SMT factory ESD specification! Understand the anti-static and grounding management requirements and inspection items of the SMT workshop in one articl
Oct 16, 2021
Basic requirements for anti-static in PCBA electronics factories:
1. The basic principles of electrostatic protection
a. Inhibit the accumulation of static charge; b. Eliminate the static charge that has been generated quickly, safely and effectively.
2. Anti-static work area
1. Floor materials
a. It is forbidden to directly use wooden floors or to lay wool, hemp, chemical fiber carpets and ordinary floor leather.
b. The ground composed of electrostatic conductor materials, such as anti-static raised floor or laying anti-static floor mats on ordinary ground, should be selected and grounded effectively.
c. It is allowed to use specially treated terrazzo ground, such as laying ground wire net, carburizing or spraying antistatic agent on the ground.
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2. Grounding
a. The anti-static system must have an independent and reliable grounding device, the grounding resistance should generally be less than 10Ω, and the burying and testing methods should meet the requirements of GBJ97.
b. The anti-static ground wire must not be connected to the neutral wire of the power supply or shared with the lightning protection ground wire.
c. The three-phase five-wire system is used for power supply, and the ground wire can be used as an anti-static ground wire (but the neutral wire and the ground wire must not be mixed).
d. The cross-sectional area of the grounding trunk line should be no less than 100mm2; the cross-sectional area of the branch trunk line should be no less than 6mm2; the grounding wire of the equipment and workbench should be a multi-strand plastic coated wire with a cross-sectional area of not less than 1.25mm2, and the color of the grounding wire should be yellow-green. Appropriate.
e. The connection method of the grounding trunk line should be brazed. f. The connecting terminals of the anti-static equipment should ensure reliable contact, easy to assemble and disassemble, and allow the use of various clip-type connectors, such as snap fish clips, plug sockets, etc.
3. Ceiling material
The ceiling material should be made of antistatic materials. Generally, gypsum board products are allowed, and ordinary plastic products are prohibited.
4. Wall fabrics
Antistatic wallpaper should be used for wall fabrics. In general, plaster paint or lime paint is allowed for walls, and ordinary wallpaper and plastic wallpaper are forbidden.
5. Humidity control
a. The relative humidity of the anti-static work area should not be lower than 50%.
b. On the premise of not causing harmful effects on the product, it is allowed to use humidifying equipment to spray formulations or water to increase the environmental humidity.
c. The humidity of the computer room should comply with the relevant regulations in GB2887, and similar computer rooms should also comply with this regulation.
6. Regional boundaries
The anti-static work area should be marked with regional boundaries, and warning signs should be hung in obvious places. The warning signs should comply with GJB1649 regulations. The entrance of the work area should be equipped with ionized air wind bath equipment.
7. It is forbidden to use and touch the charge source that is easy to generate static charge in the anti-static work area (see the table below). Paint or dipping surface, ordinary plastic veneer, ordinary vinyl and resin surface floor plastic and ordinary floor leather, polished waxed wood floor. Ordinary vinyl work clothes, caps, shoes Ordinary polyester, synthetic fibers and nylon fabrics, plastic and ordinary rubber-soled shoes Operating tools and equipment Ordinary plastic boxes, racks, bottles, tray supplies and paper products, ordinary foam and general mobile tools, compressors , Spray equipment, evaporation equipment, etc. 3. Anti-static facilities for electronic products
1. Electrostatic safety workbench
a. The electrostatic safety workbench is the basic part of the anti-static work area. It consists of a workbench, an anti-static table mat, a wrist strap connector and a ground wire.
b. There should be no less than two wrist strap joints on the anti-static table mat, one for operators and the other for technicians, inspectors or other personnel.
c. When necessary, an ion wind static eliminator should be equipped on the static safety workbench.
d. It is not allowed to stack plastic boxes (sheets), rubber, cardboard, glass, etc., which are prone to static electricity, on the electrostatic safety workbench. Drawings and materials should be placed in an anti-static document bag.
2. The anti-static wrist strap should wear an anti-static wrist strap directly in contact with electrostatic sensitive devices. The wrist strap should be in good contact with the human skin. The wrist strap must be non-irritating or allergic to the human body. The resistance value of the wrist strap system to the ground Should be in the range of 106～108Ω.
