1. Electrostatic discharge

Electrostatic discharge (ESD) is a well-known electromagnetic compatibility problem that can cause electronic devices to malfunction or damage them. When a semiconductor device is placed alone or loaded into a circuit module, permanent damage to these devices may occur even if power is not applied. An element that is sensitive to electrostatic discharge is called an electrostatic discharge sensitive element (ESDS).

       If the voltage between two or more pins of a component exceeds the breakdown strength of the component media, it can cause damage to the component. This is the main reason why MOS devices have failed. The thinner the oxide layer, the greater the sensitivity of the component to electrostatic discharge. The fault usually manifests itself as a short circuit in which the component itself has a certain resistance to the power supply. For bipolar components, damage typically occurs in the active semiconductor regions of the metallization that are separated by a thin oxide layer, thus creating a path of severe leakage.

       Another type of failure is caused by the temperature of the node exceeding the melting point of the semiconductor silicon (1420 ° C). The energy of the ESD pulse can generate localized heating, so failure of this mechanism occurs. This failure occurs even if the voltage is lower than the breakdown voltage of the medium. A typical example is that the breakdown between the emitter and base of the NPN transistor causes a sharp drop in current gain.

Functional damage may not occur immediately after the device is affected by electrostatic discharge. These potentially damaged components are often referred to as 'squats' and, once used, will exhibit greater sensitivity to later electrostatic discharges or conductive transients.

It is important to pay close attention to the damage that the component can experience under undetectable discharge voltages. When the electrostatic voltage reaches 2000 volts, the finger feels. When it exceeds 3000 volts, there will be sparks and a needle-like pain. When it exceeds 7000 volts, people have a sense of electric shock. The static voltage generated in daily life can sometimes be Up to tens of thousands of volts. However, since the frictional electrification time is extremely short, the amount of current generated is also small, so that it is generally not life-threatening to the human body. However, when the electronic component is damaged, the voltage is only a few hundred volts, and the sensitive device has only a few volts. .

The harmful effects of electrostatic discharge were recognized in the 1970s because of the development of new technologies that made components more sensitive to electrostatic discharge damage. The damage caused by electrostatic discharge can reach several million dollars per year. As a result, many large component and equipment manufacturers have introduced expertise to reduce static buildup in the production environment, resulting in a much higher product yield and reliability. Users understand the importance of preventing electrostatic discharge damage based on their own experience.

2. How to deal with electrostatic discharge?

The first step in controlling the accumulation of static electricity is to clarify the mechanism of static charge generation.

The electrostatic voltage is generated by the contact and separation of different kinds of substances. Although friction can accumulate more charge, friction is not necessary. This effect is known as triboelectric charging, and the voltage generated depends on the nature of the material itself that rubs against each other. The friction sequence book lists the ease of charging of various substances. For two substances in contact with each other, the electrons will shift from the upper part of the sequence list to the lower substance, which will cause the two substances to have positive and negative charges, respectively. The further away the substances in the sequence listing are, the greater the amount of charge they carry. The frictional electrification sequence of common substances is shown in Table 1.

The charge can also be generated by induction, which is the result of the charged body separating the charge on another object in the vicinity.

3. Practical problem solving

The solution to the problem includes: if the electrostatic discharge sensitive element (ESDS) is exposed to the outside during production and maintenance, the accumulation of charge should be prevented in the vicinity of these components, and these components should be electrostatically discharged during transportation and storage. Method packaging.

There are many ways to prevent electrostatic discharge. The best approach is to meet the requirements and the lowest cost method, which is different for different products and different occasions.

4. Electrostatic discharge protection zone (EPA)

The Electrostatic Discharge Protection Area (EPA), sometimes referred to as the Safe Operating Area, is at the heart of any ESD control measure. In this area, an Electrostatic Discharge Sensing Element (ESDS) or circuit board, or a component containing these, can work safely because the amount of charge is controlled without creating a destructive voltage. Such areas typically include work stations or workbench sets, workstations, processing equipment such as automated plug-in machines, or a production area. The scope of the EPA must be clearly marked, and it is best to have a fence to prevent unauthorized entry. Materials with the least accumulation of static charge should be used in the EPA area and allow the charge to leak into the earth in a controlled manner.

