The Four Rules Of ESD Control

Follow these simple rules of ESD control and you will never be shocked (yes, we went there) by static damage.

1^st Rule of ESD:  Ground everything you can.

If you can attached a ground cord to it, do it.  Grounding conductive materials sends any charge buildup to ground where it is neutralized.

2^nd Rule of ESD:  Avoid Insulators whenever you possibly can!

Insulators cannot be grounded and a potentially dangerous charge can build up fast.  Remove everything you can made out of insulating materials, like plastics and Styrofoam.

3^rd Rule of ESD: Use Dissipative materials whenever possible and ground them.

Get yourself a dissipative ESD Mat!  Dissipative materials allow a charge to flow slowly and controlled to ground.  Anything more conductive or more insulative is a problem.

4^th Rule of ESD: Use an ionizer on any insulators and ungrounded conductors that can’t be remove.

Ionizers blow air full of positively and negatively charged ions that neutralize static on contact.  They are the only way to control static on insulative materials.

Covering your work surface with ESD material is a basic step in ESD prevention. With a proper ESD worksurface, static is safely removed from items placed on the surface. There are several types materials used, but the most common is 2 layer ESD rubber. ESD rubber is constructed with a rugged dissipative top layer and a conductive black bottom layer. For ESD purposes, materials are classified by their resistance to the movement of electricity. The dissipative top surface allows charges to drain from its surface in a controlled manner, sending it to the bottom conductive layer. The conductive bottom layer has a low electrical resistance and quickly sends the charge to ground via a ground cord that includes a current limiting resistor for safety. The top layer is also extremely durable, offering excellent resistance to hot solders, soldering irons and most solvents.

Transforming Technologies has the most cost effective ESD rubber available.  The MT4500 series is a smooth rubber surface in light blue, royal blue, gray and green.  The MT4000 Textured series ESD rubber matting is made of 100% co-extruded rubber with a textured surface in light blue, royal blue and gray.

Contact Transforming Technologies for your work surface ESD rubber needs.

For ESD control purposes, materials are classified by how quickly electricity moves through the material. The speed is referred to as the “resistance” of the material i.e.  how strongly the material resists charge movement.

The speed is measured in Ohms and is typically displayed in powers of 10 (example 10^3). The lower the number, the more conductive the material and may be considered “Antistatic”.

The following ranges and definitions are found in ESD Association or EIA standards publications:

Conductive materials: With a low electrical resistance, electrons flow easily across the surface or through the bulk of these materials. Charges go to ground or to another conductive object that the material contacts or comes close to. Conductive materials have a surface resistivity less than 1 x 10^5 Ω/sq or a volume resistivity less than 1 x 10^4 Ω-cm.  Conductive materials are classified “Antistatic”.

Dissipative materials: For these materials, the charges flow through the material slowly and in a somewhat more controlled manner than with conductive materials. Dissipative materials have a surface resistivity equal to or greater than 1 x 10^5 Ω/sq but less than 1 x 10^12 Ω/sq or a volume resistivity equal to or greater than 1 x 10^4 Ω-cm but less than 1 x 10^11 Ω-cm.2.  Dissipative materials are classified “Antistatic” and are considered to be the ideal range for ESD materials.

Insulative materials: Insulative materials prevent or limit the flow of electrons across their surface or through their volume. Insulative materials have a high electrical resistance and are difficult to ground. Static charges remain in place on these materials for a very long time. Insulative materials are defined as those having a surface resistivity of at least 1 x 10^12 Ω/sq or a volume resistivity of at least 1 x 10^11 Ω-cm.  Insulative materials are not classified as “Antistatic”.

Anti-Static: Is a term used to describe materials that prevent the buildup of static electricity. Both conductive and dissipative materials are classified as Antistatic.  Insulative materials are not.

How to Measure Surface Resistivity
The SRM200 Surface Resistance Meter is an easy to use tester for measuring surface resistivity. This SRM200 uses parallel electrodes on the back of the meter to accurately measure RTT, RTG, or resistivity for periodic verification, factory audits or test lab evaluation of a product.

