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The harm of static electricity to the assembly of electronic equipment- Rio Tinto brand

Date: 2013/8/3 21:24

The harm caused by static electricity to the assembly of electronic equipment Hits: 3096 Published: 2009-01-18

First, electrostatic hazards can be divided into two categories: one is the adsorption of floating dust caused by electrostatic attraction; the other is the dielectric breakdown caused by electrostatic discharge.

Electrostatic adsorption

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2. Electrostatic discharge and dielectric breakdown

The starting and discharging power source of electrostatic discharge is space charge, so its stored energy is limited, so it can only provide transient breakdown energy. Although the energy of electrostatic discharge is small, its discharge waveform is very complicated and it is more troublesome to control. Soft breakdown of semiconductor devices is related to it.

The breakdown of components caused by static electricity is the main way of electrostatic hazards in electronic equipment, and it is the most common and severe hazard in the manufacture of electronic equipment.

Electrostatic discharge can cause hard or soft breakdown of the device. Hard breakdown is a permanent failure of the device, such as the output and input of the device open or short. Soft breakdown can degrade the performance of the device and reduce its index parameters and cause hidden troubles. Because the soft breakdown can make the circuit change from time to time (caused by the decrease of the index parameters), and it is not easy to find, it causes a lot of trouble for the whole machine to run and find the fault. When the soft breakdown occurs, the equipment can still work with "illness", the performance has not changed fundamentally, it is likely to pass the factory inspection, but it may cause another failure at any time. Multiple soft breakdowns can cause hard breakdowns and make electronic equipment operate abnormally.

The human body also generates static electricity. The human body's electrostatic discharge may cause the human body to be shocked and reduce work efficiency, and may cause secondary accidents (ie, device damage), so it should be paid sufficient attention.

In military training, the human body forms static electricity because the human body converts the consumed mechanical energy into electrical energy during activities. The human body is an electrostatic conductor. When the body is insulated from the ground (for example, the sole of the shoe being worn is an insulating material), the body forms a capacitance with the ground to store the charge, and its charging voltage is generally ≤50kV. When the charge accumulates to a certain level, it will discharge to form a spark once the conditions are mature, the instantaneous discharge voltage can reach thousands of kilovolts, and the discharge power can reach several thousand kilowatts.

When the human body is charged and discharged, the human body will have different degrees of reflection. This reflection is called the shock sensitivity. When the human body is subjected to electrostatic shock, although no major physical obstacles will occur, it may affect people's work efficiency, or cause mental stress and secondary damage.

Second, the anti-static of electronic equipment that should be paid attention to

Due to the rapid development of electronic equipment, small-scale and highly integrated devices have been mass-produced, which has led to smaller and smaller wire spacings, thinner insulation films, and lower breakdown voltages. However, the electrostatic voltage generated by electronic equipment in the process of production, transportation, storage, and transfer far exceeds its breakdown voltage threshold, which may cause breakdown or failure of the device, affect the technical indicators of the equipment, and reduce its reliability. .

The prevention of static electricity is mainly to suppress the generation of static electricity, accelerate the leakage of static electricity, and perform static neutralization. When a person wears non-conductive shoes, charges can be generated and accumulated due to walking and other activities, and can reach a potential of the kilovolt level. When two different objects come into contact with each other, a charge movement occurs at the interface, and positive and negative charges are arranged oppositely to form an electric double layer. If the objects are separated, an equal amount of charge of different polarity will be generated on each of the two objects. The principle to prevent this kind of static electricity is: Exclude as much as possible the main factors that generate static electricity (object characteristics, surface state, charging history, contact area and pressure, separation speed, etc.); make the objects in contact with each other in the charged sequence Position as close as possible; make the contact area and pressure between objects small, the temperature should be low, the number of contacts should be small, the separation speed should be small, and the contact status should not change sharply. Powder, liquid, and gas can generate static electricity due to friction during transportation. Therefore, measures such as limiting the flow rate, reducing the bending of the pipeline, increasing the diameter, and avoiding vibration should be adopted.

In addition to reducing the speed, pressure, friction and contact frequency, selecting appropriate materials and shapes, and increasing the conductivity, static protection can also take the following measures: ① grounding. ② Lap (or jump). ③ Shielding. ④ Use an antistatic agent on the insulator that can hardly leak static electricity to increase the conductivity and make it easy to leak static electricity. ⑤ Use methods such as spraying and watering to increase the ambient humidity and suppress the generation of static electricity. ⑥ Use a static eliminator to neutralize static electricity. This is the most effective way to eliminate static electricity.

Third, the way to eliminate static electricity in the production process of electronic equipment

Anti-static is mainly to prevent electrostatic discharge. Controlling electrostatic discharge should start from two aspects: controlling the generation of static electricity and controlling the elimination of static electricity. Controlling the generation of static electricity is mainly controlling the process and the choice of materials during the process; controlling the elimination of static electricity is mainly to accelerate the leakage and neutralization of static electricity. These two points work together to make the electrostatic voltage not exceed the safety threshold to achieve the purpose of electrostatic protection.

Static electricity can cause harm to electrostatic sensitive devices, but it is controllable and can be eliminated.

1. Elimination of static electricity:

① Prevent the accumulation of static charge in places where static electricity may be generated. That is, take certain measures to avoid or reduce the generation of electrostatic discharge. The purpose of eliminating charge accumulation can be achieved by generating while leaking.

② Existing charge accumulation should be eliminated quickly. When an insulated object is charged, the charge cannot flow and cannot leak. You can use a static eliminator to generate anisotropic ions to neutralize the static charge. When a charged object is a conductor, a simple ground leakage method is used to completely eliminate its charge. You can install an ion fan in the work area to form a complete static safe work area, which should include at least an effective static table mat and static eliminator (Siwo Technology brand): ion fan, ion wind rod, ion air nozzle, ion Wind drum, static elimination equipment, manual electrostatic dedusting gun (ion wind gun), automatic static elimination ion wind nozzle, ion wind rod, ion copper rod, ion wind copper rod, induction static elimination rod, high voltage generator. Desktop ion fan, hanging double head ion fan, hanging three head ion fan, horizontal ion fan. Anti-static equipment desktop ion fan, induction ion air nozzle, horizontal ion fan, hanging ion fan, ion air gun, ion air nozzle, ion wind rod, induction ion air nozzle, dedicated ground wire and anti-static wrist strap Wait. If you have any questions, you can consult with the Siwo Anti-static Equipment Manufacturing Center. Manufacturing Center: Pattern Fax: 010-62716673, National Sales Hotline: 013911222754.

2. Control of static electricity: The static electricity control technology adopts comprehensive measures to control static electricity hazards within the allowable range when the accumulation of electrostatic charges is unavoidable.

① The purpose of the process control method is to generate as few electrostatic charges as possible in the production process. Preventive measures should be taken in the process of material selection, equipment installation, and operation management in the process flow to control the generation of static electricity and the accumulation of charge, suppress the electrostatic potential and discharge energy, and minimize the harm.

② The purpose of the leakage method is to eliminate static charges through leakage. Generally, electrostatic grounding is used to leak the charge to the earth, and static electricity is usually leaked by increasing the conductivity of the object.

③ The electrostatic shielding method uses a grounded shield to isolate the charged body from other objects, so that the electric field of the charged body will not affect other objects around it. This shielding method is called internal field shielding. Sometimes a grounded shield is used to surround the isolated object from the external electric field. This shielding method is called external field shielding.

④The compound neutralization method and other methods can achieve the elimination of static charge through the compound neutralization method. Usually use static eliminators (Siwo Technology brand): ion wind gun, ion fan, ion wind rod, ion wind nozzle, ion wind drum, the positive and negative ions generated to neutralize the charge of the charged body, and may make the charged object The smooth surface and cleaner surroundings reduce the possibility of tip discharge.

Static Control / Static Elimination in the Electronics Industry

With the development of nanotechnology, the integration density of integrated circuits is getting higher and higher, and the corresponding breakdown voltage (Vesd) is getting lower and lower. On the other hand, some polymer materials with high surface resistivity such as plastics , The widespread application of rubber products and the destructive nature of modern production processes, the US electronics industry's losses due to ESD in the year alone amount to tens of billions of dollars. ESD Electrostatic Discharge is a "hard virus" that is common in the electronics industry. It will occur when certain internal and external conditions are met. You can follow the following four principles to establish an effective electrostatic control program.

Embrace static control in your design

To achieve more effective ESD control, first of all, in the design of devices and products, the idea of electrostatic protection should be fully reflected, and electrostatic protection elements (ESDProtectionDevice) should be set up inside the device. Use static-sensitive devices as much as possible and The electrostatic discharge sensitive (ESDS, ESD-sensitive) device provides proper input protection, making it more reasonable to avoid ESD damage.