3. Anti-static container During the development and production process of electronic equipment, all storage and turnover SSD containers (component bags, transfer boxes, printed board racks, component storage boxes, etc.) should have electrostatic protection performance. Metal and ordinary plastic containers are not allowed. When necessary, the turnover box for storing components should be grounded.
4. Ion wind static eliminator should use ion wind static eliminator to eliminate static charge on the surface of insulating material.
5. Anti-static work clothes
a. People who enter the anti-static work area or come into contact with the SSD should wear anti-static work clothes, and the anti-static work clothes fabrics should meet the requirements of GB12014.
b. In an environment where the relative humidity is greater than 50%, pure cotton products are allowed for anti-static work clothes.
6. Anti-static work shoes People who enter the anti-static work area or come into contact with the SSD should wear anti-static work shoes, and the anti-static work shoes should comply with the relevant regulations of GB4385. Under normal circumstances, ordinary shoes are allowed, but conductive shoe bundles or heel straps should be used at the same time.
7. Anti-static transport vehicles should use anti-static transport vehicles when transporting SSDs or parts and components containing SSDs.
Electrostatic protection operation specification:
1. Purpose
Clarify the anti-static operation specifications of each department on equipment, documents, materials, etc.
2 Scope of application
This operating specification applies to all departments.
3 Specific operation requirements:
3.1 Design department
3.1.1 When product designers develop products, they should formulate an ESD control outline plan to ensure that the products developed are protected from ESD.
3.1.2 When researching the design plan, it is necessary to analyze the possible electrostatic discharge failure or performance degradation. Therefore, it is necessary to remind the designer to take protective measures on the circuit, and the electrostatic sensitive device symbol ф must be marked on the design drawing. Implement key anti-static control.
3.2 Purchasing department
3.2.1 For externally purchased electrostatic sensitive devices, suppliers are required to provide antistatic protection and labels for electrostatic discharge sensitive devices in accordance with electrostatic discharge control requirements.
3.3 Quality Control Department
3.3.1 When the external inspection team inspects the delivered electrostatic sensitive devices according to the order list, to see if there are protective packaging and product marks, and products that do not meet the requirements should not be accepted.
3.3.2 The inspection of electrostatic sensitive devices should be carried out under the condition of electrostatic discharge protection (the requirements are the same as those in the "Procedures for Anti-static Operation of Components").
3.3.3 QE is responsible for the supervision of anti-static spot inspection, and report to the supervisor of each department for timely resolution of the unqualified process. If the loss caused by improper solution is solved, each department is responsible and punished; and is responsible for the production of anti-static blackboard materials publishing.
3.3.4 An anti-static training plan shall be formulated, which shall be requested by the Quality Management Department, and shall be organized and implemented jointly with the education department and the process department to ensure that all personnel handling electrostatic sensitive device products have the necessary training and reach the level of proficiency.
3.4 Personnel Department
3.4.1 Organize employees to conduct training and assessment of anti-static knowledge.
3.5 Health Management Department
3.5.1 Responsible for the provision and training of anti-static materials.
3.6 Equipment Department
3.6.1 Check the grounding wire once a week to ensure that there is no oxidation at each interface end and the grounding is good; and make a record of the spot inspection.
3.6.2 The basic production equipment such as assembly line, wave soldering, aging line, etc. shall be inspected once a week to ensure that there is no leakage of the equipment and the grounding is good; and the inspection records shall be made.
3.7 PCBA workshop
3.7.1 The operation of all electrostatic sensitive components must be carried out on an electrostatic safety workbench, and electrostatic sensitive components without anti-static packaging shall not enter the production site. For components without anti-static packaging, they should be eliminated before entering the static work site.
3.7.2 All tools, fixtures, equipment and instruments that do not have anti-static function that must be used at the operation site should be placed on the anti-static table (table) pad.
3.7.3 Outsiders who enter the site without anti-static measures are not allowed to touch the components.
3.7.4 Various equipment at the production site must take anti-static measures.
3.7.5 When manual soldering, use an anti-static low-voltage soldering iron.
3.7.6 When cleaning sensitive devices after soldering, plastic brushes cannot be used, and anti-static brushes must be used for cleaning.
