Special Considerations for Static Control Flooring for Added Risk Reduction

Static control flooring is widely used throughout the electronics industry to provide a means to ground personnel and mobile equipment to control static charge generation and build-up. While I’d argue that it is always important to control static in an electronics manufacturing environment, there are some organizations where the risk of a device failure could have more serious consequences. This raises the bar in terms of managing the risk of failure and there are some things you should keep in mind when choosing static protective flooring to help with that.

What is High Reliability?

A group of researchers at the University of California, Berkeley, in their research to understand causes of major failures, identified certain organizations that were better at handling and avoiding these failures. ^1,2 In their work, they defined a high reliability organization (HRO) as “an organization that has succeeded in avoiding catastrophes in an environment where normal accidents can be expected due to risk factors and complexity.” They further defined the five principles that HROs have in common:

• Preoccupation with failure
• Reluctance to simplify
• Sensitivity to operations
• Commitment to resilience
• Deference to expertise

These principles have been adopted across many complex industries, including aerospace, defense, nuclear power, air traffic control, automotive controls, and healthcare, in which the major failures can have catastrophic and/or life-threatening consequences. Karl Weich and Kathleen Sutcliffe have studied several of these industries and how they adopt principles of high reliability. ^3,4,5,6 

How Does this Apply to Electronics Manufacturing?

With the ubiquitous use of electronics to perform demanding and critical functions within the above-mentioned industries, high reliability is an ongoing concern. Moreover, with the growing complexity of systems used throughout our environment, coupled with the increased complexity and miniaturization of devices, the concept of high reliability is becoming more and more commonplace in the electronics manufacturing industry, in general. In fact, the IPC standard for electronics assemblies, IPC-A-610, includes the following three classes for assemblies, with the most stringent being defined as High-Reliability Electronics Products: ⁷ 

• Class 1 Electronics: General Electronics Products
• Class 2 Electronics: Dedicated Service Electronics Products
• Class 3 Electronics: High-Reliability Electronics Products

How Does this Apply to Static Control of the Manufacturing Environment?

In general, ANSI/ESDA S20.20 provides a framework for establishing a static control program for operations:

“…that manufacture, process, assemble, install, package, label, service, test, inspect, transport, or otherwise handle electrical or electronic parts, assemblies, and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts HBM, 200 volts CDM, and 35 volts on isolated conductors.” ⁸ 

Within the ANSI/ESD S20.20 framework, flooring is primarily used as a means to ground personnel and mobile equipment. For personnel, it is intended to keep the voltage on personnel to below 100 volts and thereby ensure that any potential discharge from a person to a sensitive device falls below the limit of 100 volts HBM. To meet this objective, ANSI/ESD S20.20 set standards for flooring as follows:

• The complete system must have a resistance (point-to-point and point to ground) of less than 1.0 x 10⁹ ohms as tested per ANSI/ESD STM7.1; ⁹ 
• The complete system of person-flooring-footwear must have a resistance to ground of less than 1.0 x 10⁹ ohms as tested per ANSI/ESD STM97.1; ¹⁰ and 
• The complete system of person-flooring-footwear must generate less than 100 volts as tested per ANSI/ESD STM97.2. ¹¹ 

So, ANSI/ESD S20.20 is designed to control to 100 volts HBM. But what if the devices handled are more sensitive, that is, have a lower withstand threshold? For those situations, the standard simply states:

“Activities that handle items that are susceptible to lower withstand voltages may require additional control elements or adjusted limits.”

Or, what if there is a desire to limit HBM to below 100 volts simply to increase the margin of error for preventing a failure? Or, what if there is a desire to increase the reliability of a static control program beyond what an ANSI/ESD S20.20 program would provide? 

ANSI/ESD S20.20 is viewed as providing a minimum set of standards that meet a vast majority of the needs in factory-level ESD controls. End users, trained in ESD controls, can expand upon the ANSI/ESD S20.20 controls to create more stringent requirements for their particular applications and many HROs do just that.

How Does the ESDA View High Reliability?

The issue of high reliability has become so prevalent within the ESD community that the Electrostatic Discharge Association (ESDA) has formed Working Group (WG) 19 – High Reliability to develop a guidance document to help users that may need or want to implement more stringent controls than those prescribed in ANSI/ESD S20.20. This work will include recommendations for nearly every aspect of ANSI/ESD S20.20. 

