How to protect static electricity in SMT processing

In the manufacture of electronic products, electrostatic discharge often damages the device, and even makes the device fail, resulting in serious losses, so electrostatic protection in SMT processing is very important. This magazine invited two experts from Beijing and Shanghai to introduce and analyze the electrostatic source and the principle of electrostatic protection in electronic product manufacturing, and introduced some electrostatic protection technology basis and corresponding measures in SMT production in detail. For your reference.

1, static electricity and static electricity hazards

Static electricity is a kind of electrical energy, which remains on the surface of the object, is the result of the loss of balance of positive and negative charges in a local scope, and is formed by the conversion of electrons or ions. Electrostatic phenomenon is the general name of the electrical phenomenon produced during the generation and disappearance of electric charges. Such as friction electrification, human electrification and other phenomena.

With the development of science and technology, electrostatic phenomena have been widely used in the fields of electrostatic spraying, electrostatic textile, electrostatic separation, electrostatic imaging and so on. But on the other hand, the generation of static electricity in many areas will bring great harm and loss. In the first manned Apollo spacecraft, for example, an electrostatic discharge caused an explosion that killed three astronauts. Electrostatic discharge (ESD) in the process of gunpowder manufacturing, resulting in explosion casualties often occur. In the electronics industry, with the increasing degree of integration, the internal insulation layer of integrated circuits is becoming thinner and thinner, and the width and spacing of interconnect wires are becoming smaller and smaller, such as the typical thickness of the insulating layer of CMOS devices is about 0.1μm, and the corresponding breakdown resistance voltage is 80-100V. VMOS devices have thinner insulation layers and breakdown voltages of 30V. The electrostatic voltage generated in the manufacturing of electronic products, transportation, storage and other processes far exceeds the breakdown voltage of MOS devices, which often causes hard breakdown or soft breakdown (device local damage), making it fail or seriously affect the reliability of the product.

In order 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, warehousing, inspection, storage, assembly, debugging, packaging and transportation of semi-finished and finished products of electrostatic sensitive components, and there are strict rules and regulations on the manufacturing, use and management of electrostatic protective equipment. China has also developed military and enterprise standards with reference to international standards. For example, there are space department, mechanical and electrical Department, petroleum Department and other standards.

2, electrostatic sensitive devices (SSD)

Devices that are sensitive to electrostatic reactions are called electrostatic sensitive components (SSDS). Electrostatic sensitive devices mainly refer to very large scale integrated circuits, especially metallized film semiconductors (MOS circuits). Table 1 shows the classification of electrostatic sensitive devices. According to the SSD classification table, you can take different ESD preventive measures for different SSD devices.

3. Static power supply in the manufacture of electronic products

(1) The activity of the human body, the 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. The static electricity of the human body is the main reason for the hard (soft) breakdown of the device. The electrostatic voltage generated by human activities is about 0.5-2KV. In addition, the air humidity has a great impact on the electrostatic voltage, if it is in a dry environment, it will rise by 1 order of magnitude. Table 2 shows the relation between relative humidity and electrification of human activity.

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

(2) When the chemical fiber or cotton work clothes rub with the workbench and the chair, the electrostatic voltage above 6000V can be generated on the surface of the clothing, and the human body is charged, at this time when it is in contact with the device, it will cause discharge and easily damage the device.

(3) The insulation resistance of the rubber or plastic sole is as high as 1013Ω, which produces static electricity when rubbed with the ground and electrified the human body.

(4) When the device packaged with resin, paint film and plastic film is placed in the package for transportation, the surface of the device and the packaging material can generate several hundred volts of electrostatic voltage, and discharge the sensitive device.

(5) With PP(polypropylene), PE(polyethylene), PS(polyvinyl), PVR(polyamine), PVC and polyester, resin and other polymer materials made of various packaging, material box, turnover box, PCB frame, etc., may produce 1-3.5KV electrostatic voltage due to friction, impact, discharge to sensitive parts.

(6) Ordinary work table, electrostatic generated by friction.

(7) The insulation resistance of concrete, waxing and polishing floor, rubber plate and other insulating ground is high, and the static charge on the human body is not easy to leak.