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Benefits of Sole and Full Coverage Foot Grounders

We see many companies with a conductive tile floors that measures mid-10^5 ohm resistance to ground and the operators are wearing foot grounders on each foot that passes the touch-testing, but what peak voltage on the body is generated?

Over the years, there have been independent studies conducted per ANSI/ESD STM97.2, Floor Materials and Footwear – Voltage Measurement in Combination with a Person showing that with conductive flooring measuring less than 1 x 10^6 ohm resistance and footwear measuring in the low 10^6 ohm resistance range, the following body voltage spikes were recorded:

  • Using heel grounders, body voltage spikes to ±250 volts
  • Using sole grounders, body voltage spikes were reduced to ±75 volts or less
  • Using full coverage grounders, body voltage spikes were reduced to ±25 volts or less.

Basically, the greater the footwear contact surface, the higher the probability that while walking, bending, kneeling, reaching, etc. the operator will be in contact with the ESD floor.

“Procedures For The Design, Analysis And Auditing Of Static Control Flooring/Footwear Systems” by Stephen L. Fowler, William G. Klein, and Larry Fromm includes:

“With heel grounders his potential dropped to 250 in one installation and 450 in the other, these being the peaks when both heels left the floor, as they did with nearly every step. When care was taken not to allow simultaneous contact loss with both grounders the values were 40 and 170 volts respectively. When he used a sole grounder, which is essentially a combination of heel and toe grounders, the peak voltage in both cases dropped below 30 volts.”

Charleswater Full Coverage Foot Grounders

Conductive flooring less than 1 megohm (1 x10^6 ohms) is often preferable for grounding operators wearing foot grounders. However, if the resistance upper limit is only less than 1 x 10^9 ohms, end users must add the ANSI/ESD STM97.2 test method for body voltage to the qualification of their footwear/flooring operator grounding system.  It is no longer enough to know that a standing operator is grounded. ESD flooring requires maintenance to keep them clean and effective. All ESD flooring should be cleaned with a good quality ESD floor cleaner that will not leave behind an insulative residue that can raise floor resistance. Many companies also want their floors to have a nice appearance. A good quality dissipative floor finish can improve durability and gloss while also reducing the charge generation characteristic of the floor to less than <50 volts.

Application Photo of Statguard Floor Finish

From published article “Now is the Time for ESD Control Programs to be Improved” by Fred Tenzer and Gene Felder. See full article at InCompliance Magazine- September 2012

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Continuous Monitors: What’’s Better – Single-Wire Impedance or Dual-Wire Resistance Monitors?

Dual-Wire Continuous Monitor

Fred Tenzer on Continuous Monitor History and Preference

By Fred Tenzer

First off, both technologies work fine, but one needs to understand the technologies to understand the shortcomings of each. Let me start with the following basic historical information that will make my “preferences” more clear. If you choose to just know the answer, scroll to the bottom, but I believe this information will be very helpful to you in making your decision. All the technologies will also monitor the worksurface ground circuit and while there are some differences, the worksurface part of the monitoring system is NOT discussed below.

History

When people realized that Operator Grounding was the foundation of an ESD Control Program, wristbands and coil cords came into big time use. The first ESD Association Standards meeting was in 1982 and the first standard written produced by the ESD Association was for wrist straps. The weakest part of the system was the coil cord and testing was developed for bending or flex life testing. The minimum flex life was established at 16,000 flex cycles and in the mid-1980’s that was tough to achieve.

Thus, monitoring technology was initially developed to detect initial flex fatigue while it was still in the “intermittent” stage, which is prior to a permanent open being created. Touch testing would almost never detect this “intermittent” failure mode. In addition, if a wrist strap system was touch-tested twice a day and an operator passed at 1:00 PM on Monday and failed at 8:00 AM on Tuesday, all the work that had peen performed at that station after 1:00 PM on Monday would now be suspect and would be a cause for more detailed quality inspections by many companies. Therefore, while discovering an operator grounding problem was good, it was also costly due to increased Quality Control. Thus, monitoring of the “operator ground system” grew in customer desirability and has resulted in technology improvements by some of the manufacturers and inventors of monitoring technology.

