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A wrist strap is arguably the best way to provide a safe ground connection to the operator in order to dissipate accumulated static charges with the purpose to prevent dangerous ESD exposure to sensitive ESD components.
Wrist straps must be tested to ensure that they are installed and working properly. On-demand or “touch” testers have become the most common testing method. On-demand testers complete a circuit when the wrist strap wearer touches a contact plate. One drawback with on-demand type testers is that they require a dedicated action by the wearer of the wrist strap to make the test. Also, knowing that the wrist strap has failed after the fact may possibly have exposed a highly sensitive or valuable assembly to risk. Continuous monitors eliminate the possibility of a component being exposed to ESD during the period that the wrist strap was not working properly.
Types of Wrist Straps
A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and, in turn, by itself is connected to a known ground point at a workbench or a tool. While a wrist strap does not prevent generation of charges, its purpose is to dissipate these charges to ground as quickly as possible. A single-wire wrist strap is comprised of one conductive surface contacting the wrist of an operator and providing one electrical connection to ground. A dual-wire wrist strap has two electrically-separate parts and two separate electrical connections to ground combined in one cord.
A Wrist Strap
Both types of wrist straps – when in good condition and properly worn – provide equally good connection of operator to ground. A single-wire wrist strap is undoubtedly less expensive than its dual counterpart. However, for applications where sensitive components are being handled, the share of dual-wire wrist straps is growing rapidly. The reason for this is its ability to guarantee that the wrist strap indeed provides proper dissipation of charges on the operator. The way to ensure that the wrist strap is worn properly at all times is to utilise a continuous wrist strap monitor. These units monitor proper connection of the operator to ground and alarm should this connection fail. If you want to learn more about the benefits of continuous monitoring, we recommend you read this post.
Wrist Strap Monitors
Monitoring of single-wire and dual-wire wrist straps is fundamentally different:
- Single-wire wrist strap monitors do not have a return signal path; the only physical parameter they can rely on is parasitic capacitance of operator’s body to ground.
- Dual-wire wrist strap monitors measure the resistance of the operator’s wrist between the two halves of the wrist strap.
Single-Wire Wrist Strap Monitoring
1. AC Capacitance Monitors
The first constant monitors developed made use of the fact that a person can be thought of as one plate of a capacitor with the other plate being ground. The ground and the person are both conductors and they are separated (sometimes) by an insulator (shoes, mats, carpet, etc.) thus forming a capacitor. The combined resistance of the wrist strap and person forms a resistor so that the total circuit is a simple RC circuit. A tiny AC current applied to this circuit will cause a displacement current in the capacitance to flow to ground providing a simple way to make sure the person (capacitor) resistor (wrist strap) and coil cord are all hooked up. Any break in this circuit results in a higher impedance that can be used to trigger an alarm. AC capacitance monitors have a few drawbacks:
- They do not provide a reliable way to know if the total resistance of the circuit is too low, i.e., if the current limiting safety resistor is shorted.
- Simple AC capacitance monitors can be tricked into thinking the person is wearing the wrist strap when they are not. For example, laying a wrist strap and cord on a grounded mat will increase the shunt capacitance, which allows the monitor to show a good circuit even with the person out of the circuit. Forming the cord into a tight bundle or stretching it can also provide false readings.
- Since the capacitance and therefore the impedance of the circuit will also vary with such things as the person’s size, clothing, shoe soles, conductance of the floor, chair, table mat, the person’s positions (standing or sitting), etc., these monitors often have to be “tuned” to a specific installation and operator.
This technology is still around today and is purchased by some because of its low cost and a lack of knowledge by the End-User. A big plus of this technology is the ability to use any standard single-wire wrist strap.
2. Wave Distortion Monitors
Many of the short comings of the capacitance and other earlier monitors have been overcome with the development of AC monitors that use the concept of the wrist strap wearer as a capacitor, but in a different way. The concept of the wrist strap and wearer as an RC circuit is not wrong but it is an over simplification. The total circuit actually contains resistance, capacitance and inductance (RCL). Each component value will vary with the environment, size of wearer, and the other factors that affect the accuracy of the AC capacitance monitor. What the wave form distortion monitor looks at is not the impedance level, but at the waveform generated by the circuit. Current will lead voltage at various points due to the combinations of resistance and capacitive reactance. (There is a negligible amount of inductive reactance from the coil cord.) By monitoring these distortions” or phase shifts the WDM will determine if the circuit is complete i.e.; the wearer is in the circuit and the total equivalent DC resistance is within specifications given a range of installations. Essentially, the unit will monitor the operator by sending a “signature” signal down the coil cord to the operator’s wrist. The operator acts as a load and will reflect that signal back to the monitor with a different signature. The monitor will then compare the reflected signature to its factory pre-set signatures. If the signal is within the “good” range, the operator passes and the monitor will continue its work. If the signature is “not” good, the monitor will go into an alarm-state to warn the operator to stop working and fix the problem.