This meter is designed to be used in all facets of material production including engineering, maintenance, quality control, incoming inspection, manufacturing, research, or sales departments for the testing of anti-static mats, floor finishes, paints, wrist straps, smocks, footwear, bags and containers.

The SRM200 meets periodic test requirements per Compliance Verification ESD TR53 and conforms to ANSI/EOS/ESD (S4.1, S7.1, S12.1, S2.1).

Introduction

When a tool or tester on your line stops working, everyone scrambles to correct it. Such process halts slow the manufacturing process, and cause lower product throughput. Achieving the best equipment up time is a key element in achieving profitability in any automated factory.

When tool halts are random they are quite difficult to troubleshoot. Their frequency could be every few minutes or just once a month!. Equipment diagnostics often provide no information as to the cause of these random equipment problems.

Finding the Cause of the Problem

When an automated process halts sporadically, both the hardware and the software are suspect.  Often there are lengthy finger pointing discussions between the hardware and software engineers which does not bring the problem any closer to a solution.

In some cases, the problem is neither the hardware nor the software.  Rather it can be electromagnetic interference (EMI) generated by electrostatic discharge (ESD). The EMI generated by ESD can exceed immunity levels of automation equipment. ESD discharges occur so fast (typically in nanoseconds) that they radiate EMI extremely efficiently.  This very short transient EMI is surprisingly high in amplitude because all of its energy is packed into a few nanoseconds.  Further, these short EMI transients are not detected by spectrum analyzers.

An electrostatic discharge occurs when charged objects come close enough to ground to cause a breakdown of the air gap between the object  and ground. Transferring wafers or reticles with a grounded robot or plugging a charged IC into a socket are obvious examples of such ESD events. ESD is well-known to cause damage directly to reticles, wafers and ICs.

Once an ESD event injects an EMI transient into a tool, it is efficiently transmitted through the electronic circuitry within the tool.  Circuit boards are laid out to distribute high speed clock signals and thus EMI transients are also transmitted efficiently.   It is important to realize that for EMI to cause an error it must be induced in the circuitry simultaneously with a critical circuit operation.  Thus, many EMI events can occur before one causes a tool error to occur.  This is also the reason that this type of tool halt is random in nature.

The tool halt, may be accompanied by a difficult-to-interpret error message. Equipment diagnostics succeed in making the problem look like a software bug, certainly not  EMI from an ESD event.

Conducted vs. Radiated EMI

When an ESD event occurs, the rapid transfer of charge results in currents that may be hundreds of Amperes. The entire discharge typically occurs in a time interval measured in nanoseconds. While some of the energy is dissipated in heat (which damages silicon and metallization), a significant portion of the energy becomes electromagnetic radiation. This generally occurs in the 25 MHz to 2 GHz frequency range. It may be conducted away from the site of the ESD event through a metal structure or a power line, or it may radiate through the air. In either case it may affect equipment at a considerable distance from the location of the ESD event.

Identifying ESD caused EMI

ESD diagnostics are not part of the toolkit of most production engineers. As an illustration, a manufacturer of microprocessors was experiencing random equipment problems with one of nine steppers in the photolithography area. In-house engineers and the equipment manufacturer’s field service engineers tried software upgrades and replacement of major stepper components. After 6 months of frustrating and unsuccessful investigation, a well known  ESD consultant made measurements with an antenna and a wide band digitizing oscilloscope.  He detected a random signal on the power supply line of the stepper occurring about once a minute. The random nature of the signal pointed to EMI as the possible cause of the problem.

An electrostatic field-meter found static charge developing on a panel and identified the cause of the problem in less than one hour. One of the wall panels above the stepper was not connected to ground. This large isolated conductor was becoming charged, and then discharging to the nearby grounded wall framing. The EMI from the ESD event was causing the equipment interrupts.

Diagnostic Clues

When an unexplained phenomenon occurs in a random fashion on some tools of a given tool type but not all of them that is a clue.  When a “software bug” is evident on some tools but not all but the tools have the same Rev Level of software, that is another clue.  Most importantly, do not wait six months before considering this cause for the problem.  There are tools for the detection of these transients that range from multi Giga Hertz digitizing oscilloscopes to simple EMI locators designed to be sensitive to these transients.

