Category Archives: Resources
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 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.
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!
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 if it does not meet the parameters outlined in the datasheet.
|Class 1A||250 volts to|
|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 C2||125 volts to|
|Class C3||250 volts to|
|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 M2||100 volts to|
|Class M3||200 volts to|
|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]
- 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)
Many companies implement an ESD Control Programme with the aim of improving their operations. Effective ESD control can be a key to improving:
- 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.
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 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.
|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.
|“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 220.127.116.11 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!
|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
|“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 18.104.22.168 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.
|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.
|“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.
|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.
|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.
|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.|
|“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 22.214.171.124 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.
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 . 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 . 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, .
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 . 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 .
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 .
Table I – Table of Logic Families’ Power Transition, Noise Margin, and Indeterminate Range 
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.
Table 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 . The waveform for an ESD event includes high-frequency components with a frequency range from DC to over 6GHz, . 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 .
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.
- 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.
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.
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
Let’s face it: nobody likes ‘change’! We all like our little routines and feel comfortable with what we know.
BUT: without ‘change’, everything would stay the same; ultimately humanity would stagnate and die. So, let’s think of ‘change’ as an opportunity: to improve, to progress, to be better! That’s exactly the reason behind the latest major change to the ESD Standard: ensuring your ESD Programme is the best one yet and protects your ESD sensitive devices 110%.
You will have learnt by now that a fundamental principle of ESD control is to ground conductors including people at ESD protected workstations. Wrist straps are the first line of defence against ESD. A wrist strap is the most common personnel grounding device and is required for sitting operators.
A Flooring / Footwear system is an alternative for personnel grounding for standing or mobile workers. You will know that ESD footwear must be used in conjunction with an ESD floor and needs to be worn on both feet. But did you know that the latest ESD Standard now requires an Operator Walking Test and conformance to Operator Resistance Measurements?
The importance of the Walking Test
- The Walking Test is necessary to qualify the Footwear / Flooring personnel grounding system for certification to EN 61340-5-1.
- The Walking Test can provide records to prove that the Footwear / Flooring personnel grounding system used as a static control method is providing the performance expected.
- The Walking Test is also used when testing samples for qualification of a Footwear / Flooring personnel grounding system or on an existing installed floor when evaluating a change in footwear or flooring maintenance.
Performing the Walking Test
The Walking Test is completed with a device which measures the human body voltage generated while walking. There are different units on the market: some of them will display the results on the unit itself; others connect to a computer and use software to analyse the data.
All units work in the same way though:
- You wear your ESD footwear.
- You hold a small probe (more like a rod) connected to the meter measuring your body voltage.
- While holding the probe, you walk across your ESD floor.
- You record the results and either calculate the average of the 5 highest peaks or let the software (supplied with some units) do the calculation for you.
Results of the Walking Test
The Walking Test simulates a real-world working environment with operators walking through an ESD Protected Area. The results will show the effectiveness of a Footwear / Flooring system to remove charges from the operator through the floor to ground. If the system is working properly, no more than 100 Volts will be generated on the body.
For any Footwear / Flooring system, EN 61340-5-1 requires:
- the resistance from body to ground to be <109 ohms AND
- the body voltage to be < 100 Volts (average of 5 highest peaks).
Remember that the Walking Test must be performed on ALL ESD floors using ANY ESD footwear you may be using. So, if you have 2 EPAs with different flooring and use 2 different types of footwear (e.g. shoes, foot grounders), you need to perform a total of 4 tests to cover all possible options.
Also, if you make any changes to your footwear or flooring (e.g. you change suppliers for your foot grounders or ESD floor cleaner), the Walking Test needs to be repeated to ensure compliance with the ESD Standard.
If you require further information on the changes to the ESD Standard or need the Walking Test performed in your facility, get in touch.
The control of electrostatic discharge is an important aspect when manufacturing, assembling and repairing devices that employ electronics. Electrostatic discharges can damage an electronic component at any stage of its production or application if not controlled. The primary method of control is to ground (or bring to the same potential) all conductors that come in contact or near proximity to the electronic device(s). These conductors include humans, tools, ESD mats, other electronic devices, boards, connectors, packaging, etc.
