How to reduce static electricity in SMT proofing trial production
- 2020-01-30-

In the electronics industry, as the degree of integration becomes higher and higher, the inner insulating layer of integrated circuits is getting thinner and thinner, and the width and spacing of interconnection wires are getting smaller and smaller. For example, the typical thickness of the insulating layer of CMOS devices is about 0.1 μm. The corresponding breakdown voltage is 80-100V; the insulating layer of the VMOS device is thinner, and the breakdown voltage is 30V. The electrostatic voltage generated in the manufacturing of electronic products, transportation, storage, etc. far exceeds the breakdown voltage of MOS devices, which often causes hard breakdown or soft breakdown (partial damage to the device) and makes the device invalid. Or seriously affect the reliability of the product. In electronicsSMT Proofing Trial ProductionIn environments such as, use and maintenance, various polymer materials that are prone to static electricity will be used in large quantities, which undoubtedly brings more problems and challenges to the electrostatic protection of electronic products. There are two types of damage and damage to electronic products caused by electrostatic discharge during patch processing: sudden damage and potential damage:


1. The so-called sudden damage refers to the severe damage of the device and loss of function. This kind of damage can usually be found in the quality inspection during the production process, so the main reason it brings to the factory is the cost of rework and repair.

2. The potential damage is the damaged part of the device, the function has not been lost, and cannot be found in the production process of the test, but the product will become unstable during use, good or bad, so the quality of the product constitutes More damage.

Among these two types of damage, potential failures account for more than 90%, and sudden failures account for only 10%. That is, 90% of static electricity damage is undetectable and can only be found in the user's hands, such as frequent crashes, automatic shutdown, poor call quality, large noise, time-consuming, critical errors and other problems mostly related to static electricity damage. Because of this, electrostatic discharge is considered to be a potential killer for SMT proofing and trial production, and electrostatic protection has also become an important part of the quality control of electronic products.

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

3. Static power in electronic product manufacturing

(1) The activities of the human body, the friction, contact and separation between people and clothes, shoes, socks, etc., are one of the main static electricity sources in the manufacture of electronic products. Human body static electricity is the main cause of 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 influence on the electrostatic voltage, and it will increase by an order of magnitude if it is in a dry environment. Table 2 shows the relationship between the relative humidity and the electrification of human activities.

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

(2) When chemical fiber or cotton work clothes rub against the work surface or chair, it can generate an electrostatic voltage of more than 6000V on the surface of the clothing and make the human body charged. At this time, when it comes in contact with the device, it will cause discharge and easily damage the device.

(3) The insulation resistance of rubber or plastic shoe soles is as high as 1013Ω. When rubbing against the ground, static electricity is generated and the human body is charged.

(4) When the devices encapsulated by resin, varnish film, and plastic film are put in the packaging and transported, the friction between the surface of the device and the packaging material can generate hundreds of volts of electrostatic voltage, which discharges the sensitive devices.

(5) Various packaging, boxes, and turnover made of PP (polypropylene), PE (polyethylene), PS (polyintraethylene), PVR (polyurethane), PVC, polyester, resin and other polymer materials Boxes, PCB racks, etc. may generate 1-3.5KV electrostatic voltage due to friction and impact, discharging sensitive borrowings.

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

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

(8) Electronic production equipment and tools: For example, electric soldering irons, wave soldering machines, reflow soldering furnaces, placement machines, debugging and testing equipment, high-voltage transformers, AC/blind current circuits, etc. will induce static electricity in the equipment. If the equipment electrostatic discharge measures are not good, it will cause the sensitive device to fail during the manufacturing process. The circulation of hot air in the oven and the friction of the box body, and the CO2 vapor in the cooling box of the CO2 low-temperature box can generate a large amount of static charge.

4. SMT proofing and trial production electrostatic protection principle

In the manufacture of electronic products, it is impossible not to generate static electricity. The generation of static electricity is not the hazard, the hazard lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of static electricity protection is "Meditation Elimination".

SMT proofing and trial production electrostatic protection principle:

(1) Prevent static electricity from accumulating in places where static electricity may occur. Take measures within a safe range.

(2) The existing static electricity accumulation is quickly eliminated and released immediately.

5. SMT proofing and trial production electrostatic protection method

(1) Use anti-static materials: Metal is a conductor, and the leakage current of the conductor is large, which will damage the device. In addition, since insulating materials are prone to frictional electrification, metals and insulating materials cannot be used as antistatic materials. Instead, the 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 antistatic materials. For example, the commonly used electrostatic protection materials are realized by mixing conductive carbon black in rubber, and the surface resistance is controlled below 1×106Ω·cm.