivity Calculation

where:

– k is the geometrical coefficient of the electrode assembly,
rcentre is the outside radius of the centre electrode and
router is the inside radius of the outer electrode.

For the electrodes recommended by EN 61340-5-1, the coefficient k = 10.
The unit of measure for surface resistivity is W but in practice you will often see W/square (which technically is not a physical unit).

As previously explained, the surface resistivity does not depend on shape or distance of the electrodes used when performing the test. You can compare results freely – no matter what type of electrode was used to get the measurements in the first place.

Converting from Resistivity to Resistance
Values of surface resistance and surface resistivity become comparable if the measured surface resistance value is multiplied by the geometrical coefficient of the used electrode fixture.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
If you measure surface resistance according to EN 61340-5-1, then the corresponding surface resistivity can be calculated by multiplying the resistance value by the geometrical coefficient factor k = 10. Likewise, surface resistivities can be converted to surface resistances by dividing the surface resistivity value by 10.

Per User guide EN 61340-5-2:1999 Clause 4.1.1 “Point-to-point resistance has been discussed, rather than the surface and volume resistivity which was found in previous standards and reports. This change has been made to cater for non-homogenous materials, which are becoming increasingly common in these applications, as well as ease of measurement.
Particular care is needed in interpreting results when measuring non-homogeneous materials such as multilayer mats or conductive-backed synthetic fibre carpeting containing a small amount of conductive fibre. Buried conductive layers can provide shunt paths. Be clear when stating what you have measured!

A few notes in regards to measuring surface resistance and resistivity:

  • On large surfaces, such as bench-mats, readings will sometimes vary with increasing time of measurement. This is due to the ‘electrification’ of the mat beyond the area measured. It is therefore important to measure properly and to keep the duration of measurement constant. Fifteen seconds is an arbitrary but practical duration for measurement time.
  • Moreover, the materials needing to be checked in an EPA are most of the time, non-conductive polymers that have been made conductive or antistatic by addition of conductive particles or by special treatments during manufacture. The resistivity of such materials may vary from one point to another or they may be direction dependent (anisotropic).
  • EN 61340-5-1 goes some way to specifying the procedures to be followed and test probes to be used, so that the results can be compared, at least roughly.
  • Also, the resistance of some materials may vary with humidity level and temperature. It is therefore good practice to take a note of these two parameters when measuring.

So now that we’ve identified what the difference is between surface resistance and resistivity, there is one more thing we want to cover in today’s post: the different types of surface resistances you will come across when dealing with ESD and how to measure them:

1. Resistance to Ground (Rg)
Resistance to Ground is a measurement that indicates the capability of an item to conduct an electrical charge (current flow) to an attached ground connection. The higher the resistance in the path, the more slowly the charge will move though that defined path.” [Source]
The Resistance to Ground is measured to ensure that surfaces in an EPA are correctly grounded. This is certainly one of the most useful measurements in an EPA.

Performing a Resistance to Ground TestPerforming a Resistance to Ground Test

To perform the test:

  • One 2.3kg cylindrical probe is required for this measurement.
  • Connect the probe to a megohm meter and place it on the surface to test.
  • Connect the other ohmmeter lead to earth or to an ESD ground point.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.

2. Resistance Point-To-Point (Rp-p)
A point-to-point measurement used during the qualification process evaluates floor and worksurface materials, garments, chair elements, some packaging items, and many other static-control materials.“ [Source]
Resistance Point-To-Point is used to assess the performance of an item used in an EPA.

Performing a Resistance Point-To-Point Test

To perform the test:

  • Two 2.3kg cylindrical probes are required for this measurement
  • Connect the probes to a megohm meter.
  • Place the material to be tested on an insulative surface such as clean glass and place the probes on the material.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.
  • Move the probes so as to measure in a cross direction and repeat the test.

Point-to-point measurements are important during the qualification process for proper evaluation of flooring and worksurface materials. After installation, the resistance-to-ground measurement is more applicable since it emulates how the material really behaves in practice.” [Source]

3. Volume Resistance (RV)
Although this is one of the less common measurements when it comes to ESD, it’s still worth to mention the volume resistance here. You would measure the volume resistance when a non-grounded item such as a container is to be placed on a grounded item, such as a mat. The volume resistance will indicate whether the item can be used in the desired manner.