Example of a single-wire wave distortion monitor
Wave distortion monitors solves many of the problems of the other types:
- It allows the use of any brand of single-wire wrist strap
- It cannot be tricked like the AC capacitance units
- It provides a warning if the lower (safety) resistance limits are compromised
- The tiny amount of current required to generate the waveform has never caused reported skin irritation.
As an added bonus, wave distortion monitors will also detect an open circuit or bad ground all the way back to the building ground point. This is a fundamental advantage of this kind of monitor. Other monitors may insure that the operator is connected to the monitor. No other monitor automatically ensures that the user is actually grounded.
Dual-Wire Wrist Strap Monitoring
Dual-wire resistance monitors were developed to overcome some of the problems with the AC capacitance types. By providing a second path to ground (without relying on the capacitor above) we can apply a tiny DC current. It is then simple to measure the DC resistance of the circuit and alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted). Thus, a two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement. The shortcomings with the AC capacitance monitor are eliminated.
Two-wire monitors require two wires to work. This means that the wearer must wear a dual-wire two-conductor wrist strap / coil cord which are more expensive than standard single-wire wrist straps.
Example of a dual-wire monitor
There have been some reports that a constant DC voltage applied to the wristband causes skin irritations. This has been addressed in some models by pulsing the test current and in others by lowering the test voltage.
Dual polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire systems are used to create redundancy. In critical applications, you build-in redundancy to have a backup if your primary option fails. With dual-wire wrist straps the redundancy is there as a protection rather than an alternative. If you are monitoring your dual-wire wrist strap and one wire fails, then the unit will alarm. You will still be grounded by the other wire, so there will be a significantly reduced risk of damaging ESD sensitive components if you happen to be handling them when the wrist strap fails. The wrist strap would still need to be replaced immediately. So, while both single-wire and dual-wire wrist strap monitors help to dissipate accumulated charges on an operator, only dual-wire wrist strap solutions provide assurance of a proper dissipative path from operator to ground.
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.
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 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?
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.
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 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.
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.
We already know that in an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same potential which is achieved by using ‘groundable’ materials that are then linked to ground. We have also learnt that the most common personnel grounding device to ground people to ground are wrist straps. People who are moving around should instead wear ESD footwear. So how do you know if your wrist straps and ESD footwear are working properly? Excellent question! And one we’ll answer with today’s post so let’s jump right in!
1. Purpose of Personnel Grounding Testers
Wrist Straps and ESD footwear should be part of your Verification Plan. Each component in an EPA plays a vital part in the fight against electrostatic discharge (ESD). If just one component is not performing correctly, you could damage your ESD sensitive devices potentially costing your company thousands of pounds. The problem with wrist straps and ESD footwear is that you can’t always see the damage. Just by looking at the items you would not know if they still provide sufficient protection. That’s where personnel testers provide feedback. They verify the functionality of an operator’s wrist strap and/or footwear and can determine if a person’s wrist strap and/or footwear function correctly.
Your Personnel Grounding Checklist
Your Personnel Grounding Checklist:
- Both wrist straps and footwear need to be tested at least daily before handling ESD sensitive devices and should be worn while checking.
- Verify your personnel grounding system using a wrist strap and/or footwear tester.
- Remember that a record of each test should be kept for quality control purposes.
- ONLY handle ESD sensitive components if your wrist strap and/or footwear pass(es) the test.
2. Types of Personnel Grounding Testers
Broadly speaking, personnel grounding testers can be purchased in two configurations:
- Wrist strap tester and
- Wrist strap and footwear tester.
As wrist straps are the most commonly used personnel grounding device to ground operators, you will find a lot of testers on the market that check wrist straps only.
As the name suggests, combined wrist strap and footwear testers will verify your wrist straps AND footwear.
