ESD Q&A

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1. Q: Why do some ESD grounds have impedance and some do not?
　　A: The purpose of an ESD ground is to connect a conductive surface to a place with the same potential as the power ground. A "hard ground" is a ground wire that is directly connected to the ground without additional resistance; the resistance between the power ground and the common point is basically 0Ω. A "soft ground" is a ground wire with an internal series resistance, typically 1M, designed to limit the damaging current that may be generated when the operator is exposed to an environment with 110V and a maximum of 250V. The ESD Association ANEOS/ESD S6.1-1991 recommends using a "hard ground" method to ground the ESD table or floor mat.

2. Q: I often wear one anti-static shoe, but I am often told to wear both feet. Why?
　　A: Anti-static shoes only work when they are worn correctly and connected to a conductive floor or consumable ground. Walking is an excellent example of frictional charging. If you use anti-static shoes correctly and are in close contact with the ESD floor, the charge on your body will drain into the ground. Therefore, the network between you and the ground is the same voltage, but as soon as you lift your foot wearing the anti-static shoe, you will be charged again, either by induction from your clothes or triboelectric charge generated by friction and lifting your foot. If you wear two anti-static shoes, you will further greatly reduce the chance of a net charge much higher than a few volts (typical values are 2000-5000V) because you are in a grounded state for a longer time. Therefore, it is recommended to always wear a pair of anti-static shoes when approaching moving objects.

3. Q: Is it necessary to connect a 1M resistor between the machine and the ground?
　　A: No. Referring to the manufacturer's grounding requirements for machines or equipment, it can be seen that the 1M resistor is used to protect the human body. Refer to the following questions. Side note: It is beneficial to ground all isolated conductors near ESD sensitive workstations. It can minimize unexpected electric fields or charge accumulation.

4. Q: What is the role of the 1M resistor in the semiconductor assembly process?
　　A: Assumption 1: We are talking about ESD control issues; Assumption 2: The human body is in contact with semiconductors and devices with semiconductors. 1M series resistors can be found in anti-static wrists, anti-static shoes, zippers, ground wires, etc. Their function is to limit the amount of current that can pass through the human body and protect human safety. The main limiting requirement of the 1M resistor is: at 250V AC effective value, the current is limited to 250 microamperes, which is exactly the perception level of most people (the critical value for the nervous system to reflex). The difference in the physical perception of current on the surface and in the body depends on the size, weight, moisture, skin condition, etc. of the human body.

5. Q: Do shipping boxes containing circuit boards need to be covered with lids? How do these shipping boxes work?
　　A: Generally, the lid needs to be covered. The lid properly installed on the shipping box can provide strong enough shielding for the circuit boards inside. These shipping boxes can not only provide mechanical integrity under normal use, but also provide ESD safety for the inside. If the lid is removed, any stray electric field can cause charging of many independent conductors on the circuit board. These induced charges are the source of ESD. There are many potential sources of stray electric fields: people, clothing, ungrounded two-wheeled vehicles, monitors, furniture, partitions, any non-grounded conductors, any insulators, electromagnetic interference, etc. Circuit boards in open ESD shipping boxes will pass through (or be exposed to) several power sources (electric fields) when transported by grounded vehicles, which will also cause problems for internal devices.

6. Q: If the foot pads and anti-static wrists are connected together, do they need to be connected to the earth?
　　A: No, just the equipment being handled should be unplugged and electrically connected to the ESD ground. As long as the conductive parts (skin, ESD foot pads, equipment housing, operating parts, etc.) are equipotential, ESD (discharge charge) will not occur. The earth is very suitable as a reference voltage. If you make sure that all conductive parts are connected to the ground, then it is guaranteed that these conductive parts are at the same potential. Once there is a potential difference between the two conductors, it will cause charge discharge to generate ESD.

7. Q: How does the relative temperature of the ambient air affect ESD?
　　A: Relative humidity (RH) has a direct impact on the performance of surface charge accumulation. The higher the relative humidity, the shorter the time that components can store charge. The surface charge can be reduced (due to the increase in relative humidity) by recombination or conduction. When the relative humidity increases, the conductivity of the air also increases. However, even at 100% relative humidity, the increase in charge discharge rate cannot replace proper ESD control operating procedures.

8. Q: Does the percentage of relative temperature need to be controlled at a certain value? If so, how can this value be determined?
　　A: As the air becomes drier (the percentage of relative temperature decreases), the change in the ability to generate static electricity is certain and obvious. At a relative humidity of 10% (very dry air), walking on a carpet can generate a charge of 35kV, but it will drop sharply to 7.5kV at a relative humidity of 55%. The optimal range of relative humidity in the working environment is 25%-50%. Some clean places generally require a relative humidity of 50%, and other environments require lower relative humidity due to the presence of devices that are more sensitive to corrosion and humidity. Test equipment and ESD suppression products in the optimal humidity range. Some ESD suppression products are related to humidity.

9. Q: Will strong magnetic fields (such as magnets) cause ESD? Is there any connection between magnetic fields and ESD?
　　A: As long as the magnetic field source and the object are stationary, ESD will not be generated by strong magnetic fields. If the magnetic field fluctuates or an object moves across the magnetic field lines, a current will be induced (if the object is conductive). An isolated conductor completely exposed to a pulsating magnetic field will accumulate enough charge to cause ESD problems. Magnetic fields can also be generated from electromagnetic noise (EMI). If they are not properly shielded, current (charge) will be induced on isolated conductors. Some EMI sources are PCs, ESD, transformers, fluorescent lights, etc. In a general working environment, these are likely to be less important sources of interference.

