Using a current limiting resistor in the ground cord is the user’s choice; however, the resistor is not for ESD control purposes. Either without resistor or with a one megohm resistor is acceptable, but most all Charleswater ground cords do contain a resistor at the snap end.
Here is advice from our industry’s standard:
“An easily accessible dedicated EBP [EPA ground bonding point] for the wrist strap shall be established adjacent to each working area, or working surface. A sufficient number of EBP shall be provided for operators and visitors.” (EN 61340-5-1 section 5.3.3)
“EPA ground cords shall be used to make electrical connections between groundable points and the EPA ground facility.” (EN 61340-5-1 section 5.3.4)
Regarding 1 megohm resistor, “A nominal 1 megohm resistor is commonly used in wrist straps and to ground work surfaces.” [EN 61340-5-2: Section 5.1.1] However, for Working Surfaces, it refers to “it is allowed, when approved by the ESD coordinator, to use surfaces which are “hard ground” i.e. less than 1 X 10^4 ohms to EPA ground.” (EN 61340-5-1 Note 6 of Table 1)
“The EPA ground facility shall be connected to EPA ground and provide a low resistance path to EPA ground (<2 ohms). When available, the mains protective earth shall be used. (EN 61340-5-1 section 5.3.2)
When an in-line resistor is used in Charleswater’s common point grounds it is isolated from the banana jacks
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“Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements. The test equipment selected shall be capable of making the measurements defined in the compliance verification plan.” [EN 61340-5-1 Edition 1.0 2007-08 clause 5.2.3 Compliance verification plan]
Leading companies use continuous monitors as a cost effective component in satisfying some of the audit and check requirements of EN 61340-5-1.
Wrist strap testing “Where continuous monitoring is used, no additional testing is required.” [EN 61340-4-1, per A.5.2]
“The wrist band will normally be worn for several hours at a time so it needs to be comfortable while making good contact with the skin. It is a good idea to check the wrist strap every time it is applied. Constant on line monitors can be used so that any breaks will be immediately found.” [EN 61340-5-2 clause 5.2.7]
“A properly grounded wrist strap will keep a person’s body voltage to approximately + 10 V. The main advantage to a constant [or continuous] monitor is the immediate indication that the employee receives if the wrist strap falls open. With an unmonitored system, the employee will not be aware of a wrist strap failure until the start of the next shift. This has reliability benefits for an ESD program as it might help reduce or eliminate ESD damage.
There are also other process benefits from using constant monitors such as the elimination of the need to maintain daily test logs and a reduction in the time for employees to make the daily test. For units that also monitor the connection of a work surface to protective earth, it is also possible to reduce or eliminate the checking of the work surface as part of the periodic audit of the process.
Constant monitors might be implemented by an organization due to high reliability requirements imposed by customers.” [CLC/TR 61340-5-2:2008 User guide Annex B.1.3 Constant monitors]
“In many EPAs [ESD protected areas] constant monitor wrist straps are used. These fall into two categories, dual wrist band and single wrist band. The dual wrist strap type using a split band is used with a two core cord.
The dual wrist strap type normally works using the resistance bridge method.
The single wrist strap type uses a single strap for both grounding and monitor purposes and has an a.c. signal injected which is used for monitoring purposes. This type has the advantage of using the simpler single wrist strap.” [EN 61340-5-1 A.5 Wrist straps]
See selection of Charleswater Continuous Monitors.
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.
To continue reading Enhancing Profits with Effective ESD Control Click Here
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.
- 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
- 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
To continue reading ESD Control Program Considerations when Dealing with
Class Zero Items Click Here