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Anti-static Technology in SMT Electronics Production

Time:2019/04/15 丨 Source:未知 丨 Browse times:

In the manufacture of electronic products, electrostatic discharge often damages devices, even causes device failure, causing serious losses, so electrostatic protection in SMT production is very important. This magazine invites two experts from Beijing and Shanghai to write articles to introduce and analyze the source of static electricity and the principle of static electricity protection in the manufacture of electronic products, and to introduce in detail the technical basis and corresponding measures of static electricity protection in SMT production. For your reference and study.


1. The harm of static electricity and static electricity
Static electricity is a kind of electric energy, which is retained on the surface of the object. It is the result of the unbalance of positive and negative charges in a local range. It is formed by the conversion of electrons or ions. Electrostatic phenomena are the general term of electrical phenomena produced by the generation and disappearance of electric charges. Such as friction electricity, human body electricity and other phenomena.
 
With the development of science and technology, electrostatic phenomenon has been widely used in electrostatic spraying, electrostatic textile, electrostatic separation, electrostatic imaging and other fields. 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 because of the explosion caused by electrostatic discharge (ESD); in the process of gunpowder manufacturing, accidents of explosion casualties occurred from time to time.
 
In the electronic industry, with the increasing integration, the internal insulation layer of integrated circuits becomes thinner and thinner, and the width and spacing of interconnection wires become smaller and smaller. For example, the typical thickness of the insulation layer of CMOS devices is about 0.1 micron, and the corresponding breakdown voltage is 80-100V; the insulation layer of VMOS devices is thinner and the breakdown voltage is 30V. However, the electrostatic voltage generated in the manufacture of electronic products and in the process of transportation and storage is far more than the breakdown voltage of MOS devices, which often leads to hard breakdown or soft breakdown (partial damage of devices), making them invalid or seriously affecting the reliability of products.
 
In order to control and eliminate ESD, developed countries such as the United States, Western Europe and Japan have formulated national, military and enterprise standards or regulations. The design, manufacture, purchase, storage, inspection, warehousing, assembly, commissioning, packaging and transportation of semi-finished and finished products of electrostatic sensitive components are all stipulated. There are also stricter rules and regulations for the manufacture, use and management of electrostatic protective equipment. China has also formulated military and enterprise standards in accordance with international standards. For example, there are standards of the Ministry of Space, the Ministry of Mechanical and Electrical Engineering and the Ministry of Petroleum.
 
2. Electrostatic Sensitive Devices (SSD)
Devices sensitive to electrostatic reactions are called electrostatic sensitive devices (SSDs). Electrostatic sensitive devices mainly refer to VLSI, especially metallized film semiconductor (MOS circuit). Table 1 is a hierarchical table of electrostatic sensitive devices. According to SSD classification table, different electrostatic protection measures can be taken for different SSD devices.
 
3. Electrostatic Sources in Electronic Products Manufacturing
(1) The static electricity generated by human activities, friction, contact and separation between human beings and objects such as clothes, shoes and socks is one of the main static electricity sources in the manufacture of electronic products. Human static electricity is the main reason for hard (soft) breakdown of devices. The electrostatic voltage produced by human activity 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 electrification of human activities.
 
When the human body touches the ground wire after being charged, it will produce discharge phenomenon, and the human body will produce different degrees of electric shock response, the degree of which is called electric shock sensitivity. Table 3 shows the electric shock sensitivity of human body in different electrostatic discharge processes.
 
(2) When chemical fiber or cotton uniform rubs against the table and chair, it can generate more than 6000V electrostatic voltage on the surface of the garment and make the human body charged. At this time, when it contacts with the device, it will lead to discharges and easily damage the device.
 
(3) The insulation resistance of rubber or plastic soles is as high as 1013_, which generates static electricity and electrifies the human body when it rubs against the ground.
 
(4) When the devices encapsulated with resin, paint and plastic film are put into packaging and transported, the friction between the surface of the device and the packaging material can generate several hundred volts of static voltage, which discharges the sensitive devices.
 
(5) All kinds of packaging, material boxes, turnover boxes and PCB shelves made of PP (polypropylene), PE (polyethylene), PS (polyethylene), PVR (polyamine resin), PVC and polyester, resin and other macromolecule materials may generate 1-3.5KV static voltage due to friction and impact, and discharge sensitive accessories.
 
(6) The static electricity is produced by friction on the common working table.
 
(7) The insulation resistance of concrete, waxed polished floor, rubber board and other insulating ground is high, and the static charge on human body is not easy to leak.
 
(8) In the field of electronic production equipment and tools, such as soldering iron, wave soldering machine, reflow furnace, mounting machine, debugging and testing equipment, high voltage transformer, AC/Blind Current circuit will induce static electricity in the equipment. If the electrostatic discharge measures are not good, the sensitive devices will fail in the manufacturing process. The circulating flow of hot air in the oven and the friction between the oven and the CO2 vapor in the cooling chamber of the CO2 cryogenic oven can generate a lot of static charges.
 
