1. Static Electricity and Its Hazards

Static electricity is a form of electrical energy that resides on the surface of an object. It arises from an imbalance of positive and negative charges within a localized area, resulting from the transfer of electrons or ions. The term "static electricity" encompasses various electrical phenomena that occur during the generation and dissipation of charges—for instance, electrification through friction or the accumulation of static charge on the human body. With advancements in technology, static electricity has been effectively harnessed for various applications, including electrostatic spraying, electrostatic textile processing, electrostatic separation, and electrostatic imaging. Conversely, the generation of static electricity can lead to significant hazards and losses in many fields. For example, an electrostatic discharge (ESD) triggered an explosion aboard the first Apollo manned spacecraft, resulting in the loss of three astronauts' lives. Similarly, in the manufacturing of explosives, ESD-induced explosions have occasionally caused casualties. In the electronics industry, the trend toward higher levels of integration has led to thinner internal insulating layers within integrated circuits, as well as narrower widths and smaller spacing between interconnecting wires. For instance, the insulating layer of a typical CMOS device is approximately 0.1 μm thick, with a corresponding breakdown voltage of 80–100 V; VMOS devices feature even thinner insulating layers, with breakdown voltages as low as 30 V. During the manufacturing, transport, or storage of electronic products, the static voltages generated often far exceed the breakdown thresholds of these MOS devices. This frequently results in "hard" or "soft" breakdown (localized damage), causing the devices to fail or severely compromising the reliability of the final product.

 

2. Electrostatic Sensitive Devices (SSD)

Components that are sensitive to static electricity are known as Electrostatic Sensitive Devices (SSD). These primarily consist of ultra-large-scale integrated circuits, particularly metal-oxide-semiconductor (MOS) circuits. Table 1 provides a classification system for electrostatic sensitive devices. Based on this classification, appropriate electrostatic protection measures can be implemented for specific types of SSDs.

3. Sources of Static Electricity in Electronic Products

(1) Human activity—such as the friction, contact, and separation occurring between a person and their clothing, footwear, or socks—is a primary source of static electricity in the manufacturing of electronic products. Static electricity generated by the human body is a major cause of both "hard" (catastrophic) and "soft" (degradative) breakdowns in electronic components. The electrostatic voltage generated by human movement typically ranges from approximately 0.5 kV to 2 kV. Furthermore, air humidity significantly influences this voltage; in a dry environment, the potential can increase by an order of magnitude. Table 2 illustrates the relationship between relative humidity and the level of static charge accumulated through human activity. When a person carrying a static charge touches a grounded object, a discharge occurs, which the person perceives as an electric shock—the intensity of this sensation is referred to as "shock sensitivity." Table 3 details human shock sensitivity during discharges at various electrostatic voltage levels.

 

(2) When work garments made of synthetic fibers or cotton rub against workstations or chairs, an electrostatic voltage exceeding 6,000 V can develop on the surface of the clothing, charging the person. If this person then touches an electronic component, a discharge may occur, potentially damaging the component.

 

(3) Footwear with rubber or plastic soles possesses an insulation resistance as high as $10^{13} \Omega$. Friction between these soles and the floor generates static electricity, which charges the human body.

 

(4) When components encapsulated in resins, lacquer films, or plastic films are transported in packaging, friction between the component surfaces and the packaging materials can generate electrostatic voltages of several hundred volts, leading to discharges into sensitive components.

 

(5) Various packaging materials, component trays, storage bins, and PCB racks—manufactured from polymers such as PP (polypropylene), PE (polyethylene), PS (polystyrene), PU (polyurethane), PVC, polyesters, or resins—can generate electrostatic voltages of 1–3.5 kV due to friction or impact, resulting in discharges into sensitive components.

 

(6) Standard workstation surfaces can generate static electricity when subjected to friction.

