One of the requirements of any quality ESD control program is to minimize the charge (also called voltage) on operators. When operators are charged with static electricity, they can transfer this charge to the product and possibly cause damage or destruction. One effective way to reduce static charging on people is to ground them through the use of a wrist strap and coil cord. When working properly, the wrist strap system (band and coil cord connected to common point ground) can control voltages on people down to 10 volts and less and limit damage.

Wrist straps are the most common and effective way to eliminate static on personnel, but they are also the ESD control device that is most likely to fail over time. A broken wire in the coil cord, a dirty cuff interior, incorrect wearing of the strap (too loose or placed over clothing), and high skin resistance are just a few of many causes of wrist strap failure. If wrist straps fail, the static charge on an operator will increase and may cause ESD damage without the operator even noticing. Therefore, it is imperative that measures are taken to assure that the operator is properly grounded when wearing a wrist strap.

The PDT800 is a deluxe “Near-Fail” wrist strap and foot wear combination tester with a digital read out, “near-fail” LED indicator, human body noise filtration, and options to test wrist strap only, foot only, or both wrist and feet.

ESD Test Stations

One way to assure that the wrist strap system is working correctly is to use an ESD Test Station. The ESD Test Station assesses the resistance of the wrist strap system (including the operator) and provides a pass/fail indication. Some more advanced test stations will also provide the actual resistance measurement and can even log test data on a computer. But the frequency of testing differs from company to company. Some require testing just once at the beginning of the shift. Other companies may require re-testing several times a day. Regardless of how many times an operator tests, there is a risk that the wrist strap will fail at some point during a shift.  Constant Monitors can eliminate this risk.

Transforming Technologies offers several testing station options that range from wrist strap monitors (WST200), wrist strap and footwear test stations (GTS600K), wrist strap and individual foot test stations (GTS900K), test stations with “near-fail” technology and digital display (PDT800K). These testing stations function with a standard wrist strap and coil cord using a banana jack connector and standard heel grounders.

Constant Monitoring

An ESD wrist strap plugged into a Constant Monitor. The monitor will alarm if there is a problem.

If a wrist strap fails the test at the beginning of a shift, the question arises, “When did the failure take place and how much product was handled after the failure?” Constant monitors provide assurance that the wrist strap system is working and alarms immediately if there is any disconnect from the operator to ground. There are several benefits to constant monitor systems:

• They provide immediate notification of a problem and alert the operator to not handle the product until the problem is resolved
• They monitor the wrist strap at the work location, eliminating the possibility of a functional wrist strap but a defective ground attachment
• They eliminate the need for recording and storing test results
• Some units will also monitor the work surface to verify that it is still connected to ground

The downside of constant monitors is that each operator must have their own dedicate monitor, which can be a costly initial investment.  But the savings overtime of product damage during manufacturing and lower warrant claims can make up for the constant monitor costs.

Transforming Technologies offers several low cost constant monitor options that range from monitoring a single operator (CM400), an operator and a work surface (CM410), and two operators and a single work surface (CM420). These monitors function with a standard wrist strap and coil cord using a banana jack connector.

Summary

Wrist straps are the most common method for removing charge from operators handling ESD sensitive products, but they need to be tested often to ensure they are working properly. ESD Test Stations can verify wrist strap operation but there is still a risk of wrist strap failure in between tests. Constant monitors solve this problem and can provide assurance that the system is working as designed and the products are being handled safely at all times.

This week’s ESD Q&A question::

Question: When measuring ESD Product such as ESD Mat resistance, I have noticed that the resistance measures higher on some of my older ESD rubber mats when using 10V and will sometimes be greater than 10^9 ohms. Switching to 100V will lower the resistance into the green, passable range(less than 10^9 ohms). If the mat passes at 100V, is that sufficient to give the mat a pass? When should I use 10 volts vs. 100 volts?

Answer:

Industry standards specify the test voltage required when testing or verifying ESD control items (i.e. mats).    For verification of mats, or work surfaces, you can reference the ESD Association TR53 Compliance Verification document.  This is a free download at www.esda.org (select the Standards tab and scroll down to TR53).

The following rule applies for resistance testing of ESD control items:

Perform the test at 10 volts.   If the resistance is < 1.0 x 10^6 ohms record the reading.

If the measurement at 10 volts is ≥1.0 x 10^6 ohms, switch to 100 volts, make your measurement and record the result.

Most ESD mat materials are designed to have a resistance greater than 1.0 x 10^6 ohms, so you can start your testing at 100 volts.  In the example that you stated, the mat that you tested meets the requirements.  If you are seeing the values creep higher it would be wise to test the older mats more frequently to verify that they are still under 1.0 x 10^9 ohms.

Periodically, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

Question:  What materials are the biggest cause ESD damage?

Answer:  Plastics and other synthetic materials cause the most trouble because they are insulators and trap charges on their surfaces.

Materials that are insulators hold an electric charge and cannot easily transfer the charge and cannot be grounded to earth by common means.  Conductors are just the opposite.  They are materials that easily transfer a charge.  Examples are metals, water, carbon and people. Understanding insulators and conductors is an important part of creating an ESD action plan.

Question:  What are common causes of ESD?