Performing a Volume Resistance TestPerforming a Volume Resistance Test

To perform the test:

  • Two 2.3kg cylindrical probes are required for this measurement
  • Connect the probes to a megohm meter.
  • Put the first probe upside down and ‘sandwich’ the test sample between it and the second probe placed on top.
  • Measure the resistance.

So hopefully we have put an end to any confusion in regards to surface resistivity and resistance and answered all your questions. If there is anything else you’d like to know, let us know in the comments.

References:

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]

References:

Is ESD Control a Waste of Time (and Money)?

Today’s post is going to be a bit shorter than usual. BUT: that doesn’t mean it’s going to be any less interesting. Quite the opposite! So, let’s jump right in.

A little while ago we were approached by a customer with the following statement:

Generally speaking, most IC’s these days already have adequate protection on their pins, the notable exception being discrete J-FETs, and MOSFETs, especially for RF applications.
It’s difficult to advise when these might be in use on an assembly without giving everyone in-depth training on circuit design, so to avoid trouble in the 1% of cases that matter, it’s a good idea to play safe and keep applying our procedures for the other 99% of parts too.
I am of the opinion that a PCCU in its housing does not need special treatment though. It has ESD protection, and has passed testing for this, so I am not worried about someone touching its pins without wearing a grounded wristband, etc …

So, is this statement true? Is ESD Control obsolete? Let’s find out!

Types of ESD Damage

Remember that there are two types of ESD damage:
1) catastrophic failure and
2) latent defects.

While catastrophic failures cause an ESD sensitive item to be damaged permanently, latent defects only partially degrade an ESD sensitive item that is exposed to an ESD event. It may continue to perform its intended function and may not be detected by normal inspection. However, intermittent or permanent failures may occur later.

Bottom line: Even if an ESD sensitive component has quite a high withstand voltage and no catastrophic failure has been caused, latent defects may still make your life miserable.

Continued Requirement for ESD Control

Here is a link to the ESD Association’s ESD Technology Roadmap. The document illustrates what future thresholds are expected for ESD sensitive devices and how they impact on ESD Control. The thresholds are determined by current trends in the semiconductor industry and are displayed as “roadmaps”. The aim is to predict future limitations of device protection which are driven by performance requirements and technology scaling.

You should head-over now and read through the document. But in case you haven’t got time, here are the main take-away notes:

  • Finally, these trends point to the need for continued improvements in ESD control procedures and compliance.” [section 1.0 Synopsis]
  • Therefore, the prevailing trend will be circuit performance at the expense of ESD protection levels.” [section 2.1 Overview]
  • Therefore, implementation of advanced HBM controls using the limits and qualifications requirements in ANSI/ESD S20.20, IEC 61340-5-1, or JESD625, would become necessary within the next 3 years.” [section 2.2 Device ESD Threshold Roadmaps]

Bottom line: As electronic technology advances, electronic circuitry gets progressively smaller. As the size of components is reduced, so is the microscopic spacing of insulators and circuits within them, increasing their sensitivity to ESD. Therefore, the need for proper ESD protection increases every day.

ESD control procedures and compliance continue to be requiredESD control procedures and compliance continue to be required

For more information on the ESD damage and the costly effects of ESD, check-out this post.

Protect your sensitive devices from ESD Damage

Every company should document the most ESD sensitive device that they are handling.
A prerequisite of ESD control is the accurate and consistent identification of ESD susceptible items. Some companies assume that all electronic components are ESD susceptible. However, others write their ESD control plan based on the device and item susceptibility or withstand voltage of the most sensitive components used in the facility. A general rule is to treat any device or component that is received in ESD packaging as an ESD susceptible item.
This post provides further information on how to set-up an ESD Control Plan.

So, tell us: are there instances in your company where you forego standard ESD Control practices? If so, let us know in the comments – we’d like to hear from you.

ESD Classifications

We were recently approached by a customer who wanted to know more about the different classifications of ESD products. So, we thought this would be a good opportunity to share more details with you. Be warned – this is a very theoretical post: so, loads of text but not too many pictures. We promise, we’ll have some more images in our next post!

Introduction

Part of every ESD Control plan is to identify items in your company that are sensitive to ESD. At the same time, you need to recognize the level of their sensitivity. As explained by the ESD Association, how susceptible to ESD a product is depends on the item’s ability to either:

  • dissipate the discharge energy or
  • withstand the levels of current.