MOS process is the leading technology for integrated circuit manufacturing, with metal-oxide-semiconductor field effect transistors as the basic structural element. Since the gate and source of the field effect management in the MOS device is a sub-micron-level insulating slab oxide layer, its input impedance is usually greater than 1000M, and it has an input capacitance of about 5 pF, which is extremely vulnerable to electrostatic damage. Therefore, a resistor-diode protection network is set in the input stage of the MOS device. The series resistance can limit the peak current, and the diode can limit the instantaneous peak voltage. Common protective components in the device are: capacitors, bipolar transistors, thyristors (SCR, see Figure 1), etc. When ESD occurs, they respond quickly before the protected device, absorbing and releasing ESD energy, The impact of the protected device is greatly reduced. Under normal circumstances, the protection element works in its first breakdown (FirstBreakdown) area, and will not be damaged by ESD. Once the external voltage or current is overstressed, the protection element entering the SecondaryBreakdown area will be irreversibly damaged. , Lose the protective effect on the device.

At present, many manufacturers have developed devices with internal protection circuits, and a series of corresponding test standards have also been implemented. For example, the analog switch MAX4551 developed by MAXIM has 15kV ESD protection. They must be in normal operation, shutdown mode, and off. Under electrical conditions, according to standards such as IEC1340-3-1 human body model, IEC1000-4-2 air-gap discharge, IEC1000-4-4 fast transient (FTB) discharge, and other analog ESD tests, to ensure compliance with IEC1000-4 -2Level4 requirements.

In addition, the antistatic design of the production environment is also the key to ESD control. The design is based on the electrostatic breakdown voltage (Vesd) of the insulating film of the electronic device, the Vesd of the sensitive device in the whole machine, and the antistatic performance of the production equipment. Manufacturing must define and adhere to a special ESD control level, which is determined by the most sensitive components in the production process, and the production environment must guarantee this level of security. When the level of the most sensitive components is not known, the manufacturer should implement the EIA-625 standard, which defines the ESD-protected work area as a "safe area" and does not include discharge power sources that may generate more than 200V. Many international papers have proposed using the secondary collapse current as the basis for determining the electrostatic sensitivity level. A transmission line touch wave generator (TLPG) capable of accurately measuring the secondary collapse current has also become an important tool in the development of ESD protection.

Adhere to the principle of prevention, eliminate the process of generating static electricity, and minimize the processes and materials that generate static electricity during the production, storage and transportation process, which can largely eliminate the generation and accumulation of static electricity. In order to suppress the generation and accumulation of static electricity, the use of ordinary plastics, polyethylene, and styrene products with high surface resistivity, such as chemical fiber carpets, nylon clothes, and cloth instrument covers, should be avoided in the EPA area as much as possible. Once these items are rubbed, they will generate static electricity. And it is not easy to release; to reduce the dust as much as possible, the dust particles usually adhere to the charge; the operator should avoid touching the circuit board and various IC pins with hands and clothing; when cleaning the printed circuit board (PCB), use only ESD approved natural Brush and solvent; Minimize the movement of printed circuit assembly (PCA, printed circuit assembly) during all operations and inspections, and reduce the number of operations if possible; The device should be stored in a completely closed shielded container, or with the pins facing down Dissipative grounding mats; labels on PCBs or devices that track work processes (WIP, workinprocess) should also use static dissipative labels.

The logistics of sensitive devices is also a link that cannot be neglected. In this process, the components must inevitably friction with the outer packaging to generate static electricity, and the sensitive devices exposed to the external electric field (such as near high-voltage equipment) are also vulnerable to damage, and it is likely that we have not realized Under sensitive conditions, sensitive devices have been damaged, so dissipative or anti-static shielding packaging should be used for storage and transportation. Nylon and ordinary plastic products that are prone to static electricity cannot be used, and ESD devices should be removed from the package only when they are ready to be used. take out.

Safely discharge or neutralize static electricity

Similarly, no matter how strict the measures are, it is almost impossible to completely eliminate static electricity, so our third principle is to safely discharge or neutralize static electricity, of which grounding is the most basic and effective method. Grounding provides a good discharge channel for electrostatic shock, allows the electrostatic charges accumulated on the charged body to be smoothly discharged, and is quickly introduced to the ground to avoid the discharge of sensitive components. The grounding effect directly affects the entire electrostatic protection effect. If the grounding effect is poor, the entire antistatic system will fail, and the product will be in a dangerous situation full of static electricity. Only from the test discharge point to the final ground junction Any link must be kept open to ensure the safe discharge of static electricity. IEC1340-5-1 specifies the grounding resistance, and the domestic aerospace industry standard QJ1950-90 also has similar regulations. Considering it comprehensively, the grounding resistance of production and use places in the electronics industry should be below 10, and the grounding wire must be firmly connected and sufficient. Mechanical strength, otherwise sparks may be generated in loose parts. The grounding wire of the fixed equipment should be multiple pliable braided wires of 1.25mm or more. The ground wire is provided to the anti-static wrist strap, the floor and the surface of the workbench.

The static charges on insulators such as ordinary plastics cannot be eliminated by grounding, but they can be neutralized with charges of opposite polarity. At present, a static eliminator (SVCOR) is often used. Under the action of the ground current and high voltage generated by the high-voltage generator, a stable high-intensity electric field is formed, and the ionized air forms ions. It is carried out by the air flow to reach the surface of the object and achieve the purpose of neutralizing static electricity. There is a special cleaner on the ionization device, which can remove the dust on the ionization needle and maintain the normal operation of the ionization device.

Siwo Technology ion wind static elimination equipment has a dedicated ion balance circuit, which can achieve automatic ion balance. The function of eliminating static electricity can solve production problems caused by static electricity, such as solving the problem of dust collection caused by static electricity, the problem of adhesion during the processing of plastic products, and the problem of small parts jumping caused by electrostatic discharge.

A strong electric field is formed near the tip, causing the corona discharge of the air to generate positive and negative ions. The anisotropic ions move to the charged body under the action of the electric field, so that any charge accumulation that may occur on the surface of the insulator can be continuously neutralized.

Moderate control of the temperature and humidity of the environment can also effectively curb the lethality of static electricity. Both temperature and temperature have an effect on ESD. In the same atmospheric environment, the lower temperature area will have a higher relative humidity than the higher temperature area, and the increase in humidity will increase the surface conductivity of non-conductive materials and enhance the air conductivity. The static charge accumulated on the object can leak more quickly. It can be seen that the lower the ambient temperature and the higher the humidity, the more beneficial is the protection against static electricity. Therefore, when the process conditions permit, you can use air conditioning humidification, fan sprayer spray water spray, ground spraying and other methods to increase the relative humidity of the air and reduce the harm of static electricity. Of course, usually the temperature should be controlled in the range of 18-28 degrees, and the humidity should be controlled in the range of 40-65% RH.

Real-time detection of production progress and environment

The electrostatic parameters concerned in the production process are electric field strength, electrostatic potential, charge amount and resistivity. The amount of charge is the most essential physical quantity of static electricity, but it is inconvenient to measure on-site. Usually, it is used to measure the electrostatic potential on the surface of the object. The non-contact electrostatic voltmeter is often used for measurement. Very small. The antistatic performance of an object can be identified by detecting its surface resistivity or volume resistivity, and the electric field strength statically accumulates the energy accumulated in the working environment, which is very important for the determination of EPA. Commonly used testing instruments include field strength meters, hand / wrist band detectors, surface impedance testers, electrostatic voltmeters, charge meters, etc. The latest comprehensive monitoring equipment can comprehensively monitor the static state of the area, and any parameter exceeds the standard That is, automatic alarm.

Detection is a necessary means of ESD control and should be performed in real time in all links of production. Before entering the EPA, operators need to test the performance of hand / ankle straps, anti-static shoes, and anti-static work clothes; someone must be responsible for EPA testing and maintenance, and regularly check whether various ESD equipment, appliances, and grounding devices meet anti-static requirements. Management personnel should regularly check the anti-static situation of each station, and if they do not meet the requirements, they need to make corrections in real time; the test results should be archived and verified as part of the quality management system certification.

to sum up

The above four principles are the basis for establishing ESD control procedures. An effective ESD control system is far from sufficient with anti-static hardware systems. It must also have a set of training, management systems and operating procedures that are effective and implemented. American scholar Dennis Polinski believes that an effective ESD control program should include the following elements: establish an ESD protection coordinator or team, identify losses from ESD, assess your implementation process and needs, identify ESD sensitive projects, and require support from top management , Establishing and implementing procedures and regulations, personnel training and review, review, analysis, reporting, providing feedback and improvement.