Typical ESD protection zones are shown in Figure 1, many of which are recent. The figure shows the various possible measures, but it is not necessary to use them all, which is mainly determined by the specific environment. The principle used is equipotential bonding, which connects all surfaces together to prevent potential differences between different objects.

The working surface [E1] is electrostatically lossy and is connected to the ground [C4] by a grounding facility [C3] for electrostatic discharge. The operator of the workstation is connected to the ground potential point by the wire on the conductive wrist [C1]. However, for people with frequent activities, it is best to communicate with the static loss floor [G1] through the heel and toe band [C8] (ie grounding). ). The ground wire of the wristband is terminated at the grounding point [C5].

The worn work clothes [H1, H2] worn by the operator should also be electrostatically lossy, and cover the workers' own clothes near the electrostatic discharge sensitive components. All worn gloves should also be made of conductive materials.

The swivel chair [F1] should not be regarded as the basic method for the operator to ground, but it is worth noting that the swivel chair must

Lay a layer of antistatic material so that the seat cover, backrest and armrest have a path to the ground.

The component should be stored on a shelf [I1] with a ground plane or on a grounded frame [I2]. These things and the workbench should be connected to the ground of the electrostatic discharge through the grounding wire [C2].

When a component or subassembly is shipped by hand cart, its surface conductivity should be similar to that of the work surface and the conductive frame. If the grounding wheel [A1] is electrically conductive and electrically connected to the trolley frame, the grounding slide is no longer required. If the floor of the EPA is not grounded, then when the cart is stopped for loading and unloading, connect its grounding point [C6] to the earth grounding point [C5].

During the normal operation of the operator, the effects of these measures should be evaluated by measuring their electrostatic potential and electrostatic field with an electrostatic voltmeter.

In the protected area and at the entrance and exit, the sign [J1] should be used to alert them.

The wristband and its grounding conductor should be tested regularly with a continuity tester. Conductive wheels and toe straps should also be similarly tested [B2, B3].

5. safety

There are generally powered tools and equipment within the EPA. In this environment, it is dangerous to connect a single item or device directly to the ground. For this reason, the connection of the wrist strap grounding wire, the runner and the toe band must be connected to a resistor of not less than 1M. Some wristband grounding conductors have one such resistor at each end, so even if the wristband grounding conductor is connected to the live terminal of a power-up repaired product, there is no danger.

    The Wrist Strap Ground Wire Tester is a device that detects if the resistance of the resistor is appropriate (if it is too high, it is impossible to achieve equipotential bonding; if it is too low, there will be a safety hazard).

The wrist strap grounding conductor should be equipped with a plug that can be quickly removed and is not compatible with other electrical outlets. This ensures that it will not be accidentally inserted into other electrical outlets and can be easily removed in an emergency.

6. Actual work in the ESD protection zone

In the ESD protection zone, the charge and potential cannot be kept within the allowable range if the specified operating specifications are not followed. Some examples of problems that can be caused include the introduction of documents, plastic containers, cups, etc., placed in a plastic cover that is not antistatic, into an ESD protection zone, using cleaners that can damage the electrostatic properties of the floor or work surface.

The person concerned should receive adequate training not only to learn the procedures to be followed, but also to justify the reasons for observing them. It is also useful to know the relevant parameters of components that may be damaged.

Special personnel should be assigned to take care of the maintenance and maintenance of the ESD protection zone, and also check the implementation of the procedures. These inspections should also be verified as part of the quality management system certification.

7. Transportation and storage

Conductive foam materials are commonly used when transporting components with leads. This prevents high potential differences between component leads. For dual in-line package components, static loss transistors are often used during bulk shipping.

For the circuit board assembly, when it is outside the electrostatic discharge protection zone, it should be transported in an electrostatic shielding bag or a conductive tote. Some bags are made of a conductive material that ensures that all components are at the same potential under stable conditions while dissipating the static charge that occasionally runs onto the bag. This method cannot be used for a circuit board with a battery. In this case, the lining is made of an electrostatic loss material, and the outer layer is a package of a conductive material. The bag is more expensive, but provides excellent protection for both powered and unpowered components. Similarly, the conductive box with the rails on which the circuit board is mounted must not have a bare connection to the edge.

The power-up boards are used together.