Single-Wire Monitoring Technology

Originally, simple “AC capacitance” single-wire monitors were developed. There were many shortcomings of this technology, all stemming from mostly “false negatives” (unit indicating the operator was grounded when he was not) and “false positives” (alarms going off when they shouldn’t). This technology is still around today and is purchased by some because of its low cost, around £25-£35 per operator and a lack of knowledge by the End User. A big plus is being able to use any standard single-wire wrist strap.

Wave Distortion Single Wire Continuous Monitor

The next level in single-wire evolution was “impedance” technology. However, since the capacitance and therefore the impedance of the circuit will also vary with such things as the person’s size, clothing, shoe soles, conductance of the floor, chair, table mat etc. these monitors often have to be adjusted or tuned to a specific installation and operator. Again, there were resulting “false positives” and “false negatives” though this was an improvement over the simple “capacitance” technology. This technology is also still around; the driver is low cost, £30 -£45 per operator.

The top of single-wire monitoring technology is called “Wave Distortion”. What this technology looks at is not the impedance level, but at the waveform generated by the circuit. Current will leak voltage at various points due to the combinations of resistance and capacitive reactance. There is a negligible amount of inductive reactance from the coil cord. By monitoring these “distortions” or phase shifts the Wave Distortion Monitor will determine if the circuit is complete i.e.; the wearer is in the circuit and the total equivalent DC resistance is within specifications given a range of installations. This technology is very reliable, (virtually no “false positives” or “false negatives”) and response time is very fast (<50 ms). In addition, the wrist strap open circuit test voltage is very low at 1.2 volts peak-to-peak @ 1-2 Micro Amps. Thus, a very low voltage is applied to the operator. The cost for this technology is £75 – £90 per operator.

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Images of ESD Damage

Seeing ElectroStatic Discharge (ESD) damage is basically impossible. Damage to semiconductor device structure is NOT visible at ordinary magnifications of an optical microscope. If the microscope is capable of 1000X-1500X magnifications, you just might be able to “see” something. The method used, only occasionally as there is considerable expense, is by delayering and etch enhancement producing high magnification photographs using a scanning electron micrograph (SEM). See Images of ESD Damage, photos of Human Body Model (HBM) ESD damage provided by Hi-Rel Laboratories, Inc. at 6116 N Freya, Spokane, Washington 99217 (509-325-5800 or www.hrlabs.com). Used with their permission.

Developing an ESD control program plan per CLC/TR 61340-5-2:2008

From ESD User guide CLC/TR 61340-5-2:2008 clause 4.1:

This clause outlines a step-by-step approach that can be used to establish an ESD control program.

4.1 Developing an ESD control program plan

4.1.1 Assignment of an ESD coordinator

In order to have a well thought out and implemented ESD program an ESD coordinator must be assigned. The ESD coordinator is responsible for all aspects of ESD in the facility. In order to be effective the ESD coordinator needs:

  1. the full support of management
  2. a good understanding of electrostatics and how ESD sensitive devices can be damaged. The ESD coordinator will often need to attend educational classes or seminars related to ESD in order to maintain or update their knowledge
  3. a thorough understanding of IEC 61340-5-1 and all of the organization’s processes related to the handling of ESD sensitive devices
  4. access to measuring equipment for the purposes of performing compliance verification audits as well as testing new ESD products and materials for use in the ESD program
  5. depending on the size of the facility, the ESD coordinator might also need to have auditors assigned to conduct the ESD audits

Finally, management must provide the ESD coordinator with the authority and funding necessary to ensure that the ESD control program is maintained and enforced.

4.1.2 Determination of part ESD sensitivity

The next step in developing an ESD control program plan is to determine the part, assembly or equipment sensitivity level under which the plan is to be developed. Although the requirements outlined in IEC 61340-5-1 are effective for handling parts sensitive to 100 V HBM or higher, the organization may choose to develop an ESD program based on ESD sensitivities that are greater or less than 100 V HBM. In this situation, the organization must develop an ESD control program plan that clearly states the ESD sensitivity that the program is based on. The organization can use various methods to determine the ESD sensitivity of the products that are to be handled. Some of the methods include: assumption that all ESD products have an HBM sensitivity of 100 V; actual testing of ESD sensitive devices to establish the ESD sensitivity thresholds using IEC 60749-26; referencing ESD sensitivity data in published documents such as manufacturer’s published data sheets.