ESD protection is a systematic project. It is necessary to implement the overall protection thinking and comprehensively use technologies such as voltage equalization, grounding, leakage, shielding and clamping to form a complete protection system in order to achieve obvious results.

ESD protection technology in SMT production

Editor's note: In the manufacture of electronic products, electrostatic discharge often damages the device and even causes the device to fail, causing serious losses. Therefore, electrostatic protection in SMT production is very important. This magazine invited two experts from Beijing and Shanghai respectively to write and analyze the static electricity generation sources and electrostatic protection principles in the manufacture of electronic products, and introduced some electrostatic protection technology foundations and corresponding measures in SMT production in more detail. For reference.

1. Static electricity and electrostatic hazards

Static electricity is a kind of electric energy. It is stored on the surface of an object. It is the result of the imbalance of positive and negative charges in a local range. It is formed by the conversion of electrons or ions. Electrostatic phenomenon is a general term for electrical phenomena that occur during the generation and disappearance of charges. Such as friction electrification, human body electrification and so on.

With the development of science and technology, electrostatic phenomena have been widely and effectively applied in the fields of electrostatic spraying, electrostatic textiles, electrostatic separation, electrostatic imaging and other fields. But on the other hand, the generation of static electricity will bring great harm and loss in many fields. For example, in the first Apollo manned spacecraft, three astronauts were killed due to an explosion caused by electrostatic discharge; accidents caused by explosion due to electrostatic discharge (ESD) occurred during the gunpowder manufacturing process. In the electronics industry, with the increasing integration, the inner insulation layer of integrated circuits is getting thinner and the width and pitch of interconnecting wires are getting smaller and smaller. For example, the typical thickness of the insulation layer of CMOS devices is about 0.1 μm. The corresponding breakdown voltage is 80-100V; the insulation layer of VMOS device is thinner, and the breakdown voltage is 30V. In the manufacture of electronic products and during transportation, storage and other processes, the electrostatic voltage far exceeds the breakdown voltage of the MOS device, which often causes the device to have a hard breakdown or a soft breakdown (local damage to the device), making it invalid. Or seriously affect the reliability of the product.

To control and eliminate ESD, developed countries such as the United States, Western Europe, and Japan have developed national, military, and corporate standards or regulations. There are corresponding regulations on the design, manufacture, purchase, storage, inspection, storage, assembly, commissioning, packaging of semi-finished products and finished products, and transportation of static-sensitive components. There are also stricter regulations on the manufacture, use, and management of electrostatic protection equipment. Institutional requirements. China has also formulated military and enterprise standards with reference to international standards. For example, there are standards such as the Ministry of Aerospace, the Ministry of Mechanical and Electrical, and the Ministry of Petroleum.

2. Static sensitive devices (SSD)

Devices that are sensitive to electrostatic reactions are called electrostatic sensitive devices (SSD). Electrostatic sensitive devices mainly refer to very large scale integrated circuits, especially metallized film semiconductors (MOS circuits). Table 1 is a classification table of electrostatic sensitive devices. According to the SSD classification table, different electrostatic protection measures can be taken for different SSD devices.

3. Static power in electronics manufacturing

(1) Human activities, static electricity generated by friction, contact and separation between people and clothes, shoes, socks and other objects is one of the main static power sources in the manufacture of electronic products. Human body static electricity is the main cause of the device's hard (soft) breakdown. The electrostatic voltage generated by human activities is about 0.5-2KV. In addition, the air humidity has a great influence on the electrostatic voltage. If it is in a dry environment, it will increase by an order of magnitude. Table 2 shows the relationship between relative humidity and the charging of human activities.

After the human body is charged and touches the ground wire, a discharge phenomenon will occur, and the human body will produce different degrees of electric shock response. The degree of the reaction is called electric shock sensitivity. Table 3 shows the shock sensitivities of the human body during different static voltage discharges.

(2) When the chemical fiber or cotton workwear rubs against a work surface or a chair, it can generate an electrostatic voltage of more than 6000V on the surface of the clothing and charge the human body. At this time, contact with the device will cause discharge and easily damage the device.

(3) The insulation resistance of rubber or plastic soles is as high as 1013Ω, which generates static electricity when it rubs against the ground and charges the human body.

(4) When the device encapsulated by resin, paint film, or plastic film is transported in a package, friction between the surface of the device and the packaging material can generate several hundred volts of electrostatic voltage, which can discharge sensitive devices.

(5) Various packaging, boxes, turnovers made of high-molecular materials such as PP (polypropylene), PE (polyethylene), PS (polyethylene), PVR (polyurethane), PVC and polyester, resin, etc. Boxes, PCB racks, etc. may generate 1-3.5KV electrostatic voltage due to friction and shock, which can discharge sensitive borrowings.

(6) The ordinary work surface is subject to friction to generate static electricity.

(7) The insulation resistance of insulated floors such as concrete, waxed and polished floors, and rubber boards is high, and the static charge on the human body is not easy to leak.

(8) In terms of electronic production equipment and tools: such as electric irons, wave soldering machines, reflow soldering furnaces, placement machines, debugging and testing, and other high-voltage transformers, AC / blind current circuits in the equipment will induce static electricity in the equipment. If the device's static discharge measures are not good, it will cause sensitive devices to fail during the manufacturing process. The circulation of hot air in the oven and the friction of the box, and the CO2 steam in the CO2 cryogenic box cooling box can generate a large amount of electrostatic charge.

4. ESD protection principle

It is impossible to produce static electricity in the manufacture of electronic products. The generation of static electricity is not a hazard. The hazard lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of ESD protection is "meditation elimination".

ESD protection principle:

(1) Prevent the accumulation of static electricity in places where static electricity may be generated. Take measures within safety limits.

(2) Eliminate the existing static accumulation quickly and release immediately.

5. ESD protection method

(1) Use antistatic materials: metal is a conductor, and the leakage current of the conductor will damage the device. In addition, because insulating materials are prone to triboelectricity, metal and insulating materials cannot be used as antistatic materials. Instead, a so-called electrostatic conductor with a surface resistance of 1 × 105Ω · cm or less and an electrostatic subconductor with a surface resistance of 1 × 105-1 × 108Ω · cm are used as the antistatic material. For example, the commonly used static protection material is realized by mixing conductive carbon black in rubber, and the surface resistance is controlled below 1 × 106Ω? Cm.

(2) Leakage and grounding: Ground the parts where static electricity may or may have been generated, and provide static discharge channels. The method of burying the ground is used to establish the "independent" ground. Make the resistance between the ground and the ground <10Ω. (See GBJl79 or SJ / T10694-1996)

Grounding method of static protective materials: Connect static protective materials (such as table mats, floor mats, anti-static wrist straps, etc.) to the conductors leading to independent earth wires through 1MΩ resistors (see SJ / T10630-1995). The 1MΩ resistor is connected in series to ensure that the current <5mA is discharged to the ground, which is called soft ground. Equipment enclosures and electrostatic shields are usually directly grounded, called hard ground.

The grounding method of the anti-static workbench recommended in the IPC-A-610C standard is shown in Figure 1.

(3) Elimination of static electricity on the conductor: The static electricity on the conductor can be leaked to the ground by grounding. The voltage and release time of the discharge body can be expressed by the following formula:

UT = U0L1 / RC

The voltage at the UT-T moment (V) U0-the starting voltage (V) R-equivalent resistance (Ω) C-conductor equivalent capacitance (pf)

It is generally required to discharge static electricity within 1 second. That is to reduce the voltage to a safe area below 100V within 1 second. This can prevent the leakage speed from being too fast and the leakage current from causing damage to the SSD. If U0 = 500V and C = 200pf, if you want to make UT reach 100V within 1 second, then R = 1.28 × 109Ω is required. Therefore, 1MΩ current limiting resistors are usually used in electrostatic protection systems to limit the leakage current to less than 5mA. This is designed for operational safety. If the operator accidentally touches the industrial voltage of 220V in the electrostatic protection system, it will not bring danger.

(4) Elimination of static electricity on non-conductors: As for the static electricity on insulators, since the charge cannot flow on the insulators, the static electricity cannot be eliminated by grounding. The following measures can be taken:

(A) Use of ion fan-ion fan generates positive and negative ions, which can neutralize the static electricity of static power. It can be installed in the space and near the placement head of the placement machine.