Conclusions

Many equipment failures are the result of random ESD events and a great deal of production and engineering time is wasted pursuing phantom software problems.

Charge generation is unavoidable in many high tech manufacturing processes due to the presence of easily charged insulating materials. It is impossible to assure that everything is continuously connected to ground and that ESD events will not occur. 5 Making ESD control part of your arsenal of tools to keep your factory running smoothly is an important addition.

LBL Scientific 

References

1. Levit, L., Menear, J.”Measuring and Quantifying Static Charge in Cleanrooms and Process Tools”, Solid State Technology, February 1998. PennWell.

2. Honda, M., et al, “Method of Observing ESD Around Electronic Equipment”, ESD Symposium, September 1996, ESD Association, 7900 Turin Road, Rome NY 13440.

3. Peters, L., “ESD Can Be a Tool’s Worst Enemy”, Semiconductor International, September 1997, Cahners Publishing Company.

4. SEMI E78-0998 “Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment, September 1998, SEMI.

5. Galatowitsch, S., “Terminating Static in the Cleanroom”, Cleanrooms, April 1998, PennWell.

lectro-static-discharge-basics-by-transforming-technologies

ESD Protection in the Field

Static damage is a constant threat anywhere micro-electronic com-ponents are used, but these components are at highest risk during servicing. The Transforming Technologies’ FSM2424 Field Service Kit is designed to quickly and reliably remove static charge on the service technician and provide a safe static-free surface to lay parts. The kit includes a 24” x 24” rugged mat with two 8” x 12” pockets for storing tools, an adjustable wrist band, coil cord and 15’ ground cord.

The FSM2424 Field Service Kits eliminates static charge through the principle of grounding. Both the mat and the operator’s wrist band connect to the specialized ground cord which when properly connected to ground, provides a safe path for static charges to drain to ground.

Field Service Procedure

1. Upon arrival at the service call site, unpack the field service kit in a convenient area near the equipment to be serviced.
2. Remove any accessories from the pockets and attach the ground cord to the mat via one of the two snaps found in the corner of the mat.
3. Connect the opposite end of the cord to a reliable ground. Common ground locations are water pipes, unpainted equipment frames, and building frames.
4. Slip on the wrist band and adjust the size to fit comfortably around the wrist but tight enough to make contact with the skin at all times. Snap the coil cord to the wrist band.
5. Insert the banana plug of the coil cord into the jack of the ground cord. This connects the wrist band to the same common ground as the work surface.
6. It is now safe to remove and handle static-sensitive components.

Field Service Care

The FSM2424 Field Service Kits can be cleaned with ESD-safe mat and surface cleaner or with a mild detergent and water as needed. Once clean, allow the material to air dry, removing any excess water/moisture with an absorbent cloth. DO NOT use heat of any kind to dry mat. Though inherently flame resistant, the field service products will burn with enough exposure to open flame or other heat sources of sufficient temperature. DO NOT use a clothes dryer, as the excess heat may cause the PVC material to retain the shape it held in the dryer. Most strong solvents are not recommended. AVOID: Halogens, concentrated Oxidizing Agents, Ethers, ketones, and Low-Molecular Weight Solvents derived from the Ether and ketones fami-lies. They include (but are not limited to) TetraHydroFuran (THF), Methyl Ethyl ketone (MEK), Methyl Isobutyl Ketone (MIBK) and Acetone. These chemicals will mar and/or destroy the sur-face of the PVC product (the Ketones), react with it (Oxidizing Agents), or actually dissolve it (THF and other Ethers). Wrist strap can be hand or machined washed.

Here at Transforming Technologies, we routinely receive phone calls and emails this time of year asking us how to prevent statics shocks around the home. While we love to help people when we can, the solutions and products we provide are geared more for manufacturing and the electronics fields.  This is why I was very excited to read the popular blog Lifehacker explain common ways to eliminate static shocks in the home.

