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Static Basics
Economy Heel Grounder Static Basics Technical Documents

 

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ESD and Static Basics

  How static damages
Most of us were introduced to ESD (electrostatic discharge) by the classic example of crossing the shag carpet to the doorknob.  When working with microelectronics and other ESD-sensitive items however, the static-gremlin becomes more subtle and dubious.
 

A doorknob-jolt takes hundreds of volts before it is perceptible, but many microelectronic components can be destroyed or suffer latent damage by a dozen volts or fewer.  And unfortunately the component doesn't have to be touched to be damaged or destroyed; ESD damage can occur by induction, or by the electrical field given off by any object or body with a charge imbalance brought near the product.
While training, auditing and process analysis to isolate ESD-related can be costly and time-consuming at first, the time and effort spent there is realized in fewer product rejects, fewer customer returns and ultimately a brand's reputation for high quality.  Learning the sensitivity of your product is a very good place to begin.  Contacting the National ESD Association and reading ANSI ESD S20.20-1999: Standard for the development of an ESD Control Program is another good idea.

Why grounding the body isn't enough
Wrist straps were a popular solution for years--they are inexpensive and do ground the operator's body.  But as electronics became smaller, the wrist strap became less and less effective.  The reason for this is natural fibers like cotton and hair are insulative and unfortunately quite adept at tribocharging  

How an ESD strap or garment works
    Static Shielding
    Grounding
    Ground Monitoring

 

 

 

 

Choosing the Proper ESD Garment is key to your quality control.
Static control is a crucial element to successful and quality driven electronics manufacturing. The high costs of not controlling static electricity in terms of damaged components are increasingly recognized. In fact, device damage often will not show up immediately, but instead later under normal operation of the product. The costs to manufacturers in terms of component failures, returned products, warranty costs, etc. can be tremendous. Articles published by the ESD Association state that average product losses due to static can range from 8% to 33%. Translated, the actual cost of ESD damage to the electronics industry can easily run into the billions of dollars annually. ESD garments are a critical element of an overall system designed to minimize damage caused by electrostatic discharge.

What does an ESD garment do?
ESD garments can be classified into two broad categories based on their performance; groundable ESD garments and non-groundable ESD garments. A groundable ESD garment has electrical continuity across all fabric panels of the garment through the sleeves, collar, pockets and main body parts. This electrical continuity makes it possible to ground the entire garment by connecting a ground cord to a single grounding point anywhere on the garment  (we put a 4mm snap at the hip because it's the most secure point on the garment, the most convenient from the operator's standpoint and because the snap we use is the most commonly used among wrist straps and other personnel grounding equipment. This ground connection provides a safe path to ground for any static charge that is produced on the clothing worn under the ESD garment.
A non-groundable ESD garment does not necessarily have electrical continuity between the different parts of the garment, but has a conductive grid in the fabric that acts as a shield. This protective grid draws or wicks static charges from a static generator (the clothing worn beneath the garment, in this case) and evenly discharge the static into the environment. This is called corona discharge, a steady dissipation of the static into the surrounding atmosphere. The speed at which this discharge happens is dependent upon temperature, relative humidity and the surface size of the charged object..
When a garment is not grounded, caution should be exercised to make sure that any static charge present on the garment is not discharged onto a static-sensitive device.
Due to increased competition, six sigma quality, ISO9000 and most electronic industry company requirements for ESD protection, a grounded ESD garment is the preferred choice in a majority of electronic manufacturing environments.

How can you tell if your company is using the best grounding methods available?
Static-dissipative apparel is often checked through voltage/charge decay time, surface resistivity and sometimes dissipation of triboelectric charging. Tech Wear garments are regularly monitored for resistivity, consistency and durability.

These graphs illustrate voltage generated on an operator in three different scenarios. The time of day and environmental conditions were identical in all three tests.

You can see that the bottom graph illustrates that when an operator is grounded through a Tech Wear OFX-100 garment with ESD-Cuffs, not only are the operator AND garment grounded, but the effectiveness of the ground is actually better than with a wrist strap!

 

Why select a Techwear garment?
All Tech Wear garments made of our OFX-100 and IVX-400 fabric meet or exceed the National ESD Association recommendations for protection from electrostatic discharge. The National ESD Association publishes the industry standard test method ESD STM2.1-1997, which outlines test methods for properly evaluating the resistance characteristics of garments used in the electronics industry for the control of static discharge. We encourage you to contact the National ESD Association for more information about this test method or about ESD in general at www.esda.org.

Before You Buy
Before purchasing any static control garment, be sure to ask the vendor to furnish independent test lab verification that the garment in fact will meet the standards outlined in ESD STM2.1-1997. It is important to determine that the garment will still meet these specifications after a minimum of 50 washings. Tech Wear guarantees its static control garments to meet these specifications for a minimum of 100 washings.
 

 

Copyright 2002 Production Facilitators
Last modified: 04/12/02