10. Q: Our electronic assembly workshop has ESD conductive floors, uses ESD special grounding shoelaces, and requires each operator to wear steel-toed safety shoes. If you do not wear ESD special grounding shoelaces, you cannot pass the grounding shoelace detector. Our ESD grounding shoelaces are made of #2048 plastic. Shoes made of some material have an impedance of 106-108, which can reduce static electricity. Should I buy a new tester or new shoes or something else?
　　A: First, make sure your tester has been calibrated recently. If not, you need to calibrate it. The standard for testing shoes is different from that for grounding shoelaces. Standard ESD S9.1-1995 uses a 100V impedance meter for impedances below 109. Most grounding shoelace testers use an open circuit voltage of 6-12V, which is much lower than the 100V voltage used for shoes. If the material of the shoe itself conducts electricity under high voltage, then either buy a higher voltage tester or reconsider the important factors in static discharge control. Some foot grounding (heel strap) testers are calibrated at 750k-100 M at the factory, others are only 10 M. Some new testers provide about 20-22V open circuit voltage, which may also be effective. In addition, there are some ESD shoes that are used to test foot grounding testers because they can conduct electricity very easily at lower voltages (in the consumption range).
11. Q: Are there any minimum requirements when testing anti-static wrists, and what are the minimum or more reasonable requirements?
　　A: There are no pre-defined minimum requirements for testing an anti-static wrist, but a very good rule of thumb is to test it every time you put it on or start another job. The frequency of testing depends on the ESD-sensitive equipment you are trying to protect. If the equipment is quite expensive and controlled by an operator, frequent monitoring is necessary, but if the equipment is not particularly sensitive and relatively inexpensive, then regular testing is sufficient. There are several things you should check when testing an anti-static wrist: Make sure the anti-static wrist is in close contact with the wrist; the conductive fiber or metal/conductive material is close to the skin; the snaps on the metal buckle are in good fit, the snap connectors at the coil end should be well tightened, and the banana plug spring is elastic and in close contact with the banana plug jack. Other things to check are: intermittent interruptions in the coil (usually at the end), dirt, oil, and shells welded to the inside of the strap (this will reduce the conductive properties of the strap). It is safe to apply 1-5 pounds of tension to the snaps on the metal buckle in the coil.
12. Q: If all other ESD precautions are taken, is it necessary to wear an anti-static wrist? (i.e. two grounding (foot) straps, smock, floor consumables, grounding plate, etc.), if so, why?
　　A: If the operator wears two anti-static shoes on the conductive floor and does not lift both heels at the same time, then the anti-static wrist is not needed. If the operator also wears a smock, but it is not electrically connected to the body or the ground, then the smock has only partial protection function, and the charge accumulated on it may have no place to discharge. The common method of grounding the smock is: tie a snap to the waist of the smock with a line, or use an anti-static wrist buckle to the inner cuff of the ESD smock.
13. Q: We check the ESD work surface every week to ensure that it is properly grounded through a 1M resistor. What is the general reason for unstable resistance readings between the ESD (hard ground) and the conductive work surface ground (soft ground)?
　　A: One reason for unstable resistance readings between the power ground (ESD ground) and the conductive work surface is ground current. If the foot pad is grounded at multiple points, ground current may flow through, which will cause the ohmmeter reading to be wrong. Other possible reasons include: the instrument battery voltage is too low, the ground loop between the instrument AC power supply and the ground being measured, etc. It is best to cut off the ground wire in the instrument power plug. Some digital instruments are sensitive to the magnetic field or strong electric field around them. When measuring, pay attention to checking the power distribution cable, EMI source, etc. Try to measure repeatedly in the shielded area to find out whether the problem has been eliminated.
14. Q: The answer to question 6 says: If the operator, ESD grounding plate, etc. are at the same potential, ESD cannot occur. What I want to ask is, if the grounding plate is "hard grounded" and the operator wears an anti-static wrist and is grounded through a 1M resistor, then there is still a possibility of danger, right?
　　A: Not necessarily. When current passes through a 1M resistor, there is a potential difference between its two ends. You are correct in this regard. The key to ESD control is to eliminate the charge on the body surface or other conductors within the safe discharge time of the charge. If the discharge time is a few milliseconds, then there is a potential difference between the two ends of the 1M resistor between 0 and 0.01 seconds. The typical body movement is about 500ms. Therefore, when you touch the sensitive device, you have used 490ms to decay the potential to 0.
15. Q: Why can ESD shielding bags discharge static electricity? What is the discharge process? What is the difference between antistatic bags and ESD shielding bags?
　　A: ESD shielding bags can discharge static electricity because the surface of the bag (inside, outside or both) is conductive and in contact with another conductor (ESD grounding plate). In order for the shielding bag to discharge charge, it must be conductive and connected to a conductor (usually grounded). Antistatic bags have antistatic properties and may not have shielding, conductive or MVB properties. All antistatic measures mean that the friction charging of antistatic bags will not exceed a certain voltage (usually 250V). Shielding bags should have metal sheets or conductive films in various parts of the bag, depending on the device you want to protect. Bags made of conductive, antistatic, shielding and water vapor isolation materials can protect almost all sensitive devices, but may be over-protected.
16. Q: Our users who produce printed circuit boards need an antistatic tire. Some tire manufacturers provide tires made of conductive rubber or conductive neoprene. Are antistatic and conductive the same thing?
　　A: Antistatic means that when the wheel rolls on the ground, it does not generate a voltage higher than 250V. A conductive wheel may not be antistatic, but will release it if rolled on an ESD floor. The ideal wheel is both conductive and antistatic, and when exposed to a conductor, it will neither generate excessive charge nor store charge.
17. Q: Some people bought several "wireless" ESD antistatic wrists for their PC repair station, which are said to work similarly to the corona on copiers/laser printers. I think this may be part of a composite system, but he said that the requirements for use did not mention the need for other equipment. Recently, when he opened the car door and was shocked (wearing an anti-static wrist), he was sure that the anti-static wrist was useless. There is a small rectangular piece of plastic on this elastic band, which is screwed with a brass screw, and the screw cap is exposed. I carefully opened one and found that the 1M resistor was connected between the second screw (connected to the anti-static wrist connection piece) and a soft "C"-shaped plate similar to rubber (about 1mm thick, 2cm2, hollowed out), screwed with a brass screw. Does this seem feasible?
　　Answer: No, it is not a feasible ESD control device in the ESD safe area. These passive "wireless" anti-static wrists have many limitations. If you are charged to 10kV due to friction, and you are wearing a wireless anti-static wrist, it will take several hours to drop to 5kV (or even a few days, depending on the relative humidity of the environment), but it will never be less than 10V. Most of the charge reduction is due to the natural recombination of the charge on the surface of your body (in this case, the metal shell of your "wireless" anti-static wrist) and the conductivity of the air with a certain relative humidity. (P.S. You will get the same effect if you cut your hair to about 1/4 inch. Apply conductive glue to your hair, and immediately the end of your conductive hair will act like a corona discharge tip at a fairly high voltage, causing the current to flow into the air or helping to enhance the natural recombination process. When your hair is very close to the ground, 0.1-1.5 inches, you need to consider any minimum 3kV potential difference (because of the dielectric strength of the air) or the enhanced normal air conductivity to keep you below a few kV.
18. Q: If a device is placed in a box with the leads hanging in the air, the leads will have a resistance of about several hundred ohms end-to-end and end-to-shell and a capacitance of 10pF. If the charging time is a few minutes, the hanging leads can easily be charged to 1kV (relative to ground). (The charging method can be stored in a dry N2 box close to the nozzle but not properly grounded, or wearing nylon or cloth shoes and walking through a long carpeted hall in a dry winter.) A user with grounding measures opens the ESD bag. The device is damaged because the leads charged to 1Kv discharge through other already discharged leads. The potential applied to the oxide destroys the device. All leads must be short-circuited or knotted to facilitate ESD protection, right?
　　A: For In your example, let's assume that the device is in a box (the box can be grounded and conductive) without an ESD bag. During transportation, the box may be separated from the ground and charges can be induced from the outside. Inside, the device can move around, and the friction charge ranges from a few volts to several thousand volts, depending on the materials in contact with each other. When a grounded operator opens the box and touches or approaches the hanging leads, ESD may be triggered. If the pins of the device are inserted into a conductive foam material or an ESD bypass strip is used, the device's sensitivity to discharge and ESD damage is greatly reduced. When to use a bypass strip or conductive foam depends on the type of device you want to protect, the charge storage method, and the shipping method. So be sure to ask the manufacturer for the correct precautions.
19. Q: Some ESD workbenches with anti-static wrists can be directly grounded, while others need to be first led to the ESD mat (at the opposite corner) and then grounded through a resistor. Which is correct?
　　A: Neither is necessarily correct, because it depends on the potential of the directly grounded anti-static wrist and the workbench mat. For the ESD safety table, the concept of grounding is to make all conductors at the same potential, so as to minimize the potential difference between the two conductors. Since the power ground and the earth are always in the control area, they are the most convenient connection points. If the anti-static wrist and the ESD table mat are connected to the same grounding point, then they are at the same potential, which is the correct installation method. The anti-static wrist is first connected to the ESD table mat, then passes through the table mat, then passes out from the opposite corner of the table mat, and finally connects to the ground. In this way, there is an additional series impedance (resistance or capacitance) from the table mat to the ground. Sometimes, the operator can be at a different potential from the grounded table equipment or the ESD sensitive equipment located on the table mat.

