We’ve previously published a post that explains when you need ionisation. However, following this post, we got a number of questions that prompted us to dive a bit deeper into the whole subject of ionisers. Basically with this post we’re starting right at the beginning so stay tuned…
Before talking about ionisers in more detail, we need to have a little chat about the types of materials that can be found in an EPA – conductors and insulators:
• Electrical current flows easily
• Can be grounded
Materials that easily transfer electrons (or charge) are called conductors and are said to have “free” electrons. Some examples of conductors are metals, carbon and the human body’s sweat layer. Grounding works effectively to remove electrostatic charges from conductors to ground. However, the item grounded must be conductive.
The other term often used in ESD control is dissipative which is 1 x 104 to less than 1 x 1011 ohms and is sufficiently conductive to remove electrostatic charges when grounded.
When a conductor is charged, the ability to transfer electrons gives it the ability to be grounded.
• Electrical current does not flow easily
• Cannot be grounded
Materials that do not easily transfer electrons are called insulators and are by definition non-conductors. Some well known insulators are common plastics and glass. An insulator will hold the charge and cannot be grounded and “conduct” the charge away.
Both conductors and insulators may become charged with static electricity and discharge. Grounding is a very effective ESD control tool; however, only conductors (conductive or dissipative) can be grounded.
Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.
Insulators, by definition, are non-conductors and therefore cannot be grounded. Insulators can be controlled by doing the following within an EPA:
• Keep insulators a minimum of 31cm from ESDS items at all times or
• Replace regular insulative items with an ESD protective version or
• Periodically apply a coat of topical antistat
“Process essential” Insulators
When none of the above is possible, the insulator is termed “process essential” and therefore neutralisation using an ioniser should become a necessary part of the ESD control programme.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.
An example of isolated conductors can be conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.
Reduction of charges on insulators does occur naturally by a process called neutralisation. Ions are charged particles that are normally present in the air and as opposite charges attract, charges will be neutralised over time.
A common example is a balloon rubbed against clothing and “stuck” on a wall by static charge. The balloon will eventually drop. After a day or so natural ions of the opposite charge that are in the air will be attracted to the balloon and will eventually neutralise the charge. An ioniser greatly speeds up this process.
A balloon “stuck” on a wall by static charge.
What is an ioniser?
An ioniser creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionisation can neutralise static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
An ioniser creates positively and negatively charged ions.
Note: Ionisers require periodic cleaning of emitter pins and the offset voltage must be kept in balance. Otherwise, instead of neutralising charges, if it is producing primarily positive or negative ions, the ioniser will place an electrostatic charge on items that are not grounded.
This citation from the ESD handbook provides an excellent summary:
“The primary method of static charge control is direct connection to ground for conductors, static dissipative materials, and personnel. A complete static control program must also deal with isolated conductors that cannot be grounded, insulating materials (e.g., most common plastics), and moving personnel who cannot use wrist or heel straps or ESD control flooring and footwear. Air ionization is not a replacement for grounding methods. It is one component of a complete static control program.
Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.” (ESD Handbook ESD TR20.20 Ionization, section 126.96.36.199 Introduction and Purpose / General Information)
Now that you know what conductors and insulators are, how to treat them in an EPA and when to use ionisation, the next step is to learn about the different types of ionisers available. However, as this post is already quite long, we will save that part for next week so stay tuned…. Click here to read the follow-up post.
We thought today we could focus on ESD during storage and transport. If you have read our recent post on Tips to Fight ESD, you will remember how important it is to protect your ESD sensitive items when leaving an EPA. Yet, too often we see customers who have the perfect EPA, but when it comes to transporting and storing their precious components, it’s all falling apart.
1. Packaging required for transporting and storing ESD sensitive items
During storage and transportation outside of an EPA, we recommend that ESD sensitive components and assemblies are enclosed in packaging that possesses the ESD control property of shielding.
- In ‘shielding’ we utilise the fact that electrostatic charges and discharges take the path of least resistance.
- The charge will be either positive or negative; otherwise the charge will balance out and there will be no charge.
- Charges repel so electrostatic charges will reside on the outer surface.
2. The Faraday Cage effect
A Faraday Cage effect can protect ESD sensitive items in a shielding bag or other container with a shielding layer. To complete the enclosure, make sure to place lids on boxes or containers and close shielding bags.
Cover must be in place to create Faraday Cage and shield contents.
3. Types of shielding packaging
The below list gives a few examples of what types of shielding packaging is available on the market. This list is by no means complete; there are many different options out there – just make sure the specifications state “shielding” properties.
- Metal-In Shielding Bags
ESD bags which protect ESD sensitive items. The ESD shielding limits energy penetration from electrostatic charges and discharge. They offer good see-through clarity. Available with and without dissipative zipper.
- Metal-Out Shielding Bags
Integral antistatic and low tribocharging bags which will not electrostatically charge contents during movement. Bags have an aluminium metal outer layer of laminated film. Available with and without dissipative zipper.
- Moisture Barrier Bags
Offer ESD and moisture protection and can be used to pack SMD reels or trays. Check out this post for more information on MBB and ESD Control.
