In 2011, RMV was visited by a major aerospace & defense prime due to electrostatic discharge (ESD) garment issues at one of their locations. It was understood that this site was closed for manufacturing due to ESD garment non-conformance. In addition, another satellite subcontractor’s static control garments were called into question by their customer. RMV was asked to provide guidelines on how to verify an ESD garment’s product integrity and to include testing criteria to utilize for ensuring supplier conformance. ANSI/ESD S20.20-2007 outlines some criteria for acceptance in Table 1. The testing method utilized in the evaluation of static control garments with wide acceptance is ANSI/ESD STM2.1.
In order to gain a better understanding of Table 1, ESD control garments can be identified by three specific types.
1. A static control garment (Figure 1) may or may not suppress electrostatic fields stemming from undergarments whether or not a path to ground exists. One runs the risk of charge build-up from the garment components (conductive grid, static dissipative fabric) if there is no path to ground.
2. A groundable static control garment should provide better electrostatic field suppression with a low resistance fabric connected to a groundable point.
3. A groundable static control garment system represents a system which provides a bond with the wearer’s skin to ground. This assists in suppressing undergarment charging. A groundable static control garment can be utilized with a continuous or constant monitoring system within the ESD Protected Area (EPA).
Several reputable manufacturers of ESD control garments are compliant to ANSI/ESD S20.20-2007 limits when new. After being subjected to the dynamic washing machine cycling, the thin conductive fiber network structure of a static control garment is subjected to g-forces1 which diminish electrical conductivity.
A conventional static control garment was measured, new, right out of the package and measured at 3.2 x 106 ohms. However, a sample garment taken from the production area, of the same garment type from the same garment manufacturing company, measured 1.3 x 1011 ohms. The failing garment was a few years old and had never been monitored. Likewise, the customer was unsure of how to measure for non-conformance. In Figure 4b, left, the compliant garment illustrates fiber integrity from a microscopic perspective; conversely, the older garment’s structure was less pronounced as evidenced in the photograph on the right. For best results, organizations need to subject a garment to not only qualification testing but also periodic verification using ANSI/ESD STM2.1.
Why is testing important? Aside from assurances that the supplier’s stated specifications are being met, electrical resistance testing ensures that one’s garments stay within the parameters for ANSI/ESD S20.20-2007 compliance.
What determines compliance? Many end-users and suppliers of static control garments believe that measuring surface resistance <1.0 x 1011 ohms is the upper limit for acceptance even if the ESD garment is new. That may be acceptable for a disposable ESD garment, but after prolonged use, in combination with laundering, the fabric will lose conductivity over time.
ANSI/ESD STM2.1 requires that ESD garments being considered for qualification must be washed at least three times per the manufacturer’s defined cleaning process with a submission of three samples for qualification testing. However, many companies default to acceptance testing, thus allowing the end user to determine what humidity to test and what sample size to utilize.
Some End-User or Supplier Issues with ESD garments:
- Resistance Limit is understood to be <1.0 x 1011 ohms for all garment types
- Garment Cleaning Company does not need to perform Certificate of Conformance (CoC) testing for Product
- OK to Rely upon Supplier’s Technical Data Sheet as Proof of Compliance
- ESD Garment Laundry Supplier does not have ESD instrumentation
- Garments do not undergo a formalized qualification process
- User assumption that all garment types suppress electrostatic fields
- Garments using the same base fabric from one fabricator to another will test the same
It is the author’s opinion that ESD garment suppliers should have ESD technical data and maintain calibrated instrumentation that is compliant to ANSI/ESD STM2.1 prerequisites. For qualification testing a resistance system shall be capable of making measurements from 1.0 x 103 ohms to 1.0 x 1012 ohms applying a constant voltage of 10 volts at <1.0 x 106 ohms and 100 volts for measurements greater than 1.0 x 106 ohms. ANSI/ESD STM2.1 calls out the requirements for 2-NFPA 5-lb electrodes. The focus of this article is verification of suppliers in conformance to ANSI/ESD STM2.1 specific to point-to-point resistance and resistance to a groundable snap. Moreover, ANSI/ESD STM2.1 should be required of suppliers selling ESD garments.
In evaluating an ESD garment, the qualification entity or buyer should expect to secure garments under the upper resistance limit as the laundering process will diminish performance. For instance, Table 2 illustrates findings of garments which were <1.0 x 1011 ohms when new, but insulative after ten washing machine washes. The readings were taken from sleeve to sleeve (Figure 1). Since the new garments were less than a decade from the upper limit, ten washing cycles were targeted to conduct ANSI/ESD STM2.1 sleeve to sleeve testing. If a garment was purchased with a sleeve to sleeve resistance of <1.0 x 109 ohms, one expects that fiber breakdown would take many more washing cycles.
When evaluating an ESD garment, one needs to utilize an insulative test bed to prevent bleed off. Test bed resistance requirement is called out in ANSI/ESD STM11.11-2006. A test bed per ASTM D257 of 1.0 x 1013 ohms/square or per ANSI/ESD STM11.11 of 1.0 x 1012 ohms is ideal. This ensures that the garment is isolated from ground. The test bed in Figure 5 measured 1.1 x 1013 ohms which would prevent garment bleed off. Never measure a static control garment on a static dissipative workstation.