There are other components to a good ESD Control programme including removal of unnecessary insulators, shielding, ionisation, environmental controls, training, education and top-down compliance. This post will talk about controlling discharges to a grounded ESD mat on a workstation. Watch out: it’s about to get technical!
Of specific interest in controlling an electrostatic discharge is the time rate of the discharge. A discharge will occur much quicker in/on a conductor with a surface resistance of 102 ohms than in a conductor with a surface resistance of 109 ohms. How fast or slow should the controlled discharge be? Understanding the importance of discharge times will help you choose the right ESD control materials in building, maintaining or auditing your own ESD Safe workbench(es).
The upper and lower boundaries of an ESD safe discharge rate are determined by the application and materials used. To limit the discussion, the potential energy sourced from the Human Body Model (HBM), [refer to EN 61340-5-1], is applied to an ESD sensitive work area or ESD mat.
Body and Movement
You should be familiar with the timing of the human body’s movements relative to handling or working near ESD sensitive devices to have a handle on the upper limit of the controlled discharge. To reduce the likelihood of an operator discharging onto an ESD sensitive device, they should drain any charges before bringing an ESD sensitive device in contact with themselves or another conductor, whether floating or grounded.
Table 1 – Movement times (averaged) from typical operations:
Table 1 shows averaged times (in milliseconds) for the handling of tools or devices at a work bench with a corresponding standard deviation in milliseconds. The shortest time of 153ms, or worst case, is the time that we will design our ESD sensitive workbench tabletop with. You want to be sure that your device is fully discharged well before the 153ms landing time. A good rule of thumb would be to engineer a x2 safety factor. Therefore, your device should be fully discharged before reaching 76.5ms (76.5ms x 2 = 153ms). The time constraint of 76.5ms for body movement defines the upper boundary of the controlled discharge rate (not including the standard deviation of 11ms).
Table 2 – Typical Discharge times [t = R x C x ln(V/V0)] for an RC circuit where C = 200pF and V0 = 249 Volts:
Table 2 shows calculated discharge rates for the human body model (HBM) onto an ESD grounded mat with surface-to-ground (RG) resistances from 102 to 1011 ohms. The more conductive the ESD mat on the workbench is, the faster the discharge, but there is another consideration too.
How fast is too fast? When does the discharge energy at any given time reach a critical level that can damage a semiconductor? The answer depends on several variables relative to the semiconductor’s construction such as line spacing, composition, density, packaging, etc., all leading to an ESD component classification [refer to ESD STM5.1-2007 and the manufactures’ device specifications].
For simplicity’s sake assume the worst case, class 0, which has a 0 to 249 Volt tolerance. Applying the HBM, a conservative worst case capacitance would be 200pF, twice that of the HBM and resistance of 10 kilohms. Therefore, the maximum power (P) level based on Ohm’s Law is P = V2/R (J/s) and the worst case HBM is ((249)2/10K) = 6.2 Watts or Joules per second (Js-1).
The maximum energy (E) stored in a worst case HBM capacitance (C) of 200 pF and at a maximum voltage (V) of 249 Volts, (using E = 1/2 CV2), yields 6.2mJ.
The next concern is to relate energy to time. The time constant (t) is the measure of the length in time, in a natural response system, for the discharge current to die down to a negligible value (assume 1% of the original signal). For an RC circuit, the time constant (t) is equivalent to the multiple of the equivalent resistance and capacitance. In this case, the time constant (t) of our RC circuit is (10 kilohms) x (200pF) or t = 2ms. Discharging this energy upon touching a conductor at zero volts yields a current, (using I = P/V), of (6.2Js-1)/(249V) or 24.8mA. To avoid damaging a class 0 ESD sensitive device, the discharge current must be below 24.8mA. Engineering in a “2x” safety factor, the maximum discharge current would be 12.4mA. To maintain a discharge current below 12.4mA, we need to look at our grounding equipment on the ESD sensitive workbench.
Table 3 – Discharge currents from a 6.2 mJ lossless energy source (with C = 200pF & V = 249V) dependent on the discharge time.