Whilst some items are easily damaged by discharges arising within automated equipment, others may be more susceptible to damages from personnel when being handled.

There are three main classifications based on three different ESD models. There are detailed standards available from the ESD Association:

  • Human Body Model or HBM [100 pF @ 1.5 kilohms]: ANSI/ESDA-JEDEC JS-001-2010
  • Charge Device Model or CDM [4 pF/30 pF]: ESD DS5.3.1
  • Machine Model or MM [200 pF @ 0 ohms]: ESD STM5.2

The two primary models used for ESD events today are the Human Body Model (HBM) and Charged Device Model (CDM).

The Human Body Model (HBM)

The most common model is the HBM.  This model simulates discharge occurring between a human (hand/finger) and a conductor (metal rail). For this model, a 100 picofarads (100 x 10-12 Farads) capacitor is discharged through a 1,500 ohms resistor to simulate a human body. The typical rise time of the current pulse (ESD) through a shorting wire averages 6 nanoseconds (6 x 10-9 s) and is larger for a higher resistant load.  The peak current through a 500 ohm resistor averages 463 mA for a 1,000 volt pre-charge voltage.

If a device has failed, it does not meet the parameters outlined in the datasheet.

Class Voltage Range
Class 0 <250 volts
Class 1A 250 volts to <500 volts
Class 1B 500 volts to <1,000 volts
Class 1C 1,000 volts to <2,000 volts
Class 2 2,000 volts to <4,000 volts
Class 3A 4,000 volts to <8,000 volts
Class 3B ≥ 8,000 volts

ESDS Component Sensitivity Classification for the Human Body Model (Per ESD-STM5.1)

Charged Device Model (CDM)

This is the most neglected one of the three models but it can severely compromise your ESD control programme. Here, it is the ESDS device itself that becomes charged (sliding out of a tube/bag/sorter/etc.) and when contacting a grounded conductor (table top/hand/metal tool) it will discharge to that conductor and may result in damaging ESD. The length of the discharge may be very short (less than 1 nanosecond) – however, the peak current can reach a high amperage.

The model uses a 4 pF or 30 pF verification module which can simulate from 2 to 30 Amps peak current for non-socked and up to 18 amps for socketed devices.

Class Voltage Range
Class C1 <125 volts
Class C2 125 volts to <250 volts
Class C3 250 volts to<500 volts
Class C4 500 volts to <1,000 volts
Class C5 1,000 volts to <1,500 volts
Class C6 1,500 volts to <2,000 volts
Class C7 ≥ 2,000 volts

ESDS Component Sensitivity Classification for the Charged Device Model (Per ESD-STM5.3.1)

Machine Model (MM)

This model simulates a machine discharging through a device to ground. When checking components to the Machine Model (MM), the test replicates MM failures and tells you the MM ESD sensitivity levels for your devices. The criteria is 200 pF at nominal 0 ohms.

Class Voltage Range
Class M1 <100 volts
Class M2 100 volts to <200 volts
Class M3 200 volts to <400 volts
Class M4 ≥ 400 volts

ESDS Component Sensitivity Classification for the Machine Model (Per ESD-STM5.2)

Each component in your company should be fully classified using HBM and CDM. That means an item may have a Class 2 (HBM) and Class C1 (CDM).

Bear in mind that these classifications are guides only and do not represent the real world. However, they can be used to:

  • Develop and measure suitable on-chip protection.
  • Enable comparisons to be made between devices.
  • Provide a system of ESD sensitivity classification to assist in the ESD design and monitoring requirements of the manufacturing and assembly environments.
  • Have documented test procedures to ensure reliable and repeatable results.” [Source]

References:

  • ESD Association, Inc.: Device Sensitivity and Testing
  • ANSI/ESDA-JEDEC JS-001-2010: Electrostatic Discharge Sensitivity Testing — Human Body Model
  • ESD STM5.2-2009: Electrostatic Discharge Sensitivity Testing — Machine Model
  • ESD STM5.3.1-2009: Electrostatic Discharge Sensitivity Testing — Charged Device Model
  • ANSI/ESDA/JEDEC JS-002-2014: Joint Standard for Electrostatic Device Sensitivity Testing – Charged Device Model (CDM) – Device Level
  • IEC 60749-26: Semiconductor devices – Mechanical and climatic test methods – Part 26: Electrostatic discharge (ESD) sensitivity testing – Human body model (HBM)
  • IEC 60749-27: Semiconductor devices – Mechanical and climatic test methods – Part 27: Electrostatic discharge (ESD) sensitivity testing – Machine model (MM)