Members of this working group are involved with some of the most stringent ESD control programs in the world and bring to this project their vast knowledge of ESD controls. While the work is not yet published, the working group meetings are open to guests. If you are interested in learning more about this work, go to http://www.esda.org/events for a schedule of upcoming working group meetings.

Conductive vs Dissipative Flooring – Most HROs Choose Conductive

With regard to static control flooring, the industry has historically used two “grades” of flooring, conductive and dissipative. These terms are defined in ANSI/ESD STM7.1 as:

• Conductive flooring system – has a resistance to ground of less than 1.0 x 10⁶ ohms
• Dissipative flooring system – has a resistance to ground of greater than or equal to 1.0 x 10⁶ ohms to less than 1.0 x 10⁹ ohms

But the electronics industry is moving away from classifying flooring by grade. As noted above, ANSI/ESD S20.20 simply requires that flooring have a resistance of <1.0 x 10⁹ ohms, so either of these “grades” will meet the requirement as long as they also meet the requirement for the walking voltage test when tested in combination with the footwear to be worn in the area.

While the flooring/footwear combination is extremely important in determining the body voltage generation, all else being equal, the lower the resistance of the flooring, the lower the body voltage.

Figures 1 and 2 show the test results of body voltage generated for two flooring systems. In these tests, the flooring systems were nearly identical, with the first being formulated to have a resistance in the conductive range and the second having a resistance in the dissipative range. The footwear and the person conducting the test were the same in both cases. 

As you can see, while both systems passed the requirement of ANSI/ESD S20.20, the conductive floor generated significantly lower body voltage, which could be important in a very sensitive environment and would certainly reduce the risk of a potential failure due to an ESD event. 

Also, as a floor gets dirty or ages, resistance levels may increase, potentially rendering the flooring-footwear system ineffective at achieving the desired level of protection. For these reasons, most of our HRO customers choose flooring that has a resistance of less than 1.0 x 10⁶ ohms. This provides an added measure of security that the floor will still perform when dirty and that body voltages will be kept as low as possible in the area.

Footwear

As previously noted, the ultimate test of protection and the resulting risk reduction is the amount of voltage that a person generates when walking on the floor and that this is affected by the combination of both the footwear and the flooring. There are a number of types of footwear available in the market, including:

• Heel grounders
• Sole grounders
• Conductive shoes
• Conductive booties

As mentioned, you must test the footwear that you intend to use with the floor you intend to use to ensure that you get the body voltage results that you require. The combination matters. I’ve personally seen a situation with a floor that tested consistently with a resistance in the 1.0 x 10⁵ ohms range, but the walking voltage test performed with the shoes actually used in that facility resulted in body voltage spikes greater than 100 volts. Fortunately, this was discovered in the planning stages and different flooring was chosen that worked extremely well with the footwear used.

One other thing to consider with regard to footwear is the contact that it has with the floor. Heel grounds offer the least, while conductive shoes and conductive booties have the most. The better the contact with the floor, the less likely someone will become electrically disconnected from the floor as they move across it.

Understand that Static Control Flooring is a System

For any flooring system used in an ESD control program, it is important to understand the nature of the flooring system, the components it uses, and how they interact. So, for example, a conductive vinyl flooring system that is glued down includes the conductive vinyl tile, the glue, the substrate it is adhered to, and the grounding mechanism. Many of the epoxy/resinous coating systems used include a primer layer, a highly conductive ground layer, and a more decorative finish coat. 

There have been many situations where one component in the system develops a resistance higher than desired, thereby causing the whole system to be out of compliance. In a high reliability environment, when choosing a flooring system, it would be prudent to understand all of these components and the risks associated with a potential failure in order to reduce the likelihood of a future non-compliance situation. 

Redundancy with Flooring

In any quality system, adding redundancy reduces the probability of failure. In many high reliability applications, users will control static using wrist straps, ionization, equipment grounding, and packaging, to the point where flooring may not really be necessary. The floor provides a secondary means of grounding and protection in these environments and helps to ensure charges are kept to a minimum. Moreover, many HROs will increase the areas of coverage to include more ancillary areas to help ensure that any movements of devices are within an area with static protective flooring.