21 Q: When wearing anti-static shoe covers, is it important for the shoe covers to be in close contact with the skin of the feet, and can the shoe covers be between the shoes and socks?
　　A: When wearing anti-static shoe covers to operate electronic instruments, it is not necessary to make the shoe covers directly contact the skin. The conductive connector on the anti-static shoe covers indirectly connects the conductive floor (ground) to the human body (skin). Typically, the conductive connector is placed between the insole and the sock by the foot. The skin is electrically connected to the connector through the sock. The moisture from the foot makes the sock conductive and provides a conductive path between the foot (skin) and the connector. When the shoe is first put on, it can be worn for 10 to 60 minutes to ensure that there is enough moisture to make the sock conductive so that the skin can effectively connect to the conductive connector.

22Q: Currently, my company has been using cardboard templates to check for missing components on PCBAs (integrated chips and surface mount chips). The cardboard is placed on top of the PCBA, which generates static charges. Cuts are made in the cardboard according to the size of the component to check if any components are missing. What I want to ask is, what is the appropriate material to replace this cardboard? And how can we make the cuts freely?
　　A: You can replace the cardboard template with a dissipative template as long as it can be properly connected to the same power ground as your work surface. Fiberglass is machinable and easy to drill, and may meet your requirements. Materials with extremely strong conductive properties should be avoided because they can cause ESD between the component and the conductive template.

23 Q: When testing film according to MIL-B-81705-B standard, I need a surface resistivity meter and an electrostatic decay time tester. How should I choose?
　　A: In MIL-B-81705C, for Class I shielding materials, the test procedure for electrostatic decay is specified in FTM4046 (decay rate < 2s =); the surface resistance "X" test is carried out in accordance with ASTM D257, and the applicable range is: inner surface impedance: 1×105 ≤ X ≤ 1×1012 / square, outer surface impedance: X < 1×1012 / square. According to the standard, the equipment used for these measurements can be purchased in instrument stores, including high-speed oscilloscopes, nanoammeters, nanovoltmeters, high-precision megohmmeters, high-voltage power supplies, etc. For surface resistance: Models 41262 and 41272 both meet the requirements of ASTM D257; 41272 also meets the requirements of standards EIA-541 and ESD S11.11. For electrostatic decay: We do not have equipment that complies with MIL-B-81705C. Model 42630 happens to be compatible with ESD If the requirements of S3.1 are met, some preliminary attenuation tests can be performed on the film, but they must be based on standard test methods.

24 Q: How does the "zero static" gun work?
　　A: The "zero static" gun works by mechanically compressing (stretching) the crystal material to cause charge separation and induce a high potential (2 kV-5 kV) between two wires connected to either end of the crystal (about 2 mm apart from each other). The crystal has elasticity (memory), so it can be compressed repeatedly to provide piezoelectric energy or electrostatic discharge many times with very little degradation in performance.

25 Q: What material is the surfactant coated on the surface of antistatic packaging materials? How does it work?
　　A: Surfactant is a medium that activates the surface of a substance and is used to increase or reduce surface tension to make the surface substance wet or spread continuously on the surface.

26 Q: We are trying to put together a less expensive ESD floor to improve grounding in areas of the work area where wristbands are not easily used. Is there a coating that can be used on epoxy-painted concrete floors? Mats are inconvenient to use with carts, difficult to clean, and are quite expensive. We will also change the layout of the work area, so the shape and size of the mats will not be suitable.
　　A: Yes, there are some economical ways to solve the problem of protecting your concrete floor. Conductive paint is a one-component floor coating that can dissipate static charges in a controlled manner. For electronics manufacturers, electronic assembly workshops and warehouses, anti-static conductive spray for static control floors is very effective. One gallon of paint can spray 225 square feet, with a thickness of 1-1.5 mils (1 mil = 1/1000 inch). It is recommended to spray two coats, and the electrical grounding protective coating should cover at least 60 square feet.

27. Q: Use an anti-static wristband tester to test the anti-static cup cover instead of using a Fluke 85 megohms ohmmeter, what good is that?
　　A: There are several reasons not to use an ohmmeter. First, the typical ohmmeter has a test voltage of 3V, which will give a completely different reading than a typical anti-static wristband tester (open circuit voltage 9-30 volts (depending on the manufacturer)). A lower test (open circuit) voltage will give a higher resistance reading. Second, the ohmmeter will not automatically indicate when the reading is above or below the set value. Finally, the contact structure of the electrical test board and banana jack combination on the anti-static wrist tester and the ohmmeter leads is often different. To avoid this problem, you can make a special fixture for the ohmmeter, but it will not work as well as a special fixture.

28Q: What material is the L-type three-layer rubber mat made of? Why is its service life a concern? If the wire mesh inside the mat is cut off, will it still work? If it still works, why?
　　A: The L-type material is a three-layer white or blue rubber pad material. In the welding and assembly areas, the soft loss layer plays a good role, and its top and bottom have a long service life and durability. This rubber material has a high resistance, is wear-resistant, corrosion-resistant, and is easy to clean and maintain. L-type material is heat-resistant, and will not produce toxic gases even if it is placed in a high-temperature environment. Welding and flux will not damage its surface. L-type material is also suitable for clean rooms. The conductive layer is not a metal mesh. If you cut the mat, as long as each piece is well grounded, its performance will not change.

29Q: I received some components returned from the field (chips that have been installed on the circuit board and cannot pass the power-on test), and after laboratory analysis, they were determined to be ESD/EOS. What is the probability of this potential failure? What are the signs before it occurs?
　　A: EOS and ESD are somewhat different in physical processes. Furthermore, ESD can be broken down into one-time damage and longer latent damage, depending on the energy of the ESD and the material it acts on. Studies on bipolar devices have shown that 80% of low-quality op amps are damaged by EOS or high-energy ESD, and the other 20% of op amps also show a smaller performance degradation (due to potential ESD damage).

Q30: When using a PC in the field, I noticed that wearing an anti-static wristband while using the machine can discharge static electricity even if the machine is placed on a cardboard box and the plug is not plugged into an outlet. Is this true?
　　A: Yes, connecting you to a large metal plate or other large conductor will evenly distribute the static charge on your body surface and put you at the same potential as the conductor. In some cases, you can do this to keep the power supply away from the work area. We still recommend using a separate ground wire to ground the machine to a common ground point to ensure that there is a correct electrical connection between the machine and the human body, so that all parts that can contact foreign objects that intrude on the work area are at zero volt potential.

Q31: We are working outside an old building with a three-phase four-wire AC power system. The equipment ground is connected to the neutral wire through electrical metal pipe, and the protective conduit is passed through the building. Can we use this conduit as the ESD grounding point? Or do we need to install a separate grounding system for the ESD system?
　　A: Yes, you can use conduit as an ESD ground point. The main purpose of an ESD ground or "common" point ground is to create a "common place" or to eliminate potential differences between conductors and bring all conductors in a critical work area to the same potential as the common ground point. Once the potential differences between conductors are eliminated, the ability of those conductors to generate ESD is minimized. Even if all conductors in the work area are grounded, you are still at risk of ESD. This is because external conductors will still create a certain potential difference with the ESD ground, which will generate ESD to the grounded conductor. This includes a feeder (ungrounded) feeding you (grounded) parts, IC chips coming out of the protective conduit to the grounded base, circuit boards mounted on a conductor rack, etc.