4. Principle of Electrostatic Protection
 
In the manufacture of electronic products, it is impossible not to generate static electricity. Generating static electricity is not the harm, but the harm lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of electrostatic protection is "quiet elimination".
Principle of electrostatic protection:
(1) The accumulation of static electricity should be prevented in places where static electricity may occur. Measures should be taken within the scope of safety.
(2) Quickly eliminate the existing electrostatic accumulation and release it immediately.
 
5. Electrostatic protection methods
 
(1) Use antistatic materials: metal is a conductor, because the leakage current of the conductor is large, it will damage the device. In addition, because insulating materials are prone to friction and electrification, metal and insulating materials can not be used as antistatic materials. Instead, the so-called electrostatic conductor with surface resistance less than 1 *105_. cm and the electrostatic sub-conductor with surface resistance of 1 *105-1 *108_. cm are used as antistatic materials. For example, the common electrostatic protective material is realized by mixing conductive carbon black in rubber, and the surface resistance is controlled below 1 x 106_ cm.
 
(2) Leakage and grounding: Grounding the parts that may or have produced static electricity to provide a channel for static electricity release. The "independent" ground line is established by burying the earth line. The resistance between the ground wire and the earth is less than 10.
 
Grounding method of electrostatic protective material: The electrostatic protective material (e.g. for platform cushion, ground cushion, anti-static wristband, etc.) is connected to the conductor leading to the independent earth wire through a resistance of 1M (see SJ/T10630-1995). Serial connection of 1M resistance is to ensure discharge of current less than 5mA to the ground, which is called soft grounding. Equipment housing and electrostatic shielding cover are usually grounded directly, which is called hard grounding.
 
(3) Elimination of static electricity in conductor belt: The static electricity on conductor can be leaked to the earth by grounding. The voltage and release time of discharges can be expressed as follows:
Voltage at UT-T in UT=U0L1/RC (V) U0 together initial voltage (V) R-equivalent resistance (_) C-conductor equivalent capacitance (pf)
 
It is generally required to leak static electricity within 1 second. That is to say, drop the voltage to the safe area below 1OV in one second. This can prevent SSD from being damaged by too fast leakage speed and too large leakage current. If U0 = 500V, C = 200pf, in order to make UT reach 100V in 1 second, R = 1.28 x 109_is required. Therefore, the current limiting resistance of 1M is usually used in the electrostatic protection system to limit the discharge current to less than 5mA. This is designed for operational safety. If the operator accidentally touches 220V industrial voltage in the electrostatic protection system, it will not bring danger.
 
(4) Elimination of static electricity on non-conductor strips: For static electricity on insulators, because the charge cannot flow on insulators, the static electricity can not be eliminated by grounding. The following measures can be adopted:
(a) The positive and negative ions can be produced by using the ion blower-ion blower, which can neutralize the static electricity of the electrostatic source. It can be set near the space and the mounting head of the mounter.
(b) The use of electrostatic eliminators, electrostatic eliminators, belongs to surfactants. The electrostatic eliminator can be used to wash the instrument and the surface of the object, which can quickly eliminate the static electricity on the surface of the object.
(c) Controlling ambient humidity - Increasing humidity can improve the surface conductivity of non-conductive materials and make the surface of objects difficult to accumulate static electricity. For example, humidifying and ventilating measures can be adopted in the dry environment of the north.
(d) Electrostatic shielding - shielding covers (cages) can be used for devices prone to static electricity, and shielding covers (cages) can be effectively grounded.
 
(5) Process control method: In order to produce as little static electricity as possible in the manufacture of electronic products, control the accumulation of static charge, eliminate the existing static accumulation quickly and release it immediately, effective measures should be taken in the aspects of plant design, equipment installation, operation and management system.
 
6. Electrostatic Protective Equipment
 
(1) The anti-static system of human body includes anti-static wristband, overalls, hats, 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 movable floor, etc.
(3) Anti-static operation series: including anti-static: I: as a mat, anti-static packaging bags, anti-static logistics car, anti-static soldering iron and tools.
 
7. Electrostatic measuring instruments.
(1) Electrostatic field tester: It is used to measure the surface resistance of table and ground. In plane structure and non-plane situation, different specifications of measuring instruments should be selected.
(2) Wristband tester: Measure whether the wristband is effective.
(3) Human electrostatic tester: It is used to measure the amount of static electricity carried by human body, the impedance between human feet, the electrostatic difference between human bodies, whether the wristband, grounding plug, work clothes are effective or not. It can also be used as an entry discharge to isolate human static electricity from the workshop.
(4) Megaohmmeter: It is used to measure the impedance or resistance of all conductive, antistatic and electrostatic discharge surfaces.
 