(B) Use of static eliminators-Static eliminators are surfactants. The surface of the instrument and the object can be washed with static eliminating agent, which can quickly eliminate the static electricity on the surface of the object.

(C) Control of environmental humidity—increasing humidity can increase the surface conductivity of non-conducting materials and make it difficult for objects to accumulate static electricity. For example, in the northern dry environment, humidification and ventilation measures can be taken.

(D) Adopt electrostatic shielding—For equipment that is prone to static electricity, use a shielding cover (cage), and effectively ground the shielding cover (cage).

(5) Process control method: In order to minimize the generation of static electricity in the manufacture of electronic products, control the accumulation of static charges, quickly eliminate the existing static electricity accumulation, and release it immediately, from the aspects of plant design, equipment installation, operation, management system, etc. take effective action.

6. ESD protection equipment

(1) Human body anti-static system includes anti-static wrist straps, work clothes, caps, gloves, shoes, socks, etc.

(2) Anti-static floor includes anti-static terrazzo floor, anti-static rubber floor, PVC anti-static plastic floor, anti-static carpet, anti-static raised floor, etc.

(3) Anti-static operation series: including anti-static: I: as a table mat, anti-static packaging bag, anti-static logistics trolley, anti-static soldering iron and tools.

7. Static measuring instruments.

(1) Static field tester: It is used to measure the surface resistance values of table and ground. Plane structure and non-planar situations should choose different specifications of measuring instruments.

(2) Wristband tester: measure whether the wristband is effective.

(3) Human body static tester: It is used to measure the amount of static electricity carried by the human body, the impedance between the feet of the human body, the difference in static electricity between the human bodies, and whether the wristband, grounding plug, work clothes, etc. are effectively blocked. It can also be used as a primer to discharge the static electricity of the human body from the workshop.

(4) Megohmmeter: It is used to measure the impedance or resistance of all conductive, antistatic and electrostatic discharge surfaces.

8. Technical requirements for antistatic technology in electronic product manufacturing

(1) Anti-static ground electrode ground resistance <10Ω.

(2) Ground or floor mat: surface resistance value is 105-1010Ω; friction voltage is less than 100V.

(3) Wall: resistance value 5 × 104-109Ω.

(4) Work surface or pad: surface resistance value is 106-109Ω; friction voltage is less than 100V; system resistance to ground is 106-108Ω.

(5) The resistance of the work chair to the casters is 106-108Ω.

(6) Friction voltage of work clothes, caps and gloves <300V; friction voltage of soles <100V.

(7) The resistance of the wristband connection cable is 1MΩ; the system resistance is 1-1OMΩ when the wristband is worn. The resistance of the heel belt (shoe harness) system is 0.5 × 105-108Ω.

(8) The resistance of the logistics platform facing the wheel system is 106-109Ω.

(9) The surface resistance of a logistics transfer device such as a material box, a turnover box, and a PCB rack is 103-108Ω; the friction voltage is less than 100V.

(10) The friction voltage of packaging and box 1 is less than 100V.

(11) Human body comprehensive resistance 106-108Ω.

9 Anti-static measures in electronics manufacturing and general requirements for static work areas (points)

The SMT production equipment must be well grounded. The placement machine should adopt a three-phase wireless grounding method and be independently grounded. The floor, work surface pads and chairs of the production site should meet the requirements of anti-static. Keep a constant temperature and humidity in the workshop. Facilities such as anti-static material boxes, turnover boxes, PCB racks, logistics trolleys, anti-static packaging tapes, anti-static wrist straps, anti-static soldering irons and tools should be provided.

(1) Set up anti-static area according to anti-static requirements, and have obvious anti-static warning signs. According to the electrostatic sensitivity of the devices used in the work area, it is divided into 1, 2, and 3 levels, and different protective measures are formulated according to different levels.

Level 1 electrostatic sensitivity range: 0-999V

Level 2 electrostatic sensitivity range: 2000-3999V

Level 3 electrostatic sensitivity range: 4000-15999V

Above 16000V are non-static sensitive products.

(2) The room temperature of the static-safe area (point) is 23 ± 3 ° C, and the relative humidity is 45-70% RH. It is forbidden to operate SSD (static sensitive components) in less than 30% of the environment.

(3) Regularly measure the surface resistance values of the ground, table, and turnover box.

(4) Non-production items such as tableware, tea sets, bags, woolen fabrics, ⎝⎛龙博线上娱乐⎞⎠papers, rubber gloves, etc. are prohibited on the workbench in the electrostatic safety zone (point).

(5) The workers enter the anti-static area and need to discharge. Operators must wear work clothes and anti-static shoes and socks when performing operations. Before every job, you must do an electrostatic protection safety inspection before you can pass the production.

(6) Wear an anti-static wrist strap during operation, and measure whether the wrist strap is effective every day.

(7) When testing the SSD, take one piece from the packing box, tube, and tray, measure one piece, and put one piece on it. Unqualified devices after testing should be returned to the warehouse.


(8) The power-up test must follow the power-up and power-down sequence: low voltage → high voltage → signal voltage. The order of calls is reversed. At the same time, pay attention that the polarity of the power supply cannot be reversed, and the power supply voltage must not exceed the rated value.

(9) The inspector should be familiar with the model, variety, and testing knowledge of the SSD, and understand the basic knowledge of electrostatic protection.

10. Requirements for transportation, storage, and use of static sensitive components (SSD)

(1) Do not drop onto the ground during SSD transportation, and do not arbitrarily leave the packaging.

(2) Relative humidity of the warehouse where the SSD is stored: 30-40% RH.

(3) Keep the original packaging during SSD storage. If it is necessary to replace the packaging, use an antistatic container.

(4) In the warehouse, an antistatic special label should be affixed to the place where the SSD device is placed.

(5) Apply the visual inspection method when issuing SSD devices, and count the quantity in the original packaging of the SSD devices.

(6) When writing, erasing, and information protection operations on EPROM, the writer / erase device should be fully grounded, and an antistatic bracelet should be worn.

(7) Operators in assembly, welding, board repair, and commissioning must operate strictly in accordance with electrostatic protection requirements.

(8) The printed circuit boards that pass the test and inspection shall be sprayed once with an ion spray gun before packaging to eliminate the possible static charge accumulation.

11. Management and maintenance of anti-static work area

(1) Formulate anti-static management system and have someone responsible.

(2) Spare anti-static work clothes, shoes, bracelets and other personal items for use by outsiders.

(3) Regular maintenance and inspection of the effectiveness of anti-static facilities.

(4) Check the wristband once a week (or day).

(5) Check the groundability of the table mats and floor mats and the performance of the static eliminator once a month.

(6) Anti-static components rack, printed board rack, turnover box; anti-static performance of transport vehicles, table mats, floor mats is checked every six months.

Electrostatic protection of electronic products (top)

I. Overview

In people's daily life and work, static electricity is often encountered. So, what exactly is static electricity, its mechanism of generation and what harm does it have, and how to prevent and eliminate these harms is a problem we must consider and solve.

1. What is static electricity?

Static electricity is a kind of electrical energy. It exists on the surface of an object. It is a phenomenon that occurs when the positive and negative charges are locally imbalanced. Electrostatic phenomenon refers to a general term for the phenomenon that electric charges show in the process of generation and disappearance. For example, triboelectricity is an electrostatic phenomenon.

2. Why anti-static?

Due to the rapid development of the electronics industry, small-scale, highly integrated devices have been mass-produced, which has led to smaller and smaller wire spacings, thinner and thinner insulation films, and lower breakdown voltages. However, the electrostatic voltage generated during the production, transportation, storage and transfer of electronic products far exceeds its breakdown voltage threshold, which may cause breakdown or failure of the device, affect the technical indicators of the product, and reduce its reliability. . 由此可见,静电是电子行业发展中的一大障碍。所以预防静电必须提到议事日程上来,以确保产品的质量。

为使电子器件及产品在购买、入库、发料、检验、储存、调测和安装等过程中免受静电危害,了解静电产生的机理和一些防止静电产生危害的相关知识是非常必要和重要的。

二、电子行业中静电障害的形成

电子行业中静电障害可分为两类:一是由静电引力引起的浮游尘埃的吸附;二是由静电放电引起的介质击穿;

1. 静电吸附

在半导体元器件的生产制造过程中,由于大量使用了石英及高分子物质制成的器具和材料,其绝缘度很高,在使用过程中一些不可避免的摩擦可造成其表面电荷不断积聚,且电位愈来愈高。表1列出了半导体元器件及其使用环境中部分物品表面的静电电位。