32Q: For floor mats that are only exposed to normal living environments and are covered with dust, how do you clean them? (This does not include having a qualified professional clean and test them)
　　A: The method you should use to clean ESD floor mats depends on your control plan. Assume that you have a fairly strict control plan, the ESD floor mat specifications are 106-109, and 25 people walk back and forth every day. In this case, it is best to dry clean (sweep) the floor mats every night to remove dust (which will increase its insulation and wear), and scrub them at least once a week. You can adjust according to your specific plan. It is best to give the allowable range in the control plan, measure it every day and make corresponding charts, and based on this data you can determine the appropriate period for cleaning the floor mats.
33Q: I am not very clear about the role of work clothes. The staff in my company think that work clothes are redundant if they are wearing bowl covers. This is because the charge on the human body or on the clothes will be discharged to the ground through the wristbands.
　　A: This is a very good question. This is a common misconception. Most clothes are insulating or have insulating properties (depending on the amount of sweat in the clothes, thickness, fabric, number of layers, etc.). The charge will not move on the insulator. It will remain on the insulator and neutralize after a certain period of time (usually a few hours or days), or neutralize with ions in the air (the time can be reduced to less than a few seconds when human intervention). Therefore, your sleeves, belts, etc. may carry a voltage of several thousand volts (the electric field it generates is very strong for the surrounding conductors), which may induce charges on nearby isolated conductors. Therefore, workers need to wear ESD work clothes, shielding the insulating clothing to minimize the static field generated by clothing. It is important to remember to tuck your sleeves into the work clothes.

34Q: I need to transfer circuit board assemblies from a grounded work station to a grounded wave soldering machine, but I don't have the budget for conductive wax or floor mats. The distance between the two is 20 feet. The components must be placed and transported horizontally before soldering. Can I isolate the carts carrying these components so that the operator can transport these parts?
　　A: Do not isolate the carts. If you place the components in a lossy sealed container (such as a tote), then you do not have to ground the cart to a conductive/lossy floor, although it is best to do so. Lossy sealed containers can keep the potential inside at the same level (that is, minimize or eliminate discharge) during transportation. Before opening, the suitcase needs to be placed on a lossy grounding mat to eliminate the potential difference with the ground and bring it to the same potential as the operator wearing an anti-static wristband.

35Q: The performance of the conductive floor in one department of my company is almost over the standard. I asked them how to clean the floor. They said to use a soft cleaner instead of a special cleaner. What I want to ask is, is there a special cleaner?
　　A: Soft cleaners can leave insulating materials, causing the conductive floor to fail to meet the standards. There are special conductive floor cleaners on the market that will not leave residual insulating materials. But for some conductive floors, scrubbing with water is enough.

36Q: Static charge is a common problem in the printing industry. When paper passes through the press, it will be charged. When the same paper is sent back from the press, it has a lot of charge on it, causing the paper to stick to each other. How to solve this problem?
　　A: Whenever an object contacts and separates from the surface of another object, it will cause an imbalance of charge on the surfaces of both objects, thereby generating static charge (static field). Industrial ionization is one way to solve this problem. This is to use an ion beam near the surface of the object (0.5-2 inches) to surround the surface of the object with a large number of positive and negative ions. In essence, this process neutralizes the static charge on only one side of the surface, thus reducing the induced charge. Applying the ion beam to both sides of the paper minimizes static. Generally, the ion beam should be applied to the objects after they have touched and separated, and in areas where static is particularly severe, the ion treatment should be applied first.
42. Q: What is the correct way to make the floor ESD safe? Is it feasible to use only static dissipative wax?
A: Using an anti-static dissipative floor coating (wax is usually insulative) is a good start to make the floor ESD safe. To complete the ESD safe floor system, you must have anti-static shoes or static grounding strips for the feet, drag chains on carts or swivel chairs, ESD warning signs at the entrance (have employees wear anti-static shoes) to the ESD safe area, and optional floor tape to distinguish the ESD floor. You also need to follow the manufacturer's instructions for application and maintenance. Anti-static flooring is concerned with technical details, which can be found in PDF format, including application and maintenance. We recommend at least 60 square feet of floor finish to properly couple to ground (which acts like a large capacitor) to reduce the charge imbalance to zero, and at least three layers of finish to get the correct floor impedance range (109/square) (that is, the thicker the floor, the better its conductivity).

43. Q: I have several workbenches on carpet indoors. Can I install some ESD mats and grounding strips?
A: Assuming the carpet is insulative and not antistatic, then ESD mats and grounding strips should be laid indoors to protect the workbenches, and at least 3 feet should be left outside the work area. In addition, the top of the workbench should also be protected with ESD grounding mats where ESD sensitive devices are stored. Make sure the ESD mat is lossy so that it will not cause static discharge. Depending on how often people walk on the mat, each employee should use grounding strips on both feet and one wrist (or work clothes). Minimum protection (once the floor and workbench mats are laid) is to use anti-static wrists and common grounding points in front of the workbench. (For simplicity and
convenience ).

44. Q: In order to measure or calculate the energy level of the spike discharge between two test electrodes, a high-speed oscilloscope is required. If you want to store 100 mJ or more energy, what is the range of the capacitance value? Is 500kHz fast enough to get accurate readings?
A: The response between two metal electrodes close to the charging device model has a discharge rise time of less than 200 picoseconds. This response is received by the dipole antenna at the input of the high-speed oscilloscope in the form of an electromagnetic pulse. In order to observe it accurately, at least two signal responses or a 10GHz oscilloscope are required.

45. Q: There are many electrostatic discharge products (anti-static wrist monitor model A98200). Does this anti-static wrist monitor need to be calibrated? How can it be calibrated? Is there an instrument to calibrate it? If so, how much does it cost? Can it be calibrated without calibration?
A: It should be calibrated once a year under normal use.

46. Q: In a room without electromagnetic pollution (class 100), electrostatic discharge protection is carried out by laying floor mats. What suggestions are there for this?
A: It is recommended to use blue or white mat material of type L for cleaning the house. The blue material of type L is electrically dissipative and is sold in feet straight and 2 feet wide.

47. Q: The relative humidity of the air in the printing factory is controlled at 50%. Whenever the operators touch the equipment and any metal objects such as door handles, they will feel numbness. Is the use of floor mats alone sufficient to protect against static electricity? How to use anti-static shoes or anti-static wrists?
A: There are two quite possible reasons that operators are subjected to electrostatic discharge to metal conductors. One is the friction charging caused by walking; the other is the friction charging generated by the clean house coating. The accumulation of unbalanced charges on objects generates friction electricity when they come into contact and separate, which in turn causes significant electrostatic discharge phenomena. To reduce the first problem, use electrostatic discharge floor mats with special antistatic properties. Electrostatic discharge floor finishing agent systems such as zinc-free antistatic floor finishing agents. Antistatic floor finishing agent systems have excellent antistatic properties and do not allow friction charging to exceed 100V.
To reduce the second problem, let operators wear cotton clothes instead of synthetic fibers, nylon, silk, wool, etc. To reduce the friction charging of the human body, use clean house coatings. Wearing electrostatic discharge work clothes has many benefits, which can act as an isolation between different materials, and some antistatic materials can cause charges. You can also try using floor coatings composed of different materials, because the coating materials that form the current room are at a higher level (downward or downward) in the triboelectric charging sequence list.
In short, proper human grounding (electrostatic discharge floor with antistatic wrist and foot grounding strips) can effectively eliminate this problem.