3.7.7 After employees wear an anti-static wrist strap, the wrist strap should have good contact with the skin; the resistance value of the wrist strap system, anti-static table mat, and anti-static floor mat to ground should be 1MΩ±10%. When working every day, the technician or line leader in this workshop should use a qualified multimeter to measure, and make a record of the spot inspection.
3.7.8 The enclosures of all powered tools, instruments, meters, etc. used must be well grounded. When going to work every Monday, the workshop technician or line leader shall use a qualified multimeter to measure and make a record of the inspection.
3.7.9 Electrical assemblers who have not received anti-static training shall not carry out assembly work containing electrostatic discharge sensitive devices; for employees wearing synthetic chemical fiber clothes, do not contact their clothes with electrostatic sensitive devices.
3.7.10 ESD sensitive devices are not allowed to be stacked together, and the devices cannot touch each other. The circuit boards that have been installed with sensitive devices cannot be overlapped and should be placed in an anti-static container separately.
3.7.11 When testing and repairing, when there is no electrostatic wristband, please touch the metal ground wire (board) with your hands to discharge static electricity on your body. It is strictly forbidden for people who do not take anti-static measures to touch electrostatic sensitive devices. Or circuit board.
3.8 Warehouse
3.8.1 When transporting components with pins, conductive foam materials are usually used. This can prevent a high potential difference between the component pins. For dual-in-line packaged components, static dissipative tubes are often used during bulk transportation. For circuit board components, when they are located outside the electrostatic discharge protection area, they should be transported in an electrostatic shielding bag or conductive carrying case. Some packaging bags are made of conductive materials, which can ensure that all components are at the same potential under stable conditions, and at the same time dissipate the electrostatic charge that accidentally runs on the bag. This method cannot be used for circuit boards with batteries. In this case, a packaging bag with a static dissipative material as the lining and a conductive material as the outer layer should be used. The price of this bag is higher, but it can provide excellent protection for both powered and unpowered components.
Similarly, a conductive box with a guide rail for fixing the circuit board cannot be used with a powered circuit board with exposed connectors on the edge.
Management of anti-static facilities at the production site:
1. Static safety workbench: It is composed of workbench, anti-static table mat, wrist strap connector and grounding wire.
2. There should be more than two wrist strap joints on the anti-static table mat, one for operators and one for technicians and inspectors.
3. It is not allowed to stack plastic boxes, rubber, cardboard, glass and other debris that are prone to static electricity on the electrostatic safety workbench. The drawings and materials should be placed in an anti-static document bag.
4. Anti-static wrist strap: People who directly touch electrostatic sensitive devices must wear an anti-static wrist strap. The wrist strap should be in good contact with the human skin, and the resistance of the wrist strap system to the ground should be 1MΩ.
5. Anti-static containers: The component holding bags, turnover boxes, PCB loading and unloading racks in the production site should be equipped with electrostatic protection. Metal and ordinary containers are not allowed, and all containers must be grounded.
6. Anti-static work clothes: People who enter the static work area and those who come into contact with SMD components must wear anti-static work clothes, especially in dry environments with a relative humidity of less than 50% (such as winter). Work clothes fabrics should meet the relevant national standards.
7. Personnel entering the work area must wear anti-static work shoes. Persons wearing ordinary shoes should use conductive shoe harnesses, anti-static shoe covers or heel straps.
8. The conveyor belt and drive shaft used in the production line should be equipped with anti-static grounding brushes and poles. 10. Ion wind static eliminator can be used on the surface of the conveyor belt.
10. The assembly jigs, testing jigs, welding tools, various instruments, etc. used in the production site should be equipped with good grounding wires.
11. An anti-static test bench should be installed at the entrance of the production site. Everyone who enters the production site should undergo an anti-static test. Only after passing the test can they enter the site.