In addition to WHAT they test, you will also be faced with a wide range of devices differing in HOW they test. Below you will find a (by no means complete) list of options:
- Continuous and split footplate: You will find testers with a continuous footplate which require each foot to be tested separately one after the other. Dual-footplate or independent footwear testers feature a split footplate which allows the unit to verify both feet independently at the same time. This can be a huge time-saver if you have a number of operators in your company who are required to check their personnel grounding devices.
- Portable, wall-mountable and fitted testers: Portable battery-powered (predominantly) wrist strap testers are perfect for small labs or for supervisors to spot-check workers and ensure compliance. Wall-mountable units are generally supplied with a wall plate which attaches to a wall; the tester is then mounted on to the wall plate. Some personal grounding devices are accompanied by a stand (and built-in footplate) which allow for a more freely positioning of the unit within a room.
- Relay terminal: A few testers on the market are fitted with a relay terminal that can be integrated with electronic door locks, turnstiles, lights, buzzers, etc. This can be of advantage if companies only want to allow personnel in an EPA that have passed their wrist strap and/or footwear test.
- Data acquisition: A growing number of personnel grounding devices allow for test activity data to be logged in a database. The units link to a computer which records operator identification, test results, resistance measurements, time and more. Paperless data can enhance operator accountability, immediately identifying problems while reducing manual logging and auditing costs.
Example of a Data Acquisition Tester – more information
3. Operation of Personnel Grounding Testers
Wrist strap testing:
If you are not using a continuous or a constant monitor, a wrist strap should be tested while being worn at least daily. This quick check can determine that no break in the path-to-ground has occurred. Wrist straps should be worn while they are tested. This provides the best way to test all three components: the wrist band, the ground cord (including the resistor) and the interface with the operator’s skin.
“Wrist straps should be tested periodically. The frequency of testing, however, is driven by the amount of usage, wear and ESD risk exposure that can occur between tests. For example, what is the quantity of product handled between test periods? Typical test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that a guarantee of a continuous, reliable ground is needed then continuous monitoring should be considered or even required.” (CLC TR 61340-5-2 User guide Wrist Strap clause 126.96.36.199.4 Test frequency)
“The operator shall wear the wrist strap in the normal position and plug the free end of the cord into the test apparatus. The hand contact plate shall be pressed to verify that the wrist strap system resistance is within acceptable parameters. The test apparatus can be an integrated, commercially available tester or other Instrumentation that is capable of measuring resistance from 5,0 x 104 ohms to at least 1,0 x 108 ohms. The tester open-circuit voltage is typically between 9 V d.c. and 100 V d.c.” (EN 61340-5-1 Annex A Test method A.1 Measurement method for wrist strap testing).
Example of a Wrist Strap Tester – more information
If the wrist strap tester outputs a FAIL test result, stop working and test the wrist band and cord individually to find out which item is damaged. Replace the bad component and repeat the test. Obtain a PASS test result before beginning work. For more information on troubleshooting failed wrist straps, check this post.
If using a flooring / footwear system as an alternative for standing or mobile workers, ESD footwear should be tested independently at least daily while being worn. Proper testing of foot grounders involves the verification of the individual foot grounder, the contact strip and the interface between the contact strip and the operator’s perspiration layer.
“The operator shall stand with one foot on the conductive footwear electrode. The hand contact plate shall be pressed to verify that the person footwear system resistance is within acceptable parameters. The test shall be repeated for the other foot. The test apparatus can be an integrated, commercially available tester or other instrumentation that is capable of measuring resistance from 5,0 x 104 ohms to at least 1,0 x 108 ohms. The tester open-circuit voltage is typically between 9 V d.c.and 100 V d.c.” (EN 61340-5-1 Annex A Test method A.2 Measurement procedure for footwear testing).
Example of a Wrist Strap and Footwear Tester – more information
If the footwear tester outputs a FAIL test result, stop working, and test the foot grounder and contact strip individually to find out which item is damaged. Replace the foot grounder. Obtain a PASS test result before beginning work.
We get a lot of customers asking us if they should use ESD foot grounders or ESD shoes in their EPA. And our answer is always the same: it depends! There really is no right or wrong when it comes to choosing but there are obviously a few things you need to consider before investing in one or the other.
In some cases, protective footwear (shoes, boots, etc.) is required to prevent foot injuries due to falling or rolling objects or from objects piercing the sole. Safety of the operator takes priority over ESD control at all times. If protective footwear with reliable ESD properties is not available or ESD foot grounders cannot be worn with the protective footwear in the ESD Protected Area, other personnel grounding devices such as wrist straps should be used. For more information on using wrist straps, check out this post.