The rate at which 6.2mJ of energy discharges is very important. Too fast a discharge will lead to an ESD Event, which can electromagnetically be measured using a simple loop antenna attached to a high impedance input of a high-speed storage scope. The faster the discharge the greater the discharge current becomes as well as the emf (electromotive force) on the loop antenna from the EMI (ElectroMagnetic Interference). Table 3 depicts the discharge current for 6.2mJ at varying discharge times. We are assuming lossless conditions during the discharge for the worst case. For our example, to keep the discharge current below 12.4mA, the discharge rate [from Table 3] must be no quicker than 2.01ms. This energy-based-time constraint forms the lower boundary of the controlled discharge rate.
Choosing your Matting
The upper (76.5ms) and lower (2.01ms) boundary of our controlled discharge rate are now defined and can be used to help in choosing the correct ESD mat for an ESD sensitive workstation. ESD mat materials come in many variations. In general, mats are either made from vinyl or rubber material and can be homogeneous or multi-layered. Rubber mats, in general, have good chemical and heat resistance but vinyl tends to be more cost effective. The electrical properties of an ESD mat are important to know in controlling the electrostatic discharge.
An ESD mat will be either electrically conductive or dissipative. Both terms mean that the mat will conduct a charge when grounded. The difference in the terms is defined by the materials resistance, which affects the speed of the discharge. A conductive material has a surface resistivity of less than 1 x 105 ohms/sq and a dissipative material is greater than 1 x 105 ohms/sq but less than 1 x 1012 ohms/sq. Anything with a surface resistivity greater than 1 x 1012 ohms/sq is considered insulative and will essentially not conduct charges.
Back to our example: If the maximum discharge current of our ESD sensitive device is 12.4mA, then the discharge time based on energy must be slower than 2.01ms and based on body movement must be faster than 76.5ms. Using the discharge times from Table 2 and assuming that the mat has a negligible capacitance relative to the HBM, then the mat resistance must be greater than 2.2 x 103 ohms or 2.2 x 104 ohms/sq and less than 8.3 x 107 ohms or 8.3 x 108 ohms/sq. In other words, a very conductive mat for some applications may be too quick to discharge and yield more dangerous ESD events whether properly grounded or not.
Graph 1 shows the natural response of a 249 Volt discharge in an RC circuit using a capacitance of 200 pF (HBM) into resistances (mat) of 104, 105, and 106 ohms. The natural response of the104 ohms curve is below 1% of its’ initial voltage in less than 10ms where the 106 ohms curve takes less than 1ms to discharge to less than 1% (V < 2.49V) of its initial value (V0 = 249V).
The role of Wrist Straps
Another defence, and the most common method, to reduce the risk of creating an ESD event is wearing a grounded wrist strap at the workstation. The wrist strap connects the skin (a large conductor) to a common potential (usually power ground). Properly worn, the wrist strap should fit snugly, making proper contact with the skin, to reduce contact resistance.
The wrist strap, since it is connected to ground, will quickly discharge any charge the body either generates through tribocharging or becomes exposed to through induction. Any time the body directly touches a charged conductor, a discharge will occur because the body is at a different potential (0 Volts). Controlling this discharge is important if the conductor is an ESD sensitive device and in minimising induced charges through EMI onto nearby ungrounded ESD sensitive devices.
The electrical properties of the skin of an operator can have a wide range in both resistance and capacitance depending on several variables. An operator’s hand touching a charged device will initiate a discharge at the rate of the time constant of the skin before including the RL properties of the wrist strap. To reduce the potential of an unsafe discharge from a device to a very conductive operator, adding resistance to the operator at the interface from skin to device may be necessary. Some solutions are static dissipative gloves or finger cots, which if worn properly, may add from 1 to 10 megohms to the RC circuit of the skin. This, in turn, slows down the discharge rate to well over 2ms.
The upper and lower boundaries of a safe discharge rate are determined by the application and materials used. The movements of the operator define our upper boundary and the max energy, as defined by the ESD sensitive component classification, dictates our lower boundary. We want to design an ESD sensitive workbench to control the discharge rate (via the circuit’s time constant) of our grounded or conductive materials within these limits.