8. Requirements for Anti-static Technical Indicators in Electronic Products Manufacturing
(1) Grounding resistance of anti-static ground pole is less than 10.
(2) Ground or mattress: surface resistance 105-1010_; and friction voltage < 100V.
(3) Wall: Resistance value 5*104-109_.
(4) Work table or pad: surface resistance value 106-109_; friction voltage < 100V; ground system resistance 106-108_.
(5) The resistance of the working chair to the casters is 106-108_.
(6) The friction voltage of work clothes, caps and gloves is less than 300V, and the friction voltage of soles is less than 100V.
(7) Wristband connects cable resistance 1M; system resistance 1-1OM when wearing wristband. The resistance of heel strap (shoe bundle) system is 0.5 x 105-108_.
(8) Resistance 106-109_of the wheel system on the logistics platform.
(9) The surface resistance of material box, turnaround box and PCB frame is 103-108_, and the friction voltage is less than 100V.
(10) Packing substitution, box-to-box friction voltage < 100V.
(11) Human body resistance 106-108_.
 
9 Anti-static Measures and General Requirements for Static Operating Areas (Points)
 
SMT production equipment must be well grounded. Mounter should adopt three-phase wireless grounding method and grounding independently. The ground of the production site, the cushion of the working table, the chair and so on should meet the anti-static requirements. Maintain a constant temperature and humidity environment in the workshop. It should be equipped with anti-static material box, turnover box, PCB frame, logistics car, anti-static packaging belt, anti-static wristband, anti-static soldering iron and tools.
 
(1) Anti-static area should be set according to anti-static requirements, and there are obvious anti-static warning signs. According to the electrostatic sensitivity of the devices used in the working area, they are divided into 1, 2 and 3 levels, and different protective measures are formulated according to different levels.
Level 1 Electrostatic Sensitivity Range: 0-1999V
Level 2 Electrostatic Sensitivity Range: 2000-3999V
Level 3 Electrostatic Sensitivity Range: 4000-15999V
More than 16000V is a non-electrostatic sensitive product.
 
(2) The room temperature and relative humidity of the electrostatic safety zone (point) are 23 (+3 C) and 45-70% RH. It is forbidden to operate SSD (electrostatic sensitive components) in less than 30% of the environment.
 
(3) Measure the surface resistance of the ground, desktop and switchbox regularly.
 
(4) Non-productive items such as tableware, tea sets, bags, wool fabrics, newspapers and rubber gloves are prohibited from being placed on the workbench in the electrostatic safety zone (spot).
 
(5) When staff enter the anti-static area, they need to discharge. Operators must wear work clothes and anti-static shoes and socks when operating. Safety check of static electricity protection must be done before each operation, and production can only be carried out after passing the standard.
 
(6) Wear anti-static wristband during operation, and measure the effectiveness of wristband every day.
 
(7) When testing SSD, one piece should be taken from the package box, tube and plate, one piece should be measured and put on one piece, not piled on the table. Unqualified devices should be withdrawn from storage after testing.
 
(8) The order of power-on and power-off must be followed: low voltage high voltage signal voltage. The order of power off is the opposite. At the same time, it should be noted that the polarity of the power supply should not be reversed and that the voltage of the power supply should not exceed the rated value.
 
(9) Inspectors should be familiar with the type, variety and testing knowledge of SSD, and understand the basic knowledge of static electricity protection.
 
 
10. Requirements for Transportation, Storage and Use of Static Sensitive Devices (SSD)
 
(1) SSD shall not fall to the ground during transportation and shall not be separated from the package at will.
(2) Storage relative humidity of SSD: 30-40% RH.
(3) The original package should be maintained during the storage of SSD. Containers with anti-static properties should be used when packaging should be replaced.
(4) In the warehouse, special anti-static labels should be affixed to the positions where SSD devices are placed.
(5) Visual measurement is used to count the number of SSD devices in the original package.
(6) Writer/eraser should be grounded sufficiently and anti-static Bracelet should be brought when writing, erasing and protecting EPROM information.
(7) Operators of assembling, welding, repairing and debugging must strictly operate in accordance with the requirements of electrostatic protection.
(8) Printed circuit boards (PCBs) which are qualified for testing and testing shall be sprayed with an ion spray gun once before packaging to eliminate the possible accumulation of static charges.
 
11. Management and Maintenance of Antistatic Workspace
 
(1) To formulate an anti-static management system and have a special person in charge.
(2) Standby anti-static work clothes, shoes, bracelets and other personal goods for use by foreign personnel.
(3) Regular maintenance and inspection of the effectiveness of anti-static facilities.
(4) The wristband is examined once a week (or every day).
(5) The grounding property of the mattress and floor mattress and the performance of the electrostatic eliminator are checked once a month.
(6) Anti-static components rack, printed board rack and turnover box; anti-static performance of transport vehicle, table mat and floor mat is checked every six months.

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