 

(7) Insulating floor surfaces—such as concrete, waxed or polished floors, or rubber matting—exhibit high insulation resistance, which prevents the static charge accumulated on the human body from dissipating (leaking away). (8) Regarding electronic production equipment and tools: High-voltage transformers and AC/DC circuits within equipment—such as soldering irons, wave soldering machines, reflow ovens, pick-and-place machines, and testing/debugging apparatus—can induce static electricity on the equipment itself. If static discharge measures are inadequate, this can lead to the failure of sensitive components during the manufacturing process. Furthermore, the circulation of hot air within ovens (causing friction against the chamber walls) and the presence of CO2 vapor in cryogenic cooling chambers can both generate significant amounts of static charge.

 

4. Principles of Electrostatic Protection

In the manufacturing of electronic products, it is impossible to completely prevent the generation of static electricity. The danger does not lie in the generation of static electricity itself, but rather in its accumulation and the subsequent electrostatic discharge (ESD). The core of electrostatic protection is the "elimination of static charge."

Principles of electrostatic protection:

(1) Prevent the accumulation of static electricity in areas where it is likely to be generated, ensuring that levels remain within a safe range.

(2) Rapidly eliminate any static charge that has already accumulated by providing an immediate discharge path.

 

5. Methods for Electrostatic Protection

(1) Use of anti-static materials: Metals are conductors; however, because they allow for high leakage currents, they can damage electronic components. Conversely, insulating materials are prone to generating static electricity through friction. Therefore, neither metals nor standard insulating materials are suitable as anti-static materials. Instead, "electrostatic conductors" (with a surface resistance of < 1×10⁵ Ω·cm) and "electrostatic semi-conductors" (with a surface resistance of 1×10⁵ to 1×10⁸ Ω·cm) are used. For example, a common anti-static material is created by incorporating conductive carbon black into rubber, bringing the surface resistance to below 1×10⁶ Ω·cm.

 

(2) Leakage and grounding: Grounding areas where static electricity may be—or has already been—generated provides a path for static discharge. This is achieved by installing a dedicated "independent" ground wire, ensuring that the resistance between the ground wire and the earth is less than 10 Ω. Grounding methods for electrostatic protective materials: Electrostatic protective materials (such as workbench mats, floor mats, anti-static wrist straps, etc.) should be connected to a conductor leading to an independent earth ground via a 1 MΩ resistor (refer to SJ/T 10630-1995). This 1 MΩ resistor is placed in series to ensure that the discharge current to the ground remains below 5 mA; this is known as "soft grounding." Equipment enclosures and electrostatic shielding covers are typically connected directly to the ground, which is known as "hard grounding."

 

(3) Eliminating static electricity from conductors: Static electricity on a conductor can be dissipated into the ground through a grounding connection. The voltage on a charged object as a function of discharge time can be expressed by the following equation: $U_T = U_0 e^{-t/RC}$, where $U_T$ is the voltage at time $t$ (V), $U_0$ is the initial voltage (V), $R$ is the equivalent resistance (Ω), and $C$ is the equivalent capacitance of the conductor (pF).

Generally, it is required that static electricity be dissipated within 1 second—that is, the voltage should drop below the 100 V safety threshold within 1 second. This prevents damage to static-sensitive devices (SSDs) that could otherwise be caused by an excessively rapid discharge or an overly high discharge current. For example, if $U_0 = 500\text{ V}$ and $C = 200\text{ pF}$, and the goal is to reach $U_T = 100\text{ V}$ within 1 second, a resistance of $R = 1.28 \times 10^9\text{ Ω}$ is required. Therefore, electrostatic protection systems typically employ a 1 MΩ current-limiting resistor to keep the discharge current below 5 mA. This is a safety design for operators; should an operator accidentally come into contact with a 220 V industrial power source while within the electrostatic protection system, it will not pose a danger.