电子设备制造防静电技术要求

2.5.4重要工位上应配备腕带监视器,以随时监视腕带是否处于正常状态。

2.5.5在装配未采用防静电包装的零部件时,应消除静电后再进入防静电工作区。

2.5.6手拿SSD时,应避免接触其引线和接线片。

2.5.7服装、图纸资料等物品不得接触SSD。

2.5.8清洗SSD及其部件时,不得使用塑料刷。

2.5.9不具备防静电功能的必备工具、用具,应放在防静电桌垫上,并置于离子风静电消除器的作用范围内。

2.5.10操作时应尽量减少对SSD及其部件的接触次数。

2.5.11装联电子设备时,应使用有接地线的低压直流电动起子。

2.5.12在手工焊接时,应采用防静电低压恒温烙铁。

2.5.13在任何场合均不允许未采取防静电措施的人员接触SSD及其零部件。

2.5.14生产过程中使用的设备(成型机、插件机、波峰焊机、贴片机、切脚机、清洗机等),必须采取降静电措施。

3、关联部门的责任

3.1采购、检验

采购、检验人员应具备SSD一般保护常识,遵守静电防护操作规程

3.2贮存

3.2.1元器件库房管理人员应掌握SSD的一般保护常识

3.2.2库房必须是静电安全工作区,防静电基本设施见表2。

3.2.3 SSD入库、出库都必须装在防静电包装内,并遵守基本操作规程

3.3运输

运输SSD及其组件时,应将其放入防静电包装内进行,必要时,应使用离子空气来消除运输过程中产生的静电。

3.4调试

调试部件、整件、整机技术指标时,操作人员应遵守本标准中的有关规定。

3.5维修

3.5.1外出维修时应使用防静电维修箱(包)。

3.5.2维修时应遵守本标准有关操作规程。

贮存、运输SSD及其部件时,应妥善进行防静电包装。

4.5.2 拒绝接收未包装在静电防护容器里的SSD。

4.5,3 当需要把SSD及其部件从静电防护容器中取出时,必须在静电安全工作台上进行,并遵守以下事项,

a. 操作人员必须穿防静电工作服,

b. 戴上防静电腕带,腕带必须与皮肤有良好的接触并将腕带接入可靠的防静电接地系统中。

4.5.4 重要工位上应配备腕带监视器,以随时监视腕带是否处于正常状态.

4.5.5 在装配未采用防静电包装的零部件时?应消除静电后再进入防静电工作区。

4.5.6 手拿SSD日才,应避免接触其引线和接线片;

4.5.7 服装、图纸资料等物品不得接触SSD。

4.5.8 清洗SSD及其部件时,不得使用塑料刷.

4.5.9 不具备防静电功能的必备工具、用具,应放在防静电桌垫上

的作用范围内。

4.5,10 操作时应尽量减少对SSD及其部件的接触次数。

4.5.11 装联电子设备时,应使用有接地线的低压直流电动起子。

并置于离子风静电消除器

4.5.12 在手工焊接时,应采用防静电低压恒温烙铁。对GJBl649规定的I类SSD的焊接还应在拔掉烙铁电源插头后进行。

4.5.13 禁止重复使用器件包装管包装SSD。

4.5.14 在任何场合均不允许未采取防静电措施的人员接触SSD及其零部件。

4.5.15 生产过程中使用的设备(成型机、插件机、波峰焊机、贴片机、切脚机、清洗机等),必须采取降静电措施。

4.6 安全警告

在接通电源的电子设备上工作时,必须遵守有关安全操作规程。

4.7包装及标志

电子设备的防静电包装及标志应符合GJB 1649规定。

5 关联部门的责任

5.1 采购、检验

采购、检验人员应具备SSD一般保护常识,遵守静电防护操作规程。

5.2 贮存

5.2.1 元器件库房管理人员应掌握SSD的一般保护常识.

5.2.2 库房必须是静电安全工作区,防静电基本设施见表2。

5.2.3 SSD入库、出库都必须装在防静电包装内,并遵守基本操作规程。

5.3 运输

运输SSD及其组件时,应将其放入防静电包装内进行。必要时,应使用离子空气来消除运输过程中产生的静电。

调试

调试部件、整件、整机技术指标时,操作人员应遵守本标准中的有关规定。

5.5设计研制

5.5.1 应把防静电意识贯穿于新产品设计研制全过程中.

5.5.2 设计电路时,应尽量选择不易受到ESD损害的元器件,并尽可能采用静电抑制技术(例如开关接地,涂导电涂料等).

5.5.3 委托研制SSD时,应向承制方提出防静电要求

5.6 维修

5.6.1 外出维修时应使用防静电维修箱(包) .

5.6.2 维修时应遵守本标准有关操作规程

电子元器件的静电解决办法1.静电放电

 

静电放电(ESD)是大家熟知的电磁兼容问题,它可引起电子设备失灵或使其损坏。当半导体器件单独放置或装入电路模块时,即使没有加电,也可能造成这些器件的永久性损坏。对静电放电敏感的元件被称为静电放电敏感元件(ESDS)。

如果一个元件的两个针脚或更多针脚之间的电压超过元件介质的击穿强度,就会对元件造成损坏。这是MOS器件出现故障最主要的原因。氧化层越薄,则元件对静电放电的敏感性也越大。故障通常表现为元件本身对电源有一定阻值的短路现象。对于双极性元件,损坏一般发生在薄氧化层隔开的已进行金属喷镀的有源半导体区域,因此会产生泄漏严重的路径。

另一种故障是由于节点的温度超过半导体硅的熔点(1415℃)时所引起的。静电放电脉冲的能量可以产生局部地方发热,因此出现这种机理的故障。即使电压低于介质的击穿电压,也会发生这种故障。一个典型的例子是,NPN型三极管发射极与基极间的击穿会使电流增益急剧降低。

器件受到静电放电的影响后,也可能不立即出现功能性的损坏。这些受到潜在损坏的元件通常被称为“跛脚”,一旦加以使用,将会对以后发生的静电放电或传导性瞬态表现出更大的敏感性。

要密切注意元件在不易察觉的放电电压下发生的损坏,这一点非常重要。人体有感觉的静电放电电压在3000—5000V之间,然而,元件发生损坏时的电压仅几百伏。

静电放电的危害效应是在二十世纪七十年代开始认识到的,这是由于新技术的发展导致元件对静电放电的损坏越来越敏感。静电放电造成的损失每年可达到几百万美元以上。因此,许多大型的元件和设备制造厂引进专业技术以减小生产环境中的静电积累,从而使产品合格率和可靠性提高了许多。用户根据自己的经验也懂得了防治静电放电损害的重要性。

2.如何对付静电放电?

控制静电积累的第一步是要弄清楚静电荷的产生机理。

静电电压是由不同种类的物质相互接触与分离而产生。尽管摩擦能够使电荷积累得更多,但是摩擦并不是必要的。这种效应即是大家熟知的摩擦起电,所产生的电压取决于相互摩擦的材料本身的特性。磨擦起电序列表列出了各类物质的带电难易程度。对于相互接触的两种物质,电子会从序列表较上的物质转向较下的物质,这样就会使两种物质分别带正负电荷。序列表中的物质离得越远,各自所带的电荷数量也越大。常见物质的磨擦起电序列如下表所示:

表1摩擦起电序列

● 人体+

● 玻璃

● 云母

● 聚酰胺

● 毛织品

● 毛皮

● 丝绸

● 铝

● 纸

● 棉花

● 钢铁

● 木头

● 硬橡胶

● 聚脂薄膜

● 聚乙烯

● 聚氯乙烯

● 聚四氟乙烯(PVC)大多数是负极性的

电荷也可通过感应产生,这是带电体使其附近的另一物体上的电荷发生分离的结果

3.实际问题解决

问题的解决包括:如果静电放电敏感元件(ESDS)在生产和维护期间暴露在外面,那么在这些元件附近,应防止电荷的积累,并且在运输和保管过程中,将这些元件按防静电放电的方法包装。

防止静电放电,有许多方法可以采用。最好的办法是满足要求且成本最低的方法,这样的方法对于不同的产品和不同的场合都是不同的。

4.静电放电保护区域(EPA)

静电放电保护区域(EPA),有时指安全操作区,是任意一种静电放电控制措施的核心所在。在此区域中,静电放电敏感元件(ESDS)或电路板,或包含这些的组件,都可以很安全地工作,因为电荷的数量得到控制,而不会产生破坏性电压。这种区域中通常包含工作台或工作台组、工作站、自动插件机一类的处理设备或者一块生产区。EPA的范围必须清楚的标明,最好设置一围栏以防止未经允许的无关人员入内。EPA区域内应使用静电荷积累最小的材料,并且可使电荷以受控制的方式泄入到大地中。