48. Q: What is the generally accepted standard for an ESD safe workstation? That is, all objects in the workstation are below 100V or other potential? What standard specifically describes this requirement?
A: ESD safe workstations are based on the sensitivity of the product. For some users, this may be 200V, while for others it may be 20V! You can refer to the relevant data of the component manufacturer.

49. Q: We are developing a prototype with a large video monitor inside. Its glass window surface generates an electrostatic field, which occasionally causes small sparks, which quickly jump to the vicinity of metal parts and also cause sparks on people who come into contact with metal parts. Is there a simple way to solve this problem, such as grounding?
A: The surface of the glass window is charged to a very high potential, most likely due to the collision of high-energy electrons on the inner surface. This high potential will generate a high electric field, causing the discharge to move. Grounding will be very helpful. Covering the outer surface of the glass window with a conductive material and grounding it at the same time can reduce the accumulated net charge to a very low level. The electrostatic discharge system can be tried, which has static dissipation properties on the outer surface of the glass, and its surface should be grounded (the braided conductor can be stripped off 1/4 inch and this end is connected to one side or corner of the glass).

50. Q: How to handle disposable electrostatic discharge grounding strips?
A: The instructions for using disposable anti-static wrist grounding wires are as follows:
1. Wear an anti-static wrist before handling electrostatic sensitive parts.
2. Open the first 12 inches of the grounding strip and peel off the protective layer to the appropriate length so that it can fit the wrist well. At the same time, firmly
stick the exposed part around the wrist.
3. Completely untie the grounding strip, peel off 4 inches of the protective layer from the end, and connect the exposed adhesive surface to the electrical ground.

51. Q: We design and produce switching power supplies, and its internal circuit has EPOM. The product consists of a main PCB (double-sided board), the bottom side is the welding side, covered with a 1 mm thick aluminum shell (only partially shielded, with ventilation slots, etc.), and the back is completely open. What I want to ask is, what kind of electrostatic discharge bag also has a packaging function?
A: The exact details depend on the sensitive equipment you are trying to protect and the procedures you have in place. You can also call up the first level and refer to the MIL-STD-1686 standard, which classifies components that are extremely susceptible to damage caused by electrostatic discharge voltages greater than 0-1999V as ESD sensitive.
For Class I devices, you should use an ESD bag made from an ESD system that strictly complies with the electronic and mechanical performance requirements of MIL-B-81705C, which also has antistatic internal features. In addition to its packaging protection, the ESD bag can be used as ESD protection so that the switching power supply can be used normally.

52. Q: Part of my company's business is to manufacture blank plastic cards for financial and health care purposes. These cards are made into sheets by heating and extrusion, 10 at a time. Each sheet is formed as follows: A PVC core (usually 0.66 mm thick) is placed between two PVC cover sheets. This is then placed between two stainless steel transfer plates. The 10 PVC sheets and transfer plates are sandwiched between two 6.35 mm thick rubber sheets, and then two standard steel plates are placed on the outside. One of the cards was placed on the laminator's rollers, and the other nine cards were heated together for about half an hour. However, when separating each card, I encountered static electricity. The stainless steel conveyor plate was constantly vibrating, and I was shocked by static discharge arcs about a foot away from the cards. After laying anti-static mats in the factory and the operators wearing anti-static gloves, the conveyor plate still vibrated a little. In fact, the shocks they received when they touched the new stainless steel plate were very weak. The workers were shocked about nine times for each card. I think the static electricity is generated by the friction between the PVC sheets due to heat and pressure during lamination. Some of the static charge is transferred from the plastic card to the steel plate, thus conducting the static charge to the operator. I don't know if the static charge can be quickly removed from the surface of the plastic card through the stainless steel conveyor plate. I speculate that the static charge on the card is negative polarity, so can I draw the negative charge from the surface of the card by generating some intermediate charge to the stainless steel conveyor plate and then remove or reduce it?
A: The problem you raise seems to be due to the charge accumulation during the thermoelectrochemical heating and compression process or the friction electricity generated when separating the cards. What you are talking about (the card) looks like a large capacitor. When you produce multiple layers of alternating conductive and insulating (or very high resistance) materials, in fact, the multiple layers of alternating conductive materials have formed a capacitor. Therefore, you are experiencing an "arc" of electrostatic discharge at a distance of one foot. The electric field between the cards must be very large, generally up to 300,000V, and it is likely to reach 600,000V when the atmospheric pressure is 760 mmHg and the temperature is 25 degrees. It is best to ground all the steel plates at the same point, and the operators are also grounded at the same point. This will reduce the huge potential difference between the operators and the steel plates. However, further measurement of the electric field is still required. During the process of separating the cards, use an electric field meter to measure to see if the static charge generated has a grounding effect. If it is indeed the case, it is necessary to further use compressed air ions on the cards to neutralize the charge on the PVC core.

53. Q: I use an air freshener that produces negative ions. Whenever I touch the front (metal part) of it, it will spray a little freshener. How can I solve this problem?
A: The air freshener you use uses high-voltage corona discharge to emit freshener ions. The metal part may be a transmitter or an eccentric plate. If this plate is not grounded, you will get a "shock" or current flowing through your body. High voltage transmitter leads can also give you an electric shock. It is recommended to take the following measures to solve this problem:
· Do not touch the metal parts to check the circuit and make sure that the metal parts are connected to the power ground
· Cover the front of the metal parts with an insulating object (mesh) to keep the human body away from the metal parts (need to check whether its use effect is affected)
· Reposition the freshener can so that it is not easy for the human body to touch it, and put a warning sign.
· Check the polarity of the power ground and try to reverse the positive and negative
poles · Make sure the can is properly grounded
54. Q: The anti-static wrist tester has a common ground, and we have connected the workbench to the ground. When the anti-static wrist is on the common ground, can they work the same as when they are connected to the ground?
A: Yes. In fact, compared with the power ground, it is better to connect to the ground to reduce electronic noise when establishing a common point ground. Depending on several factors of the equipment (electronic), the power ground may be feasible.

55. Q: What materials are the antistatic bags made of for electronic components? Does it work under positive or negative polarity conditions?
A: There are several types of antistatic bags: shielded (embedded metal, attached metal, vapor shield), antistatic and conductive. The most commonly used "static" or ESD bag is a shielding bag, which has all three of the above characteristics. The shielding bag has a layer of metal, usually aluminum (similar to aluminum foil), which has the "shielding" effect of the Faraday cage effect. ESD shielding bags have the following characteristics:
· Mechanical (mechanical parts inside)
· Electronic shielding: (Faraday cage) to minimize the current (voltage leakage) or electrostatic discharge through the membrane. Therefore, it can protect the inside from electrostatic
discharge interference.
· Antistatic: Non-triboelectric charging, or prohibition of static electricity generated by contact and separation, that is, when objects move around in the antistatic bag. When
walking on the floor, touching metal objects, such as door handles, you will feel the impact of static spark discharges (when walking on carpet).
· Conductivity: Allows charge to move or surface charge to move to a lower or higher potential, that is, when you connect the surface to the ground point (zero potential). It is best to use conductive materials with electrostatic discharge properties (105-1010). Materials with excellent insulation properties exceeding 1011 and any materials with poor conductivity should not be used.
　　Typical shielding bags are composed of the following parts:
· Polyester (0.0127 mm thick)
· Aluminum deposited on polyester material (10-25 angstroms, 1 angstrom = 10-10 meters)
· Polyethylene (0.0635 mm)
56. Q: In winter, electrostatic discharge occurred 17-20 feet away from the computer and caused it to restart. Have you heard about it? (Is there any relevant data to prove that electrostatic discharge at a distance will damage the relevant equipment?) How to solve this problem?
Answer: A. People who are active often carry a lot of static electricity, which can be solved by laying anti-static discharge floors and wearing anti-static shoes.
　　　　B. Conductive surfaces can provide a path for rapid high-energy discharge. Static loss mats and spray coatings can be laid on each conductive surface and
　　　　　 grounded.
　　　　C. Dry environment will further increase the risk of electrostatic discharge. The humidity can be controlled for the main parts or air ion beams can be used.