4.1.3 Initial process and organizational assessment

Before the ESD control program plan can be developed, an initial assessment of the processes and organizations impacted by an ESD control program should be conducted. Organizations and processes that might be affected include:

  • purchasing
  • design engineering
  • receiving inspection
  • quality assurance
  • manufacturing
  • testing
  • maintenance
  • packaging and shipping
  • field service
  • failure analysis
  • repair services
  • spare parts storage
  • material handling and parts conveyance
  • receiving

An assessment of each area where ESDS parts are handled should be conducted in order to determine ESD hazards and possible ESD process procedures. The information accumulated throughout these steps forms the basis for developing the ESD control program plan.

4.1.4 Documentation of ESD control program plan

After gathering the above information, the organization is in a position to begin documenting the program plan. The plan should state the scope of the program which includes the tasks, activities and procedures necessary to protect the ESD sensitive items at or above the ESD sensitivity level chosen for the plan. Although the primary focus of the plan is to outline strategies for meeting the administrative and technical elements of IEC 61340-5-1, other items may be beneficial to incorporate as well. These additional items might include:

  • organizational responsibilities
  • defined roles and responsibilities between the organization and subcontractors and suppliers
  • strategies for monitoring product yields and processes that might be important in determining the effectiveness of ESD control measures currently in place or in assessing whether additional measures should be taken
  • approaches for ensuring continual improvement of the ESD program
  • a list of approved ESD control products and materials.

The administrative and technical elements of IEC 61340-5-1 that need to be addressed in the plan (unless tailored) include:

  • training plan
  • compliance verification plan
  • technical requirements
  • grounding bonding systems
  • personnel grounding
  • protected areas
  • packaging
  • marking

Charleswater – your ESD Control Experts. Contact Customer Service for help with your ESD Control Programme.

Enhancing Profits with Effective ESD Control

Our thanks to Conformity Magazine Published in December 2004 issue

Accurate process evaluation provides real answers

Provided by the ESD Association
by Stephen Halperin, in collaboration with Ron Gibson

“We need to spend HOW MUCH?”
Recently, a company experienced several large losses due to electrostatic discharge (ESD) and had a very unhappy customer on their hands. The manufacturing vice president now faced a substantial expenditure for new ESD loss prevention equipment. The company’s first step had been to hire an ESD consultant who recommended the purchase of several thousands of dollars in ionization equipment and monitoring instruments for several of the company’s facilities.

The troubled VP read the report several times looking for justification of the expense. However, the report did not define how the recommended equipment would meet the VP’s specific needs. Other than describing how ionization reduced electrostatic charge after it is generated and that the instruments could confirm that a discharge occurred, the report did not identify the actual cause of the process problem. No ESD measurements were described. There were no details related to cause of product loss, device sensitivity concerns, value issues, process and handling details, examination and description of existing controls, or rationale for how the recommended tools would solve the problem in question. The report was clearly based on the consultant observing the process of a single manufacturing environment. In effect, the report made a purchasing recommendation based on a “blanket” opinion, not on facts specific to the needs of the company or their customer. Such an approach typically makes a bad situation worse. While the recommended tools may have been very useful for investigating a process or for solving defined problems, they are expensive Band Aids“ when used in undefined problem situations.

Today’s electronic manufacturing environment demands that minimal ESD controls be in place to provide fundamental protection for electrostatic discharge sensitive (ESDS) devices. When basic ESD controls are employed and losses still occur, manufacturing and quality managers face more difficult problems., In assessing the problem, companies struggle with a variety of major questions concerning a specialized technology, while having minimal information and available skills. To avoid the risk of making the wrong investment decision without solving the initial problem, management needs a way to select and implement the most effective ESD controls that fit their financial situation, solve their specific problems, and provide a respectable return on their investment.

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ESD Control Program Considerations when Dealing with Class Zero Items

ANSI/ESD S20.20 Foreword states:

  • “This standard covers … electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts Human Body Model (HBM).”
  • “When handling devices susceptible to less than 100 volts HBM, more stringent ESD Control
    Program Technical Requirements may be required, including adjustment of program Technical
    Element Recommended Ranges.”