Answer: Just about any contact and separation of two materials will generate static.

Opening a common plastic bag. Removing adhesive tape from a roll or container. Walking across a floor and grabbing the door knob. Sliding circuit boards on a work bench.  You get the idea.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

Question: Why do I only get a static shock when I touch metal?

Answer: Shocks are usually only felt if your body is charged to over about 4000V, and you touch something conductive (like metals or water, or other people).

If the wall or door is made of wood, concrete or some other material that has low or intermediate conductivity, any static charge on your body escapes slowly and usually does not cause a shock. If you touch metal, water, or another person when your body is highly charged, the charge is discharged quickly as the material is highly conductive. In this case you may feel a shock.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

Question:  Does the weather affect static electricity?

Answer: You’ll often find dry weather will affect static electricity.

Shocks are worst under certain weather conditions. Static shocks will increase during the winter when the air outside is cold and dry. They may disappear in warmer damper weather.

Static charge build-up is enhanced when the weather is dry so static problems are often noticed more in dry air of the winter. The air outside can be very dry when the weather is cold. Indoors, central heating or air conditioning can give very dry conditions which promote static electricity. Heating warms the air and reduces its humidity.

Static shocks are often noticed in cold dry weather, especially when in a centrally heated environment, and may disappear when the weather gets more humid. Static shocks may also be encouraged under air conditioning in hot weather.  Many people choose to use a humidifier to increase air moister which helps dissipate static charges.

When there are ESD concerns in environments such as semiconductor or electronics manufacturing, the ANSI20.20 requires that temperature and humidity readings are taken when measuring static.  Typically a static field meter or a surface resistance meter are use to measure static and many take these readings as well as measuring static.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

Question: Why do I experience static shocks in the office, when my colleagues do not?

Answer:  Short answer, there are lots of reasons. Lets review them:

There are many reasons why you might get static shocks in the office. Firstly, some people are just more sensitive to shocks than others. It like in life, people experience things differently, for example, some people can have music up louder than others. For most people, the threshold for feeling shocks is in the range  2,000-4,000V.

Secondly, you may be storing more static electricity than others. This depends on the size of your body and feet, and the thickness of your shoe soles! A bigger body, bigger feet, and thinner shoe soles, means more charge has to be stored to produce the same voltage. Try changing your shoes if you begin to experience static shocks regularly or consider a ESD chair mat, or an ESD chair.

Thirdly, you may be generating more charge than others. This may be due to the material of your shoe soles, or the way that you walk. If it happens when sitting, it may be due to the material of your clothes, and the amount of static they generate against your chair.  Insulators like plastics, polyester or nylon generate huge amounts of static.  Choose natural fibers or in extreme cases, try an ESD jackets which are used in manufacturing environments.

Question:  Are static shocks a health risk?

Answer: Static shocks can be a nuisance – but are not generally a health risk.

Fortunately there is little risk attached to such electrostatic discharges. In most cases they are just a common nuisance. The biggest risk is that a shock could cause you to have an accidental injury. For example, you might withdraw your arm suddenly and hit it against something.

Household static shocks, for example, are very high frequency and have a very short duration. According to LiveScience.com, it’s possible to generate up to 25,000 volts by dragging your feet across a carpet, though  the voltage of a typical carpet shock between a few hundred to a few thousand volts. People can typically feel static at 2000 volts. Either way, household static shocks are almost always harmless. Most of the shocks that electrocute people are at a much lower frequency, a much longer duration, and have more energy creating a much more significant driving force.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

 Question: Why do I get static shocks when I touch the door knob?

Answer: Static charges build up on shoes when you walk.

Most modern shoes have highly insulating rubber or plastic soles. As you walk, static charges can build up on the soles of the shoes. This is especially true if the floor is also insulating. Some older nylon carpets are particularly good at generating static electricity.  The charge on the shoes soles induces static electrical charge on your body, and when you touch something conductive, such as metal, the static will discharge and cause a the shock. If you are indoors, the point can be proved by walking around for a while with no shoes on – you will probably not experience shock.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.

Q:  What is “ESD”?

A: ESD is short Electrostatic Discharge.

Electrostatic Discharge, or ESD, is a single-event, rapid transfer of electrostatic charge between two objects, usually resulting when two objects at different potentials come into direct contact with each other.  ESD can also occur when a high electrostatic field develops between two objects in close proximity.  ESD is one of the major causes of device failures in the semiconductor industry.

Electrostatic charge build-up occurs as a result of an imbalance of electrons on the surface of a material.  Such a charge build-up develops an electric field that has measurable effects on other objects at a distance.  The process of electron transfer as a result of two objects coming into contact with each other and then separating is known as ‘triboelectric charging’.

This charging process results in one object gaining electrons on its surface, and therefore becoming negatively charged, and another object losing electrons from its surface, and therefore becoming positively charged. A person can get triboelectrically charged in a number of ways, even by just walking across a room.

Every Thursday, Transforming Technologies will answer questions concerning all things ESD: static causes, threats,  ESD prevention, best practices and all things static in a feature we call ESD Q&A.  If you have ESD questions that you would like to be answered, email info@transforming-technologies.com  with Q&A in the subject line.