5 common mistakes in ESD Control & how to avoid them

Many companies implement an ESD Control Programme with the aim of improving their operations. Effective ESD control can be a key to improving:

  • Productivity,
  • Quality and
  • Customer satisfaction.

However, problems arise when an organisation invests in ESD protective products and/or equipment and then misuses them. Not only do these companies waste a lot of money but they could also be causing more harm than good. So, with today’s blog post we want to highlight some of the major issues we have come across and how you can avoid or fix them.

Introduction

Remember that for a successful ESD Control Programme, ESD protection is required throughout the manufacturing process: from goods-in to assembly all the way through to inspection. Anybody who handles electrical or electronic parts, assemblies or equipment that are susceptible to damage by electrostatic discharges should take necessary precautions.

Think of viruses or bacteria that can infect the human body. Just like ESD, they are invisible. Yet, in hospitals the defence against this hidden threat is controlled by extensive contamination control procedures including sterilisation. The same applies to ESD Control: you should never handle, assemble or repair electronic assemblies without taking adequate protective measures against ESD.

Treat ESD like Viruses & Bacteria
Treat ESD like you would Viruses and Bacteria

For an ESD Control Programme to be successful, there is discipline required; basic ESD Control principles should be followed:

  • Ground conductors.
  • Remove, convert or neutralise insulators with ionisers.
  • Shield ESD sensitive items when stored or transported outside the EPA.

For more information on how to get your ESD Control Programme off the ground (no pun intended) and create an EPA, check this post.

Common Mistakes in ESD Control

1. Poorly maintained or out-of-balance Ionisers

If an ioniser is out of balance, instead of neutralising charges, it will produce primarily positive or negative ions. This results in placing an electrostatic charge on items that are not grounded. These could then discharge to nearby sensitive items potentially cause ESD damage.

 Check Remember to clean emitter pins and filters using appropriate tools. Create a regular maintenance schedule which will extend the lifespan of your ionisers tremendously.
Consider using ionisers with “Clean Me” and//or “Balance” alarms. These will alert you when cleaning is required.
 Standard All ionization devices will require periodic maintenance for proper operation. Maintenance intervals for ionizers vary widely depending on the type of ionization equipment and use environment. Critical clean room use will generally require more frequent attention. It is important to set up a routine schedule for ionizer service.”
[CLC TR 61340-5-2 User guide Ionization clause 4.7.6.7 Maintenance and cleaning]

This post covers in detail how ionisers work and what type of ioniser will work best for your application.

2. Ungrounded ESD Garments

We’ve seen it so many times: operators wearing an ESD coat (without appropriate wrist straps and/or footwear/flooring) thinking they are properly grounded. Well, here is some news for you: you are not!

 Check Every ESD garment needs to be electrically bonded to the grounding system of the wearer. Otherwise it just acts as a floating conductor. There are a few options to choose from:
·         Wrist Straps
·         ESD footwear/flooring
·         Hip-to-Cuff grounding
 Standard The ESD risk provided by everyday clothing cannot be easily assessed. The current general view of experts is that the main source of ESD risk may occur where ESDS [ESD sensitive items] can reach high induced voltage due to external fields from the clothing, and subsequently experience a field induced CDM [Charged Device Model] type discharge. So ESD control garments may be of particular benefit where larger ESDS having low CDM withstand voltage are handled, and operators habitually wear everyday clothing that could generate electrostatic high fields.
[CLC TR 61340-5-2 User guide Garments clause 4.7.7.1 Introductory remarks]

Another thing to remember with ESD clothing is that they do lose their ESD properties over time. So make sure you incorporate periodic checks (see #3 below).

If you need more information on ESD coats, we recommend having a look at this post.