For the HBM and a class 0 device, the materials chosen for a safe ESD workbench should have electrical properties to support discharge rates between 2ms and 76.5ms. These discharge rates, using worst case assumptions, equate to an ESD mat surface with a Resistance-To-Ground (RG) between 2.2 x 103 ohms and 8.3 x 107 ohms. This controlled discharge rate window will vary depending on the class of semiconductor components used (class 0 to class 3B per ESD STM5.1-2007) and the properties of production resources used (human vs. automated).
Please note that the numbers calculated were based on assumptions used to simplify the explanation of the material. Real-world applications are much more complex and require a more detailed analysis, which was beyond the scope of this blog post.
The purpose of an ESD protective working surface is to aid in the prevention of damage to ESD sensitive items (ESDS) and assemblies from electrostatic discharge. An ESD protective working surface provides protection in the following two ways:
- Providing a low charging (antistatic) working surface area that will limit static electricity to be generated below potentially damaging levels.
- Removing the electrostatic charge from conductive objects placed on the working surface.
1. Types of ESD working surfaces
ESD protective working surfaces are categorised into two general categories: conductive and dissipative.
A conductive working surface is defined by most documents as a material that has a surface resistance of less than 1 x 104 ohms. Conductive materials are the quickest to ground a charge but they can also cause damage by discharging too rapidly. Conductive materials are usually used as floor mats or flooring products.
A dissipative working surface is defined as being materials having a surface resistance of at least 1 x 104, but less than 1 x 109 ohms. Dissipative materials will dissipate a charge slower and are recommended for handling electronic components. Dissipative materials are usually the preferred choice for bench top working surfaces.
Most people in the industry consider working surfaces to be the second most important part of an ESD Control Programme, with personnel grounding being most important.
2. Grounding Methods for working surfaces
Method 1: Grounding via ground cords
- Vermason recommends using an earth bonding point cord when grounding via ground cords. Most earth bonding point cords will ground an ESD protective working surface and provide banana jacks for two wrist strap grounds.
Earth Bonding Point for each workstation
- An earth bonding point should be installed at each workstation and should be connected directly to a verified electrical system ground or to a verified grounding bus which is connected to the protective earth ground. Only one groundable point should be installed on a working surface.
- Wrist straps should never be grounded through a working surface, as the added resistance of the working surface material will prevent the wrist strap from operating properly.
Proper Grounding of Wrist Straps
Method 2: Grounding via a grounded conductive surface
- This alternate form of grounding should only be employed when using a homogeneous dissipative material with a volume resistance of less than 1 x 108
- The dissipative working surface may be placed on a properly grounded laminate, metal or other conductive surface. The working surface will electrically couple to the grounded surface and may not require a separate ground cord.
- When using this type of grounding method be sure to test that the working surface Rg is less than 1 x 109 ohms, tested per IEC 61340-2-3. Also consider increasing Compliance Verification test frequency.
Alternate Grounding Method via a Grounded Conductive Surface
3. Groundable Point Installation
Before installing a groundable point on your work surface you must first determine whether you will need a male stud or female socket, the type of snap hardware and the desired location.
There are generally 3 types of groundable points available for working surface mats: screw-on snap kits, push & clinch snaps (with prongs) or stud & posts sets (requiring installation using a punch and an anvil).
Snap Kits and Tools
- Determine the position of the grounding snap (one only per mat). Punch a hole through the material with a small Phillips screwdriver or awl.
- Insert the screw through the bottom on the snap fastener, the washer and the material. Affix the assembly with the conical nut supplied with the kit and tighten down the screws.
Installing a screw-on Mat Grounding Snap
Push & clinch snaps:
This snap is designed for use with any type of soft mat material: dissipative, conductive or multi-layered. It is recommended for use with three-layered material, because it provides better contact with the internal conductive layer. It is recommended that before inserting this snap, the mat be punctured with a sharp tool where the snap will be placed.
Centre the prongs on the snap assembly. Apply pressure to the snap until the prongs come through the back of the mat, then clinch over prongs making flat to the mat’s bottom side to secure snap as shown in the below picture.
Installing Push & Clinch Mat Grounding Snap
Stud & post sets:
This type of groundable point must be riveted through bench and floor mats to connect ground cords. A punch and anvil are simple but effective tools to achieve a neat finish with firm materials no more than 4mm thick.