　　Product solution: Use anti-static floor finishing agents with better static loss and effect, or apply coating protection to all areas susceptible to interference. It is recommended to use zinc-free floor finishing agents with good anti-static properties and use them with anti-static shoes (such as anti-static shoes with model 24802 for electrostatic discharge systems). The use of these two products will greatly reduce friction charges, which may be one of the more important factors in the process of electrostatic shock. Apply typical anti-static materials to all non-lossy surfaces, replace all conductive surfaces with lossy materials (104), or cover conductive surfaces with lossy materials, that is, use model T2 or model L floor mat materials. Make sure all conductive surfaces and operators (anti-static shoes, anti-static wrists, etc.) are grounded. Use materials with higher resistance on conductive surfaces, so that the energy transferred during electrostatic discharge can be minimized and electromagnetic interference can be reduced or eliminated.

57. Q: During chip processing, the circuit chip
A: First, it is necessary to minimize the exposure time of the semiconductor material to a very high energy electron pulse. The chip of the highly integrated microcircuit is sensitive to high energy pulses just like other packaged chips. The metal pad can be smaller and less easy to touch. But if a charged conductor discharges one of the micro pads and a current is transmitted or there is a potential difference between the two points, the nearby semiconductor material may be damaged. We know that the logic and sensitive semiconductor materials (for a certain device) that apply electrostatic discharge, when combined, may cause a reduction in product quantity or quality, thereby reducing the efficiency of the production line.

58. Q: What are the differences between different types of ESD floor mats (i.e. rubber, vinyl, 3-layer board, polypropylene fiber, laminated insulation board, etc.)? In what environments can they be used and how much do they cost? A: There are several factors to consider
when choosing the mat material:
First, the application must be determined. Is there a wire bond on the mat? Will it be used in a harsh chemical environment? Drop strength, is it for flooring or work surface? What are the internal parameters of the mat resistance? Once you have these factors in mind, the rest of the process will be simple. How many workstations do you plan to mat? Or how many square feet do you want to mat? Is it permanent or temporary? Mica and fiberglass reinforced plastics can be used as permanent coverings or as car mat materials. Rubber mats have good heat and chemical resistance, and vinyl is more effective. Do you need fatigue-resistant materials or not? Do you need floor mats that work well with electronic components? Or do you want floor mats that can be divided as needed? Finally, what color do you want? If you know these questions, you will make the right choice.

59. Q: If you have a large truck with a printed circuit board rack and it is placed in a garage without static discharge. But you still have to wear an anti-static wrist and connect it to the banana plug with a pliers. Are the following two points feasible? Or neither of them will work?
　　A. Connect the cable of the anti-static wrist to the same truck shelf as the computer assembly rack, making sure that you are at the same electrical potential as the truck shelf before working.
　　B. Connect the pliers to a grounding box at the end of the anti-static wrist cable, then place your hands on the shelf, computer assembly rack to obtain equipotential, and start checking the assembly rack.
2. Is it bad to use pliers? If so, what are the harms?
Answer: When working on a circuit assembly bench, it is beneficial to connect the anti-static wrist to a rack similar to the car shell. However, it must be done strictly in accordance with the following requirements: the assembly bench must be conductive (<1011), the rack must also be conductive and connected to the entire car body, and the connection point of the anti-static wrist to the car body is connected to this circuit. Due to contact impedance, grounding integrity, friction charging and electrostatic induction problems, it is not advisable to ground the car body through your hands. The non-conductive parts of the car body may also have an electric field (wheels, car box, etc.), so if you let go just before handling the ESD sensitive part, it is possible to cause an imbalance in the charge distribution on the opposite side of the conductive part of the car body.
There is no harm in connecting the pliers on the anti-static wrist to the common ground point (CPG). Before use, you should check the quality of the connection both mechanically and electronically to ensure that there is a good electrical bond between the pliers and the common ground point, and that the breaking force (mechanical integrity) is greater than or equal to 1 pound and less than the full 5-pound pull force.

60. Q: Is it correct to connect the ESD floor system to the house lighting protection or grounding system?
A: We strongly recommend that the ESD grounding system be connected to the house lighting protection grounding system. Typically, the lighting grounding system has a direct and separate path to ground without sudden conductor turns. When the lighting is interrupted, there will be a large current pulse in this path. If the ESD grounding system is also in this path, it is important to isolate the lighting ground circuit from the house/earth grounding circuit. They can be connected to the 8-foot grounding pole outside the house at the same point, but the paths should be independent of each other. It should be checked by a professional electrician and the electrical code should be maintained.
61. Q: We have a class 10,000 clean tent, inside which we plan to build a device to explore space. The device is designed based on optical principles and is very sensitive to particles and outgassing. There are also a lot of electrons in the tent, and I want to know what is the best material to use for the clean room and static loss requirements?

A: The best material for cleanrooms is Type L floor mat material. Each roll is 0.60 cm wide and 122 m long. Wristbands should be used to connect floor mats at the request of the cleanroom. These materials can be ordered from the relevant manufacturers.

62. Q: We have discovered the effect of magnetic therapy. At work, we are in an environment of electrostatic discharge. Work clothes, shoes and socks, wristbands, chairs, ion generators, almost everything is static. We want to know if wearing these shoes with magnetic insoles will damage the finished products. We have installed electronic sensor elements in the air chamber. Magnetic therapy works for so many people that we don’t want to give it up. Any information will be greatly valued. Is it right?

A: The insoles with magnetic therapy you are talking about will not interfere with the electrostatic discharge process. The only exception is: if the magnetic field at the electrode tip is very strong and your foot moves very slowly over a low impedance conductor, then you will generate a current. If the energy cannot be discharged, it will cause failure. Over time, the discharge will accumulate to a very high level.

63. Q: After going through the MIL-883 model, we found a different situation where the shielding component can be damaged by electrostatic discharge when it is removed from the shielding bag. I am also looking for books on this subject, that is, the leads of the components in the electrostatic discharge bag need to be shortened when they are working in a protected state. How to explain this phenomenon?

A: When the shielding component is removed from the shielding bag, it may be damaged. The static electricity generated can charge an object (like the component in the shielding bag), which can cause electrostatic discharge problems during movement. You need to specify the antistatic characteristic parameters when ordering shielding bags. Antistatic refers to the triboelectric charging behavior exhibited by the material.

64. Q: I am trying to find a company that can comply with the MIL-W-80 standard, which is a 1966 MIL standard related to static dissipative acrylic fibers or acrylic coatings. Please explain in detail.