ESD
Q&A knowledge
1. Question: Why do some ESD ground wires have impedance and some do not?
Answer: The purpose of the ESD ground wire is to connect a conductive surface to a place with the same potential as the power ground. The "hard ground" is directly connected to the ground with a ground wire without additional resistance; between the power ground and the common point contact The resistance is basically 0Ω. "Soft ground" is a ground wire with internal series resistance, a typical value of 1M. The purpose of this design is to limit the damage current that may be generated when the operator is exposed to an environment of 110V and 250V. ESD Federation ANEOS/ESDS6.1-1991 recommends using a "hard ground" method to ground ESD countertops or floor mats.
2. Question: I often wear an anti-static shoe, but the defendant is often told to wear both feet. Why?
Answer: Anti-static shoes only work when they are worn correctly and are connected to conductive floors or consumables. Walking is an excellent example of generating electricity by friction. If you use anti-static shoes correctly and connect them tightly to the ESD floor, the charge on your body will leak to the ground. Therefore, the network formed between you and the ground is the same in voltage, but as soon as you lift your feet with anti-static shoes, you will be charged again, either from your clothes or because of friction and lifting your feet. Generate triboelectricity. 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 value is 2000-5000V), because you are in the grounded state for a longer time, so it is recommended to When approaching moving objects, be sure to wear a pair of anti-static shoes.
3. Question: Do I need to connect a 1M resistor between the machine and the ground?
Answer: No need. According to the manufacturer's grounding requirements for machines or equipment, 1M resistors are used to protect the human body. Refer to the following questions. Side note: It is good to ground all isolated conductors close to ESD sensitive workstations. Can reduce accidental electric field or charge accumulation to the smallest amount.
4. Q: What is the role of 1M resistor in the semiconductor assembly process?
Answer: Hypothesis 1: We are talking about ESD control issues; Hypothesis 2: The human body is in contact with semiconductors and devices with semiconductors. 1M series resistance can be found in places such as anti-static wrists, anti-static shoes, zippers, and ground wires. Its function It is to limit the amount of current that can pass through the human body to protect the safety of the human body. The main limitation requirement of 1M resistor is: when the effective value of 250V AC, the current is limited to 250 microamperes, which is exactly the perception level of most people (the critical value for the reflection of the nervous system). The difference in the physical perception of electric current in the rest of the body and in the body depends on the size, weight, moisture, and skin condition of the human body.
5. Q: Does the shipping box containing circuit boards need to be covered? How do these shipping boxes work? Answer: Generally, the cover needs to be closed. The cover correctly installed on the shipping box can provide strong enough shielding for the circuit board inside. These shipping boxes can not only provide mechanical integrity under normal use, but also provide ES for the inside.
D security. If the cover is removed, any stray electric field can cause the charging of many individual conductors on the circuit board, and these induced charges are the source of ESD. The potential sources of stray electric fields are many: people, clothing, ungrounded motorcycles, displays, furniture, compartments, any ungrounded conductor, any insulator, electromagnetic interference, etc. The circuit board in the open ESD shipping box will pass through (or be exposed to) several charged power sources (electric fields) when transported by a grounded vehicle, which will also cause problems with internal equipment.
6. Q: If the foot pad and the anti-static wrist are connected together, do they need to be connected to the ground?
Answer: No, it’s just that the equipment being handled should be unplugged and electrically connected to the ESD ground. As long as the conductive parts (body surface skin, ESD foot pads, equipment casing, operating parts, etc.) are equipotential, ESD (discharge of charge) ) Will not happen. The ground is very suitable as a reference voltage. If you confirm that all conductive parts are connected to the ground, then you can ensure that these conductive parts are at the same potential. Once there is a potential difference between the two conductors, it will cause the charge to discharge. Generate ESD.
7. Q: How does the relative temperature of ambient air affect ESD?
Answer: Relative humidity (RH) has a direct impact on the performance of surface accumulating charges. The higher the relative humidity, the shorter the time for the parts to store charge. The surface charge reduction (because of the increase in relative humidity) can be through recombination or conduction. When the relative humidity increases, the conductivity of the air also increases. But even at 100% relative humidity, the increase in charge discharge rate cannot replace proper ESD control procedures.
8. Question: Does the percentage of relative temperature need to be controlled at a specific value? If so, how can this value be determined?