3. Not Checking ESD Control Products

A lot of companies waste thousands of pounds by buying and installing ESD Control products but then never check them resulting in ESD equipment that is out of specification. They haven’t got the tools in place to check their ESD items and have no idea if they are actually working correctly. Remember, ESD products (just like any other) are subject to wear and tear, workstations get moved, ground cords get disconnected…. The list goes on.

 Check When investing in ESD Control Products, make sure you also establish a Compliance Verification Plan. This ensures that:
·         ESD equipment is checked periodically and
·         Necessary test equipment is available.
 Standard A compliance verification plan shall be established to ensure the organization’s fulfilment of the requirements of the plan. Process monitoring (measurements) shall be conducted in accordance with a compliance verification plan that identifies the technical requirements to be verified, the measurement limits and the frequency at which those verifications shall occur. The compliance verification plan shall document the test methods used for process monitoring and measurements. If the organization uses different test methods to replace those of this standard, the organization shall be able to show that the results achieved correlate with the referenced standards. Where test methods are devised for testing items not covered in this standard, these shall be adequately documented including corresponding test limits. Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements.
The test equipment selected shall be capable of making the measurements defined in the compliance verification plan.

[EN 61340-5-1 clause 5.2.4 Compliance verification plan]

For detailed instructions on how to create a Compliance Verification Plan, have a read through this post.

4. Re-Using Shielding Bags with Holes or Scratches

ESD Shielding Bags are used to store and transport ESD sensitive items. When used properly, they create a Faraday Cage effect which causes charges to be conducted around the outside surface. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’. However, if the shielding layer of an ESD Shielding Bag is damaged, ESD sensitive items on the inside will not be protected anymore.

 Check Re-using shielding bags is acceptable as long as there is no damage to the shielding layer. Shielding bags with holes, tears or excessive wrinkles should be discarded.
Use a system of labels to identify when the bag has gone through five (5) handling cycles. When there are five broken labels, the bag is discarded.
 Standard ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area. Per Packaging Standard EN 61340-5-3 clause 5.3 Outside an EPA “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.

This post provides further “dos and don’ts” when using ESD Shielding Bags.

5. Using Household Cleaners on ESD Matting

A lot of people use standard household cleaners on their ESD matting not realising how damaging this is to their ESD Programme. Many household cleaners contain silicone which creates that lovely shine you get when wiping surfaces in your home. The problem is that on an ESD working surface mat, that same silicone creates an insulative layer which reduces the grounding performance of the mat.

 Check Don’t spend all this extra money on ESD matting and then coat it with an insulative layer by using household cleaners. There are many specially formulated ESD surface and mat cleaners available on the market. Only clean your ESD working surfaces using those cleaners.
 Standard “Periodic cleaning, following the manufacturer’s recommendations, is required to maintain proper electrical function of all work surfaces. Ensure that the cleaning products used to not leave an electrically insulative residue which is common with some household cleaners that contain silicone.”
[CLC TR 61340-5-2 User guide Work surfaces clause 4.7.1.5 Maintenance]

This post covers everything you need to know about ESD protective working surfaces.

Now – the above list is by no means complete. There are many more issues we see when setting foot into EPAs but we think it’s true to say that these issues are some of the ones we encounter more often.

What issues have you come across before? Leave us a comment below.

The link between ESD and EMI

Introduction to ESD and EMI

Current Electrostatic Discharge (ESD) control practices have substantially minimised the dangers from unwanted electrical overstresses that are known to haunt semiconductors and other micro-electrical devices at all stages of their manufacturing, handling and applications.
The act of grounding an ungrounded ESD Sensitive (ESDS) device can trigger an ESD event, yielding latent or catastrophic damage by means of an energy or voltage failure mechanism in the ESDS device. To minimise this potential problem, the rate of discharge must be controlled during grounding and the work potential at the grounding electrode must be increased [1]. Decreasing the rate of discharge will limit the current density of a potential electrical arc (ESD event). Any combination of an increase in resistance or capacitance in the contacting electrodes (the two materials that sustain a discharge) can decrease the rate of discharge and lessen the effects of an ESD Event.
One of the side effects from an electrostatic discharge (ESD) is an induced EMI (Electromagnetic Interference). An ESD-induced EMI in the near-vicinity of mission-critical equipment can cause data errors, temporary resets or even power-up resets requiring operator intervention [2]. This is caused by the EMI undergoing conversion to a voltage or current, which in turn corrupts the operation of the circuit/logic inputs.

