Routine compliance verification testing can reveal degradation of a workstation mat’s electrical dissipative properties over time. Chronic failures occurring sooner than expected warrant root-cause investigation. This article describes a different approach taken to identify the cause of this elusive problem.
Problem Background
Early life failure rates of ESD workstation mats prompted an in-depth investigation at one of our manufacturing facilities. “Failure” in this case refers to a mat’s electrical resistance drifting to 1×109 ohms or above and therefore not meeting the work surface limit requirement of ANSI/ESD S20.20, using the test procedure in ESD TR53. Our ESD control program compliance verification testing revealed that the resistance measurements were changing in some mats, however not consistently as one would expect. The resistance drift in the affected mats occurred over six to nine months, spanning several audit cycles.
This was elusive and frustrating for two reasons:
- Not all of the 2-layer rubber mats shifted in resistance; only some did.
- Since the matting we used at that time did not contain a manufacturing date or lot number printed on the mat, it was difficult to identify exactly how old the mat material was and if the resistance shifting was a lot problem or completely random.
We somewhat assumed that all 2-layer rubber ESD matting was pretty much the same, mainly because the electrical specifications from various
manufacturers are typically the same: 1.0 X 106 to <1.0 x 109 ohms.
First Considerations
Initially we turned to our suppliers for assistance in solving our problem. The first (and logical) question an ESD mat manufacturer asks when confronted with a high resistance mat reading is, “How was it cleaned?” The use of common household kitchen cleaners, which often contain silicone to achieve a glossy finish, can leave an insulative layer on ESD dissipative matting. This could lead to high resistance readings. Other unapproved cleaning products contain ammonia or citric acid, which can “attack” the rubber in the mat and cause degradation. A review of our ESD mat cleaner inventory showed that we were indeed using only approved products, thereby eliminating the suspected root-cause of improper mat cleaning.
A New Theory
What other factor could there be that would reduce the effective life of the mats? Environmental conditions such as temperature and humidity were in good control, relative humidity typically being held to 40% (+/- 10%) in the manufacturing area year round. Air quality was consistent with all EH&S standards for industry. The only other variable that we could think of was the factory lighting. Could it be that normal factory fluorescent lighting was affecting the mat material, causing it to degrade prematurely over time? Since all fluorescent lights emit some ultraviolet (UV) radiation, might this be affecting the ESD mats?
We decided to stage an experiment to test this theory. Our test set up would use standard light fixtures positioned very close to mat samples in order to accelerate any affect that the fluorescent light might have. Periodic measurements would track each mat’s performance over time.
Material Selection
Our factories primarily use 2-layer rubber matting because of its high-temperature resistance and ability to clean better than vinyl. For comparative purposes, it was decided that a variety of mat materials from different manufacturers would be included in the experiment.
The following mat types were obtained:
2-layer rubber: 6 different P/N’s
2-layer vinyl: 1 P/N
3-layer vinyl: 1 P/N
All of the mat materials were supplied new from the manufacturers. There was no treatment such as pre-cleaning done to them.
Test Set-up
Several work benches were set up off-site, with 48-inch standard fluorescent light fixtures mounted four inches from the work surface of each bench (see Figure 1). As stated previously, this close distance was intended to accelerate any effects that the lighting might have on the mats. In the factory, we typically experienced mat resistance shifting over six to nine months. If the lighting is truly a factor, any shift in resistance should be seen more quickly in this set-up.
Figure 1: Fluorescent light test set-up (one of three)
A 14” by 14” square was cut from each mat roll to use as a sample. Each sample was marked with an ID number in the corner for tracking. The resistance of each sample was then measured point-to-point, as described in ESD S4.1. Two such measurements were taken for each mat; first with the probes placed on opposite diagonal corners, and then again on the other pair of opposite corners. Both measurements and their average were recorded on a data collection tracker. The mat samples were then placed next to each other beneath the light fixtures. (Note: The temperature beneath the lights measured between 75°F – 80°F, so extreme heat was not a factor.)
Monitoring and Collecting the Data
Periodic resistance measurements were taken over time (see Figure 2), with the same test procedure used as in the test set-up. The intervals began at one week to start, then were extended (for logistical reasons) as the study progressed. The samples’ positions were shifted between each measurement session, in order to assure similar exposure conditions for all the samples. Ambient temperature and relative humidity percentage were recorded for each set of measurements taken.
Figure 2: Data collected over the 10-week test of fluorescent lighting on our eight mat samples (click image for larger version)
Results
- There was significant variation in the different mats’ tolerance to fluorescent light exposure relative to their ability to retain specified electrical properties (see Figure 3).
- Both vinyl mat samples appeared to be unaffected by the extended exposure.
- Only one, Manufacturer #4, out of the six rubber mat samples retained its electrical resistance properties throughout the duration of the experiment.
- The final measurement session was taken after removing all the mats from the light source for 24 hours, to see if there was a rebound effect. The data shows that the failed mats did not recover their original properties; their degradation appeared permanent.
Figure 3: Graph depicting results of our workstation mat test
Conclusions
- Compliance verification periodic testing with data collection is a critical part of an ESD control program.
- Likely from ultraviolet (UV) radiation, fluorescent lighting can have a measurable effect on the electrical properties of 2-layer rubber ESD dissipative mats over time.
- Different mats in the sample tolerated the effects of fluorescent lighting by varying degrees.
- Interestingly, neither vinyl mat tested showed any significant change to their resistive properties.
- Since this experiment did not take into account lot to lot variation at the supplier for each mat P/N, future mats purchased will require lot traceability information to be stamped on the bottom of the roll, every 3 feet minimum.
- The ESD Association technical report ESD TR4.0-01-02 Worksurfaces and Grounding Survey committee should deliberate on whether to add ultraviolet light immunity as a property to consider in the Worksurface Selection Guide.
The manufacturer of Mat ID #4 stated that the 2-layer rubber material composition is proprietary, but they advise us that their formula includes UV stabilizers. They were originally added to the rubber to stabilize color retention, ensure ESD performance over time, and eliminate surface-layer deterioration that results in cracking when the product is flexed. 
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Sam Theabo is a staff Quality Engineer and corporate ESD Coordinator at Plexus Global Headquarters in Neenah, Wisconsin, with 24 years of career service. Previously, Sam was a Quality Assurance Engineer at AT&T – Western Electric in Lisle, IL, supporting the manufacture of ESS telephone switching systems. Theabo is ASQ-CQA and CQT certified. |
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Brian Retzlaff is a Process Engineer and ESD Subject Matter Expert at Plexus Manufacturing Solutions – Neenah Operations in Neenah, Wisconsin. He has 20 years of career service with Plexus. Since 2006, Brian has been an active member of the ESD Association, where he serves as a committee member and volunteer. |
