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Debugging the Bunnysuit
John Schuch
Testing ESD-protective cleanroom garments doesn't have to be difficult.
Here's how
manufacturers can use existing standards to ensure a successful garment program.
Operators working in cleanroom environments wear a wide variety of garments ranging from frocks to full bunnysuits. Although manufacturers may believe that such garments provide protection against electrostatic charge (ESC) and electrostatic discharge (ESD), it is often unclear how they ensure that the garments can do so.
Testing a garment for particulate performance is pretty straightforward, in that industry has accepted standard test methods and procedures. When selecting cleanroom garments, companies often conduct body-box tests, Helmke Drum tests, and a number of other expensive evaluations. Many labs provide this type of testing, and even some laundry services can now test the performance of cleanroom garments.
ESD testing of cleanroom garments, however, is a trickier affair—not because the tests are complicated (in fact, they are very simple), but because there are really no cleanroom-specific standards for such testing. Often, cleanroom engineers are reduced to reliance on statements in the
manufacturers' catalogs for proof of ESD resistance. The absence of a cleanroom-specific standard for ESD testing has left many engineers in a situation where they are buying an ESD-protective product with no way to test it, and other engineers looking for any test method that would allow them to evaluate the performance of their garments. Luckily, an applicable standard does exist, and although it is not cleanroom-specific, many engineers use it for cleanroom applications.
A Charge Is a Charge There is only one kind of electrostatic charge (ESC).
It's the same in a cleanroom as it is in a printed circuit board assembly factory, in a bank lobby, or on a living room doorknob. The overall impact may be greatly different, but a charge is a charge. The laws of physics are the same everywhere. With that in mind,
it's pretty easy to see that a set of tests to evaluate the ESD performance of garments for use in nonclean environments can easily be applied to cleanroom garments.
The Institute of Environmental Sciences and Technology (IEST) has a committee working to create cleanroom-specific garment testing standards, but it is expected that the publication of any workable standards document is a long way off. In the meantime, many cleanroom engineers are applying a test standard created and published by the ESD Association (ESDA) for evaluating the electrical performance of ESD-protective garments.
The ESDA standard (ESD-STM2.1-1997: Garments) details the equipment, tools, and methods to be used in testing the electrical performance of garments.1 This document provides a step-by-step series of tests that can help cleanroom engineers determine whether their garments perform as the manufacturers assert they do. The tests are simple and straightforward, and
don't require an electrical engineer to accomplish. ESDA also publishes a handbook and glossary, which are invaluable resources for any manufacturer seeking to develop an ESD protection plan for its facility.
Recently, the Honeywell Space Systems Operations facility in Arizona decided to completely reevaluate its garments, clean and nonclean, in terms of electrical performance and protection from ESC. The bases for this evaluation were the ESD-STM2.1-1997 test methods, with a few additions. Under the direction of Ron Roden of Honeywell, garments from many major manufacturers were obtained and run through an exhaustive battery of tests. The tests were intended to determine how well the garments perform initially, and how they perform after numerous laundry cycles. Results of the tests indicate that garments on the market today have a very wide range of electrical performance (see Table I). Such results demonstrate the need for evaluation testing as well as for ongoing performance auditing once the garments are in use.
Performance Testing
Manufacturers develop ESC/ESD protection programs because some product, material, or equipment within their processing area needs to be protected from the discharge of electrical energies. In order to understand the significance of the tests available for garment evaluations, it is important to understand in electrical terms what an ESD/ESC-protective garment is supposed to do.
Sources of ESC. In the context of garments, there are three basic sources of electrical charge. The first is a charge generated within the garment, that is, by the rubbing of a silk blouse, polyester golf shirt, or other clothing made from artificial fibers. When the cleanroom garments are not ESD-protective, they can easily generate such a charge all by themselves. The second source of charge is the garment rubbing against other nonconductive surfaces within the workspace, such as Plexiglas panels. The third charge source is direct induction. If an operator passes his arm through the electrical field of an old video monitor or high-voltage power supply, for example, a charge can be induced on his arm. There are other ways an operator and garment can become electrically charged, but these examples illustrate that any operator and garment moving through and working within a controlled environment will inevitably become charged.