The effects from undesired electromagnetic radiation, EMI, on ungrounded or unshielded conductors is commonly underestimated. An ESD event occurring outside an ESDS protective work area can still pose a risk to unshielded and ungrounded conductors within the critical work or ESDS area.

Case Examples of ESD/EMI Problems

Some examples of ESD/EMI problems reported from the Center for Devices and Radiological Health (CDRH) databases are listed by product recall numbers. Recall numbers M485337, M485338, M562311 (March 1994) state that static from bed sheets when a nurse was making the bed caused infusion pumps to sound a “processor lock-up alarm”. Recall number M249358 (October 1991) states that a discharge from an operator to the timer of a radiation therapy system caused the timer’s display to blank just as treatment began. Recall numbers Z3112, Z3212, Z3132, Z142 (January 1992) state that ESD affected infant radiant warmers, causing the heater to turn on or off, the alarm not to activate, and the display to become blank or corrupted, [9].
Today’s TTL and CMOS logic states have a logic “0” at 0.8 Volts or lower and a logic “1” at 2.0 Volts or higher. This leads to a smaller indeterminate range of 1.2 Volts for most TTL and some CMOS logic circuits and places the logic inputs from these circuit traces or cable connections susceptible to induced EMI voltages exceeding this range. One example of an ESD-induced EMI was characterised from office chairs [3, 4]. Induced voltages over 2 Volts have been measured on a printed circuit assembly (PCA) 90 cm from the furniture ESD [4]. 2 Volts is enough voltage to easily drive a TTL circuit let alone an ECL circuit into a logic error.

Table I lists some logic devices and their potential susceptibility to EM energies. Noise margin is a quantitative measure of a device’s noise immunity. The high-level DC noise margin in Table I is the difference between the minimum device output levels for a logic high VOH of the driving gate and the minimum input level VIH required by the driven gate to recognise a “1” logic state. The Indeterminate Range is the difference between the logic inputs’ low level maximum and high level minimum to differentiate between a logic “0” or “1”.
Some types of common lab stools and office chairs can radiate a series of impulsive fields from metal legs due to internal ESD when a person rises from the chair. As many as 12 pulses have been recorded within a 10 second period after a person rises from a chair [3].
Smith stated that a value of tens of millivolts per inch (~2 V/m) is generally not enough to affect digital logic whereas values over one volt/inch (40 V/m) are potential problems. One example observed induced voltages of over 4 volts/inch (>160 V/m) in cables one foot from one type of office chair [3].

Table1Table I – Table of Logic Families’ Power Transition, Noise Margin, and Indeterminate Range [8]

What often looks like software errors in process equipment may actually be caused by an external static charge (or discharge) problem. An ESD event anywhere in a room can cause an EMI. That EMI can couple into a system through cables or open chassis and induce a noise voltage greater than the logic inputs’ indeterminate range and cause a single event upset. EMI effects to microprocessors or other circuit logic latch-ups in process equipment can manifest itself in several ways, such as random hangs, robotic malfunction, or software errors, all resulting in downtime and reduced throughput.

Theoretical Energy Analysis

1. Mechanism of an ESD Event
There are three well-known methods to simulate an ESD Event: the human body model, the machine model and the charge device model. Each has its place to aid in designing the proper ESD Control Programme depending on the application.
Induced voltage from an EMI energy transfer to a logic input trace with typical area of 40mm2 could be as high as 485mV with an ESD-induced 100 MV/m field at 33cm as depicted in Table II. 485mV is enough voltage to flip a logic state of an ECL device as depicted in Table I. From the same ESD event, a data input cable with a receiving area of 40 cm2 can have an EMI-induced voltage of 4.85 which is enough voltage to drive a logic error in any family or subfamily of logic circuits; TTL, CMOS, & ECL.

Table2Table II – EMI Energy Transfer from an ESD to an Isolated Conductor using antenna theory where: the area of the conductor is A=variable, the distance from the source is R=1/3 meter, ESD has a 1 ns rise time and a 3 ns pulse width.