8. On-site repair

An electrostatic connection point should be placed on the product that needs to be repaired on site, so that the service technician can connect the grounding wire of the wristband before opening the cover of the device. Spare parts should be transported in an electrostatically shielded bag or case unless the electrostatic discharge sensitive component is not included in the spare part. If the module is exposed to the exposed condition, the static loss floor mat should be attached to the electrostatic junction of the product for use as a work surface.

9. Relevant standards

In 1987, the United Kingdom conducted its first attempt to document practical procedures, the result of which was BS5783. Rather than saying that it is a standard for which tests should be carried out, it is better to call it a practice norm. The second phase of this work is to convert this standard into a specification in the European organization, numbered CECC 000151, entitled: “Basic Specifications: Protection of Electrostatic Sensitive Components Part 1: General Requirements”. The standard was published in 1991 and renumbered EN 1000151 in 1992. Other parts were published in 1993 (Part II: Low Humidity Conditions Requirements) and 1994 (Part III: Cleaning Area Requirements, Part IV: High Pressure Environmental Requirements). The content of these sections is beyond the scope of this article.

The standard includes not only the requirements for installation, maintenance and inspection of the measures described herein, but also the detailed requirements of the electrostatic protection device itself, including the test method.

The continuous development of technology and processes and the experience gained in the implementation of standards, as well as the widespread use of automated machinery and equipment, have led to the continuous improvement of these standards, including the rationalization of their structure, while separating the user guide from the standardized version. The revision work has been incorporated into the international forum organized by the International Electrotechnical Commission, and the newly developed standards will be published in the IEC 1340 series, which is undoubtedly complementary to European standards. The relevant standard parts are shown in Table 2:

Anti-static principle

The main factors causing static electricity (characteristics of the object, surface state, history of charge, contact area and pressure, separation speed, etc.) are excluded as much as possible; the positions of the objects in contact with each other in the charged sequence are as close as possible; The contact area and pressure should be small, the temperature should be low, the number of contacts should be small, the separation speed should be small, and the contact state should not be changed abruptly. Powder, liquid, and gas generate static electricity due to friction during transportation. Therefore, it is necessary to limit the flow rate and reduce the bending of the pipe. Measures to increase the diameter and avoid vibration.

In addition to reducing speed, pressure, friction and contact frequency, using appropriate materials and shapes, and increasing conductivity and other suppression measures, the following measures can be taken: 1 Grounding. 2 lap (or jump). 3 shielded. 4 Antistatic agents for insulators that can hardly leak static electricity to increase the conductivity and make static electricity easy to leak. 5 Use spray, watering, etc. to increase the ambient humidity and suppress the generation of static electricity. 6 Use a static eliminator to perform static neutralization.

ESD

ESD English means Electro Static Discharge means 'electrostatic discharge'. We all know that static electricity can be generated by contact, separation or mutual friction of different substances. For example, extrusion, cutting, handling, agitation and filtration in the production process, as well as walking, standing, undressing, etc. in the living, generate static electricity. It can be seen that static electricity can be said to be ubiquitous in our daily life. There are high electrostatic voltages on our bodies and around us, thousands of volts or even tens of thousands of volts. These static electricity may have little effect on the human body, but for some ESDS (electrostatic sensitive components), it can directly lose its normal performance and even completely lose its normal function. This ESD protection is very necessary.

Overview of human body static

When a person wears a non-conductive (referred to as an object with a conductivity less than 1×10-6 s/m), it generates and accumulates electric charge due to walking and other activities, and can reach a potential of a kilovolt level (such action is a dangerous action). At this time, when someone touches other objects, a spark discharge is generated and an electric shock is received. The experiment conducted in China yields the E-t (potential-time) curve as shown in Fig. 1. In the figure, ×, 墷, etc. represent different individuals. Walking on the blanket, undressing, etc., produced a maximum potential of 2,450 volts. These activities were repeated on the terrazzo floor and the highest potential was measured at 3500 to 6000 volts. It can be seen that the human body potential is related to the ground material. In addition, it is also related to the relative humidity of the air, the inner and outer clothes and the material of the footwear. Experiments have shown that in flammable and explosive places, not only work clothes (coats) should not be made of chemical fiber materials, but underwear should not be used. The anti-static measures in human activities mainly include: wearing conductive shoes, wearing anti-static overalls mixed with conductive fibers or treated with antistatic agents, with a human body grounding wrist strap, or touching the ground by hand before some operations Copper wire; if necessary, the working ground should be made conductive, such as conductive whetstone or conductive rubber (sticking surface) with conductivity of 10-6 s/m.