HMB Classification Class 0 is:
Per ESD-STM5.1 Human Body Model (HBM) Table 1 Class 0 has ESD Voltage Range < 250 Volts
Basically, to control the environment to decrease the probability of ESD damage in “Class Zero”
situations, involves increasing ESD protective redundancies and periodic verifications to those ESD
Control technical elements.

Improved Grounding

  • Personnel: Decrease Wrist Strap and ESD Footwear upper limit permitted (The ESD Association has test data showing charge on a person is less as the path-to-ground resistance is less) The use of continuous monitors, smocks, use / increased use of ESD flooring, sole or full coverage foot grounders (HBM & CDM)
  • Worksurfaces: Dissipative (CDM) i.e. change < 10^9 to a requirement of 10^6 to 10^8 ohms
  • Bonded grounds – Carts, shelves, equipment
  • Conductors: Minimizing isolated conductors like devices on PC Boards (CDM)

To see examples of Wrist Straps capable of dealing with class zero environments Click Here

To see examples of Grounding Cords capable of dealing with class zero environments Click Here

Minimize Charge Generation
The best form of control is to minimize charge generation. Grounding and ionization eliminate charges once generated. Shielding protects from generated charges.

  • Personnel – Low Charging floor finish
  • Surfaces – Use of low charging (anti-static) topical treatments

Insulators

  • Eliminate as best as possible all non-process necessary insulators
  • Topically treat where ever possible insulators that cannot be removed
  • Consider use of ESD Chairs or treat to reduce charge generation
  • Shield charges on clothing by using ESD Smocks

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Class Zero Items Click Here

ESD Control, Return on Investment

Ryne C. Allen

Desco Industries Inc. (DII), Employee Owned

November 1999

Reproduced with Permission, EE-Evaluation Engineering, November, 1999

INTRODUCTION

I. Introduction to ESD Control Programs

ESD Control programs are an essential part of a quality process and are always needed when handling ESD sensitive electronic/semiconductor devices. The extent of the ESD Control program is determined by the ESD Sensitive (ESDS) devices themselves and how they are handled. Refer to article “How to Set Up an ESD Control Program” [1] for additional information.

One of the main reasons that companies deploy ESD Control programs is to save money. Increased throughput and decreased scrap can yield a Return On Investment (ROI ) of up to 1,000% per [2]. A secondary reason is to comply with their customers’ and ISO 9000 type programs’ requirements. Whatever reason, setting up and implementing an ESD Control program will almost always produce favorable financial results.

II. Cost Reduction via ESD Control Programs

Having ESD awareness and following through with an ESD Control program is essential in reducing quality failures due to ESD. ESD can affect product reliability with catastrophic damage which is readily apparent to latent degradation. Latent degradation is particularly expensive requiring costly inspection and rework cycles in-house or product failure in the field. Maintaining good ESD controls will improve product throughput or yield, increasing reliability in the field which improves customer satisfaction leading to increased future business.

One test equipment manufacturer noted that GMR heads were being damaged during or after testing. These heads are extremely sensitive to ESD, and require additional handling precautions.

It is very important when designing and implementing an ESD Control program to know the ESD susceptibility of the ESD Sensitive (ESDS) devices you are trying to protect. Classification of these devices should include all simulation models human body model (HBM), Machine Mode (MM), and Charged-device Model (CDM) that will properly characterize the devices’ sensitivity when handled at various locations within the facility [6]. This will allow for the most economical program design.

Gene Chase, an ESD Consultant with ETS Inc., is quoted as saying “Millions of dollars are lost every year due to ESD [4]. Many of these incidents occur within the computer and communications industry.” Examples of losses from ESD may be any of the following:

  • Lost Time
  • Loss of Connection
  • Loss of Data
  • Shocks to Personnel
  • Upset to A System Requiring A Re-Boot
  • Damage to Equipment
  • Equipment Hardware Failure

To properly determine the return on investment (ROI) from your ESD Control program, you must collect return, repair and scrap cost data before and after implementation.

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