表2 静电放电保护区

A1接地轮

A2接地滑片

A3接地面

B1腕套测试器

B2脚跟接地测试器

B3脚跟接地底板

C1腕套和腕套绳

C2接地线

C3静电放电接地设施

C4地

C5接地搭接点

C6大地接地点

C7手套

C8脚趾和脚跟带箍

D1电离剂

E1工作面

F1腿和座套已接地的转椅

G1人体接地地板

H1工作服

H2工作帽

I1具有接地面的搁板

I2接地机架

J1静电放电保护区标志

 

典型的静电放电保护区如图1所示,此图摘自EN100015-1,其中许多是新近的。该图给出了各种可能的措施,但并没有必要全部使用这些措施,这主要由特定的环境决定。所采用的其本原理就是等电位搭接,即将所有表面连接在一起,防止不同物体之间产生电位差。

工作面[E1]是静电损耗性的,通过静电放电的接地设施[C3]连接到地[C4]。工作站的操作人员通过导电腕[C1]上的导线和地电位点相连,然而,对于活动频繁的人员,最好是通过脚跟和脚趾带箍[C8]与静电损耗地板[G1]相通(即接地)。腕套的接地导线在接地点[C5]处进行端接。

操作人员所穿的破旧工作服[H1、H2]也应是静电损耗性的,并在靠近静电放电敏感元件附近遮住工作人员自己的衣服,所有破旧的手套也应是导电材料的。

转椅[F1]不应视为操作人员接地的基本方法,但值得注意的是,转椅上必须铺一层抗静电的材料,使座套、靠背和扶手均有与地相通的路径。

元件应存放在带有接地面的搁板[I1]上,或者是接地的机架[I2]上。这些东西与工作台都应通过接地导线[C2]与静电放电的地面相接。

当用手推车装运元件或子配件时,其表面导电性能应与工作面和导电机架的导电性能相似。如果接地轮[A1]导电性良好,且与手推车车架电气连接,那么不再需要使用接地滑片了。如果EPA的地板没有接地,那么当手推车停下来装卸东西时,则应将其接地点[C6]与大地接地点[C5]相连。

在操作人员的正常工作期间,对所采用的这些措施的效果应该用静电伏特计测量其静电势和静电场来评估。

在保护区内和进出口处,应使用标志[J1]来提醒他们注意。

应对腕套及其接地导线用导通测试仪定期进行检测。导电轮和脚趾带箍也应作类似的检测[B2,B3]。

5. safety

EPA内一般有加电的工具和设备。在这种环境中,将单个物件或设备直接连接到地是很危险的。正是由于此原因,腕套接地导线、转轮及脚趾带箍的连接处均要串入一只不低于1M的电阻。有些腕套接地导线的每端均有一只这样的电阻,因此,即使腕套接地导线接在加电维修的产品的带电接线端,也不会有危险。

腕套接地导线测试仪是一种检测电阻的阻值是否合适(如果太高,不可能实现等电势搭接;如果太低,会出现安全危害)的仪器。

腕套接地导线要配以可快速拔下的与其它电气插座不兼容的插头,这样可以保证它不会误插到其它电气插座上,并且,在紧急情况下容易拔下。

6.静电放电保护区内的实际工作

在静电放电保护区内,如果不遵守明确的工作规范,电荷和电势就不能保持在允许的范围内。一些会导致问题的例子包括:将装在不抗静电的塑料封面内的文件、塑料容器、杯子等带进静电放电保护区内,使用会破坏地板或工作表面静电特性的清洁剂。

有关人员应接受足够的训练,不仅学习需要遵守的规程,还要了解必须遵守这些规程的理由。了解可能损坏的元件的有关参数也是有用处的。

应指定专人负责静电放电保护区的保养与维护,同时还要对规程的执行情况进行检查。这些检查也应作为质量管理体系认证的一部分加以核查。

7.运输与存放

运输带引脚的元件时,通常使用导电泡沫材料。这可以防止元件引脚间出现较高的电势差,对于双列直插式封装的元件,在散装运输过程中常采用静电损耗性管。

对于线路板组件,当位于静电放电保护区外时,应将其置于静电屏蔽袋或导电搬运箱内进行运输。有的包装袋使用导电材料制成,它可确保所有元件在稳定条件下处于同一电势,同时将偶然跑到袋上的静电荷耗散掉。这种方法不能用于带电池的电路板,对于这种情况,应采用衬里是静电损耗性材料的,而外层是导电材料的包装袋。这种袋子的价格更高,但可对加电和未加电的组件提供极好的保护作用。同样,内部装有固定电路板的导轨的导电箱不能与边缘上有裸露连接器的加电电路板一起使用。

8.现场维修

An electrostatic connection point should be set on the product to be repaired on site, so that the maintenance technician can connect the ground wire of the wristband before opening the cover of the device. Spare parts should be transported in static shielding bags or cases unless they do not contain electrostatic discharge sensitive components. If the module is working in an exposed state, connect a static-dissipative floor mat to the product's electrostatic bonding point and use it as a work surface.

9. Related standards

In 1987, the United Kingdom made its first attempt to document practices, with the result being BS5783. Rather than calling it a standard about what tests should be performed, it is better to call it a code of practice. The second phase of this work is to translate this standard into a specification in the European organization, whose number is CECC000151, and its title is: "Basic Specification: Protection of Electrostatic Sensitive Components Part 1: General Requirements." This standard was published in 1991 and renumbered as EN1000151 in 1992. The other sections were published in 1993 (part two: requirements for low humidity conditions) and 1994 (part three: requirements for clean areas, part four: requirements for high pressure environments). The content of these sections is beyond the scope of this article.

The standard not only includes requirements for installation, maintenance, and inspection of the measures described in this article, but also details the detailed requirements of the electrostatic protection device itself, including test methods.

The continuous development of technology and processes and the experience accumulated in the implementation of standards and the widespread use of automated machinery and equipment have led to the continuous improvement of these standards, including the rationalization of their structure, and the separation of user guides from standardized versions. The revision work has been included in the international forum organized by the International Electrotechnical Commission. The new standards will be published in the IEC1340 series. There is no doubt that this is complementary to European standards. The relevant standards are shown in Table 2:

Table 2 International Electrotechnical Commission 1340 scheme structure

IEC 1340-1

summary

IEC 1340-1-1

Guide to electrostatic principles

IEC 1340-1-2

Definitions and terms

IEC 1340-2

Static measurement method

IEC 1340-2-1

Consumption of electrostatic charge

IEC 1340-2-2

Rechargeability

IEC 1340-2-3

Resistance and Resistivity

IEC 1340-3

Static Effect Simulation Method

IEC 1340-3-1

Mannequin

IEC 1340-3-2

Machine model

IEC 1340-3-3 charging element model

IEC 1340-3-4

Field effect model

IEC 1340-4

Standard test methods for special occasions

IEC 1340-4-1

Estimates of floor mats

IEC 1340-4-2

package

IEC 1340-4-3

Footwear

IEC 1340-5

Protection details of electrostatic sensitive components

IEC 1340-5-1

Overall demand

IEC 1340-5-2

User guide

IEC 1340-6

Static control technology and method for evaluating its effect

IEC 1340-6-1

Ionizer

Anti-static measures for the electronics industry

Static electricity is ubiquitous in life and production. It is from the rubbing of hands and feet to the flow of dry air. The static electricity is the last thing. If the conditions are suitable, the starting point is about a few volts and the peak is over a few hundred. Thousands of volts can be realized in an instant. These all pose a great threat to static-sensitive circuits such as CMOS, let alone the damage caused by equipment leakage. Therefore, the electronics industry takes static electricity as its enemy and makes every effort to open the door outer.

Static electricity is relative to "dynamic electricity", that is, the flowing charge in a conductor, which is a charge that does not flow under normal circumstances. It is mostly caused by friction between insulators or dry air and friction between insulators. When its energy has accumulated to a certain degree When the insulator that prevents it from being neutralized can no longer be blocked, a severe discharge occurs, that is, electrostatic discharge (ESD), and the maximum voltage at this time can reach thousands or even tens of thousands of volts. It is bound to cause damage to static-sensitive components (see Table 1 and Table 2)

Table 1.Electrostatic voltages that are easily generated at the production site.

Production occasion

Electrostatic voltage

Humidity 10 ~ 20%

Humidity 65 ~ 90%

When walking on the carpet

35000V

1500V

When walking on vinyl floors

12000V

250V

When holding a vinyl plastic bag in your hand

7000V

600V

When contacting polyester bags at the assembly line

20000V

1200V

When contacting the polyurethane with the operating station

18000V

1500V

Table 2 Breakdown voltage of static electricity on some electronic devices.