One major drawback of ESD garments is the joining of similar fabric materials. Primary areas of concern are between the cuff and sleeve (Figure 6, left) and shoulder and sleeve (Figure 6, right). Many garments are manufactured with base fabrics that exhibit favorable electrical conductivity. However, breaks in conductivity due to improper stitching at the fabric joints because of inexperienced or untrained personnel can compromise resistance measurements. One garment company using the same ESD fabric will produce a better product than another less skilled in the art of ESD garment manufacturing.
Taking measurements from cuff to cuff represents a very important performance criteria and this test can be an effective tool in verifying a supplier’s proficiency in manufacturing an ESD garment to the required conductivity levels.
GROUNDABLE STATIC CONTROL GARMENT SYSTEM EVALUATION
Three unnamed ESD garments were evaluated per ANSI/ESD STM2.1 “for the Protection of Electrostatic Susceptible Items – Garments.” In Figure 8, the standard test method contact points served as the reference. The findings at low (11.9%RH) and moderate (51.2%RH) are illustrated in Table 3 and summarized in Table 4 of this article.
The overall findings of three ESD garments under test were in compliance to ANSI/ESD S20.20-2007 requirements for a Groundable Static Control Garment System as referenced in Table 1 of this article. If placement of ground points is compromised between the sleeve and cuff or sleeves and main garment at the shoulder area, then the resistance readings could easily be over the acceptable limit of 3.5 x 107 ohms. A few years ago, a static control cleanroom garment stainless steel ground points had been replaced with nylon snaps. Consequently, a path to ground between the nylon contact points and the static dissipative cleanroom boots were placed at risk. For a cleanroom static control garment, a left cuff to nylon ground point measured 3.4 x 1013 ohms while the exact style of ESD garment with stainless steel ground snaps produced favorable reading at 1.4 x 106 ohms. This problem could have been prevented if the supplier had been required to submit a CoC (Certificate of Conformance) for resistance testing per ANSI/ESD STM2.1 between the cleanroom garment leg and the static dissipative boot when snapped together. Moreover, any modifications to a static control garment system must have customer buy in and be verified using ANSI/ESD STM2.1.
WHAT SHOULD A CUSTOMER EXPECT FROM A VALUED SUPPLIER?
An organization which provides cleaning services for garments should be required to provide performance CoC testing per laundered lot of ESD garments. In the event that a smaller company has only a few garments, then testing can be conducted for each garment after an agreed upon time frame, for example, every three wash cycles. One organization requires sleeve to sleeve testing of six Static Control Garments per 100 washed. Thus, ESD garments are selected at random for testing to ensure supplier compliance.
It is very important that one drive down the requirements to a supplier. For instance, one supplier claims to conduct Electrostatic Testing by calling out “Surface Resistivity” for electrical resistance of a six inch square fabric specimen with a limit of <1012 ohms. Surface Resistivity is measured in ohms/square (not in ohms) for DC conductance on insulators. Thus, the supplier is conducting improper testing for static control garments. The same supplier calls out sleeve to sleeve testing (electrostatic decay) from 1000 volts without listing cut-off time and without referencing the standard.
Fed-TM-STD-191A, METHOD 5931, Notice 7, 9 August 2000 provides guidance of decay testing for garments.
Depending on the garment type shown in Table 1 of this article, the limits range from <1.0 x 1011 ohms for Static Control Garments, <1.0 x 109 ohms for Groundable Static Control Garments and <3.5 x 107 ohms for a Groundable Static Control Garment System per ANSI/ESD S20.20-2007 and ANSI/ESD STM2.1. Therefore, the type of garment utilized by an organization will dictate the upper limit of resistance. Ideally, a cleaning apparel company should conduct ANSI/ESD STM2.1 CoC testing from cuff to cuff or sleeve to sleeve or leg to leg. This would represent a good indicator of a garment’s performance while identifying potential threats in advance of developing into real issues.
- “Laundry-spin-g-Force, how to judge the quality of your spin drying cycle and what it means,”
- NASA-STD-8739.21, 18 June 2010
- ANSI/ESD S20.20-2007
- ANSI/ESD STM2.1-1997
- ANSI/ESD STM11.11-2006
- ANSI/ESD S6.1-2005
- ESD TR20.20-2008
- Fed-TM-STD-191A, METHOD 5931, Notice 7, 9 August 2000
- ESD from A to Z, Kolyer & Watson, Chapman & Hill, 1996
- Figure 3, right Simco Website
Bob Vermillion, CPP/Fellow, Certified ESD & Product Safety Engineer- iNARTE provides proven expertise in advanced materials testing and evaluation, troubleshooting robotics and electronic systems for the aerospace, disk drive, medical device, pharmaceutical, automotive and consumer electronics sectors. As a member of the ESD garment working group, Bob also serves on the ESDA Standards Committee and is a BoD member of iNARTE. Just recently, Bob was invited to join the SAE G-21 Committee. Speaking engagements include ESD Seminars in the U.S. and abroad including guest lecturer seminars for California State Polytechnic University, San Jose State University, University of California at Berkeley, and Clemson University. In 2012, Bob will conduct a Suspect Counterfeiting of Materials/ Packaging Seminar for Oxford University. Bob is CTO of RMV Technology Group, LLC, a NASA Industry Partner at NASA-Ames Research Center and 3rd Party ESD Materials and Systems Level Testing, Training and Consulting Company. You can reach Bob at 650-964-4792 or [email protected].