Most people do not start feeling the effect of an ESD event until the discharge is at least 2000 volts. It typically takes a ESD discharge of greater than 2,000 or 3,000 volts for a person to feel the “zap”. The number varies because the sensitivity of people is different and measuring the voltage is imprecise, so neither 2,000 nor 3,000 is an exact number. However, because an operator does not feel a charge, does not mean that they are not putting an component at risk. Common components used in modern electronic devices are susceptible to 100 volts or less.
The ESDA Handbook ESD TR20.20 provides guidance that can be used for developing, implementing, and monitoring an electrostatic discharge control program in accordance with ANSI/ESD S20.20. It also mentions ESD that a person can fill a few different places.
Wrist Strap section 184.108.40.206
“Static electricity is a natural phenomenon that occurs in all climates and at all levels of relative humidity year round. Most people cannot feel an electrostatic discharge unless the static voltage is greater than 2000 volts.”
Nature of Static Electricity Section 2.3
“The quantity, charge, is difficult for most people to visualize without some reference. As an example, an average person has a capacitance of about 100 picofarads (pF) and can feel a static discharge at their fingertips when the potential difference between their body and a grounded conductive object is about 3,000 volts (3 kV).”
ionizer Discharge Time and Product Sensitivity Section 220.127.116.11.3.3
“Most personnel will not notice static discharges from the human body until they exceed 3,000 volts.”
Q: What is a wrist strap?
A: A wrist strap is the combination of a wrist band and a cord (usually coiled) that is used to ground an operator that handles ESD susceptible devices.
Q: Metal wrist straps last many times longer than fabric. I know it is dependent on conditions, but what is an average life expectancy of both in your best judgement?
A: Fabric: Under normal conditions, we expect the clasp portion of a fabric band to last forever – a lifetime. The band material can be replaced or washed as needed.
Metal: 3 years–5 years, but we rarely see one of the these that has failed due to normal use. The bands are designed with a series of springs and those springs can break if over-stretched, but that is about the only wear point on a metal band. See a video on how the metal bands are made HERE
Q: Do cords break/wear out faster than the wrist straps?
A: Yes. the cord will be subject to far more wear than a wrist band in most applications.
Q: Can I buy new cords or bands separately?
A: Yes. Customers also find that they need a different style cord (length, material, banana angle) or band (metal, fabric, color) after their initial purchase.
Q: Are ALL fabric wrist straps considered to be elastic?
A: Desco’s are, but no, not ALL fabric wrist straps are elastic. Desco uses 36 strands of lycra spandex in the fabric to maintain contact with the wearer skin throughout normal motions. Some low quality wrist strap bands have minimal or no elastic fibers in them and only rely on a “tight cinch” of the band to maintain contact with the wearer’s skin. These types of bands are subject to intermittencies because the “tight cinch” is uncomfortable for the wearer and will loosen during normal (un-normal) movements by the operator.
Q: When looking at all that is available, why the big disparity in wrist strap pricing with yours and your competitors?
A: The disparity in price is largely based on the quality of the raw materials and workmanship in manufacturing Desco wrist straps. Some differentiating features are:
- 300 series stainless steel machined snaps on wrist band
- Silver plated polypropylene yarn used in band material
- Lycra spandex elastic
- 300 series stainless steel used on metal clasp parts
- Replaceable band
- Patented barrel type banana plug with machined contour spring support.
- Cord bending Life Test average >1,000,000 cycles
- Cord break load average 30 pounds.
- T-2940 tinsel conductor and 7 strands of polyester wrapped with copper for flexibility and strength.
- Date coded
- UL Listed
- Made in Chino, CA USA
Q: Is anyone’s quality really that much better?
A: Yes. Anybody that moves to continuous monitor system or an ESD Protected Area access control system that does not allow an operator into the ESD Protected Area or to continue working in the ESD Protected Area realizes that there is a massive difference in the quality. Quality assurance departments that track performance of wrist straps quickly realize that putting the product that they are handling at risk in favor of an ill-functioning wrist strap is not worth it.