- Punch a 5mm diameter hole at the desired location of the mat.
- Insert the post from underneath and apply the stud over the protruding post on the top side.
- Fit the anvil under the post and place the punch inside the stud and hammer the post (or use an arbor press) until it rolls and a tight assembly is achieved.
Using a Punch and Anvil to install Stud & Post Sets
4. Selection of Common Point & Floor Mat Grounding Systems
- Determine the type of common point grounding system you will use: barrier strip, bus bar, grounding block or common point ground cord. Vermason recommends the use of common point ground cords and earth bonding bars.
- If you determine that you will use ground cords, you must now determine the type of ground cord you will use for your workstation grounds. It is the user’s preference to use a ground cord with or without a current limiting 1 megohm resistor to ground working surfaces or floor mats. Selection of the ground cord is determined by user needs and specifications; the resistor is not for ESD control.
Examples of Grounding Cords
- Earth bonding point bars allow the grounding of multiple operators at one common ground point. They also mount easily under the front edge of a workstation benchtop.
Earth Bonding Point Installation
5. Mat Installation
- For best results, allow the mats to lay flat for about four hours at room temperature before installing. This will give the material time to flatten out from being rolled for shipment.
- Test all workstation grounds for proper resistance to ground.
- Lay the mat in position and snap the ground cord to it. Bring the other end of the ground cord to the common ground point (or earth bonding bar) and attach it using the ring terminal (or other termination device). The electrical systems junction box and connecting conduit should also connect to earth protective ground. Tie the ground wire to the bench to keep it out of the way and neat. You may cut and strip the ground wire to a shorter length and attach it with an extra ring terminal if required.
Note: DO NOT DAISY CHAIN. Because of the high resistances inherent to many types of protective surfaces, daisy chaining of these materials can cause the overall resistance to exceed the required limit of EN 61340-5-1.
ESD working surface should never be grounded in series, i.e. daisy chained
- If your kit includes a floor mat, you should duplicate step 2 and attach the floor mat ground to the same ground point as the working surface ground.
- Measure the resistance from the ground snap on the mat to the common ground point. It should read 1 megohm ±20 percent if you are using a ground cord with a resistor, and less than 10 ohms if you are using a ground cord without a resistor.
- If you have a surface resistance or resistance to ground tester available, you may wish to test the resistance to ground from the mat surface. Note: depending upon the accuracy of the instrument you are using, you may get a wide range of results in resistance to ground tests. In order to get the electrical readings specified per EN 61340-2-3, two 2.2kg electrodes are to be used. This will require a megohmmeter with 100 volt open test circuit voltage and two 2.2kg electrodes.
- If you are using a mat kit that includes the wrist strap, install the wrist strap directly to the common point mat ground cord. Again, test the resistance from the backplate of the wrist strap to the common ground point. It should read 1 megohm ± 20 percent.
Adding a Wrist Strap
- Your completed installation of an ESD workstation should comply with one of the electrical diagrams illustrated below.
Proper wiring diagrams for conductive and dissipative ESD workstations
6. Maintenance and Cleaning
For optimum performance, periodic cleaning is required following the manufacturer’s recommendations.
BE SURE YOU TEST ALL GROUNDS AND THE WRIST STRAP FREQUENTLY
So you’ve identified ESD sensitive items in your factory and you realise that you need to implement ESD Control measures. But where do you start? There is so much information out there and it can be completely overwhelming. Don’t panic – today’s blog post will provide you with a step-by-step guide on how to set-up a suitable ESD Control Plan.
“The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
• compliance verification
• grounding / equipotential bonding systems
• personnel grounding
• EPA requirements
• packaging systems
• marking” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.1 ESD control program plan]
“Each company has different processes, and so will require a different blend of ESD prevention measures for an optimum ESD control program. It is vital that these measures are selected, based on technical necessity and carefully documented in an ESD control program plan, so that all concerned can be sure of the program requirements.” [EN 61340-5-1 Edition 1.0 2007-08 Introduction]
1. Define what you are trying to protect
A prerequisite of ESD control is the accurate and consistent identification of ESD susceptible items. Some companies assume that all electronic components are ESD susceptible. However, others write their ESD control plan based on the device and item susceptibility or withstand voltage of the most sensitive components used in the facility. A general rule is to treat any device or component that is received in ESD packaging as an ESD susceptible item.