Compliance Verification of Flooring

ESD TR53 ¹² provides compliance verification procedures for ongoing verification of control items used in an ANSI/ESD S20.20 control program. For flooring, some things to take into consideration for ongoing compliance verification include:

• The periodicity of testing: This is not prescribed in either ANSI/ESD S20.20 or TR53.The periodicity of testing should be regular enough to head off non-compliance, as determined by:
  • Use and maintenance: If the floor is subjected to a lot of traffic and dirt, it may need to be checked more regularly. Likewise, the regularity and extent of floor cleaning will impact how often the floor should be checked.
  • Life of electrical properties: Some flooring systems have lifetime electrical properties, while others, such as applied finishes, only last a few months.
  • Any changes: The floor should be checked if there are any changes in use or maintenance practices and materials. Any of these could impact the performance of the floor.
  • Past results: By monitoring results over time, you can get a sense of if or how the floor is changing over time and adjustments can be made to periodicity as appropriate.
• Incorporating regular walking body voltage tests: The procedure in ESD TR53 for verifying flooring is a simple check of resistance to ground. As noted previously, the body voltage generation is a critical measurement of the effectiveness of a flooring-footwear system. As such, walking body voltage measurements should be taken periodically following the procedure of ANSI/ESD STM97.2. Doing so will help ensure that that flooring system and the flooring-footwear combination is still providing the desired static control.

Conclusion

While ANSI/ESD S20.20 provides a very strong framework for establishing an ESD control program, it does recognize that some organizations may need to enhance their program to meet their particular needs. HROs fall into this latter category and often will take added steps in their ESD control programs to reduce any potential risk of an ESD event leading to a failure. There are several things that an HRO can consider with regards to their static protective flooring system, including the resistance of the system, the footwear used in conjunction with that system, the extent of area covered, and the ongoing verification of the system. 

Endnotes

1. Rochlin, Gene I., (1996-06-01), “Reliable Organizations: Present Research and Future Directions,” Journal of Contingencies and Crisis Management, 4 (2): 55–59. doi:10.1111/
   j.1468-5973.1996.tb00077.x. ISSN 1468-5973.
2. Roberts, K.H., (1989), “New Challenges in Organizational Research: High Reliability Organizations,” Organization & Environment,
   3 (2): 111–125. doi:10.1177/108602668900300202.
3. Weick, K. E., Sutcliffe, K. M., and Obstfeld, D., (1999), “Organizing for High Reliability: Processes of Collective Mindfulness,” In B. M. Staw & L. L. Cummings (Eds.), Research in Organizational Behavior (Vol. 21, pp. 81-123). Greenwich, CT: JAI Press, Inc.
4. Weick, K. E., and Sutcliffe, K. M., (2001), Managing the Unexpected: Assuring High Performance in an Age of Complexity, First Edition,
   San Francisco: Jossey-Bass.
5. Weick, K. E., and Sutcliffe, K. M., (2007), Managing the Unexpected: Resilient Performance in and Age of Uncertainty, Second Edition,
   San Francisco, CA: Jossey-Bass.
6. Weick, Karl E., Kathleen M. Sutcliffe, 2001, Managing the Unexpected – Assuring High Performance in an Age of Complexity,
   San Francisco, CA, USA: Jossey-Bass. pp. 10–17. ISBN 978-0-7879-5627-1.
7. IPC International, “IPC-A-610H: Acceptability of Electronics Assemblies,” 3000 Lakeside Drive, 105N, Bannockburn, IL 60015.
8. Electrostatic Discharge Association, “For the Development of an Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices),” 7900 Turin Road, Bldg. 3, Rome, NY 13440.
9. ANSI/ESD STM7.1, EOS/ESD Association, Inc. “Flooring Systems Resistive Characterization.”
10. ANSI/ESD STM97.1, “Floor Materials and Footwear-Resistance Measurement in Combination with a Person STM97.1.”
11. ANSI/ESD STM97.2, EOS/ESD Association, Inc. “Floor Materials and Footwear – Voltage Measurement in Combination with a Person.”
12. ESD TR53, EOS/ESD Association, Inc., “Compliance Verification of ESD Protective Equipment and Materials.”