During PCB焊接, electrostatic discharge often damages components, sometimes rendering them inoperable, resulting in significant losses. Therefore, static protection during PCB welding is of utmost importance.
Static Electricity and Its Hazards
Static electricity is an electrical energy that remains on the surface of objects, resulting from an imbalance of positive and negative charges within a localized area, and is formed through the conversion of electrons or ions. The term "static electricity" refers to the general term for electrical phenomena that occur during the generation and disappearance of charges, such as phenomena like frictional charging and human body charging.
With technological advancements, electrostatic phenomena have been widely and effectively utilized in fields such as electrostatic painting, electrostatic spinning, electrostatic sorting, and electrostatic imaging. However, on the flip side, the generation of static electricity can lead to significant harm and losses in many industries. For instance, in an Apollo manned spacecraft, an explosion caused by static discharge resulted in the deaths of three astronauts; accidents involving explosions and casualties due to electrostatic discharge (ESD) during the manufacturing process of explosives are not uncommon. In the electronics industry, as integration levels rise, the internal insulation layers of integrated circuits become thinner, and the widths and spacings of interconnect lines decrease. For example, the typical thickness of the insulation layer in CMOS devices is about 0.1μm, with a corresponding breakdown voltage of 80-100V; the insulation layer of VMOS devices is even thinner, with a breakdown voltage of 30V. During the manufacturing, transportation, and storage of electronic products, the static voltages generated are often far beyond the breakdown voltage of MOS devices, which can lead to hard breakdown or soft breakdown (localized damage to the device), rendering the device inoperable or severely affecting product reliability.
To control and eliminate ESD (Electrostatic Discharge), developed countries such as the United States, Western Europe, and Japan have established national and corporate standards or regulations. There are corresponding provisions for the design, manufacturing, purchase, storage, inspection, warehousing, assembly, debugging, packaging, and transportation of electrostatic-sensitive components, as well as stringent rules and regulations for the manufacture, use, and management of static protective equipment. China has also formulated corporate standards in accordance with international standards, such as those from the Ministry of Aerospace, the Ministry of Mechanical and Electrical Engineering, and the Ministry of Petroleum.
2. Static-Sensitive Devices (SSD)
Electrostatically sensitive devices are referred to as Static-Sensitive Devices (SSD). These primarily include ultra-large scale integrated circuits, particularly metalized film semiconductors (MOS circuits). Table 1 provides a classification chart for static-sensitive devices. Different electrostatic protection measures can be adopted for various SSD devices based on the SSD classification chart.
3. Static Power in Electronics Manufacturing
(1) Static electricity generated from human activities, such as friction, contact, and separation between humans and objects like clothes, shoes, and socks, is one of the main sources of static electricity in electronic product manufacturing. Human static electricity is a primary cause of hard (soft) breakdown in devices. The static voltage produced by human activity is approximately 0.5-2KV. Additionally, air humidity has a significant impact on static voltage; in a dry environment, it can increase by an order of magnitude. Table 2 shows the relationship between relative humidity and the charging of human activities.
Upon contacting the ground wire after a person becomes electrified, a discharge phenomenon occurs, causing the person to experience varying degrees of electric shock sensations, which is referred to as the shock sensation threshold. Table 3 presents the shock sensation thresholds of humans during the discharge process at different static voltages.
(2) When fiber or cotton work clothes rub against work surfaces or chairs, static voltages over 6000V can be generated on the clothing surface, charging the human body. Contact with components during this time can cause discharge, easily damaging the devices.
(3) Insulation resistance of rubber or plastic soles reaches up to 10^13Ω, generating static electricity upon friction with the ground, thereby charging the human body.
(4) When devices encapsulated with resin, lacquer film, or plastic film are transported in packaging, friction between the device surface and the packaging material can generate several hundred volts of static electricity, which can discharge onto sensitive devices.
Various packaging, bins, turnover boxes, PCB racks, and more made from high polymer materials such as PP (polypropylene), PE (polyethylene), PS (polystyrene), PVR (polyurethane), PVC, and polyester resins may generate static voltages of 1-3.5kV due to friction and impact, which can discharge onto sensitive components.
(6) Standard desk surfaces generate static electricity due to friction.
(7) Insulating groundings such as concrete, waxed and polished flooring, and rubber mats have high insulation resistance, making it difficult for static charges on the human body to leak.
(8) In electronic production equipment and tools, such as soldering irons, wave soldering machines, reflow ovens, pick-and-place machines, and debugging and testing equipment, high-voltage transformers, AC/DC circuits within the equipment, can induce static electricity. Poor static discharge measures in the equipment can lead to failure of sensitive devices during the manufacturing process. The circulation of hot air in the oven and friction with the housing, as well as the cooling of CO2 vapor in the CO2 refrigeration box, can generate a large amount of static charge.
4. Static Protection Principles
During the PCB welding process, it is impossible not to generate static electricity. The danger does not lie in the generation of static electricity itself, but in the accumulation of static and the resulting static discharge. The core of static protection is "quiet elimination."