2. ESD Event
An ESD event can have a fast rise time, especially for low voltage discharges [5]. The waveform for an ESD event includes high-frequency components with a frequency range from DC to over 6GHz, [4]. This electromagnetic radiation (EM) can readily couple to circuit traces (conductors acting as antennae). For ungrounded conductors coupled within a capacitive circuit, this EM wave can induce a static charge, building until a discharge, breakdown, recombination or neutralisation occurs. High-speed circuits, by their nature, tend to be very susceptible to high-frequency signals such as those from a nearby ESD event.
The electrostatic field strength (Eo) just before an ESD is proportional to the charged voltage (V) at gap width δ. The gap width, δ, is defined by Paschen’s Law, but may vary in each discharge condition. The electric field strength Eo = V/δ where V is from 0.5kV to 30kV and δ is from 5μm to10mm, can yield an electric field strength as high as 6 GV/m. This extremely high field strength is attributed to a smaller gap width, δ = 5μm. It is important to note that the arc length of an ESD is of greater influence to its disturbance than its voltage [7].

3. EMI
An Electromagnetic Interference (EMI) is an unwanted electromagnetic energy, (whether intentionally or unintentionally generated), of almost any frequency and energy level. EMI is defined to exist when undesirable voltages or currents are present to adversely influence the performance of an electronic circuit or system. Sources of radiated electromagnetic energy from ESD are very common in today’s factories from furniture ESD, raised flooring ESD, Human Body ESD, hand held toolbox ESD and metal-to-metal ESD [3, 4, 6, 7]. An EMI, or summation of EMIs, can over time induce a charge (static voltage) on an ungrounded conductor coupled in a capacitive circuit, i.e., an isolated capacitor. An even more common occurrence is a single ESD induced EMI that can upset a logic circuit and cause systems errors. The very fast rise time of an ESD may be preserved if it flows through a metal conductor, resulting in radiated EMI.

Solutions

  • Assume that all electronic devices are susceptible to damage or logic error states from ESD and EMI, respectively; and take the proper precautions.
  • Proper grounding of isolated conductors and use of ground-planes near active conductors will minimise some of these effects.
  • Shielding the known emitting devices will help, but it is the unknown emitters that will cause the most problems. Thus, shielding the receptors, sensitive logic devices, will help combat EMI-induced logic errors. Start shielding at the device level, for it is less costly than at the system level.
  • Reduce ground-loop areas between interconnected equipment and systems. Route interconnected cables inside conduit, cable trays or raceways when possible. Do not coil excess cable into a helix, but rather fold back and forth to foil antenna gains.
  • Metal-to-metal discharges will always derive the largest current derivatives (di/dt) and hence generate the strongest EMI fields. Treat isolated conductors as charged devices and ground them with an electrically dissipative material (R > 104 Ohms). This will slow down the energy transfer from the conducted ESD causing the resultant EMI to be negligible to any active near or far field system.

Conclusion

A high energy ESD can drive a substantial EMI energy to couple and charge passive circuits or energise active circuits with significant system problems. EMC practices involving shielding designs typically account for EMI from known sources, but should also consider unplanned sources such as ESD events in the near vicinity of the active or sensitive system(s).
With today’s logic devices having smaller noise margins and indeterminate ranges, susceptibility to ESD-induced EMI should be accounted for in the design and implementation of the systems incorporating logic circuits.

White-Space

References:
R. C. Allen, “Controlling Workstation Discharge Times”, Evaluating Engineering, Jan. 1998, pp. 88-92.
G. Chase, “EMI from ESD – An Insidious Alliance”, NARTE News, Vol. 14, No 1, 1996, p 22.
Smith, “A New Type of Furniture ESD and Its Implications”, EOS/ESD Symposium Proceedings, EOS-15, 1993.
Y. Tonoya, K. Watanabe, and M. Honda, “Impulsive ESD Noise Occurred from an Office Chair”, EOS/ESD Symposium Proceedings, EOS-15, 1993.
S. Podgroski, J. Dunn, & R. Yeo, “Study of Picosecond Rise Time in Human-Generated ESD”, Proc. IEEE Int. Symp. Electromagnetic Compatibility, Cherry Hill, NJ, Aug. 12-16, 1991, pp. 263-264.
Y. Tonoya, K. Watanabe, and M. Honda, “Impulsive EMI Effects from ESD on Raised Floor”, EOS/ESD Proceedings, EOS-16, 1994.
D. Pommerenke, “Transient Fields of ESD”, ESO/ESD Symposium, pp. 150-159, 1994.
R. C. Dorf, The Electrical Engineering Handbook, 2nd Edition, CRC Press, pp. 1773-1777, 1997
J. Silberberg, “What Can/Should We Learn from Reports of Medical Device Electromagnetic Interference?”, FDA, Rockville, EMBC95 paper 10.2.1.3, 1995