All US based Desco Industries locations will be closed on Monday July 4, 2016 in observance of the 4th of July Holiday. Customer service, production, shipping, and all other departments will be closed. No orders will be processed for the day. Normal operations will resume on Tuesday July 5, 2016.
We wish you a safe and happy 4th of July Holiday.
Education is the ESD Association’s first priority. By offering convenient training, the Association provides ESD professionals with the knowledge and tools needed to meet the challenges of ESD in their companies.
The 2007 version of the ANSI/ESD S20.20 was updated in 2014. The ESDA now offers a guided presentation of the changes on their website HERE
The JAN/FEB edition of the ESDA’s Threshold details the changes – Significant Changes Made From the 2007 Version To The 2014 Version Of ANSI/ESD S20.20
To download a complimentary .pdf of ANSI/ESD S20.20-2014, go to the ESD Association web site www.ESDA.org.
The 6th generation of Desco dataloggers, the SmartLog® Pro, began shipping this week. The basic function of a datalogger has always been to record the data obtained from testing an operator’s wrist strap and/or foot grounders, but as the need for ESD control has advanced, so have the requirements of the testers. The SmartLog® Pro identifies the operator, controls access to the ESD Protected Area (EPA), and logs test data to a network accessible database. Each log entry includes operator identification, test results, resistance measurements, time, temperature and humidity. The paperless data enhances quality control and operator accountability by immediately identifying problems while reducing logging and auditing costs.
For more information on the SmartLog® Pro and to sign up of the for a demo click HERE.
All US based Desco Industries locations will be closed on Monday May 30, 2016 in observance of the Memorial Day Holiday. Customer service will be closed for the day and no orders will be processed for the day. Normal operations will resume on Tuesday May 31, 2016.
Desco Industries, Inc. manufactures ESD control products and other production supply products at six USA factories. Products include ESD bags, wrist straps, worksurface and floor matting, laminates, foot grounders, smocks, gloves, flooring, ionizers, continuous monitors, testers, tape, dispensing bottles, document holders and other workbench accessories.
The major advantage* of ESD shoes is that they do not require a tab to connect to the operator. Conductive additives are blended into the sole (inside to outside) of the ESD shoes and connects (through the sweat layer) to the operator’s feet. However, there are a number of other considerations to when selecting ESD footwear:
Does the footwear meet the ESD Association (ESDA) standards?
Many manufactures of ESD shoes often reference ASTM standards for their ESD spec, but state nothing about ESDA standards. The ESDA standards are written specifically for electronics manufacturing and handling. The walking test defined in the ANSI/ESD STM97.2-2006 is one of the most important methods for qualifying ESD footwear for used in ESD Protected Areas.
Not all ESD footwear is created equal.
There are different styles of both ESD shoes and ESD footwear. In most cases the specs of each style will vary. While one style of ESD footwear may retain its ESD properties for 6 months or longer, another will start failing within 90 days. The performance of all styles of ESD footwear should be verified on an on-going basis (twice daily testing is the norm) and records should be kept for quality control purposes.
Involve operators in the selection process.
The intentions of a ESD Control Plan are always to improve the quality control process of a facility. However, getting employees to participate and support the program is not always that simple. When selecting an ESD footwear is a good idea to consider the opinion on the operators. They may not find the style of ESD shoe being considered to be comfortable or they may become frustrated that the ESD foot grounder that has been selected does not stayed secured properly. In some facilities, many operators are temporary or on a flexible schedule that would not justify certain types of ESD footwear and it is never recommended that operators share footwear due to hygiene issues.
Consider the cost/value of the footwear program.