Principle of Electrostatic Protection:
Prevent the accumulation of static electricity in areas where it may be generated. Take measures to ensure safety within the designated range.
(2) Rapidly dissipates existing static buildup, releasing immediately.
5. Electrostatic Protection Methods
(1) Utilizing anti-static materials: Metals are conductors, and due to the large leakage current of conductors, they can damage devices. Additionally, insulating materials are prone to static charge generation through friction, so they cannot be used as anti-static materials. Instead, materials known as electrostatic conductors with surface resistances below 1×10^5 Ω·cm and electrostatic semi-conductors with surface resistances between 1×10^5 and 1×10^8 Ω·cm are used. For instance, commonly used anti-static materials are achieved by blending conductive carbon black into rubber, controlling the surface resistance to below 1×10^6 Ω·cm.
(2) Leaking and Grounding: Ground areas that may generate or have already generated static electricity to provide a static discharge path. Establish a "separate" grounding wire using the buried earth wire method. Ensure the resistance between the grounding wire and the earth is less than 10Ω. (Refer to GBJl79 or SJ/T10694—1996)
Electrostatic Protection Material Grounding Method: Connect the electrostatic protection material (such as anti-static desk mats, floor mats, and anti-static wrist straps) to a conductor leading to an independent earth line through a 1MΩ resistor (refer to SJ/T10630-1995). The series connection of a 1MΩ resistor is to ensure that the current discharged to the ground is less than 5mA, known as a soft grounding. The equipment casing and electrostatic shielding covers are usually grounded directly, referred to as hard grounding.
(3) Elimination of static electricity on conductors: The static electricity on a conductor can be discharged to the earth by grounding. The voltage of the discharger and the release time can be expressed by the following formula:
UT=U0L1/RC
At time UT-T, the voltage (V) is U0, the initial voltage (V), R - equivalent resistance (Ω), C - equivalent capacitance (pf) of the conductor.
To prevent damage to the SSD due to rapid leakage and excessive leakage current, it is generally required to dissipate static electricity within 1 second, reducing the voltage to below 100V. This ensures that the leakage rate is controlled and does not exceed safe levels. If U0 is 500V and C is 200pF, and you want to reduce UT to 100V within 1 second, the required resistance R would be 1.28×10^9Ω. Therefore, in static protection systems, a 1MΩ current-limiting resistor is typically used to limit the discharge current to less than 5mA, ensuring operational safety. If an operator accidentally comes into contact with a 220V industrial voltage while in the static protection system, it will not pose a hazard.
(4) Elimination of Static Electricity on Non-Conductors: For static electricity on insulators, as charges cannot flow on insulators, grounding is not an effective method to dissipate static electricity. The following measures can be taken:
(a) Utilizing an Ion风机—Ion wind generators produce positive and negative ions to neutralize static electricity. They can be set up near the space and the tape applicator's tape head.
(b) Utilize static dissipating agents—static dissipating agents are a type of surfactant. They can be used to wipe instruments and object surfaces, quickly removing static electricity from them.
Regulate Environmental Humidity – Increasing humidity can enhance the surface electrical conductivity of non-conductive materials, making it less likely for static electricity to accumulate on surfaces. For instance, in dry northern climates, measures such as humidified ventilation can be adopted.
(d) Utilize electrostatic shielding—use shielding enclosures (cages) for equipment prone to static buildup, and ensure the enclosures are effectively grounded.
(5) Process Control Method: To minimize the generation of static electricity in electronic product manufacturing, control the accumulation of static charges, and quickly dissipate existing static accumulations for immediate release, effective measures should be taken in aspects such as factory design, equipment installation, operation, and management systems.
6. Electrostatic Protection Equipment
(1) The anti-static personal protective equipment includes anti-static wrist straps, workwear, hats, gloves, shoes, and socks.
(2) Anti-static flooring solutions include anti-static terrazzo, anti-static rubber flooring, PVC anti-static plastic tiles, anti-static carpets, and anti-static modular flooring.
(3) Antistatic Operation Series: Includes antistatic items such as workbench mats, antistatic packaging bags, antistatic logistics carts, antistatic soldering irons, and tools.
Static Measurement Instruments
Electrostatic Field Tester: Used for measuring surface resistivity of countertops, floors, and other surfaces. Different models of measuring instruments should be selected for flat and non-flat structures.
(Wristband Tester: Measures the effectiveness of the wristband.)
(3) Body Static Electricity Tester: Used to measure the amount of static electricity carried by the human body, the impedance between the feet, the static electricity difference between individuals, and the effectiveness of protective devices such as wrist straps, grounding plugs, and workwear. It can also serve as an initial discharge method to keep static electricity from the body out of the workshop.
(4) Megohmmeter: Used for measuring the impedance or resistance of all conductive, antistatic, and static dissipative surfaces.