Device type

Minimum sensitivity for EOS / ESD

(Expressed by electrostatic voltage V)

VMOS

MOSFET

GaAs FET

EPROM

JFET

SAW (Surface Acoustic Wave Filter)

Operational Amplifier

CMOS

Schottky diodes

SMD film resistor

Bipolar transistor

Emitter-Coupled Logic Circuit

Thyristor

Schottky TTL

30 ~ 1800

100 ~ 200

100 ~ 300

100 or more

140 ~ 7000

150 ~ 500

190 ~ 2500

250 ~ 3000

300 ~ 2500

300 ~ 3000

380 ~ 7800

500 ~ 1500

680 ~ 1000

100 ~ 2500

Lightning is formed by high-voltage electrostatic discharge caused by friction between air currents and water droplets in clouds. When high-voltage charged clouds pass near buildings, some of the charges can be neutralized by the "tip discharge" effect of the lightning rod; when the amount of charge in the clouds is too large, or the clouds move When it is too fast to fully neutralize, a lightning strike will be formed by a lightning discharge from the lightning rod. In both cases, especially during the lightning strike, the entire building and the nearby ground are charged, and the damage caused by the lightning strike is mainly direct lightning and lightning induction. Because people are in the "equal-potential" state in buildings, like birds falling on high-voltage lines, they are generally not subject to lightning strikes. However, lightning induction (ultra high-voltage electrostatic induction and strong electromagnetic induction) can cause damage to electrostatic sensitive devices.

Equipment leakage, especially the small leakage that does not cause electric shock to people is not static electricity. Although most people can hardly feel it, but due to its universality (any electrical equipment always has some leakage) and high internal resistance It has the characteristics of generating peak electrical pulses with a maximum approximate power supply voltage (100 ~ 400V) and a short time, which is still enough to cause electrical overload (EOS) damage to electrostatic sensitive devices. Therefore, it is also an extremely important aspect in electrostatic protection systems.

The main mechanisms of electrostatic discharge (ESD) and electrical overload (EOS) causing damage to electronic components are: thermal secondary breakdown; metal coating melting; dielectric breakdown; gas arc discharge; surface breakdown; body breakdown and so on. See Schedule 3

Component category

Component components

Failure mechanism

Invalidation flag

MOS structure

MOSFET (discrete)

MOS integration

Digital integration

Linear integration

Hybrid circuit

Voltage-induced dielectric breakdown and subsequent high-current phenomena

Large short circuit leakage

Semiconductor junction

Diode (PN.PIN Schottky)

Bipolar transistor

Junction field effect tube

Thyristor

Bipolar integrated circuits, MOSFET and MOS integrated circuits

Secondary breakdown and micro-diffusion of microplasma caused by excess electrical energy and overheating

Increase in current beam caused by diffusion of Si and AL (electrothermal migration)

Lapse

Thin film resistor

Hybrid integrated circuit (thick film, thin film) resistor monolithic integrated circuit thin film resistor

Hermetic film resistor

Dielectric breakdown, destruction of voltage-dependent current paths and micro-current paths related to Joule heat

Resistance drift

Metallized strip

Hybrid IC

Monolithic IC

Comb cover transistor

Burnout of metal related to Joule heat energy

open circuit

Field effect structure and non-conductive cover plate

Memory

EPROM, etc.

Barrier of positive ions and surface area due to ESD, causing surface inversion or grid threshold voltage drift

Performance degradation and failure

Voltage transistor

Crystal Acoustic Surface Wave

Crystal breaks due to mechanical force caused by excessive voltage

Performance degradation and failure

Small distance between electrode valves

Surface acoustic wave device

Various microcircuits in IC

Electron discharge causes electrode material to melt

Performance degradation and failure

The basic principle of antistatic should be to prevent and suppress the generation and accumulation of electrostatic charges, and to quickly and safely and effectively eliminate the generated electrostatic charges. However, many measures of antistatic are a set of system engineering, and the omissions in one link may be thousands of miles away. The embankment is broken in the anthill's urn, so don't be careless.

1. Buried anti-static ground wire:

(1). The lightning rod of the factory building is generally welded with the reinforced concrete of the building and properly grounded. When a lightning strike occurs, the grounding point and even the entire building floor will become the discharge point of high voltage and large current. "Step voltage" will be generated within the range of 20M, that is, the ideal zero potential is no longer within this range. In addition, as the neutral line of the three-phase power supply cannot be absolutely balanced, an unbalanced current will also be generated and flow into the neutral line. Grounding point, so the buried point of the anti-static ground wire should be 20 meters away from the building and equipment ground (see Figure 1)

(2). Buried method: In order to ensure reliable grounding, at least three points should be grounded, that is, a pit deeper than 1.5m every 5m, and an iron pipe or angle iron of more than 2m is driven into the pit (that is, the angle iron is inserted above 2m underground) ), Then these three places are welded together with a 3mm thick copper bar, and 16m㎡ insulated copper core wires are welded and introduced into the indoor trunk line.

(3). Appropriate amount of charcoal powder and industrial salt shall be applied in the pit to increase soil conductivity. After the landfill is measured with a ground resistance tester, the ground resistance shall be <4ω. (See Figure 2) and tested at least once a year. <>

2. Laying and testing of anti-static ground wire:

(1). All anti-static ground wires use 6m㎡ multi-strand copper-core insulated wires. Each floor or appropriate section uses copper bars or more than 40A switches, and the gate is connected to the main line to facilitate inspection and maintenance.

(2) The anti-static ground cable should be well insulated from the equipment case, workbench iron frame, work light frame, etc. to prevent short circuit, overlap or broken connection.

(3). Paving another inspection line at the "trunk end" of the segmented copper bar or switch. (1.5 ~ 2m2 is enough). Each workshop shall have 2 ~ 3 inspection points, fixed and clearly marked.

(4). Measurement: use pointer multimeter, resistance file.

a). The resistance between each antistatic test point and the antistatic ground wire is 5 ~ 15Ω, ideally it should be 0Ω. But the actual measurement is 2m㎡, the resistance from the test point to the summary point + 6m㎡, the conductor from the summary point to the measured point The sum of the point resistances, this value is about 5-15Ω and is basically unchanged.If the measurement result tends to infinity, it is because the anti-static ground line or the measurement line has a broken wire, and it should be repaired in time.

b) The resistance between the anti-static ground and the equipment ground. This resistance value is composed of the line resistance of the anti-static ground wire itself + the line resistance of the equipment ground wire + the ground resistance between the two ground wires. However, due to the dryness and wetness of the ground between the two ground wires The influence of ground current is very complicated, especially the current, the frequency of the direction, etc. are changing every moment, and the measurement result is mainly determined, so it can only be measured with a pointer meter, and its value ranges from a dozen ohms to hundreds of K It is normal, only that there is no short circuit or open circuit between the two places.

3. Anti-static floor (see Figure 4):

The most standardized anti-static floor is a composite structure similar to anti-static rubber.The lower layer is a conductive layer connected to the anti-static ground, and the upper layer is an insulating anti-static generation layer, which will not generate static electricity due to walking friction. The conductive layer should be insulated when laying. The mat is separated from the building floor and wall to prevent the floor from static electricity during a lightning strike, and the conductive layer is connected to the anti-static ground through a 1MΩ20W resistor. It plays a role of electrostatic shielding and electromagnetic shielding. This floor is too expensive, but it can be effective Prevent various hazards of lightning and static electricity.

General electronic factories often use simple anti-static floors (only the insulation anti-static generation layer is mostly paint or floor glue), which is directly laid on the building floor, which greatly reduces the cost and can also prevent the static electricity generated by walking. The ultra-high voltage electrostatic induction and strong electromagnetic induction generated by lightning strikes have poor protection.

4.Anti-static work surface: as shown in Figure 4

The green surface of the anti-static rubber is an anti-static generation layer, which has a large resistance and a surface resistance of 108 ~ 1010Ω.

The black surface of the antistatic rubber has a small resistance, the surface resistance is 104 ~ 106, and it is well connected to the green surface, which can ensure a good ground. It can act as an electrostatic shield and discharge. It can be connected by a buckle, and a dedicated electrostatic bracelet wire (included) 1M resistor) ground. Or put a 0.2mm thick iron plate or copper foil on the insulating table, solder the wires to the electrostatic ground wire through the 1MΩ resistor, and then flatten the antistatic rubber (black side down, close to the conductive sheet). The 1MΩ resistor also provides a static discharge path to prevent over-speed discharge from sparking and isolating.