A: MIL-W-80 refers to solid materials in sheet or tape form and cannot be applied to floor coatings. The floor coating also complies with several industry recognized standards (ANSI/ESD-S7.1 floor coating surface impedance standard, AATCC TM-134-1979 is the standard for static electricity generated by floor coatings, FTMS 101C, M-4046 is the charge decay standard for floor coatings).

65. Q: My company produces computers, and one of its reliability tests is electrostatic discharge. The standard used in the test is IEC-810-4 L3, air: 8kV, lap voltage: 6kV. But we want to sell to a purchasing unit in California, USA. How many kilovolts is the electrostatic discharge voltage used for testing?

A: There are three test methods used to test ESD sensitivity:
　1. Human body model (+4000V and -4000V voltage waveform, 0 and 500 loads)
　2. Charged component model (voltage waveform up to 2000V)
　3. Mechanical model (+400V and -400V voltage waveform, 0 and 500 loads)
　　This includes the voltage limit, note that there will be different waveform requirements depending on the load impedance. In Marlboro, Massachusetts, and Rome, New York, the ESD Association, these standards are valid for ESD systems.

66. Q: I have a dry powder fire extinguisher. When the powder is pushed upward by a spiral conveyor, it is injected into a stainless steel tube with a diameter of 15.2 cm and a length of 91 cm through a metal detector. When the firefighter touches the lid of the fire extinguisher, an ESD phenomenon occurs. We try to connect all metal parts to the same single point grounding point and place an antistatic mat on the floor. Any good suggestions for firefighters wearing all-cotton clothing so they can move around without wrist guards?

A: As the dry powder passes through the stainless steel tube and fills the extinguisher, the dry powder and the extinguisher are generating friction electricity. The operator is most likely to feel the static discharge due to this process. You can try to connect all metal parts to a single point ground (CPG) and use anti-static floor mats. You have taken some important ESD control procedures, but it still has not solved your problem. You need to check the following points:
　1. The fire extinguisher cover should be conductive and connected to the same single point ground.
　2. The fire extinguisher body should be conductive and also connected to the same single point ground.
　3. If neither of the above is conductive, then ionization should be considered.
　4. Anti-static floor mats can be conductive or dissipative and should be connected to the same single point ground.
　5. All operators should wear a pair of 2M anti-static shoes and connect to a conductive ground mat so that they will not become the source of excitation for ESD events at the fire extinguisher cover.
　　As long as the above steps are followed correctly, your problem should be under control.

67. Q: Recently, I was checking the ion beam using two metal plates with different charges (one small and one large and regular), i.e., a charge plate analyzer. I took readings over the same area and beam spacing and found that the smaller one was about 6V while the larger one was -8V. Which value should I take when trying to keep the beam spacing the same? When the readings for the plates show +/-X volts, does this represent X% of the total voltage or is it related to the charge being detected? (e.g., 1% of 1500V or 1% of 8V?)

A: First, check the two sections of the charge plate analyzer separately to calibrate them to ensure their accuracy. Second, check the product specifications for measurement accuracy. Third, the charge plate areas can be different. The ion beam has several sources along its axial length. Make sure you are at the same distance and covering the same area. If the source is DC and changes polarity every inch, your measurements will be affected by the different positions of the charge plates. The accuracy should be related to the voltage reading measured by the charge plate. Check the product data sheet to confirm that it applies to your unit circuit.

68. Q: Most of the floors in the factory production hall are covered with a layer of static dissipative tiles, but there are some corners that are not covered. The floor is made of concrete. I wonder if this method is dissipative enough? Would applying a conductive coating such as conductive wax to the floor work?

A: Concrete alone will not form a complete static discharge floor. Even in the floor, groundwater can seep through the seepage holes and make the concrete conductive, but there will always be some inconsistent wet and dry areas. Obviously, conductive wax is useful. We are better to think of conductive wax as a coating for the floor because there is no wax in the coating. Wax is insulating. Before applying wax to the floor, you should cover the concrete floor tightly to avoid water-induced poor adhesion problems. This will also increase the coverage of the floor coating (by reducing the porosity of the floor). Other solutions include: coating the concrete floor with a conductive paint; using conductive tiles or floor mats; laying conductive carpets or marking off an area as a non-static discharge safe zone.

69. Q: I am very interested in anti-static shoes, but I don't know what the ESD floor must have or the instructions. If I want to test the ESD plasticity of the floor, what measures must be taken? Please give some suggestions. In addition, what is the specific implementation plan for the floor to meet the requirements?

A: According to the standard EIA-625, the resistance of the ESD floor should be greater than 105 and less than 109. The Electrostatic Discharge Association has a trial standard ESD DSTM54.1-1997, which combines floor materials and footwear items together when measuring resistance. This is actually a measurement method that does not stipulate or recommend working in the resistance range. In fact, in order to test the floor, the standard ANSI/ESD-S7.1-1994 has a clear test method, which is to drive a voltage of 10V or 100V with two five-pound probes. The instrument used is model 41273, which is specifically used for this measurement.

70. Q: Will the reliability of CMOS integrated chips be degraded after being subjected to severe electrostatic discharge shock?

A: If the discharge path includes one of the CMOS device input (pin solderable) terminals, then the severe electrostatic discharge will cause instability in the CMOS unit circuit. This will shorten the life of the related components and cause intermittent failures. You can cut open the package of the faulty component and put it under a high-power microscope or scanning electron microscope, and you can see the damaged area.

71. Q: What is wireless electrostatic discharge protection?

A: The only applicable measure is to force the air corona discharge and ionization, which can be widely used as an "air ionizer". There are several models of electrostatic discharge systems to choose from. The only thing that is eliminated is the discharge time, which depends on the charge discharge rate, air circulation speed and proximity. It is close to 2 seconds at most. Protection measures are urgently needed during product production, but the second protection measure is also feasible when using grounded wristbands and electrostatic discharge floor mats.

72. Q: How to judge the failure of anti-static shielding bags, for example, there are creases or small holes in the bag.

A: All your answers can be found in the MIL-B-81705C standard document. In particular, an antistatic shielding bag (Type III) can be considered invalid if it meets any of the following conditions:
　* The antistatic bag delaminates as detailed in Section 4.8.4.1 of the MIL-B-81705C standard.
　* The electrostatic discharge decay rate is greater than 2 seconds.
　* The electromagnetic interference is attenuated by less than 10dB.
　* The diameter of the holes is greater than 2mm.
　* It cannot shield electrostatic discharge events below 30kV.
　* The surface resistance per square area inside and outside is greater than 1012, and the surface resistance per square area inside is less than 105.
　　In short, due to the Faraday effect, if there are creases or holes in the shielding bag, it still has a shielding effect. However, it is safer to throw away these antistatic bags, which may cost more testing fees, and just replace them after they are qualified.

73. Q:
　　1. What is the nominal humidity level in a Class 100 clean room used to produce hard drives?
　　2. Pomalux is the only plastic material I know of that is the best in terms of ESD safety, but it is quite expensive. Are there other similar materials?
　　3. Are there any universal ionizers? If so, what manufacturers make them? How can I verify that their results are correct?
　　4. Why do the readings of the static sensor vary greatly? (That is, there is no repeatability or large jumps.)