Answer: When the air is gradually drying (the percentage of relative temperature decreases), the change in the ability to generate static electricity is definite and obvious. When the relative humidity is 10% (very dry air), when walking on the carpet, a charge of 35kV can be generated, but when the relative humidity is 55%, it will drop sharply to 7.5kV. The relative humidity of the working environment is in the best range of 25%-50%. Some clean places generally require a relative humidity of 50%. Due to the presence of devices that are more sensitive to the effects of corrosion and humidity, other environments require lower relative humidity. Test equipment and ESD suppression products in the best humidity range. Some ESD suppression products are humidity-related.
9. Q: Will strong magnetic fields (such as magnets) cause ESD? Is there any connection between magnetic field and ESD?
Answer: As long as the source of the magnetic field and the object are stationary, ESD will not be generated due to the strong magnetic field. If the magnetic field fluctuates or the object moves across the magnetic field lines, an electric current will be induced (if the object is conductive). Isolated conductors that are completely exposed to the 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, currents (charges) will be induced on the isolated conductors. ), some EMI sources are PC, ESD, transformer, fluorescent lamp, etc. In the general working environment, these are likely to be less important sources of harassment.
10. Question: Our electronic assembly workshop has an ESD conductive floor, and we use ESD special grounding shoelaces. Every operator is required to wear steel toe safety shoes. If you do not wear the ESD dedicated grounding shoelace, you cannot pass the grounding shoelace detector. Our ESD dedicated grounding shoelace is made of #2048 plastic, shoes made of a certain material, with an impedance of 106-108, can reduce static electricity, should I buy a new tester or new shoes or whatever Something?
Answer: First of all, make sure your tester has been calibrated, if not, you need to calibrate it. The standard for testing shoes is different from that of grounded shoelaces. The standard ESDS9.1-1995 uses a 100V impedance meter for impedance below 109. Most grounded shoelace testers use 6-12V open circuit voltage, which is much lower than the 100V used for shoes. Voltage. 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 electrostatic discharge control. Some foot ground (heel strap) testers are calibrated at 750k-100M before leaving the factory, others are only 10M, some new testers provide about 20-22V open circuit voltage, which may have a certain effect. In addition, there are some ESD shoes 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 lower requirements when testing anti-static wrist straps, and what are the lower or more reasonable requirements?
Answer: There are no pre-specified minimum requirements when testing an anti-static wrist, but a very good experience is: every time you wear an anti-static wrist or start another work, test it. The frequency of the test depends on The ESD-sensitive equipment that you do your best to protect. If the equipment is quite expensive and controlled by an operator, regular monitors are necessary, but if the equipment is not particularly sensitive and relatively inexpensive, then regular testing is sufficient. There are a few things you should check when testing an anti-static wrist: make sure that the anti-static wrist is in close contact with your wrist; conductive fiber or metal/conductive material is close to the skin; the snap on the metal buckle is suitable, and the snap on the coil end The connector should be tightly fastened, the spring of the banana plug has good elasticity, and it is in close contact with the plug hole of the banana plug. Other things that should be checked are: intermittent interruptions in the coil (usually at the end), dirt, oil and welding in the belt For the outer shell (this will reduce the conductivity of the strap), it is safe to apply 1-5 pounds of tension to the snap buckle 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 strap? (That is, two grounding (foot) straps, blouses, floor consumables, grounding plates, etc.). If so, why?
Answer: If the operator wears two anti-static shoes on the conductive floor and does not raise both heels at the same time, then the anti-static wrist is not needed. If the operator also wears a blouse, but it is not electrically connected to the body or the ground, so The blouse has only a partial protection function, and the accumulated charge on it may not be released. The common grounding method of the blouse is to tie a button to the waist of the blouse with a string, or use an anti-static wrist to buckle the inner cuff of the ESD blouse. .
13. Question: We check the ESD work surface every week to ensure that it is properly grounded through the 1M resistor. What is the general cause of unstable resistance readings between ESD (hard ground) and conductive work surface ground (soft ground)?
