Measuring electrostatic discharge
By Hank Hogan
Rome, NY — Any engineer will say that to overcome a problem you first have to measure it. Unfortunately, measurement in the case of electrostatic discharge (ESD) means techniques have to be developed, data gathered, and results interpreted.
That`s one of the reasons why new standard test methods covering garments, seating, and work surfaces were recently released by the ESD Association of Rome, NY. The Association also currently has a draft standard practice in circulation that covers air ionizers. Furthermore, the group recently revised its standard test method for the human body ESD model. At the same time, the Association extensively revamped its document classifications.
All of this activity on the ESD front is important to the cleanroom industry.
“In the cleanroom environment, controlling static electricity is just as important for the contamination aspects of the environment as for the protection of the product being worked on,” remarks Kay Adams, chair of the ESD Association`s working group on garments.
However, the garment document neither specifically includes nor excludes cleanrooms. Instead the specification, STM 12.1-1997, lists two approved test methods, as well as the need to precondition the garment at particular humidity levels and temperatures for 48 hours prior to testing. The test consists of a resistance measurement from sleeve to sleeve and panel to panel of the garment. An upper limit for the resistance is 1011 ohms while a lower limit, set by safety considerations, is 105 ohms.
According to Adams, many but not all cleanroom garments have trouble meeting these limits. Because of that, the working group is now looking into other test methods. One possibility would be the development of a charge decay test in which a garment would be charged up to a known potential and then the dissipation of that charge over time monitored. Such a test might not be as reproducible as one based on resistance but could offer meaningful relative data. The group is also investigating a collaboration with the Institute of Environment Sciences and Technology (IEST) on a joint method/specification.
As for seating, that can literally be quite shocking.
“Seating is the biggest static generator. If you have a static control floor and a standard chair that`s rolling around, you can generate quite a few thousand volts,” notes Stan Weitz, chair of the ESD Association`s seating working group.
According to Weitz, the standard test method, STM 12.1-1997, specifies that during certification by a manufacturer testing be done at both a room temperature 12 percent and 50 percent relative humidity environment. Users of a chair are allowed to do testing under ambient conditions. The document specifies that periodic retesting be done at a workstation in a facility`s normal environmental conditions.
The method recommends a maximum resistance to ground for any component — seat, back, arm rests, etc. — of 109 ohms. At that resistance, charge decays in less than two seconds. In this case ,ground means quite literally the ground — the floor that the chair, bench, or stool rests upon. In the case of a chair with conductive casters, each caster must be tested independently. For chairs with drag chains, the chain must also be tested. The minimum resistance is the standard 105 ohms.
As for the final new standard test method, STM 4.2-1997, unlike the others ,it does not measure resistance. Instead it tackles a different problem. The standard test electrode used in all these resistance measurements has five-pound weighted tips covered by conductive rubber. Such an electrode will always have low contact resistance to a surface. However, when a component is placed upon a hard work surface, contact resistance may actually be quite high. The standard resistance measurement test for work surfaces will not reliably detect such a condition.
To overcome this problem, the new work surface test calls for charging up a six-inch metal plate, lowering it onto a surface for five seconds, and then raising it. In this laboratory-only procedure, the voltage before and after contact is measured. Just what the maximum voltage can be is not specified. According to Steve Gerken, former senior vice president of the ESD Association and past chair of the work surface committee, the test may eventually be supplanted by one based on low voltage resistance.
As for the other documents, the revised test method for the human body model (HBM) contains changes that reduce the overall test time.
A second change revises HBM tester specifications in a way that will also cut the qualification cycle.
The final draft document covers techniques intended to replace a difficult to perform laboratory test with one more suitable for widespread use in facilities. Ron Gibson, chairman of the standards committee and senior vice president of the ESD Association, thinks the draft may be approved by February 1999. However, he also says it will likely never be more than a standard practice because of problems with the reproducibility of the data.
Hank Hogan is a freelance writer in Austin, TX.
The ESD Association`s Four Document Classifications
Standard: A precise statement of a set of material, product, system or process requirements and measurement procedures. Designated as Sxx.xx-yr.
Standard Test Method: A definitive procedure for the identification, measurement and evaluation of a material, product, system or process that yields a reproducible result. Designated as STMxx.xx-yr.
Standard Practice: A procedure for performing one or more operations or functions that may or may not yield a test result. Note: Any test results are not reproducible. Designated as SPxx.xx-yr.
Technical Report: A collection of technical data or test results published as an informational reference. Designated as TRxx.xx-yr.
Any documents issued as drafts carry a D prefix to indicate draft status. These are published for and subject to industry review.
Founded in 1982, the ESD Association is an ANSI recognized standards-development organization.