Introduction to ESD Foot Grounders
ESD foot grounders are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic charges from personnel. They are easy to install and can be used on standard shoes by placing the grounding tab in the shoe under the foot.
Example of ESD Foot Grounder – more info
Guidelines for ESD foot grounders:
- It is recommended that ESD foot grounders are worn on both feet in order to ensure that a continuous path to ground is maintained at all times (even when lifting one foot).
- Contact strips should be tucked inside the shoe with as much contact area as possible to the bottom of the stockinged foot. ESD foot grounders rely upon the perspiration layer inside of the shoe to make contact through the stocking.
- A current limiting one or two megohm resistor in series with the contact strip is recommended but not required.
- ESD foot grounders should be tested independently at least daily while being worn.
Advantages of ESD Foot Grounders
ESD foot grounders are often preferred over shoes because one size fits many foot sizes, thereby reducing stock holdings and simplifying operations.
ESD foot grounders also usually pass the mandatory resistance test as soon as worn, whereas some ESD shoes require a ‘warm-up period’ in order for the operator’s RG to drop below 35 megohms.
They are easily replaceable, quick to put on and less bulky than ESD shoes. They can also be easily taken off when leaving an EPA.
Disadvantages of ESD Foot Grounders
A common complaint with ESD foot grounders is that they don’t last very long. However, there are a few simple tricks to avoid a quick ‘burn-out’:
- The useful life of an ESD foot grounder will depend a lot on the floor and its surface roughness: the rougher the floor the greater the wear. We recommend ESD foot grounders only to be used indoors where floors are usually smoother (and where the ESD foot grounder is less likely to become wet, thereby short circuiting the resistor).
- The manner in which the wearer walks can also affect the life span of the grounder.
In summary, with reasonable care and if used only indoors, ESD heel and toe grounders can last several weeks.
Introduction to ESD Shoes
Conductive additives are blended into the sole (inside to outside) of ESD shoes and connect to the operator’s feet. ESD shoes provide an electrically conductive path from the wearer to the floor – from the operator’s socks (through the sweat layer), to the insole and then to the other outer sole.
Examples of ESD Shoes
There are a number of considerations when selecting ESD shoes:
- Does the ESD shoe meet the ESD Association (ESDA) standards?
Many manufactures of ESD shoes often reference ASTM standards for their ESD specification but state nothing about ESDA standards. The ESDA standards are written specifically for electronics manufacturing and handling. The walking test defined in the ANSI/ESD STM97.2-2006 is one of the most important methods for qualifying ESD shoes for use in ESD Protected Areas.
- Not all ESD shoes is created equal.
There are different styles of ESD shoes. In most cases the specifications of each style will vary. While one style of ESD shoes may retain its ESD properties for 6 months or longer, another will start failing within 90 days. The performance of all styles of ESD shoes should be verified at least daily on an on-going basis and records should be kept for quality control purposes.
Advantages of ESD Shoes
The major advantage of ESD shoes is that they do not require a tab to connect to the operator. There won’t be any issues with the tab not staying inside the shoe – as soon as ESD shoes are put on, the operator is grounded.
ESD shoes are unlikely to be put on incorrectly and have a lower chance of breaking compared to ESD foot grounders. They are generally more reliable and durable.
Disadvantages of ESD Shoes
A big drawback with ESD shoes is obviously the larger initial investment cost. Especially, if you have a large number of operators working in your EPA, it will be costlier to equip everyone with ESD shoes.
Now you know all the ins-and-outs of ESD foot grounders and ESD shoes, you’re probably more confused than ever. So what should you go for? Well, as we said right at the beginning of this post: it depends! It depends, e.g.
- On your budget: are you prepared to initially invest a larger amount of money or would you prefer to spread the costs evenly over time? Also consider the cost over a longer period of time (i.e. 2 or 5 years). Whilst ESD shoes are more expensive initially, ESD foot grounders have to be replaced regularly which adds up, as well.
- On your operators: When selecting your ESD grounding device, it is a good idea to consider the opinion of the operators. They may not find the style of ESD shoe being considered to be comfortable or they may become frustrated that the ESD foot grounder that has been selected does not stay secured properly. In some facilities, many operators are temporary or on a flexible schedule that would not justify certain types of ESD footwear and it is never recommended that operators share footwear due to hygiene issues.