Answer: One reason for the unstable resistance reading between the power ground (ESD ground) and the conductive work surface is ground current. If the foot pad is grounded at multiple points, there may be ground current flowing, which will make the ohmmeter reading wrong. Other possible reasons are: the battery voltage of the meter is too low, the ground loop between the meter's AC power supply and the ground being measured, etc., it is best to cut off the ground wire in the power plug of the meter, and some digital meters may have strong magnetic fields or strong magnetic fields around them. The electric field is sensitive. Pay attention to check the power distribution cables, EMI sources, etc. during the measurement, and try to repeat the test several times in the shielded area to find out whether the problem has been eliminated.
14. Question: The answer to question 6 says: If the operator and the ESD grounding plate are at the same potential, ESD is unlikely to occur. What I want to ask is if the grounding plate is "hard grounded" and the operator wears an anti-static wrist, And grounding through a 1M resistance, then there may still be danger, right?
Answer: Not necessarily. When the current passes through a 1M resistor, there is a potential difference between its two ends. You are correct on this point. 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. A typical body The action is about 500ms, so when you touch the sensitive device, you have used 490ms to decay the potential to 0.
15. Q: Why can the ESD shielding bag discharge static electricity? What is the release process? What is the difference between an antistatic bag and an ESD shielding bag?
Answer: The ESD shielding bag can discharge static electricity, because the surface of the body bag (internal, external or both) is conductive and is in contact with another conductor (ESD grounding plate). If the shielding bag can discharge the charge, it must be conductive. , And connected to a conductor at the same time (usually grounded), the anti-static bag has anti-static properties, and can have no shielding, conductive or MVB properties. All anti-static measures mean that the friction charging of the anti-static bag will not exceed a certain voltage (usually 250V). The shielding bag should have metal foil or conductive film on all parts of the bag, depending on the device you want to protect. Bags made of conductive, antistatic, shielding and water vapor barrier substances can protect almost all sensitive devices, but they may be overprotected.
16. Question: Our users who produce printed circuit boards need an antistatic tire. Some tire manufacturers provide tires made of conductive rubber or conductive neoprene. Is antistatic and conductive the same thing? Answer: The meaning of antistatic means that when the wheel rolls on the ground, it does not generate a voltage higher than 250V. The conductive wheel may not be antistatic, but it will be released if it rolls on the ESD floor. The ideal wheel is both conductive and antistatic. When exposed to a conductor, it neither generates excessive charge nor stores charge.
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17. Q: Some people have bought several "wireless" ESD anti-static wrist straps for their PC repair stations. It is said that their working principle is similar to the corona on a copier/laser printer. I think this may be part of the composite system, but he said that other equipment was not mentioned in the usage requirements. It was close. When he opened the door and was shocked (wearing an anti-static wrist), he was convinced that the anti-static wrist was useless at all. There is a small rectangular piece of plastic product on this elastic band, which is screwed with a brass screw with the screw cap exposed. I carefully opened one and found that the 1M resistor was connected to the second screw (connected to the anti-static wrist It seems to be feasible to screw it with a brass-colored screw between a rubber-like soft "C"-shaped plate (about 1mm thick, 2cm2, formed after hollowing out)?
Answer: Not feasible. In the ESD safe area, this is not a feasible ESD control device. These passive "wireless" anti-static wrist straps have many limitations. If you are electrified up to 10kV due to friction, and you are wearing a wireless anti-static wrist The relative humidity of the environment), but never less than 10V, most of the charge reduction is due to the natural recombination of the surface charge of your body (this time is the metal shell of your "wireless" anti-static wrist) and the air with a certain relative humidity Conductivity. (Attachment: Cut the hair to about 1/4 inch, you will get the same effect. Brush the conductive glue on the hair, and the end of your conductive hair will be like a corona discharge tip at a relatively high voltage. The current flowing into the air may help enhance the natural recombination process. When your hair is very close to 0.1-1.5 inches from the ground, you need to consider any minimum 3kV potential difference (because of the dielectric strength of the air) or enhanced normal air conduction Rate so that you are below a few kV.