The purpose of an ESD-protective garment is to eliminate these charges, as quickly and safely as possible, before they can affect any materials or equipment within the process. This is accomplished by providing a path to ground for the charge. For a charge to move from the
operator's arm to ground, for instance, there must be an electrically conductive path from the location of the charge to ground. To ensure that the path is complete, testing must be conducted on three aspects of the garment.
Testing. The first element of testing is designed to ensure that the fabric of the garment is able to conduct electricity. This is easily accomplished with a simple surface resistivity meter touched to the fabric in several places. The ESDA standard details where and how to make these measurements. Many people conduct this test, and in many cases it is the only test used to qualify the ESD performance of a garment. Unfortunately, while this test can tell whether the fabric can conduct electricity, it cannot tell whether there is a good path to ground. This would be the only test required if each garment were made from a single piece of fabric; however, this is not the case.
Because virtually all cleanroom garments are constructed of individual panels of fabric sewn together along seams, the second element of testing is designed to test the conductivity of the garment from panel to panel. Even presuming that the fabric is conductive, there is no path for an electrical charge to move to ground unless there is electrical continuity between the panels of fabric. Imagine an operator whose garment is grounded at the waist on the left side, and whose right arm brushes against a Plexiglas machine panel. A large charge can be generated on the arm, but if the panels of the garment are not electrically connected, there is no way for the charge to get from the arm to ground. So the operator just walks around with a well-charged arm until it comes into contact with some other material that can absorb some or all of the charge—and that material may be the product being processed. To ensure panel-to-panel continuity, the ESDA standard tests require a sleeve-to-sleeve measurement of resistance. That is, one probe of the measurement instrument is connected to the right sleeve and the other to the left. Measuring resistance this way measures the continuity of the seams as well as of all the panels of fabric between the right and left sleeve.
Since the ESDA standard was written for garments worn in noncleanroom areas, some modification is necessary to adapt the standard for use in testing cleanroom garments. Noncleanroom garments most often take the form of a jacket or smock, while cleanroom garments are often made up of a coverall, hood, and boots or shoe covers. For the same reasons that the ESDA standard calls for testing of the garment from sleeve to sleeve, cleanroom garments should be tested for continuity from hood to shoes. To perform such testing, the garment is assembled and resistivity measurements are taken from the top of the hood to the soles of the boots.
The final element of testing is designed to establish that the ground point used by the garment is satisfactory. In most applications one of three grounding methods is used. The first method is to ground the operator and garment through conductive boots or shoe covers to a conductive floor. In this instance the hood to shoe sole measurement mentioned above will ensure that charges on the garment can indeed make it to the floor, and thus to ground. The second grounding method is to use a grounding cord attached to the garment at the waist and plugged into a grounding jack at the workstation. To test this method, measurements should be taken from the garment fabric to the far end of the grounding wire. The testing should also ensure that the connection on the garment is rugged and cannot easily be ripped out.
The third method of grounding makes things a little more tricky: connecting the garment to the body of the operator, and grounding the operator through the use of a common wrist strap and cord. Most garments of this type make their electrical connection to the operator by means of conductive wrist cuffs or wrist straps hidden under the ends of the sleeves. For such garments, testing should ensure that there is a good and constant electrical connection to the body of the operator. A testing method for wrist cuffs is not provided in the ESDA standard, but should include testing for conductivity of the fabric as well as for a snug fit that will ensure a constant electrical connection.
Although some engineers also test the continuity of the garment to ground path through an actual person, the risks of such testing are obvious. Such testing is not recommended. However, there are very good dual-wire constant monitors that can measure continuity through the person as well as the garment, and they do it very safely.
