ESD

Ionizers remove static from surfaces that cannot be grounded. You will find two common types: AC or Alternating Current and DC or Direct Current. This presentation reviews the pros and cons of each technology and will help you decide which ionizer will work best for your application.

For more information, contact Transforming Technologies 419-841-9552.

This week’s ESD Q&A question comes from StaticCare reader Stephanie, she asks:

Question: Is there really a need for an ESD wastebasket?

Answer:

Hello Stephanie,

Thank you for taking the time to submit your question to Transforming Technologies.

It is fair to wonder why there is a need for ESD wastebaskets. If the basket resides underneath a workstation and stays there permanently, the need may not exist. It is extremely unlikely that a wastebasket that stays underneath a workstation could present a danger to ESD sensitive devices, regardless of the field voltage that may exist on the basket.

There are a number of circumstances however, when the wastebasket could present a significant hazard to electronic products. The following are just a few examples of potentially risky scenarios:

Work surface cleaning:   The operator lifts the wastebasket to the edge of the work station to collect the debris being removed. Should product be present on the work surface and not contained in a shielding container, there is a good opportunity for exposure to electrostatic fields that exceed safe limits.

Lead trimming/board cleaning operations:   When trimming component leads or using aerosol cleaners on PCBs (Printed Circuit Boards), some operators will hold the PCB over the wastebasket to catch the clippings or solvent run-off. Many times the PCB will be placed well into the waste container to assure catching the unwanted materials. This operation puts PCBs and ESD sensitive devices very close to potentially significant electrostatic fields.

Aisle proximity:  Some wastebaskets have been observed in close proximity to production floor aisles where product transfer carts pass or may be parked. ANSI/ESD S2020 requires items that generate 2000 volts or more (measured at 1” from the item) be kept at least 12” from ESD sensitive devices. Using ESD wastebaskets eliminates the risk of carts being parked next to static generating baskets.

While company ESD policies may prohibit some of the actions mentioned above, it does not mean that they will not occur from time to time. ESD wastebaskets provided assurance that the wastebasket will never be an ESD threat to your product.

Transforming Technologies offers ESD wastebaskets in static dissipative polypropylene and in stainless steel for clean room operations. If a liner is to be used for easier maintenance, Transforming Technologies offers them in both conductive and static dissipative.

WBAS 28

• Volume: 28 Quart (7 gallon)
• Height: 15″, Opening: 10.5″ X 14.75″
• Carbon Loaded

Q:  What is “ESD”?

A: ESD is short Electrostatic Discharge.

Electrostatic Discharge, or ESD, is a single-event, rapid transfer of electrostatic charge between two objects, usually resulting when two objects at different potentials come into direct contact with each other.  ESD can also occur when a high electrostatic field develops between two objects in close proximity.  ESD is one of the major causes of device failures in the semiconductor industry.

Electrostatic charge build-up occurs as a result of an imbalance of electrons on the surface of a material.  Such a charge build-up develops an electric field that has measurable effects on other objects at a distance.  The process of electron transfer as a result of two objects coming into contact with each other and then separating is known as ‘triboelectric charging’.

This charging process results in one object gaining electrons on its surface, and therefore becoming negatively charged, and another object losing electrons from its surface, and therefore becoming positively charged. A person can get triboelectrically charged in a number of ways, even by just walking across a room.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

Q:  What is the greatest static threat to electronics and other materials?

A:  YOU!

The human body can generate the biggest charge of anything likely to come near these devices.

Charge can often build-up on people and reach levels that give uncomfortable shocks, can damage sensitive electronic parts or give fire risks when handling solvents and other flammable materials. Dry air humidity encourages static charge build-up, and under dry external conditions the atmosphere in a building can become even drier. Electrostatic charge build-up can be far worse under these conditions. For example, on a dry day, humans can generate a static field of more than 100 volts by just the slight raising of an arm.  During normal, everyday activities, it is common to generate 6,000 volts or more.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

The following is an excerpt from the article “Static Electricity in Cleanrooms” by Lawrence B. Levit, Ph.D., founder of LBL Scientific. He has worked in the static charge control industry for over 15 years. Levit is a senior member of the ESD Association, a senior member of the IEST and sits on the Board of Directors of the SiVa ESD Society and is the chair of WG22 of the IEST. He holds 6 patents in the area of ESD control.

Introduction
Static electricity is often overlooked in cleanroom environments and the results of this oversight can cause a reduction in profitability. In some factories where the effects are not studied and understood, they are dealt with by budgeting a reduction in production yield for unforeseen issues. Static charge and its effects certainly can be understood and it can be kept in check. The purpose of this article is to remove the “Black Magic” from the issue, explain how it affects the manufacturing process and discuss remedies

What is special about a cleanroom?
A cleanroon is a unique environment. For many applications it is regulated to a low relative humidity (RH) to optimize the process. Much of the particulate contamination is excluded by massive amounts of air filtration involving high efficiency particulate air (HEPA) filters. Objects in the cleanroom are wiped down before they can enter into the room. As it turns out, these actions are very good for maintaining an ultra clean environment but they are also the prescription for achieving massive levels of static charge in a cleanroom.