Case Study: Vanilla Electronics & the SmartLog

Vanilla Electronics Ltd provide supply chain, kitting, manufacturing and logistics services to technology companies worldwide. A flexible and professional approach helping OEM’s seamlessly transition from design and prototyping, through to final box build, test, then direct end customer order fulfilment and reverse logistics.
In 2014, following another year of growth, Vanilla had the need to expand its manufacturing area and enhance all stores, picking and kitting areas. In addition, the construction of a new 1,500 sq ft cleanroom was required.

Vanilla Electronics - EPAVanilla’s ESD Protected Area (EPA)

As part of the £500k expansion project, Vanilla were looking to install an access control system for their ESD Protected Area (EPA) – the aim was three-fold:

  • Verify the functionality of an operator’s wrist strap and footwear.
  • Log the test results electronically.
  • Control access to the EPA.

Vanilla looked at different solutions, compared features and benefits of available systems and attended various demonstrations and presentations.

In the end, Vanilla decided to install the SmarLog V5™ which is designed for fast, frequent and accurate testing of ESD personnel grounding items. Its unique design embeds an ESD tester, time clock, keypad, barcode scanner, Ethernet module and optional proximity badge reader into a compact stainless steel enclosure. The included relay terminal can be used to promote access control for passed tests.

Dan Croft, Managing Director at Vanilla Electronics, explains the decision: “The Smartlog’s capabilities were very impressive and we decided to install two systems: one at the entry to our kitting and picking area and another to the manufacturing area.” Both units were connected to a door entry system which allows Vanilla to control access to these areas: “Employees and visitors are unable to enter our EPA without successfully passing the necessary footwear and wrist strap tests.

Vanilla Electronics - Testing

A Vanilla employee performing a test using the SmartLog V5™

By touching the solid-state switch, the SmartLog V5™ independently tests the resistance path limits of the worn wrist strap and both worn ESD footwear in less than 2 seconds. It may also test a worn ESD garment if it is used as part of personnel grounding path. Test results are then electronically stored in the SmartLog V5™ and easily downloaded to a PC for logging records and evaluation (using Team5 software).
Paperless data can enhance operator accountability, immediately identifying problems while reducing logging and auditing costs. Operator identification can be accomplished using the keypad, scanning a barcode badge or waving a proximity badge (verify compatibility with the factory).

The SmartLog V5™ can test either single or dual-wire wrist straps and its split footplate design provides individual footwear testing all in a single test. With the use of TEAM5 Software, the SmartLog V5™ can be programmed to assign unique test requirements to personnel. An individual can be required to test either wrist strap only, footwear only or a combination of the two.
If a resistance path is below or exceeds the set limits, the SmartLog will indicate failure via audio and visual alarms. The included relay terminal allows access control for passed tests.
The SmartLog V5™ can also be networked to a company’s Intranet using its embedded Ethernet module.

After installation, Vanilla very easily assigned each user their own unique Barcode IDs and enabled automatic recording of both wrist and footwear test results. They then set the test frequency for each operator. The Team5 software automatically sends an email to line managers should any individual miss their scheduled test. “As well as passed results, the system also logs failures. This allows us to monitor wear and tear of ESD personnel grounding items and replace any that produce common failures.
The Team5 software also allows Vanilla to quickly and efficiently produce accurate reports proving that each member of staff has met their ESD test requirements. Vanilla can align test data with their other systems such as pick and build information and environmental data including temperature and humidity. “The SmartLog system offers our customers full traceability throughout the process.

Vanilla have recently arranged for the installation of the successor of the Smart V5™, the SmartLog Pro™, at one of their manufacturing facilities in Wiltshire – a true testament to the capabilities of SmartLogs. “They protect the EPAs much more effectively than a traditional manual system and really look the part during customer visits!”.

The all new SmartLog Pro™

The all new SmartLog Pro™ features a touch-screen display fulfilling two functions:

  • Allows keypad entry and
  • Visual colour-coded display of results.
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