SMT Production Electrostatic Discharge Protection Technology
In electronic product manufacturing, electrostatic discharge (ESD) often damages components, even causing them to malfunction and resulting in serious losses. Therefore, ESD protection in SMT production is crucial. This journal invited two experts from Beijing and Shanghai to write articles introducing and analyzing the sources of static electricity and the principles of ESD protection in electronic product manufacturing. It provides a detailed introduction to some basic ESD protection technologies and corresponding measures in SMT production for your reference.
I. Static Electricity and its Hazards
Static electricity is a form of electrical energy that resides on the surface of an object. It results from an imbalance of positive and negative charges within a localized area and is formed through the conversion of electrons or ions. Static electricity is the general term for electrical phenomena generated during the generation and disappearance of charges, such as triboelectric charging and human body static electricity.
With technological advancements, static electricity has been widely and effectively applied in fields such as electrostatic spraying, electrostatic textiles, electrostatic sorting, and electrostatic imaging. However, on the other hand, the generation of static electricity can cause significant hazards and losses in many fields. For example, in the first Apollo manned spacecraft, an explosion caused by electrostatic discharge (ESD) killed three astronauts; accidents resulting in injury or death due to ESD are also common in gunpowder manufacturing.
In the electronics industry, as integration density increases, the internal insulating layers of integrated circuits become thinner, and the width and spacing of interconnect wires become smaller. For example, the typical thickness of the insulating layer of a CMOS device is about 0.1 μm, with a corresponding breakdown voltage of 80-100V; the insulating layer of a VMOS device is even thinner, with a breakdown voltage of 30V. The electrostatic voltage generated during the manufacturing, transportation, and storage of electronic products far exceeds the breakdown voltage of MOS devices, often causing hard breakdown or soft breakdown (localized damage) phenomena, leading to device failure or severely affecting product reliability.
To control and eliminate ESD, developed countries such as the United States, Western Europe, and Japan have established national, military, and enterprise standards or regulations. There are corresponding regulations governing the design, manufacturing, purchase, warehousing, inspection, storage, assembly, debugging, packaging, and transportation of electrostatic sensitive components (ESS). Strict regulations also govern the manufacturing, use, and management of ESD protection equipment. my country has also formulated military and enterprise standards based on international standards. For example, there are standards from the Ministry of Aerospace, the Ministry of Machinery and Electronics, and the Ministry of Petroleum.
II. Electrostatic Sensitive Devices (SSDs)
Devices sensitive to electrostatic reactions are called electrostatic sensitive components (SSDs). ESD-sensitive devices mainly refer to very large-scale integrated circuits, especially metallized film semiconductors (MOS circuits). Different ESD protection measures can be taken for different SSD devices according to the SSD classification table.
III. Static Electricity Sources in Electronic Product Manufacturing
(1) Static electricity generated by human activity, such as friction, contact, and separation between people and objects like clothing, shoes, and socks, is one of the main static electricity sources in electronic product manufacturing. Human static electricity is a major cause of hard (soft) breakdown in devices. The static voltage generated by human activity is approximately 0.5-2KV. Furthermore, air humidity has a significant impact on static voltage; in a dry environment, it can increase by an order of magnitude. (2) Friction between synthetic fiber or cotton work clothes and work surfaces or chairs can generate a static voltage of over 6000V on the clothing surface, charging the human body. Contact with such charged clothing can lead to discharge, easily damaging components.
(3) Rubber or plastic shoe soles have an insulation resistance as high as 10¹³ Ω. Friction with the ground generates static electricity, charging the human body.
(4) When components encapsulated in resin, paint, or plastic films are transported in packaging, friction between the component surface and the packaging material can generate a static voltage of several hundred volts, discharging sensitive components.
(5) Various packaging materials, boxes, turnover boxes, PCB racks, etc., made of PP (polypropylene), PE (polyethylene), PS (polyvinyl chloride), PVR (polyurethane), PVC, and other polymer materials such as polyester and resin can generate a static voltage of 1-3.5KV due to friction and impact, discharging sensitive components.
(6) Ordinary work surfaces generate static electricity through friction.
(7) Concrete, waxed and polished floors, rubber sheets, and other insulating surfaces have high insulation resistance, making it difficult for static charges on the human body to leak.
(8) Regarding electronic production equipment and tools: For example, high-voltage transformers and AC/DC circuits in equipment such as soldering irons, wave soldering machines, reflow ovens, mounting machines, debugging and testing equipment can induce static electricity. If the equipment's static discharge measures are inadequate, it can cause sensitive components to fail during the manufacturing process. The circulation of hot air in ovens and friction with the oven body, as well as CO2 vapor in CO2 low-temperature cooling chambers, can all generate large amounts of static charge. Static Electricity Protection Principles
It is impossible to completely eliminate static electricity in the manufacturing of electronic products. The generation of static electricity is not the problem; the problem lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of static electricity protection is to eliminate it quietly.
(1) Prevent static electricity accumulation in areas where it may be generated. Take measures within safe limits.
(2) Quickly eliminate existing static electricity accumulation and release it immediately.
In SMT production, the use of these anti-static products is essential, such as ionizers, ion bars, and static eliminators.