An operator handling an ESD susceptible item
2. Become familiar with the industry standards for ESD control
A copy of EN 61340-5-1:2007 can be purchased from British Standards: “BS EN 61340-5-1:2007 applies to activities that: manufacture, process, assemble, install, package, label, service, test, inspect, transport or otherwise handle electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 V human body model (HBM). BS EN 61340-5-1 provides the requirements for an ESD control program. The user should refer to IEC 61340-5-2 for guidance on the implementation of this standard.
3. Select a grounding / equipotential bonding system
The elimination of differences in potential “can be achieved in three different ways:
- grounding using protective earth:
the first and preferred ESD ground is protective earth if available. In this case, the ESD control elements and grounded personnel are connected to protective earth;
- grounding using functional ground:
the second acceptable ESD ground is achieved through the use of a functional ground. This conductor can be a ground rod or stake that is used for grounding the ESD control items in use at a facility. In order to eliminate differences in potential between protective earth and the functional ground system it is highly recommended that the two systems be electrically bonded together;
- equipotential bonding:
in the event that a ground facility is not available, ESD protection can be achieved by connecting all of the ESD control items together at a common connection point.” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.3.1 Grounding/equipotential bonding systems]
Example of a grounding/equipotential bonding system
4. Determine the grounding method for operators (Personnel Grounding)
The two options for grounding an operator are:
- a wrist strap or
- foot grounders/footwear.
In some cases, both (wrist strap and foot grounders) will be used.
5. Establish and identify your ESD Protected Area (EPA)
ESD Control Plans must evolve to keep pace with costs, device sensitivities and the way devices are manufactured. Define the departments and areas to be considered part of the ESD Protected Area. Consider if customers and/or subcontractors should be included. Implement access control devices, signs and floor marking tape to identify and control access to the ESD Protected Area.
Example of an ESD Protected Area including signs and floor marking tape
6. Select ESD control items to be used in the EPA based on your manufacturing process
Elements that should be considered include: working surfaces, flooring, seating, ionisation, shelving, mobile equipment (carts) and garments. Check-out this post for more information.
7. Develop a Packaging (Materials Handling & Storage) Plan
When moving ESD susceptible devices outside an ESD protected area, it is necessary for the product to be packaged in an enclosed ESD Shielding Packaging.
8. Use proper markings for ESD susceptible items, system or packaging
From EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.1: “The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include: …marking”.
If you are handling ESD sensitive devices, there are 3 symbols you need to know.
The ESD Susceptibility (left) and ESD Protective Symbol (right)
9. Implement a Compliance Verification Plan
Developing and implementing an ESD control programme is only the first step. The second step is to continually review, verify, analyse, evaluate and improve your ESD programme.
“Process monitoring (measurements) shall be conducted in accordance with a compliance verification plan that identifies the technical requirements to be verified, the measurement limits and the frequency at which those verifications must occur. The compliance verification plan must document the test methods used for process monitoring and measurements… Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements. The test equipment selected shall be capable of making the measurements defined in the compliance verification plan.” [EN 61340-5-1 Edition 1 2007-08 clause 5.2.3 Compliance verification plan]
Regular programme compliance verification and auditing is a key part of a successful ESD control programme.
10. Develop a Training Plan
“The training plan shall define all personnel that are required to have ESD awareness and prevention training. At a minimum, initial and recurrent ESD awareness and prevention training shall be provided to all personnel who handle or otherwise come into contact with any ESDS [ESD sensitive] items. Initial training shall be provided before personnel handle ESD sensitive devices. The type and frequency of ESD training for personnel shall be defined in the training plan. The training plan shall include a requirement for maintaining employee training records and shall document where the records are stored. Training methods and the use of specific techniques are at the organization’s discretion. The training plan shall include methods used by the organization to ensure trainee comprehension and training adequacy.” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.2 Training Plan]
11. Make the ESD Control Plan part of your internal quality system requirements
A written ESD Control Plan provides the “rules and regulations”, the technical requirements for your ESD Control Programme. This should be a controlled document, approved by upper management initially and over time when revisions are made. The written plan should include following:
- Qualified Products List (QPL): a list of EPA ESD control items is used in the ESD control Plan
- Compliance Verification Plan: includes periodic checking of EPA ESD control items and calibration of test equipment per manufacturer and industry recommendations.