Limits of human discharge and electric shock

The limits of human body discharge and electric shock are: 1 The discharge amount reaches (2~3)×10-7 or more, that is, the discharge of electrostatic shock occurs. As shown in Table 1, an electrostatic shock is generated when the potential is about 3 kV. This is when the human body electrostatic capacitance is equal to or less than the standard value of 100 picofarads, that is, when the value is generally 90 to 100 picofarads. When sitting in a chair or wearing a thin-soled shoe or an old leather shoe, the capacitance is greater than 100 picofarads, and sometimes even hundreds of picofarads, and the limit potential of the electrostatic shock generated at this time is smaller than the value in Table 1. 2 If the charged object is a conductor, when the charged potential is greater than or equal to the value in Table 2, there is a discharge of (2 to 3) × 10-7 or more of the discharge charge on the human body, and the human body is subjected to an electric shock. 3 In the case of more than 100 picofarads, when there is an electrostatic field near the human body, and when there is a charged body with a large energy nearby, the human body is continuously induced, which may be enough to make the human body with a height greater than 100 picofarads higher. The different potentials cause greater danger in flammable and explosive places. Therefore, the upper limit of the human body capacitance should be supplemented. 4 If the charged body is a non-conductor, the limit of electrostatic shock can not be expressed as a charged potential as for a conductor. However, in most cases, an electrostatic shock occurs when the charged potential is about 30 kV or more and thus discharge is generated to the human body. Therefore, the limit of electrostatic shock occurring at this time is as a rough standard with a charged potential of about 10 kV and a charged charge density of about 10-5 cc/m2. However, when the charged state is particularly uneven, the above-mentioned determination cannot be made when there is a portion having a high conductivity locally in the non-conductor and a grounding body in or near the charged object.

Static Protection

The static electricity generated by the solid and the two different objects are in contact with each other, and a charge shift occurs at the interface thereof, and the positive and negative charges are arranged to form an electric double layer. At this time, if the object is separated, an equal amount of charge of different polarity is generated on each of the two objects. Generally, the greater the distance between the two objects in contact with each other in the 'charged sequence', the greater the amount of static electricity generated. The polarity of the charge is based on the relative position in the charged sequence.

During the mutual contact and separation process, the excess charge of the positive (+) or negative (-) volume of the object is neutralized by discharge and conduction, or tends to decrease toward space and earth leakage. This process is called charge relaxation. In general, it begins to moderate while generating static electricity. Since any object that is in contact with and separate generates static electricity or strong static electricity, modern electronic devices that are extremely sensitive to static electricity, new gunpowders, and flammable and explosive gases with low flash point (such as at normal temperature and normal pressure) That is, low-flashing special rocket fuel such as liquid hydrogen which can be volatilized, etc., as long as weak static electricity may cause an accident, fire or explosion, it is a difficult technical problem to prevent the generation of static electricity or the elimination of static electricity. As for the discharge caused by static electricity, the film is sensitized during the production process, the electronic components are damaged, the fiber entanglement caused by the mechanical action, and the printing paper are not uniform, etc., which is also a complicated problem.

Principle of preventing static electricity

The principle of preventing static electricity generation is: 1 Try to eliminate the main factors that generate static electricity. The main factors affecting the generation of static electricity are: the characteristics of the object; the surface state of the object; the charged history of the object; the contact area and its pressure; the separation speed. 2 The charging sequence shown in Table 3 should be referred to so that the objects in contact with each other are as close as possible in the charged sequence. 3 Make the contact area and pressure between the objects small, the temperature should be low, the number of contacts should be small, the separation speed should be small, and the contact state should not change sharply. Static Protection