Effects of Laundering
Different garments and fabrics react to the laundering process differently. Washing will inevitably reduce the conductivity of all garments, but how much and how quickly can vary considerably. Some garments become completely insulative and will fail electrically after just a few washings. Others may hold their conductivity through 100 or more laundry cycles.
This
degradation is dependent on the laundry's process and chemistry as much as
on the garment's fabric and construction, and is usually measurable
after just a few washes. For Honeywell's evaluation, all the test garments were fully tested electrically and were then sent to be laundered a minimum of three times. Once they were returned to Honeywell all of the electrical tests were repeated. After just three washings some remarkable changes in conductivity were evident (see Table I).
Manufacturer A |
| Sample Number |
1 |
2 |
3 |
| |
Prewash |
Postwash |
Prewash |
Postwash |
Prewash |
Postwash |
| Sleeve to Sleeve |
2.1 X 105 |
2.1 X 105 |
1.8 X 105 |
1.7 X 105 |
2.4 X 105 |
1.7 X 105 |
| R Sleeve to L Panel |
2.0 X 105 |
2.0 X 105 |
1.8 X 105 |
1.7 X 105 |
2.4 X 105 |
1.7 X 105 |
| L Sleeve to R Panel |
2.4 X 105 |
2.0 X 105 |
1.9 X 105 |
1.8 X 105 |
2.4 X 105 |
1.6 X 105 |
| Surface (right front) |
4.3 X 105 |
4.5 X 105 |
3.9 X 105 |
3.9 X 105 |
3.5 X 105 |
4.4 X 105 |
| Surface (left front) |
3.4 X 105 |
4.5 X 105 |
3.3 X 105 |
3.3 X 105 |
3.5 X 105 |
4.4 X 105 |
| Surface (rear) |
3.2 X 105 |
3.3 X 105 |
3.5 X 105 |
4.3 X 105 |
3.1 X 105 |
5.1 X 105 |
| Manufacturer B |
| Sample Number |
4 |
5 |
6 |
| |
Prewash |
Postwash |
Prewash |
Postwash |
Prewash |
Postwash |
| Sleeve to Sleeve |
1.7 X 1012 |
4.1 X 1012 |
1.1 X 1012 |
1.3 X 1012 |
8.8 X 1011 |
Infinite |
| R Sleeve to L Panel |
7.3 X 1012 |
3.0 X 1012 |
1.7 X 1012 |
1.6 X 1012 |
3.0 X 1012 |
1.8 X 1012 |
| L Sleeve to R Panel |
No Test |
Infinite |
No Test |
2.8 X 109 |
No Test |
3.0 X 1012 |
| Surface (right front) |
3.2 X 108 |
9.1 X 108 |
1.7 X 108 |
5.6 X 108 |
5.6 X 108 |
5.6 X 108 |
| Surface (left front) |
1.7 X 108 |
1.5 X 108 |
4.1 X 1012 |
8.3 X 108 |
9.1 X 108 |
7.0 X 109 |
| Surface (rear) |
3.2 X 108 |
4.0 X 108 |
2.6 X 1010 |
2.5 X 109 |
2.8 X 108 |
7.3 X 108 |
| Table I. Electrical performance of cleanroom garments before and after laundering (in ohms). |
Conclusion
As a result of its exhaustive testing, Honeywell was able to select the best garment for its process, qualify a laundry service, and do so with the confidence that its decisions were based on real test results rather than marketing materials.
Manufacturers make huge investments in their cleanroom garments, and it only makes sense that they should perform to requirements in every way. The lack of a cleanroom-specific test standard for ESD performance does not mean that manufacturers have to fly blindly. There are tests that can be performed, and resources that manufacturers can tap into, to help ensure the success of their garment programs.
References
1. ESD-STM2.1-1997: Garments (Rome, NY: ESD Association, 1997).
2. ESD Handbook (ESD Association).
3. ESD-ADV1.0-1994: ESD Glossary (ESD Association, 1994).
John Schuch is principal of ESD Resources (Mesa, AZ), and president of the Arizona chapter of the ESD Association. He can be e-mailed at schuch@esdres.com.
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