- Training Plan: an ESD Programme is only as good as the use of the products by personnel. When personnel understand the concepts of ESD, the importance to the company of the ESD control programme and the proper use of ESD products, they will implement a better ESD control programme improving quality, productivity and reliability.
We’re talking about Electrostatic Discharge (ESD) on this blog all the time. But what exactly does it mean and why is it so dangerous? Today’s post will answer those questions!
All matter is constructed from atoms. These atoms have negatively charged electrons circling the atom’s nucleus which includes positively charged protons. As the atom has an equal number of electrons and protons, it balances out having no charge. So far, so good!
The problem is that all materials can tribocharge or generate ElectroStatic charges. Most commonly, this happens through contact and separation – examples are:
- Unwinding a roll of tape
- Gas or liquid moving through a hose or pipe
- A person walking across a floor and soles contacting & separating from the floor.
Unwinding a roll of tap can generate an electrostatic charge
The simple separation of two surfaces can cause the transfer of electrons between surfaces resulting in one surface being positively and the other one negatively charged. With that we’ve just generated an ElectroStatic charge!
The amount generated varies and is affected by materials, friction, area of contact and the relative humidity of the environment. At lower relative humidity, charge generation will increase as the environment is drier. Common plastics generally create the greatest static charges.
ELECTROSTATIC DISCHARGE (ESD)
If two items are at the same electrostatic charge or equipotential, no discharge will occur.
However, if two items are at different levels of ElectroStatic charge (i.e. one is positively and the other one negatively charged), they will want to come into balance. If they are in close enough proximity, there can be a rapid, spontaneous transfer of electrostatic charge. This is called discharge or ElectroStatic Discharge (ESD). Examples in daily life:
- Lightning, creating lots of heat and light
- The occasional zap felt when reaching for a door knob
- The occasional zap felt when sliding out of a car and touching the door handle
Have you felt the zap before?
In a normal environment like your home, there are innumerable ESD events occurring, most of which you do not see or feel. It takes a discharge of about 2,000 volts for a person to feel the “zap”. It requires a much larger ESD event to arc and be seen (e.g. lightning). While a discharge may be a nuisance in the home, ESD is the hidden enemy in a high tech manufacturing environment. Modern electronic circuitry can be literally burned or melted from these miniature lightning bolts. ESD control is therefore necessary to reduce and limit these ESD events.
TYPES OF ESD DEVICE DAMAGE
ESD damage to electronic components can lead to:
- Catastrophic Failures
- Latent Defects
Catastrophic failure causes a failure in an ESD sensitive item that is permanent. The ESD event may have caused a metal melt, junction breakdown or oxide failure. Normal inspection is able to detect a catastrophic failure.
A latent defect can occur when an ESD sensitive item is exposed to an ESD event and is partially degraded. It may continue to perform its intended function, so may not be detected by normal inspection. However, intermittent or permanent failures may occur at a later time.
COSTLY EFFECTS OF ESD
A catastrophic failure of an electronic component can be the least costly type of ESD damage as it may be detected and repaired at an early manufacturing stage.
Latent damage caused by ESD is potentially costlier since damage occurs that cannot be felt, seen or detected through normal inspection procedures. Latent defects can be very expensive as the product passes all inspection steps and the product is completed and shipped. Latent defects can severely impact the reputation of a company’s product. Intermittent failures after shipping a product can be frustrating, particularly when the customer returns a product, reporting a problem which the factory again fails to detect. It consequently passes inspection and the product is returned to the customer with the problem unresolved.
The worst event is when the product is installed in a customer’s system, and performs for a while and then performs erratically. It can be very expensive to troubleshoot and provide repairs in this situation.