“One of the easiest ways to avoid static shock is to pay attention to what you’re wearing and what kind of fabrics make up the furniture in your house. For example, Electrostatics.net notes that rubber-soled shoes are great insulators, and will build up a lot of static in your body when combined with a wool or nylon carpet. Instead, try walking around in leather soled shoes, or cotton socks instead of wool socks. Leather soled shoes are also great for grocery shopping, since shopping carts can often cause lots of static electricity.

Similarly, wool sweaters are common offenders, especially in the dry winter (when you usually wear them). If you sit in a chair made out of the right fabric, you’ll build up quite a bit of static. Again, cotton is going to be much more friendly, so try wearing cotton clothes when you want to avoid nasty shocks. Certain furniture covers or antistatic sprays can help alleviate this problem, too.

You may have also noticed that often, when you get out of your car, you get a shock when you touch the door. You might have even heard that touching the door frame as you get out of the car can help, and that’s true. Make sure you start holding the metal frame before you get out of the car, and you keep touching it until you’re out of the seat completely. If you forget to do this, you can also touch the car door with your keys. Since the electricity will discharge through them, you won’t feel a shock”

Read the full article for more tips on avoiding static shocks in the dry air of the winter months.

An ESD Workstation is a work area that is free of static and safe from ESD (electrostatic discharge). The workstation is made up of materials and equipment that limit voltage generation and eliminate static if it is present.

For an ESD workstation to function properly, all components of the work area (work surfaces, people, equipment, etc.) must be grounded to the same electrical ground point called the “common point ground.”

We list the steps and components necessary to an ESD workstation. While it is recommended to take every precaution available to control static, not all the ESD steps below may be necessary for basic ESD protection. Your workstation may not require all these items. Call Transforming Technologies for help designing your ESD workstation

ESD Protection Steps

Stationary Personnel

Mandatory Steps: 1,2,3

Suggested Steps: 7,9

Optional Steps: 4,5,6,7,10

Mobile Personnel

Mandatory Steps: 4,5,6

Suggested Steps: 9

1. ESD Table Mat

A work surface that removes static from conductive items placed on it. Commonly made from durable materials such as rubber, vinyl or laminate. A 10mm grounding snap should be installed in the mat or every 10 ft if it is long.

2. Wrist Band and Coil Cord Set

Safely grounds a person working at a workstation.  It is worn around the wrist and is connected to ground through a long coiled cord and the common point ground.

3. Common Point Ground

Used for grounding ESD mats and provide a path-to-ground for the person wearing wrist straps.  It attaches to the ESD worksurface via the 10mm grounding snap.

4. ESD Floor Mat

ESD floor mats remove static charges from conductive items placed on it. ESD footwear such as a Heel Grounder must be worn to remove static from the person standing on the mat. It is best used for mobile personnel that cannot be tethered to one location by a wrist strap.   A grounding snap should be installed in the mat or every 10 ft if it is long.

5. ESD Floor Mat Ground Cord

Used for grounding the floor mat by providing a path-to-ground for an ESD floor mat.  Connects to the mat via a grounding snap.

6. ESD Heel Grounders

Worn on each shoe to connect a walking or standing person to ground.

7. Constant Monitors

These electronic devices continuously monitor the path to ground of a wrist strap, work surface and other grounded components on a workstation.  Eliminates the need to test wrist bands and protects against ground failures.

8. ESD Jackets

Provide shielding from static charges on your clothing. A “Hip-to-Cuff” grounding system ensures proper grounding without requiring wristbands being worn that can inhibit your movement.

9. ESD Equipment Testers

ESD wrist strap and heel grounders, like any equipment used every day, can wear. Ensure these items are in proper working order with ESD Equipment testers.

10. ESD Air Ionizers

Materials, called insulators, cannot be grounded by typical means.  Ideally insulators should be removed from the workstation, but if they must stay an ESD Air ionizer can neutralize static buildup.  Delivering a balanced stream of ionized air, ionizers remove static charges from insulating materials and isolated conductors.

Every situation is unique and requires different steps to properly manage a static situation.