Static electricity generated by powder and suppressed

During the air transport, belt transport or sieving process, the powder generates static electricity due to friction between the powder or between the powder and the tube wall. Therefore: 1 The conveying speed inside the pipeline should not exceed a certain limit, and the diameter of the pipeline should not be less than a certain minimum. There shall be no objects such as nets or grids that obstruct the transport and generate static electricity. The size and shape of the powder should be preferred. 2 Minimize the bending and contraction of the pipeline; avoid drastic changes in wind speed and delivery. 3 The internal surface of the pipe wall shall be regularly cleaned and repaired by appropriate air vibration and other measures to prevent the powder from being piled up. 4 The pipeline should be made of conductive material as much as possible and grounded. 5 The shape of the spiral blade and the upper limit of the number of revolutions of the spiral should be preferred; vibration of the conveyor belt or abnormal vibration due to improper conveyance should be avoided, and the powder should not be suspended and scattered. 6 In bucket conveying, the wall slope of the hopper and the funnel should be close to vertical to reduce the friction area; the bucket wall should not disturb the powder falling process; it should be cleaned regularly; no metal sliding should be installed on the hopper Holder. 7 should preferably be the size and shape of the powder, as well as the material of the hopper to minimize static electricity. 8 hoppers and funnels should use conductive materials as much as possible and ground them.

Static electricity generated by liquids and suppressed

The liquid generates static electricity during pipeline transportation, or when flowing through the hose, due to friction between the liquids or due to friction between the liquid and the pump. When the other conditions are the same, the static electricity is proportional to the flow rate of 1.8 to 2 power. In order to limit static electricity, it should be noted that: 1 the flow rate of hydrocarbon oil should not exceed the values listed in Table 4. 2 Under the same conveying capacity, the diameter of the piping and hose should be increased to reduce the flow rate. 3 There should be no turbulent or drastic changes in the conveying state. The piping should be as small as possible to reduce the bending and contraction. The inner wall of the piping should be smooth. Do not install metal mesh, protrusions, etc. in the pipe. The filter should be placed on the flow source side as much as possible. 4 The flow rate should not change sharply at any part and any time. The initial flow rate should be controlled at a small flow rate, and the medium flow rate should not exceed the specified value. 5 Do not mix air, water, dust and oxides (rust, etc.) into the liquid. 6 A damper pipe section and a mitigation tank for reducing the flow rate should be installed at the end of the piping and the hose. 7 When transporting liquids with tankers, tankers, tank cars, tanks and other containers, it should be noted that due to the vibration of the tank, the liquid rubs against the walls to generate static electricity. The moving speed should not change sharply during transportation. It should be moved as evenly as possible. There should be partitions in the tank to separate them. The liquid should not be waved or splashed; no impurities should be mixed in the liquid; the inside of the tank should be cleaned regularly.

Static electricity generated by gas and suppressed

During the flow and discharge of the gas, the particles and the wall surface are generated when the high-pressure air contains the compressor oil and the condensation water mist generated by the compression, and the particles such as the pipe rust and dust flow in the pipe or are ejected from the opening. Static electricity is generated by collision and friction with nearby objects. Therefore: 1 Apply an air filter to filter out mist and particles, etc. before flowing and ejecting. 2 The discharge flow should be small, the discharge pressure should be low, and special attention should be paid to the explosion caused by hydrogen gas injection. 3 Pipes and hoses should be cleaned before use to remove rust and dust. 4 When condensed carbon dioxide is ejected, dry ice should be avoided at the opening because it collides with the liquid phase components and rubs against each other, or collides with the wall surface, rubs and splashes to generate static electricity. 5 The liquefied petroleum gas cylinder, the opening of the pipe and the flange should be cleaned and kept clean. 6 Hydrogen, acetylene, propane, city gas and nitrogen gas cylinders, pipes, hoses, etc. should be cleaned before use to remove rust and moisture. Try not to use a rubber tube, but use a metal tube and ground it. 7 The opening of the steam pipe is prone to static electricity. Dry steam should be used as much as possible. The discharge amount should be small. The discharge pressure should be limited to 98 N/cm2 or less, and the nozzle with less static electricity should be used. There should be enough distance. 8 When spraying aerosols and paints, do not spray large and intense objects at close range. 9 Aircraft and spacecraft generate static electricity by friction with air during flight. Figure 2 shows the relationship between rocket height (km) and potential (kV). With an antistatic needle in place, static electricity can be vented to the atmosphere to prevent excessive rise in potential. China's bundled rockets that launch synchronous satellites are equipped with anti-static needles.