- Bubble Shielding Bags
These bags combine the “Faraday Cage” and mechanical protection. They shield about twice as well as normal shielding bags of equivalent size.
- Component/Circuit Boards Shippers
These boxes offer an efficient way of shipping or storing ESD sensitive circuit boards and other items. They provide ESD shielding with the lid closed. The foam cushioning reduces stress from physical shock.
- In-Plant Handlers/Storage Containers
Shield ESD sensitive items from charge and electrostatic discharges (with lid in place). They provide ESD and physical protection for ESD sensitive circuit boards.
4. Additional options for storing ESD sensitive items
Do you have the following in place?
- ESD flooring
- Grounded personnel (using foot grounders). Read this post for more information on how to ground moving personnel.
- Grounded racking
Operator wearing foot grounders
IF (and this is a BIIIG IF) the above requirements are fulfilled, you can use conductive bags or containers to store your ESD sensitive items. Conductive materials have a low electrical resistance so electrons flow easily across the surface. Charges will go to ground if bags or containers are handled by a grounded operator or are stored on a grounded surface.
Conductive materials come in many different shapes and forms:
- Conductive Black Bags
Tough and puncture resistant bags which are made of linear polyethylene with carbon added. The bags are heat sealable.
- Rigid Conductive Boxes
Provide good ESD and mechanical protection. Boxes are supplied with or without high density foam for insertion of component leads or low density foam which acts as a cushioning material.
- PCB Containers
Are flat based and can be stacked. They are made of injection moulded conductive polypropylene.
Again, there are many more options available on the market so make sure you do your research.
Note: we do not recommend using conductive packaging to transport ESD sensitive devices. Also, pink antistatic and pink antistatic bubble bags are not suited for storing or transporting ESD sensitive components.
5. Final thoughts
Packaging with holes, tears or gaps should not be used as the contents may be able to extend outside the enclosure and lose their shielding as well as mechanical protection.
Also, do not staple ESD bags shut. The metal staple provides a conductive path from the outside of the ESD bag to the inside. The use of a metal staple would undermine the effectiveness of the ESD bag making a conductive path for charges outside the bag to charge or discharge to ESD sensitive components inside the bag. To close an ESD bag, it is recommended to heat seal or use ESD tape or labels after the opening of the bag has been folded over. Alternatively, you can use ESD bags with a zipper.
Sealing ESD Bags the correct way
One final word of warning:
When ESD sensitive items are unpackaged from shielding bags or other containers, they should be handled by a grounded operator at an ESD workstation
An ESD Protected Area (EPA) is a defined space within which all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same potential. This is achieved by simply using only ‘groundable’ materials, i.e. materials with an electrical resistance typically of less than 109 ohms, for covering of surfaces and for the manufacture of containers and tools. All surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Movable items, such as containers and tools, are bonded by virtue of standing on a bonded surface or being held by a bonded person. Everything that does not readily dissipate charge must be excluded from the EPA.
Example of an EPA Area:
|1.||Bench Top Ionisers||8.||Ground Cords||15.||Floor Mats|
|2.||Packaging Containers||9.||Overhead Ionisers||16.||Wrist Straps|
|3.||Earth Bonding Point Bar||10.||Shielding Bags||17.||Foot Grounders|
|4.||Testers and Monitors||11.||Data Acquisition||18.||Lab Coats|
|5.||ESD Tape||12.||PCB Storage||19.||Floor Maintenance|
|6.||Worksurface Mats||13.||Signs and Labels||20.||Waste Bins and Liners|
|7.||Surface Resistance Meters||14.||Floor Marking Tape||21.||Document Handling|
Application Photo of Surface Resistance Test Kit
A significant increase in the discipline of implementing the fundamentals of ESD control noted in ANSI/ESD S20.20 Foreword, calls for:
- Ground all conductors in the EPA including people
- Remove all insulators from the EPA or use ionizers for process necessary insulators
- Package ESD sensitive items going outside the EPA in packaging that provides electrostatic discharge shielding
We encourage developing a hatred for insulators. The alternatives are:
Remove the insulative item from the EPA
- Substitute the item with an ESD protective version (such as tape, document holders, material handling containers, plastic bottles, etc.)
- Periodically treat insulative surface with a topical antistat
- Neutralize electrostatic charges using ionization
Other ESD Control Items
Other EPA ESD control items to add to the ESD control program might include shelving, mobile equipment (carts), gloves, and/or seating.
Improve Compliance Verification Plan
- Consider greater frequency of internal audits per ESD TR53
- Use of computer data collection system for wrist straps and footwear testing, continuous monitors, and ionizers
- Use of ground continuous monitors for worksurfaces and other ESD elements
- Test ionizers more frequently, consider self monitoring ionizers, consider computer based data collection
- Increased testing using static field meter to verify that automated processes (like auto insertion, tape and reel, etc) are not generating charges above acceptable limits.
Application Photo of Volt Meter and Software in Factory STM97.2 Testing Voltage Charge on Person (Photograph courtesy of TREK, INC.)