It is well known that static charge is created efficiently in a low humidity environment. It is commonplace for individuals to get stung by a spark when they reach for a doorknob in the winter time. Buildings are heated and lower RH results from the temperature rise. This same effect causes the rate of static charge generation to be higher in a cleanroom, especially a low RH cleanroom, than in a conventional room.

Once static charge has been generated by contact with other materials, nature has several methods to dissipate the charge. The first mechanism is conduction of this electrical charge to ground through any surface contamination on the object. Normally, such contamination, for example oil from a person’s hands, is removed from the object before it enters the cleanroom and the object remains clean because the products are only handled by gloved hands.

The only mechanism remaining for the natural dissipation of surface charge is the presence of ions in the air. These ions are created by naturally occurring radioactivity and by other items like runner water and electrical motors. Unfortunately, ions cannot pass through a HEPA filter – they are attracted electrostatically to the materials of the HEPA filter – so they are removed from the air entering the cleanroom.

To complicate matters further, most cleanrooms employ many insulators such as glass and plastic. Teflon, for example, is an incredibly effective insulator and holds onto its static charge aggressively. Glass and other plastics are also very effective insulator. As a consequence of the above argument, cleanrooms allow static charge to be generated very efficiently and dissipated very poorly. The result is levels of static charge which far exceed those in conventional rooms.

For the full article and to learn how to control ESD in a cleanroom, follow this link to “Static Electricity in Cleanrooms.”

Levit graduated from Case Institute of Technology, Cleveland, Ohio with a BS degree in physics with honors.  In 1970, he earned a Ph.D. in Experimental High Energy Physics from Case Western Reserve University. He can guide you to a more profitable factory by eliminating the sources of ESD damage, mitigating microcontamination issues and identifying EMI from ESD that is effecting your robotics and automated test systems. Contact LBL Scientific by Phone at 925-788-2969   or    EMail

One Meg Vs Two Meg Heel Grounders

Introduction

Have you ever wondered what it means when a heel grounder described as having a “1 meg resistor”? or a 2 meg resistor?  The size of a current limiting resistor is often included in the description  of personal grounding products, but what does it mean and how concerned should I be with the size of the resistor when I am buying heel grounders?  Today we will answer these questions but first some background:

What is a Resistor?

A common resistor

A resistor is a component of an electrical circuit that resists the flow of electrical current. A resistor has two terminals across which electricity must pass, and is designed to drop the voltage of the current as it flows from one terminal to the next. A resistor is primarily used to create and maintain a known safe current . Most grounding products have a current limiting resistor and it is most commonly one megohm, rated at least 1/4 watt with a working voltage rating of 250 volts. This ensures that the flowing current will be within safe levels.

Heel Grounders and ESD Protection?
Heel grounders discharge static from a person to ground by connecting the person to a grounded walking surface. A heel grounder is made from a conductive material and is worn on each shoe.  A conductive ribbon placed inside the wearer’s shoe or sock makes electrical contact with the skin through perspiration. The ribbon is joined to a resistor which limits current should accidental exposure to electricity that may  occur. The other end of the resistor is joined to a conductive sole. This sole contacts a grounded ESD floor mat or ESD flooring system. Heel grounders should be worn on both feet to maintain ground contact while walking. UL and OSHA recommends a minimum of 1 megohm resistance to ground (RTG) in order to limit current for safety purposes.

Types of Heel Grounders?
Heel Grounders come in a variety of styles:

• Cup Style: Made with two reversible soles for longer life span.
• Foot Grounder: Provides a more complete path-to-ground due to wider contact area and heel-to-toe coverage.
• Disposable: Strip of conductive material is applied to shoe. Economical and perfect for one time use.
• Toe Grounder: Ideal for high heels when standard heel grounders won’t fit properly.

Resistors and Heel Grounders
Heel grounders come with either a 1 megohm or 2 megohm resistor. When one foot is on the ground, a 1 megohm heel grounder gives you an RTG of 1 megohm and a 2 megohm heel grounders results in 2 megohm RTG. But when both feet are on the ground, the sum of the resistors yield a RTG of 1/2 that measurement.
For example:

• When you wear two, 1 megohm heel grounders and have both feet on the floor, your RTG is only 1/2 megohm, NOT 1 megohm!
• If you wear two, 2 megohm heel grounders an have both feet on the ground, your RTG is 1 megohm.
• By wearing two heel grounders with a 2 megohm resistor you are complying with UL and OSHA at all times.

This can be sometimes confusing.  So we made a diagram to illustrate:

Conclusion

Heel grounders are an effective and popular way to ground mobile personnel in ESD areas. Heel grounders use built in resistors to limit any potential current that flows through the device to ensure operator safety.  The size of the resistor that you use depends on the standards of the specific operation,  but by wearing two heel grounders with a 2 megohm resistor you are complying with UL and OSHA at all times.

To learn more about ESD heel grounders and Resistors, visit Transforming Technologies.com