Even chairs (stools) should be paid attention to. Most production lines use ordinary plastic stools, which are prone to generate static electricity through friction with clothing. If possible, use anti-static chairs and connect them to static ground through 1MΩ resistors. Put on a static cloth.

5.Grounding and testing of electric equipment such as electric irons, small tin furnaces, test instruments:

Electric soldering irons, small tin furnace testing instruments, etc. must be properly connected to the equipment ground with a three-terminal plug. It is not difficult to do so, but because of frequent occurrences such as: the socket ground terminal is loose, the wire is disconnected, and the soldering iron tip is oxidized and connected to the casing (Ground) disconnection and other phenomena, so it should be tested every shift, and can be tested with self-made simple on-off indicator lights, and immediately replace the problem if found.

6. Anti-static clothing (clothing, shoes, gloves, etc.):

The so-called anti-static clothing is woven with special synthetic fibers. Generally, rubbing and friction will not generate static electricity. However, it is not a static shielding clothing. It cannot eliminate the static electricity generated by other clothing on the body. Therefore, it should be worn only inside. A shirt or underwear, wearing an anti-static clothing. Wearing multiple pieces of chemical fiber in winter, and wearing anti-static clothing with wool clothing is not very useful. Therefore, it is more important to control the temperature and humidity of the environment, and wearing an electrostatic bracelet is more important than electrostatic clothing. Anti-static gloves are used to prevent the generation of static electricity; to isolate hands from products (insulation); to prevent sweat from contaminating the product, and so on, are necessary.

7.Anti-static bracelet:

The anti-static wristband is grounded by a stainless steel case that is close to the wrist, and is grounded by a wire and an iron clip through a 1MΩ resistor in the wire. The purpose is not only to discharge the static electricity on the human body at any time, but also to prevent sparks caused by rapid discharge. Causes damage and acts as an isolation. And broken wires or poor contact can make the static hand ring virtual. The so-called wireless bracelet can't actually discharge the static charge carried by the human body.

(1). Standardize the wearing and clamping method of electrostatic bracelet:

a. The stainless steel case of the electrostatic wristband should be worn on the inside of the left wrist, where the contact resistance is the smallest.

b. It should be in close contact with the skin, not loose, and not separated by clothes.

c. Alligator clips should be used to hold the exposed part of the electrostatic ground wire at the root instead of using the front teeth.

d. When leaving work or walking, the operator can take off the bracelet, mobile staff (cadres, quality control) should remove the clip and wrap it around the wrist for mobile use.

(2). The static electricity ring should be tested and recorded in the morning and afternoon each shift. The tightness is subject to passing the test. Any unqualified should be adjusted or replaced immediately.

(3). Do not wear wireless bracelet.

8.Install ion fan:

The temperature of the pre-heating part of the wave furnace is 80 ~ 120 ℃, and static electricity is easily generated under such high temperature and dry hot air. The ion fan uses high pressure to ionize the air into positive and negative ions, and the fan blows the air containing a large number of positive and negative ions into the furnace. In order to neutralize the static electricity generated by the hot air on the PCB and components, an ion fan should be installed at the entrance of the wave furnace. At the entrance of the anti-static work area, the starting point of the conveyor belt or above can also be installed with an ion fan according to actual needs.

9.Add anti-static cleaning roller to the conveyor belt:

Self-made simple device: Wrap a hard plastic tube with a length slightly smaller than the width of the conveyor belt around a towel cloth (it should be flat), and then use iron rods (as shafts) to fix it at both ends of the conveyor belt after wetting. The bottle method is constantly humidified. When the conveyor belt is started, the roller rotates with the conveyor belt under its own weight to clean, humidify and prevent static electricity. This simple self-made device can function as an ion fan of the conveyor belt in certain occasions.

10. PCB on the assembly line (mainly refers to post-DIP repair welding, testing, assembly and other processes) should be equipped with an anti-static sponge pad to prevent static electricity and board surface scratches. Transfer between processes using anti-static board storage trucks Or the card box (the surface resistance is below 106Ω), and the ground is properly connected through a 1MΩ resistor.

It is important to distinguish between static shielding materials (bags) and antistatic materials (bags). Antistatic materials (bags and pads) are mostly pink. They are only used as cheap pads and intermediary packaging for electrostatic sensitive devices. Only static electricity is generated, and if there is an electrostatic discharge, it can pass through these anti-static materials to cause harm. Static shielding packaging is mostly silver, black, gray, aluminum foil-shaped opaque and black, gray translucent materials, and now has a grid-like shape. Full transparent materials, etc., the basic principle is to vacuum-coat a layer of aluminum on the outside of the antistatic material as a conductive electrostatic shielding layer. When an electrostatic potential is generated, the shielding layer will evenly distribute the induced electrostatic potential across the entire packaging surface, reducing the surface potential difference. Prevents local high-potential electrostatic discharge, and also has a good shielding effect on high-frequency and strong electromagnetic fields. With different protection levels, the conductive layer of the electrostatic shielding packaging material is also divided into: outer resistance layer-insulation film-aluminum foil layer-insulation film; insulation film ─Aluminum foil layer─Insulation film; Vacuum coating layer─Insulation film; Printed conductive grid layer─Insulation film and other grades. A little attention can be found when the IC leaves the factory. The outer packaging is 2 ~ 3 layers of electrostatic shielding material, and the internal supporting materials (such as push plates and feeding tapes) are black high-resistance conductive materials, about 106MΩ, which only serves to shield and neutralize the electrostatic potential of each pin. The role of static discharge path. When static-sensitive components (such as ICs) and products are shipped, they must be packaged with static shielding materials instead of anti-static bags.

11. Regulation of temperature and relative humidity:

Electronic operations, especially SMT, have high requirements on temperature and humidity.The general temperature is controlled at 18-28 ° C. Too high or too low will affect the normal operation and accuracy of the equipment; the relative humidity should be 50% to 85%, If it is too low, it is easy to generate static electricity (see Table 1). If it is too high, the equipment is prone to dew condensation and the solder paste contains more water, so monitoring and regulation should be strengthened. For antistatic, when the relative humidity in autumn and winter is low, you can use a humidifier or wet cloth Local solution.

12. Other

(1) The electric soldering iron should be an antistatic low voltage constant temperature soldering iron as far as possible and well grounded.

(2) Low-voltage DC electric screwdriver (electric driver) using a ground wire.

(3) Anti-static brushes should be used for small batch cleaning of PCBs. Do not use ordinary plastic brushes.

(4) In some occasions, ceilings and walls should use antistatic materials. Generally speaking, even ordinary gypsum boards and lime-painted walls can be used, but the use of plastic ceilings and ordinary wallpapers and plastic wallpapers is prohibited.

13.Static tester:

If conditions are available, consider adding a "hand-held non-contact static tester", so that you can monitor the generation, size, and tracking of the defects caused by static electricity in real time to understand the improvement effect. Because its price is too expensive, most companies dare not ask.

14.Monitoring and recording:

The anti-static measures must be implemented by a special person and a system can be implemented. Otherwise, all hardware investment may not be effective.

(1). Personnel: Two people should manage, test, and record part-time. In most cases, two people are required to cooperate and prevent the flow of personnel.

(2) Tests and records: In summary, the following tests and records should be completed every day.

a. Static test point -------- Electrostatic ground Analog multimeter

Electrostatic ground ----------- Equipment ground Resistance measurement

b. Grounding the tip of the soldering iron / temperature measurement of the soldering iron tip.

c. Tin furnace grounding / tin furnace temperature measurement.

d. Test instrument ground measurement.

e. Electrostatic wristband grounding test. Electrostatic wristband tester

f. Indoor temperature / relative humidity measurement and regulation. Temperature / hygrometer.

(3) Check the implementation of the staff's dress and anti-static regulations in the anti-static work area.

(4). If necessary, use a static tester to measure the static voltage in various situations at the work site and on the assembly line.

The electrostatic voltage should generally be less than 100V, and in special cases should be less than 25V.

15.Training and literacy: Anti-static knowledge / measures should be taken as an important content of all staff training so that every employee can understand and form good professional habits. For example:

Clothing, drawings and other materials must not touch components, drawings and materials should be placed in anti-static document bags and hung;

Plastic boxes, such as leather, cardboard, glass, etc., which are prone to static electricity, are not allowed to be stacked on static safety workbenches;

You must load the electrostatic ring and gloves before you can touch the components. Hold the PCB or sensitive devices by the edges as much as possible to avoid touching their leads and terminals;

Consciously abide by and implement anti-static rules and regulations.


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