- ESD awareness training for all in the EPA or who may come into the EPA including suppliers
- Testing to verify comprehension and training adequacy
- Training on the proper use of test equipment
- Training on proper compliance verification test procedures
Application Photo CD-ROM ESD Training
Just to maintain a company’s current level of quality and reliability may require a substantial improvement in a company’s ESD control program. Now is the time for improvement as ESD sensitivity withstand voltages continue to get lower and companies may soon be handling class 0A HBM items. To combat HBM failures improved personnel grounding is required. For example, heel grounders should be replaced with full coverage foot grounders. However, most failures are CDM. To combat CDM failures, ionization should be added or improved, and conductive surfaces should be covered with dissipative material. In general, disciple should be enhanced implementing ESD control fundamentals, compliance verification testing should be increased, and training should be improved.
From published article “Now is the Time for ESD Control Programs to be Improved” by Fred Tenzer and Gene Felder. See full article at InCompliance Magazine- September 2012
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- Single-wire and Dual-wire Wrist Strap Compatible
- Embedded CCD Barcode Scanner
- Ethernet Communication
- Adjustable Resistance Limits
- Programmable Test Requirements
- Compatible with Proximity Readers
- Embedded relay terminal for use with electronic
TEAM5 is the most powerful and accurate ESD Test Acquisition Software on the market. Use TEAM5 to automate and manage the ESD test data collected by the SmartLog V5™.
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|50762||Smartlog V5™, Europe, w/o Software||£2,294.72|
|50763||SmartLog V5™, Euro, 10MM Adapter, w/o Software||£2,294.72|
|50764||Reader Only, SmartLog V5™, HID ProxPoint Plus||£ 370.28|
|50765||Reader Only, SmartLog V5™, HID iClass||£ 383.04|
|50491||TEAM5 Enterprise Software with 1 Year Service||£3,886.16|
|50493||TEAM5 Software with 1 Year Service||£ 623.36|
“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 1.0 2007-08 clause 5.2.3 Compliance verification plan]
- One Size Fits All Adjustable Heel Grounder
- 1 Megohm resistor
- Conductive tab length = 432 ± 2mm
– Black rubber surface: 8 x 10^4 ohms nominal
– White rubber surface: 1 x 10^12 ohms nominal
- Date coded for traceability
- Made in the Philippines
|249270||Trustat™ Heel Grounder, Single Rubber, Velcro, 1 Meg||£3.31|
|249271||Trustat™ Heel Grounder, Single Rubber, Quick Release, 1 Meg||£3.03|
|249272||Trustat™ Heel Grounder, Double Rubber, Velcro, 1 Meg||£4.42|
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|249274||Trustat™ Toe Grounder, Adjustable, Quick Release, 1 Meg||£4.28|
**The prices in the Vermason Online Catalogue supersede all pricing on new product announcements and may change without notice
“Most people do not stand solidly on both feet, it is important that paths to ground are made in the heel and toe area of both feet. Where toe and heel straps are used as ESD footwear, once these are worn outside the EPA, particularly on carpets, they are likely to accumulate fluff and become ineffective; this requires that they be checked or replaced on every visit to the EPA [ESD Protected Area].
When ESD footwear is used, it should be noted that ESD footwear alone cannot achieve protection, but needs to be used in conjunction ”[EN 61340-5-2 clause 5.2.8 Footwear]
Leading companies use continuous monitors as a cost effective component in satisfying some of the audit and check requirements of EN 61340-5-1.
Wrist strap testing “Where continuous monitoring is used, no additional testing is required.” [EN 61340-4-1, per A.5.2]
“The wrist band will normally be worn for several hours at a time so it needs to be comfortable while making good contact with the skin. It is a good idea to check the wrist strap every time it is applied. Constant on line monitors can be used so that any breaks will be immediately found.” [EN 61340-5-2 clause 5.2.7]
“A properly grounded wrist strap will keep a person’s body voltage to approximately + 10 V. The main advantage to a constant [or continuous] monitor is the immediate indication that the employee receives if the wrist strap falls open. With an unmonitored system, the employee will not be aware of a wrist strap failure until the start of the next shift. This has reliability benefits for an ESD program as it might help reduce or eliminate ESD damage.
There are also other process benefits from using constant monitors such as the elimination of the need to maintain daily test logs and a reduction in the time for employees to make the daily test. For units that also monitor the connection of a work surface to protective earth, it is also possible to reduce or eliminate the checking of the work surface as part of the periodic audit of the process.
Constant monitors might be implemented by an organization due to high reliability requirements imposed by customers.” [CLC/TR 61340-5-2:2008 User guide Annex B.1.3 Constant monitors]
“In many EPAs [ESD protected areas] constant monitor wrist straps are used. These fall into two categories, dual wrist band and single wrist band. The dual wrist strap type using a split band is used with a two core cord.
The dual wrist strap type normally works using the resistance bridge method.
The single wrist strap type uses a single strap for both grounding and monitor purposes and has an a.c. signal injected which is used for monitoring purposes. This type has the advantage of using the simpler single wrist strap.” [EN 61340-5-1 A.5 Wrist straps]
See selection of Vermason Continuous Monitors.