18. Question: If a device is put into the box and the lead is suspended, the lead has an end-to-end, end-to-case resistance of about several hundred ohms and a 10pF capacitance. If the charging time is several minutes, the dangling lead can easily Charge to 1kV (relative to ground). (The charging method can be stored in a dry N2 box close to the nozzle but incorrectly grounded, or wearing nylon or cloth shoes to walk through a long carpeted hall in a dry winter) Users with grounding measures open In the ESD bag, the lead when charged to 1Kv is discharged through other discharged leads, causing damage to the device. The potential applied to the oxide destroys the device. All leads should be short-circuited or knotted to facilitate ESD protection, right?
Answer: For the example you gave, we 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 electric charges can be induced from the outside. Inside, the device can move anywhere, and the triboelectric charge ranges from a few volts to a few thousand volts, depending on the substances in contact with each other. When a grounded operator opens the box, touches or approaches the dangling lead, it may cause ESD. If the pin of the device is inserted in a conductive foam material or an ESD bypass strip is used, the sensitivity of the device to discharge and ESD damage is greatly reduced. When to use the bypass strip or conductive foam depends on the device you want to protect Type, charge storage method, and shipping method. So be sure to ask the manufacturer for the correct precautions.
19. Q: Some ESD workbenches with anti-static wrist straps can be directly grounded, while others need to be led to the ESD bench pad (on the opposite corner), and then grounded through a resistor. Which is correct?
Answer: Both are not necessarily correct, because it depends on the potential of the directly grounded anti-static wrist and the workbench pad. For ESD safety benches, the concept of grounding is to make all conductors at the same potential, so that the two conductors are at the same potential. The potential difference is minimized. Since the power ground and the earth are always in the control area, it is the most convenient connection point. If the anti-static wrist and ESD table pad are connected to the same grounding point, they have the same potential, which is the correct installation method. The anti-static wrist strap is first connected to the ESD table mat, then passes through the table mat, then passes through the table mat diagonally, and then grounds, so that the connection between the table mat and the ground has an additional series impedance (resistance or capacitance), sometimes, The operator can be at a different potential from the grounded desktop device or the ESD sensitive device on the bench pad.
21 Question: When putting on anti-static shoe covers, is it important that the shoe cover is in close contact with the skin of the foot, and can the shoe cover be between the shoe and the socks? Answer: When wearing anti-static shoe covers to operate electronic instruments, it is not necessary that the shoe covers are in direct contact with the skin. The conductive joint on the anti-static shoe cover indirectly connects the conductive floor (ground) to the human body (skin). Typically, the conductive joint is stepped between the insole and the socks. The skin is electrically connected to the joint through the sock. Moisture from the foot makes the sock conductive and provides a conductive path between the foot (skin) and the joint. When you put on the shoes for the first time, you can wear them for 10 to 60 minutes to ensure that there is enough moisture, so that the socks have good conductivity, so that the skin can be effectively connected to the conductive joints.
Question 22: Currently, my company has been using cardboard templates to check the missing components on the PCBA (manifold block and surface mount chip). The cardboard is placed on top of the PCBA, which will generate static charges. Make an opening on the cardboard according to the size of the component to check if there is any missing component. What I want to ask is, what is the suitable material that can replace this cardboard? At the same time how can we speak freely?
Answer: You can replace the cardboard template with a dissipative template, as long as it can be properly connected to the same power ground on your work surface. FRP is machinable and easy to drill, maybe it can meet your requirements. Avoid using highly conductive materials because they can cause ESD between the component and the conductive template.
23 Question: When testing film according to MIL-B-81705-B standard, I need a surface resistance meter and an electrostatic decay time test table. How should I choose? Answer: in MIL-B-81705In C, for Type I shielding materials, the test procedure for electrostatic attenuation is specified in FTM4046 (attenuation 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 bought in instrument stores, including high-speed oscilloscopes, naammeters, and navoltmeters , High-precision megohmmeter, high-voltage power supply, etc.. For surface resistance: models 41262 and 41272 meet the requirements of ASTMD257; 41272 also meets the requirements of standards EIA-541 and ESD S11.11. For electrostatic attenuation: we have no relationship with MIL-B- 81705C compatible equipment. Model 42630 just meets the requirements of ESDS3.1, and can do some preliminary attenuation tests on the film, but it must be based on standard test methods.