One study indicated the cost to be:
- £7 Device
- £7 Device in board – £700
- £7 Device in board and in system – £7,000
- £7 Device and system fails – £70,000
Industry experts have estimated average electronics product losses due to static discharge to range from 8 to 33%. Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of dollars annually.
It is critical to be aware of the most sensitive items being handled in your factory. 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. As you can predict, the need for proper ESD protection increases every day.
If you’re new to ESD and ESD Control, we suggest you read this article for more information on how to protect your ESD sensitive devices.
Today’s post concludes our 2-part series on periodic verifcation. If you have missed the first part, you can catch-up on it here. As a reminder, it is recommended to regularely check all ESD Protected Area (EPA) products to ensure they are working correctly. After covering working surface matting and wrist straps in last week’s post, we’ll jump right in to discuss the remaining components in your EPA.
A flooring / footwear system is an alternative for personnel grounding for standing or mobile workers. Foot grounders quickly and effectively drain the static charges which collect on personnel during normal, everyday activities. Foot grounders should be used in conjunction with floor surfaces which have a surface resistance of less than 1010 ohms.
As ESD floors get dirty, their resistance increases. For optimum electrical performance, floor matting must be cleaned regularly using an ESD mat cleaner, such as Reztore™ Surface & Mat Cleaner. Do not use cleaners with silicone as silicone build-up will create an insulative film on the surface.
Dissipative floor finish can be used to reduce floor resistance. Periodic verification will identify how often the floor finish needs to be applied. As the layer(s) of dissipative floor finish wear, the resistance measurements will increase. So, after some amount of data collection, a cost effective maintenance schedule can be established.
Testing floor matting
Floor matting can be checked using a resistance meter. Surface resistance meters are designed to measure resistance point-to-point (Rp-p) or surface to ground (Rg) in accordance with EN 61340-5-1 Electrostatics and its test method IEC 61340-2-3.
ESD Shoes or Foot Grounders play an essential part in the flooring/footwear system. For more information on how to ground moving personnel effectively, check this post.
Before handling ESD sensitive devices, visually inspect your ESD footwear for any damage. Just like wrist straps, footwear should be checked while being worn using a wrist strap/footwear tester.
Checking foot grounders using 222567
Records of each test should be kept. Analysis and corrective action should take place when a footwear tester indicates a failure. Footwear needs to be checked daily.
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.
Make sure your ESD shielding bags are un-damaged
It is up to the user to determine if a shielding bag is suitable for re-use or not. The testing of every bag before re-use is not practical. Many companies will discard the shielding bag once used and replace it with a new one. Others will use a system of labels to identify when the bag has gone through five handling cycles:
- Non-reusable labels are used that require the label be broken to open the bag.
- The bag is then resealed with a new label.
- When there are five broken labels, the bag is discarded.
The same principle applies to other ESD packaging, e.g. component shippers.
Ionisers are intended to neutralise static charges on insulators thereby reducing their potential to cause ESD damage. However, poorly maintained ionisers with dirty emitter pins and out-of-balance ionisers can put a charge on ungrounded items.
Remember to clean ioniser emitter pins and filters regularly. You can now even purchase ionisers that will alarm when emitter pins need to be cleaned or the ioniser is out of balance.
Checking ionisers using 50598
The EMIT Ionisation Test Kit 50598 allows the Digital Static Field Meter 50597 to be used to measure the offset voltage (balance) and charge decay of ionisation equipment. The Test Kit also includes a Charger used to place a ±1000V charge on the 50567 Conductive Plate, making it possible to measure the discharge times of air ionisation equipment per ANSI/ESD SP3.3 Periodic Verification of Air Ionizers.
Wrist Strap/Footwear and Resistance Testers etc.
So you check your wrist straps and/or footwear and bench and/or floor matting regularly. But have you remembered the testers themselves? What good do all the checks do, if the testers you use are out-of-spec and show you incorrect results?
Yearly calibration is recommended – many manufacturers offer a calibration service or alternatively you can purchase calibration units from them and perform the calibration yourself.
So there you have it – a list of the most commonly used products in your ESD Protected Area (EPA) that you should check on a regular basis.
Questions for you: Do you have a verfication plan in place? If so, how often do you check your ESD protection products?