Static power supply in electronic product manufacturing

(1) Static electricity generated by human activities, friction, contact and separation between people and objects such as clothes, shoes and socks is one of the main static power sources in electronic product manufacturing. Human static electricity is the main cause of hard (soft) breakdown of devices. The electrostatic voltage generated by human activities is about 0.5-2kv. In addition, air humidity has a great impact on electrostatic voltage, which will increase by an order of magnitude in a dry environment. Table 2 shows the relationship between relative humidity and electrification of human activities.

 

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

 

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

 

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

 

(4) When the devices encapsulated with resin, paint film and plastic film are placed in the packaging for transportation, the friction between the device surface and the packaging material can produce hundreds of volts of electrostatic voltage and discharge the sensitive devices.

 

(5) Various packages, material boxes, turnover boxes and PCB racks made of PP (polypropylene), PE (polyethylene), PS (polyvinylene), PVR (polyurethane), PVC, polyester, resin and other high molecular materials may generate 1-3.5kv electrostatic voltage due to friction and impact, and discharge power to sensitive tools.

 

(6) Ordinary worktable, static electricity is generated by friction.

 

(7) The insulation resistance of insulated floors such as concrete, waxed and polished floors and rubber plates 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 iron, wave soldering machine, reflow soldering furnace, mounting machine, commissioning and testing, high-voltage transformer and AC / blind current circuit in the equipment will induce static electricity in the equipment. If the electrostatic discharge measures of the equipment 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 box and the CO2 steam in the cooling box of CO2 low-temperature oven can produce a large amount of static charge.