Answer:
　　1: Good nominal humidity is between 40% and 55%. You need to minimize corrosion while maximizing ESD protection. There are papers that can be tested for humidity that you can use at your convenience.
　　2: Other materials with good static safety include vinyl, rubber, cardboard, and polyethylene, depending on your application. ESD safety also means antistatic properties, loss properties, and shielding properties, which also depends on your application.
　　3 and 4: The charge plate analyzer is the preferred instrument for monitoring corona discharge type air carrier ionizers. Also, the static sensor measures a small area at a time, and the static charge does not have to be balanced on the conductive or insulating surface. If a material is subjected to an electric field or triboelectric excitation, an imbalance of free electrons occurs. The balance is not always the same, and the distance of the electrometer sensor from the surface being measured can further change the balance. The geometry of the surface being measured can also cause changes in field strength, for example, curved surfaces (cylindrical pyramids) have stronger fields than flat surfaces because charge always accumulates at the tip of the object.

74. Q:
　　1. Do conductive finger cots reduce loading? What is the benefit of using conductive finger cots over lossy gloves?
　　2. We have many 3M 961 ionizers, are these good enough for zero-level stations?
A:
　　1: The benefit of conductive finger cots depends on the following questions: What are the specifications of the conductive finger cots? What problems are you dealing with? What is the operator most comfortable with? Are you going to work in a clean room or not? In summary, a disposable glove will also reduce the chance of electrostatic discharge that may occur with a conductive finger glove (<104=), but there is no difference when using a disposable glove. Gloves cover a larger area of the hand than finger gloves, and if you need to wear gloves in more than 12 occasions, then wearing gloves will be more effective.
　　2: For a zero-level station, the tolerance for any particle cannot exceed 2.5 microns anyway. The only way is to get a zero-level clean room with an ionizer to ensure that the only component in the clean room is a corona discharge emitter. You may know of some systems that have corona discharge emitters, I am not familiar with the 961 equipment. If it is an overhead or other "blower" type equipment, then it will not meet your standards. For class 100, our overhead lines are good, once you start to prepare,...

75. Q: Let's assume that our company's ESD measures are very good, except for one thing: our company's employees like to put pink antistatic foam under the computer motherboard. They think that this will prevent the motherboard from sliding around on the ESD workstation. Because they believe that this will extend the life of the workstation surface. However, I think this is unlikely because the foam is insulating. On the other hand, because of the measurement, I brought an electrometer, and things placed on this foam (placed on the work surface) will consume charge. So, is this situation acceptable?

A: If the pink foam is not dissipative and is electrically connected to ground, then you are ready (usually you can use it). In fact, it depends on the ESD sensitivity of the product you are trying to protect. In order to measure the surface to the foam ground To measure surface resistance, you need to use a megohmmeter. Measuring at both 10V and 100V, the surface resistance should be between 105 and 109, which is safe.
　　You said you used an electrostatic field meter to measure, which is good, but it will not completely guarantee that the foam is safe. One of the characteristics of the electrostatic meter you tested was the discharge time. The most important thing is that the charge on the work surface should be eliminated in less than 100ms. If it takes several seconds to clear all the charge, this will extend the charge discharge time and may expose ESD sensitive components to ESD attack.
　　There are other materials available that can be used to place your work on an ESD protected workstation without sliding properties. Model T2 floor mat material is the best choice in this case.
Here are some questions to ask:
　a. How sensitive are the components I will be protecting?
　b. Is the foam conductive at safe loss levels?
　c. Can the charge in the foam be discharged in about 100ms?
　d. Does the foam need to be grounded (if it is conductive)?
　e. Do operators need to wear ESD wristbands?
　　If you agree with all of the above questions, then you are already using some safe ESD prevention measures.

76. Q: My company is trialing ESD shoes, wristbands, and floor mats on workbenches. If the operator is soldering components and other parts that can cause mess or heat, is there any other material that can be placed on the floor mat without compromising the ESD performance? When space is limited, does the operator not have to move the floor mat?
A: Yes, your best option is a fiberglass solder filler (FRP), which is a high heat resistant solder ESD floor mat. This can be checked off your standard rubber or vinyl ESD workbench floor mat, or you can order a larger fiberglass solder filler, which is a workbench floor mat with a grounding cord. Rubber mats also have good heat resistance characteristics (although not as good as FRP). Model T2 or Model L workbench floor mats are recommended.

77. Q: If we are using wrist guards and floor mats, do we still need to wear heel grounders? If using heel grounders, do we still need to use floor coatings?
A: If operators are walking around and close to sensitive components, then heel grounders must be used. It will keep you safe when handling ESD sensitive components. If you are transferring ESD sensitive products or components between workstations, the cart and the person should be grounded through the heel grounder or shoes (person), and the trailing cord (cart). And connected to a properly grounded floor. It is a good idea to use grounded floor mats or antistatic floor coatings near ESD sensitive areas because these materials have excellent antistatic properties that limit triboelectric charging. If workers must move around frequently, then in order to take full advantage of safe ESD solutions, an antistatic floor coating should be placed on the floor and connected to ESD shoes.

78. Q: I work in computer networks and we know that metals in contact with each other cause oxidation at the joints, which can cause problems in the system. We are trying to take the best approach to eliminate it. Some technicians recommend wiping the joints clean with an eraser, while others claim that this will cause ESD events. Is this possible?
A: Yes, erasers can triboelectrically charge metal contacts and therefore have the potential to cause ESD, although this is a common method of cleaning joints in the workshop. However, the risk of ESD can be minimized by grounding the joints when wiping them, or using a conductive rubber and wearing a wristband to ground them when you work with your bare hands.
　　First, you need to evaluate each circuit board by the most sensitive component on the board, so that you can determine its sensitivity level (ESD sensitivity). This will tell you if the board is sensitive to ESD. If not, you may need to take some simple precautions to avoid potential ESD hazards.
　　The best way to test ESD sensitivity is to find an old board, use an eraser to clean the solder joints, and then use an electrometer to measure the electric field (measure voltage) generated during the scrubbing process. The first step is to zero the electrometer, then take two readings before and after. If the value you read is over 50V, then it is recommended that you use this method to clean the joints.
　　Note that not all boards are like this. Some boards may have embedded components that are very sensitive to ESD (level 0), or are made of different materials (so each material has different triboelectric priority), etc. Erasers also have different triboelectric charging order/antistatic properties, so each board should be treated as a separate module.

79. Q: Can we connect the ground of the ESD to the ground of the power line?
A: Yes, the ground of the power supply is sufficient. The power ground terminal can be the building ground, the power ground, or the earth, as long as the potential between the power ground terminal and the workstation ground is equal (i.e. the potential difference is 0V). The best ground is the earth at this time, but any ground will not be disturbed by electrostatic discharge. The power ground transmits electromagnetic interference noise, which may cause a series of problems, mainly depending on the type of load on the line.

80. Q: Our production personnel's protection measures include wearing electrostatic discharge work clothes. If the work clothes are not buttoned in the front, we have always had differences on the shielding effectiveness. Is it necessary to button the work clothes? In other words, if the work clothes are not buttoned, will it have a shielding function?
A: It is good to tie the work clothes, and the advantage is to protect the non-conductive and non-grounded clothing outside. Even if the work clothes are not tied, it still has some protection because the cuffs, back and sides of the work clothes cover the clothes you usually wear, but the front is not covered. The front part of the body is the part that most sensitive products are easy to approach/contact.

ses2007

Most of them are related to production. It would be nice if there are some related to static electricity in the system.