Protective measures

In addition to reducing speed, pressure, friction and contact frequency, selecting appropriate materials and shapes, increasing conductivity and other suppression measures, the following measures can be taken:

1 Grounding, that is, the metal conductor is electrically connected to the earth (grounding device) to leak the charge to the earth. This method is suitable for eliminating static electricity on conductors and resistivities below 108 ohms, and should not be used to eliminate static electricity on insulators, because the grounding of insulators is prone to spark discharge, causing ignition or igniting of flammable and explosive liquids or gases. Interference from electronic facilities. A resistance of 106 to 109 ohms should be maintained between the insulator and the ground. It is only for eliminating the grounding for static electricity on the conductor. The resistance value should not exceed 100~1000 ohms. The non-metallic conductor grounding should be covered with reliable metal or conductive paint, and the contact area should be no less than 10 cm2. Mobile devices cannot be grounded frequently, and grounding operations should be selected in no dangerous situations and times.

2 lap (or jump). Two or more separate metal conductors are electrically connected such that they are substantially at the same potential with each other.

3 shielded. The surface of the charged object is covered with a grounded metal wire or a metal mesh to limit the electrostatic hazard to such an extent that it does not occur. Shielding measures also protect the electronics from static electricity.

4 For insulators that can hardly leak static electricity, antistatic agents are used to increase the electrical conductivity, making static electricity easy to leak.

5 Use spray, watering and other methods to increase the relative humidity of the environment to 60-70% to suppress the generation of static electricity and solve the problem of static electricity in the production of textile mills.

phenomenon

In the sixth century BC, humans discovered that the amber friction can attract the 'static phenomenon' of light and small objects. This is the electrical properties that are presented when free charges are transferred between objects. In addition, when the silk or wool is rubbed, the small spark generated is the effect of charge neutralization. 'Thunder' is in nature, because the positive and negative charges accumulated in the clouds are intensely neutralized, resulting in electro-optic, thunder, and heat.

Electrostatic phenomena include many examples of nature, such as the attraction between plastic bags and hands, the seemingly spontaneous barn explosion, the destruction of electronic components during the manufacturing process, the working principle of the photocopier, and so on. When the surface of an object contacts other surfaces, the charge builds up on the surface of the object to become static. Although the charge exchange is caused by the contact and separation of the two surfaces, the effect of charge exchange is noticed only when the resistance of one of the surfaces is high and the current becomes small. Because the charge will be trapped on the surface, where it will last for a long time, enough for this effect to be observed for a while.

Electrostatic phenomena are caused by electrostatic forces that point charges interact with each other. Coulomb's law specifically describes the physical properties of electrostatic forces. In a hydrogen atom, the electrostatic force of electrons and protons interacting with each other is greater than the gravitational force, and the magnitude of the electrostatic force is about 40 times that of the order of gravity.

Elimination method

1. Wash your hands before going out, or put your hands on the wall to remove static electricity, and try not to wear chemical fiber clothes.

2. In order to avoid static electricity, you can use small metal devices (such as keys), cotton rags, etc. to touch the door, door handle, faucet, chair back, bed rail, etc. to eliminate static electricity, and then touch with your hands.

3. Wear cotton underwear.

4. When getting ready to get off the bus, hold the file with your right hand, then touch the iron part with your finger, then open the door, put the left hand on the iron door, but don't loose your left hand, then let the right hand drop and get off. At this time, if you hold the door with your right hand, you will not be charged~~ha. Next, force a level, get it~~

5. To deal with static electricity, we can take both 'defense' and 'put'. 'Prevention', we should try to use pure cotton products as the fabric of clothing and home accessories, try to avoid the use of chemical fiber carpets and furniture with plastic surface materials to prevent frictional electrification. Keep away from electrical appliances such as televisions and refrigerators as much as possible to prevent inductive electrification. 'Put' is to increase the humidity, so that the local static electricity is easy to release. When you turn off the TV and leave the computer, you should wash your face immediately and let the static charge on the surface of the skin be released in the water. In the winter, try to use high moisturizing cosmetics. Commonly used humidifiers. Some people like to raise ornamental fish and daffodils indoors is also a good way to regulate indoor humidity.

In addition, recommend an economical and practical method of humidification: place a basin of water under the heater, use an old towel (or a cloth that absorbs water), put it in the water, and put it on the heater so that it can evaporate into the house around the clock. Three liters of water.