 

(4) Eliminating static electricity from non-conductors: Because electric charges cannot flow across insulators, grounding cannot be used to eliminate static electricity from them. The following measures may be taken: (a) Use an ionizer (ion fan)—an ionizer generates positive and negative ions that neutralize static electricity at the source. These can be positioned in the workspace or near the placement heads of surface-mount (SMT) machines. (b) Use of antistatic agents—Antistatic agents are a type of surfactant. They can be used to wipe the surfaces of instruments and objects, enabling the rapid elimination of static electricity from those surfaces. (c) Control of environmental humidity—Increasing humidity enhances the surface conductivity of non-conductive materials, making it more difficult for static electricity to accumulate on surfaces. For example, in the dry environments often found in northern regions, measures such as humidification and ventilation can be implemented. (d) Use of electrostatic shielding—For equipment prone to generating static electricity, shielding covers (or cages) can be employed, provided that these shields (or cages) are effectively grounded.

 

(5) Process control method: To minimize the generation of static electricity during the manufacturing of electronic products, control the accumulation of static charges, and ensure the rapid elimination or immediate dissipation of any existing static charge, effective measures must be taken across various areas, including facility design, equipment installation, operational procedures, and management systems.

 

6. Electrostatic Protection Equipment

(1) Personal antistatic systems include items such as antistatic wrist straps, work garments, caps, gloves, footwear, and socks.

 

(2) Antistatic flooring options include antistatic terrazzo, antistatic rubber flooring, PVC antistatic plastic flooring, antistatic carpeting, and antistatic raised (access) flooring.

 

(3) Antistatic operational equipment: This includes items such as antistatic workbench mats, antistatic packaging bags, antistatic carts for material handling, and antistatic soldering irons and other tools.

 

7. Electrostatic Measuring Instruments

(1) Electrostatic field testers: Used to measure the surface resistance of items such as tabletops and floors. Different specifications of measuring instruments should be selected based on whether the surface is flat or non-planar.

 

(2) Wrist strap testers: Used to verify the effectiveness of wrist straps.

 

(3) Human body electrostatic testers: Used to measure the amount of static electricity carried by a person, the impedance between a person’s feet, the potential difference between individuals, and the effectiveness of protective gear such as wrist straps, grounding plugs, and work garments. They can also function as an entry-point discharge system, preventing static electricity carried by personnel from entering the production workshop. (4) Megohmmeter: Used to measure the impedance or resistance of all conductive, anti-static, and static-dissipative surfaces.

 

8. Anti-Static Technical Requirements for Electronics Manufacturing

 

(1) Grounding resistance for the anti-static grounding electrode: <10Ω.

 

(2) Floors or floor mats: Surface resistance: 10⁵–10¹⁰Ω; triboelectric voltage: <100V.

 

(3) Walls: Resistance: 5×10⁴–10⁹Ω.

 

(4) Work surfaces or mats: Surface resistance: 10⁶–10⁹Ω; triboelectric voltage: <100V; system resistance to ground: 10⁶–10⁸Ω.

 

(5) Resistance from the seat surface of a work chair to its casters: 10⁶–10⁸Ω.

 

(6) Triboelectric voltage for work garments, caps, and gloves: <300V; for shoe soles: <100V.

 

(7) Resistance of the wrist strap connection cable: 1MΩ; system resistance when wearing a wrist strap: 1–10MΩ. System resistance for heel straps (foot grounders): 0.5×10⁵–10⁸Ω.

 

(8) System resistance from the platform of a material handling cart to its wheels: 10⁶–10⁹Ω.

 

(9) Material handling equipment (such as component bins, totes, and PCB racks): Surface resistance: 10³–10⁸Ω; triboelectric voltage: <100V.

 

(10) Packaging bags and boxes: Triboelectric voltage: <100V.

 

(11) Total human body resistance: 10⁶–10⁸Ω.