So have a hard look at the numbers and an honest conversation with your employees and then take it from there!
Following our post on how to make sure that your ESD Flooring is working properly, we thought today would be a good opportunity to talk about the difference between conductive and dissipative flooring.
Let’s first define what “conductive” and “dissipative” is:
- Electrostatic conductive materials have a resistance to ground (RG) of greater than 1 x 103 ohm but less than 1 x 105 ohm. A “Conductive ESD Floor” is defined by IEC 61340-4-1 (the flooring standard) as a floor that measures less than 1 x 106 ohm (RG).
- Electrostatic dissipative materials have a resistance to ground (RG) of greater than 1 x 105 ohm but less than 1 x 1012 ohm.
So now we’ve clarified that, you’re probably dying to know what type of flooring you should be using… So here it goes: If you use ESD flooring for a person/footwear/flooring personnel grounding system, we recommend conductive flooring.
Person/Footwear/Flooring Personnel Grounding System
Want to know why? Easy!
EN 61340-5-1 requires ESD flooring to be less than 1 x 109 ohms (RG). The same standard requires a person/footwear/flooring to be less than 3,5 x 10 7 ohms (resistance in series of operator plus footwear plus floor). Remember that floors get dirty which can raise floor resistance. Therefore it is good to start off with a floor that is conductive (less than 1 x 106 ohm). So even if the resistance increases, you’re within the required limits of the ESD Standard.
Make sure you check our Floor Mat Selection Chart.
“For standing operations, personnel can be grounded via a wrist strap system or by a flooring-footwear system”. [EN 61340-5-1 clause 5.3.2 Personnel grounding]
“When the use of a wrist strap system is impractical, the [ESD] floor and [ESD] footwear shall be the primary means of ESD control.” [EN 61340-5-1 clause 5.5 EPA working practices]
Antistatic is an ESD control property properly referred to as “low charging”. This is an important property for all bags used inside or outside of an ESD protected area (EPA). Specifically, it is important that the inside of the bag is low charging. This ensures that when the ESD Sensitive Device is inserted into the bag or otherwise moves, there is minimum electrostatic charge generation.
Dissipative is the electrical property of resistance so when grounded electrostatic charges will be removed to ground. This is an important ESD control property as the bag can be grounded; being dissipative provides an electrical path for an electrostatic charge to dissipate from the bag.
Shielding Bag used in an ESD Protected Area (EPA)
All bags inside the ESD Protected Area [EPA] should be both low charging and dissipative whether packaging ESD sensitive item or non-ESD sensitive items.
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:
a) dissipative or conductive materials for intimate contact;
b) a structure that provides electrostatic discharge shielding.
NOTE 1: If electrostatic field shielding materials are used to provide discharge shielding, a material that provides a barrier to current flow should be used in combination with the electrostatic field shielding material.
NOTE 2 Dissipative materials are preferred for intimate packaging in situations where charged device model (CDM) damage is a concern.”
View our offering of ESD Bags HERE.
While there is no direct link between protection from ESD and protection from moisture, many devices and components require protection from both. Statshield Moisture Barrier Bags (MBB) are designed to protect the contents from both moisture and ESD events. MBBs protect ESD susceptible devices by forming a faraday cage around the contents and they protect the contents from moisture with specialised layers of film that control the Moisture Vapour Transfer Rate (MVTR). In addition to the bags, Desiccant Packs and Humidity Indicator Cards must be used for proper moisture protection (read our article “How to use desiccants with Moisture Barrier Bags” for more information). The bag must be heat sealed with a vacuum-sealer to eliminate the amount of “moisture laden air” within the 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 standardized 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 MBB in section 188.8.131.52.2 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.”
Further information on using MBBs:
- Surface Mounted Devices
Surface Mounted Devices (SMDs) 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. For more information on SMDs and moisture sensitivity, see the following articles:
- Lead-Free Solder
The introduction of lead-free soldering has increased the problem because of the increased reflow temperatures required. Am article from www.circuitnet.com called “Procedure for handling moisture sensitive PCB’s” details the new risks.
- PCB Laminate
The moisture problem is no longer only isolated to the SMD, but is also a problem for the PCB laminate itself. Moisture in the PCB laminate can result in delaminating problems. For further information about moisture concerns in PCB laminates see Ray Prasad’s post on SMTonline.com “Moisture Sensitivity Concerns in PCBs for Lead-free Assemblies“.