In most cases a ESD footwear-flooring system is an ongoing program that will need to be continuously managed. With ESD footwear there are is two major value components of an ESD footwear product- ESD properties and the durability of the footwear. ESD footwear must maintain a consistent Point to Groundable Point Resistance of < 1 x 109 (ANSI/ESD STM97.1) and Walking Voltage Generation of <100 volts (ANSI/ESD STM97.2) throughout the useful life of the product. The concern with the ESD footwear’s durability is that a breakdown can contribute to foot, leg, back pain or other discomfort in the operator. Initial qualification of footwear for both the ESD properties and physical structure is an important factor in the ESD footwear selection process.
Desco offers samples of foot grounders for product qualification and sizing purposes.
*In some cases protective footwear (shoes, boots, etc) is required to prevent foot injuries due to falling or rolling objects, or from objects piercing the sole. Safety of the operator takes priority over ESD control at all times. If protective footwear with reliable ESD properties is not available or foot grounders cannot be worn with the protective footwear in the ESD Protected Area, other personnel grounding devices such as wrist straps should be used.
Most people notice a difference in the ability to generate static electricity when the air gets dryer (the % RH decreases). As an example, walking across a carpet can yield a charge of 35kV at 10% RH [very dry air], but will drop significantly to 7.5kV at 55% RH. In an electronics manufacturing environment lower humidity may result in lower yields from production due to an increase in ESD events during manufacturing processes.
A normal range for humidity in an electronics manufacturing environment is be between 30%RH and 70%RH. Some facilities try to maintain a constant moderate RH (~50%), whereas other environments may want lower %RH due to corrosion susceptibility to humidity sensitive parts. The recommended humidity range is usually determined by the specs of the devices and components being assembled. Increasing the humidity in a an electronics manufacturing facility can help to reduce ESD events, increased humidity can lead to other unwanted quality issues in electronics manufacturing environment such as corrosion, soldering defects and the popcorn effect on moisture sensitive devices.
While not a replacement for grounding, shielding, or ESD worksurfaces, ionization can mitigate ESD events in areas where dry air is normal*. Worksurface ionizers (Benchtop and Overhead Ionizers ) produce positively and negatively charged ions that are moved to the controlled area with fan driven airflow. Point-of-use air ionizers use compressed gas to combat electrostatic attraction neutralizing charges on particles causing contamination or visual defects on products. Laminar Flow Ionization Bars are used with Laminar Flow Hoods, controlled chambers and other contained applications. Room Ionization reduces electrostatic discharge (ESD) and electrostatic attraction (ESA) in cleanroom environments.
The ESDA’s TR20.20-2008 discusses RH in a few different places. Some significant statements are listed below.
ESD Handbook ESD TR20.20-2008 section 2.3 Nature of Static Electricity:
“The moisture level in the air, or relative humidity in the environment, are important considerations in the liberation and accumulation of static electricity. It is well known that static electricity in the form of static cling and static shocks are more prevalent when the air is dry. Heating interior air in the winter months dries out the already dry air in the higher latitudes. Static charge accumulation is easier on dry materials since moisture on surfaces tends to allow charges to slowly dissipate or recombine.
However, it is impractical to use humidity control alone to provide static control since static charges are developed even at relative humidity levels of 90% and greater. For most situations, 30 to 70% RH is considered the appropriate range. Special areas, such as wafer fabrication, may require lower humidity control for processes that are affected by moisture (e.g., photoresist application). Soldering is known to be affected by high relative humidity conditions (>70%). For areas that have low ambient humidity, ionization is an important consideration to aid in reducing charge accumulation levels and provide neutralization of charges after they are developed but before they can cause difficulties.”