 

9. Anti-Static Measures and General Requirements for Electrostatic Protected Areas (EPAs)

SMT production equipment must be properly grounded; pick-and-place machines should utilize a three-phase, four-wire (or similar) grounding configuration with an independent ground connection. Floors, work surface mats, and chairs within the production area must all comply with anti-static requirements. The workshop environment must be maintained at a constant temperature and humidity. Facilities such as anti-static component trays, turnover boxes, PCB racks, logistics carts, anti-static packaging materials, anti-static wrist straps, and anti-static soldering irons and tools must be provided.

 

(1) Establish anti-static zones based on anti-static requirements, and ensure they are clearly marked with anti-static warning signs. Classify work areas into levels 1, 2, and 3 according to the electrostatic sensitivity of the components used, and implement specific protective measures for each level:

Level 1 electrostatic sensitivity: 0–1999V

Level 2 electrostatic sensitivity: 2000–3999V

Level 3 electrostatic sensitivity: 4000–15,999V. Components with a sensitivity threshold above 16,000V are considered non-electrostatically sensitive.

 

(2) The ambient temperature in an electrostatic safety zone (or station) should be 23±3°C, with a relative humidity of 45–70% RH. Handling electrostatic-sensitive devices (ESDs) in environments with a relative humidity below 30% is prohibited.

 

(3) Periodically measure the surface resistance of floors, tabletops, turnover boxes, and other surfaces.

 

(4) Non-production items—such as tableware, tea sets, handbags, wool fabrics, newspapers, or rubber gloves—must not be placed on workstations within an electrostatic safety zone (or station).

 

(5) Personnel must discharge static electricity before entering an anti-static zone. Operators are required to wear appropriate work uniforms, anti-static footwear, and socks. Before beginning any operation, a safety check of anti-static protective measures must be performed; production may only commence once these checks are passed.

 

(6) Operators must wear an anti-static wrist strap, and the effectiveness of the strap must be verified daily.

 

(7) When testing ESDs, remove one component at a time from its packaging (tray, tube, or reel), test it, and then set it aside; do not allow components to accumulate on the tabletop. Components that fail testing must be returned to storage.

 

(8) When performing powered testing, follow the correct power-on and power-off sequences: apply power in the order of low voltage → high voltage → signal voltage. The power-off sequence should be the reverse. Additionally, ensure that power supply polarity is correct and that the supply voltage does not exceed the rated value. (9) Inspection personnel must be familiar with the models, types, and testing procedures for electrostatic-sensitive devices (SSD), and possess a fundamental understanding of electrostatic protection.

 

10. Requirements for the Transport, Storage, and Use of Electrostatic-Sensitive Devices (SSD)

(1) During transport, SSDs must not be dropped, nor should they be removed from their packaging.

 

(2) The relative humidity in storage areas for SSDs must be maintained between 30% and 40% RH.

 

(3) SSDs must remain in their original packaging during storage; if repackaging is necessary, anti-static containers must be used.

 

(4) In storage areas, locations where SSDs are kept must be marked with specific anti-static labels.

 

(5) When issuing SSDs, the quantity should be verified by visual inspection while the devices remain in their original packaging.

 

(6) When performing write, erase, or data protection operations on EPROMs, the programmer/eraser must be properly grounded, and the operator must wear an anti-static wrist strap.

 

(7) Personnel involved in assembly, soldering, board repair, or debugging must strictly adhere to all electrostatic protection requirements.

 

(8) Printed circuit boards (PCBs) that have passed testing and inspection should be treated with an ion air gun prior to final packaging to eliminate any accumulated electrostatic charge.

 

11. Management and Maintenance of Anti-Static Work Areas

(1) Establish an anti-static management policy and designate specific personnel to oversee it.

(2) Maintain a supply of anti-static garments, footwear, and wrist straps for use by visitors or external personnel.

(3) Regularly maintain and inspect anti-static facilities to ensure their effectiveness.

(4) Inspect wrist straps on a weekly (or daily) basis.

(5) Check the grounding of table and floor mats, as well as the performance of static eliminators, on a monthly basis.

(6) Evaluate the anti-static properties of component racks, PCB racks, transport containers, carts, table mats, and floor mats every six months.