8. Static Dissipation Technical Requirements in Electronic Product Manufacturing
Static Dissipative Grounding Electrode Resistance < 10Ω.
(2) Floor or Mat: Surface Resistance: 105-1010 Ω; Friction Voltage: <100V.
(3) Wall: Resistance value 5x10^4 - 10^9 Ω.
(4) Worktop or Pad: Surface resistance 106-109 Ω; Friction voltage < 100V; Ground system resistance 106-108 Ω.
(5) The working chair's resistance to caster wheels is 106-108Ω.
(6) Workwear, hats, and gloves friction voltage < 300V; shoe soles friction voltage < 100V.
(7) Wristband cable resistance: 1MΩ; system resistance while wearing the wristband: 1-10MΩ. Heel strap (shoe strap) system resistance: 0.5×10^5-10^8Ω.
Logistics vehicle faces resistance of 106-109Ω in the wheel system.
(9) Packaging materials such as bins, turnover boxes, and PCB racks have a surface resistivity of 103-108Ω; friction voltage is less than 100V.
(10) Packaging: Friction voltage of boxes < 100V.
(11) Total human body resistance: 106-108 Ω.
General Requirements for Anti-Static Measures in Electronics Manufacturing and Static-Sensitive Work Areas (Points)
Printed Circuit Board (PCB) welding equipment must be properly grounded, with the placement machine employing a three-phase AC grounding method and a separate grounding system. The ground surface, workbench pads, chairs, and other areas in the production facility must meet anti-static requirements. The workshop should maintain a stable temperature and humidity environment. Anti-static material boxes, trolleys, PCB racks, logistics carts, anti-static packaging tapes, anti-static wrist straps, anti-static soldering irons, and tools should be provided.
(1)静电-sensitive areas are established according to static dissipative requirements, with clear static dissipative warning signs. The work areas are divided into levels 1, 2, and 3 based on the static sensitivity of the devices used, with different protective measures implemented for each level.
1-Level ESD Sensitivity Range: 0-1999V
2-Level ESD Sensitivity Range: 2000-3999V
3-Level ESD Sensitivity Range: 4000-15,999V
Products with a non-static sensitive rating of 16,000V and above.
(2) The ambient temperature in the electrostatic safe area (point) is 23±3°C, with a relative humidity of 45-70% RH. Operation of SSDs (Static Sensitive Devices) is prohibited in environments below 30% humidity.
Regularly measure the surface resistivity of surfaces such as floors, desks, and周转箱.
(4) Non-production items such as utensils, tea sets, bags, woolen fabrics, newspapers, and rubber gloves are prohibited on the workbenches in the electrostatic-safe area (point).
(5) Employees must be discharged when entering the antistatic area. Operators must wear work uniforms and antistatic shoes and socks when performing operations. A static protection safety check must be conducted before each shift, and only upon passing the check can production commence.
(6) Wear anti-static wrist straps during operation and check their effectiveness daily.
(7) When testing SSDs, remove one from the packaging box, tube, or tray, test it, and then place it back. Do not pile them on the table. Disqualified components should be returned to the inventory.
(8) During power-on testing, it is essential to follow the correct sequence of powering up and shutting down: start with low voltage, then high voltage, and finally signal voltage. The shutdown sequence is the reverse. Additionally, be cautious not to reverse the polarity of the power supply, and ensure the voltage does not exceed the rated value.
Inspectors should be familiar with the models and varieties of SSDs, possess knowledge of testing, and understand the basics of electrostatic protection.
10. Requirements for the Transportation, Storage, and Use of Electrostatic Sensitive Devices (ESD)
(1) SSDs must not be dropped during transportation and must remain in their packaging at all times.
(2) Relative humidity in the storage room for SSDs: 30-40%RH.
(3) Keep SSDs in their original packaging during storage, and use an anti-static container if a change in packaging is necessary.
(4) In the storage room, anti-static labels should be affixed to the locations where SSD devices are stored.
(5) During the distribution of SSD devices, count the quantity by visual inspection within the original packaging of the SSD devices.
When performing write, erase, and information protection operations on EPROMs, the writer/eraser should be fully grounded and a static-dissipative wristband must be worn.
(7) Operators of assembly, welding, sheet repair, and debugging must strictly adhere to static protection procedures.
Prior to encapsulation, qualified printed circuit boards undergo a single ion spray treatment to dissipate any static charge that may have accumulated.
11. Managing and Maintaining Antistatic Work Areas
Establish anti-static management policies and assign dedicated personnel for oversight.
(2) Backup antistatic workwear, shoes, bracelets, and other personal items are available for use by visitors.
(3) Regularly maintain and inspect the effectiveness of static-dissipative equipment.
(4) Check the bracelet weekly (or daily).
(5) Monthly inspections of the grounding for table and floor mats, as well as the performance of static eliminators.
(6) Anti-static component racks, PCB racks, and周转boxes; transport vehicles, desk pads, and floor mats are to be inspected for anti-static properties every six months.