ESD Handbook ESD TR20.20-2008 section 5.3.16 Humidity:
“Humidity is beneficial in all ESD control program plans. Contact and separation of dry materials generates greater electrostatic charges than moist materials because moisture provides conductivity that helps to dissipate charge. For this reason, ESD effects are most noticeable in the winter since heating systems reduce building environment moisture. Geographic location (desert vs. coastland) is also a major contributor to ambient conditions inside buildings. Any circumstance that results in a low relative humidity will permit a greater accumulation of electrostatic charges. Relative humidity above 30% in ESD protective areas is desirable as long as other adverse conditions are not created as a result of humidity levels. Generally speaking an upper limit of 70% is desirable to prevent corrosive effects on the metal portions of electronic devices and assemblies.
Besides the increasing propensity to generate electrostatic charges on dry materials in general, performance of many ESD protective materials degrade. In fact, when exposed to low humidity conditions, some ESD protective materials become totally ineffective or become sources of electrostatic charges. Therefore, evaluation of ESD control materials should include performance testing in controlled environments at the lowest expected operating relative humidity level. Manufacturers of ESD protective materials should be able to provide performance data in regards to relative humidity. Likewise, materials should be tested in moderate humidity conditions as well to ensure they do not become “too conductive” and present a potential safety hazard to personnel working with substantial voltages. See the Personnel Safety section of this handbook for further guidance in this area.
Humidity control in factories or physically large areas or buildings can be difficult and expensive. In smaller rooms or areas, it may be possible to use portable humidifiers to raise the immediate area humidity. However, in large facilities and factories the environmental systems many need to include steam generation and monitoring equipment to control humidity. This type of equipment is expensive to install and purchase especially in pre-existing facilities. To reduce the total cost impact, companies should consider the need for humidification equipment when planning new facility construction.”
A complete static control program must also deal with isolated conductors that cannot be grounded and insulating materials (e.g., most common plastics). If an insulator cannot be removed from the ESD Protected Area, substituted with an ESD protective version, or controlled with a topical antistat, ionization must be used. These insulators are often referred to as process necessary insulators. Common process necessary insulators used for manufacturing electronics include flex cables, product enclosures, test fixtures, and the PCB substrate substrate itself*.
Neutralization of charges on insulators does occur naturally as positive and negative charged ions are in the atmosphere. As opposite charged ions are attracted to the charge on the insulator, the charges will be neutralized over time (same charged ions will be repelled). However, the process is much too slow for ESD susceptible components.
Examples of some common process necessary insulators are PC board substrate, insulative test fixtures, and product plastic housings. Examples of isolated conductors can be conductive traces or components loaded on a PC board that are not in contact with ESD worksurface.
The primary functions of ionizers are to:
- Discharge / Neutralize process necessary insulators that can charge ESDS devices / assemblies via induction, thus creating potential CDM damage, and/or
- Discharge / Neutralize ESDS devices / assemblies that are not grounded, thus are an isolated conductor, thus creating potential CDM damage, and/or
- Discharge / Neutralize insulators where particulate contamination can cause visual defects (for example – dust on plastic or glass lenses, attracted to / held in place by Electrostatic Attraction [ESA])
*Note. The PCB substrate is a process necessary insulator and can become charged during automated handling processes and the ESD susceptible components on the PCB may be isolated or floating conductors as well.
Desco offers a variety of ionizers:
Contact Desco for more information about when you need to ionize or how to choose the right ionizer for your application.
Lenovo Service posted a video on their youtube site called “Understanding Electrostatic Discharge (ESD) for Technicians.” We encourage you to take a look.
The video is very informative and does a good job of covering the basics of ESD control when working with ESD susceptible devices or components. The basic practices of controlling ESD when handling ESD susceptible items are pretty straight forward.
- Establish and control access to an ESD Protected Area. It can be one bench or a whole facility (or a field service kit that a Lenovo Service person might use)
- Ground all conductors including operators.
- Use a dissipative grounded surface for ESD susceptible products to be placed on if needed.
- Remove all non-process essential insulators from the ESD Protected Area. Use ionization to control charges on process essential insulators.
- If an ESD susceptible item needs to be removed from the ESD Protected Area, place it in a shielding bag or storage container that forms a faraday cage.