1. Electrostatic protection principle

In the manufacture of electronic products, it is impossible not to generate static electricity. The generation of static electricity is not a source of harm. The harm lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of ESD protection is “Static Electricity Elimination.”

Electrostatic protection principle:

 (1) Prevent static electricity accumulation in places where static electricity may be generated. Take measures within the safety range.

 (2) The existing electrostatic accumulation is rapidly eliminated and released immediately.

(1) Use of anti-static material: Metal is a conductor, because the leakage current of the conductor is large, and the device may be damaged. In addition, since the insulating materials are prone to frictional electrification, metal and insulating materials cannot be used as antistatic materials. Instead, a so-called electrostatic conductor having a surface resistance of 1×10 5 Ω·cm or less and an electrostatic sub conductor having a surface resistance of 1×10 5 -1×10 8 Ω·cm are used as the antistatic material. For example, the commonly used electrostatic protection material is realized by mixing conductive carbon black into the rubber, and the surface resistance is controlled to be less than 1×10 6 Ω•cm.

(2) Leakage and grounding: Ground the part where the static electricity may be generated or has been generated, and provide an electrostatic discharge passage. The “ground” method is used to establish “independent” ground lines. Make the resistance between ground and earth <10Ω.

  Electrostatic protective material grounding method: the static protection material (such as for the table mats, ground mats, anti-static wrist strap, etc.) is connected to the conductor leading to the independent earth ground through 1MΩ resistance. The 1MΩ resistor is connected in series to ensure a current of <5mA to ground, which is called soft ground. The device housing and the electrostatic shield are usually directly grounded, which is called hard ground.

(3) Elimination of conductor static electricity: Static electricity on the conductor can be grounded to leak static electricity to the earth. The voltage and release time of the discharge body can be expressed by the following formula:

      UT=U0L1/RC

       Where UT-T voltage (V) U0 a starting voltage (V) R-equivalent resistance (Ω) C-conductor equivalent capacitance (pf)

  It is generally required to leak static electricity within 1 second. That is, within 1 second, the voltage is reduced to a safe area below 100V. This will prevent the SSD from being damaged due to excessive leak speed and excessive leakage current. If U0 = 500V, C = 200pf, want to make UT reach 100V within 1 second, need R = 1.28 × 109Ω. Therefore, the electrostatic protection system is usually used 1MΩ current limiting resistor, limiting the discharge current to 5mA or less. This is designed for operational safety. If operators in the electrostatic protection system accidentally touch the 220V industrial voltage, it will not bring danger.

(4) Elimination of non-conductor with static electricity: For static electricity on the insulator, since the charge cannot flow on the insulator, static electricity cannot be eliminated by a grounding method. The following measures can be taken:

      (a) Use an ion fan- Ion fan to generate positive and negative ions, which can neutralize static electricity from the static power supply. Can be placed near the space with placement machine patch heads.

     (b) Use of an antistatic agent – An antistatic agent is a surfactant. The static elimination agent can be used to wash the surface of the instrument and the object, and the static electricity on the surface of the object can be quickly eliminated.

     (c) Controlling the humidity of the environment – Increasing the humidity can increase the surface conductivity of the non-conductor material so that the surface of the object is unlikely to accumulate static electricity. For example, the northern dry environment can take humidification and ventilation measures.

    (d) Electrostatic Shielding – For devices that are prone to static electricity, use a shield (cage) and effectively ground the shield (cage).

 (5) Process control method: In order to minimize the generation of static electricity in the manufacture of electronic products and control the accumulation of electrostatic charges, the existing electrostatic accumulation is rapidly eliminated and released immediately, and the plant design, equipment installation, operation, and management systems should be adopted. take effective action.

  (1) The human body anti-static system includes anti-static wristbands, work clothes, caps, gloves, shoes, socks, etc.

  (2) Anti-static ground includes anti-static terrazzo floor, anti-static rubber floor, PVC anti-static plastic floor, anti-static carpet, anti-static raised floor and so on.

  (3) Anti-static operation series: Including anti-static: I: as a desk mat, anti-static bags, anti-static logistics car, anti-static soldering iron and tools.

  (1) Electrostatic field tester: It is used to measure the surface resistance value of the mesa and the ground. Plane structure occasions and non-planar occasions to choose different specifications of the measuring instrument.

  (2) Wristband tester: Whether the wristband is effective or not.

  (3) Human body electrostatic tester: It is used to measure the amount of static electricity carried by the human body, the impedance between the human body’s feet, measure the static electricity difference between the human body, and whether the wristband, grounding plug, work clothes, etc., are effective for protection. It can also be used as an entry discharge to keep the body electrostatically separated from the workshop.

  (4) Megohmmeter: It is used to measure the impedance or resistance of all conductive type, antistatic type, and electrostatic discharge type surfaces.

  (1) Antistatic grounding electrode resistance <10Ω.

  (2) Floor or floor mats: surface resistance 105-1010 Ω; friction voltage <100V.

  (3) Wall: The resistance value is 5×104-109Ω.

  (4) Worktable or pad: Surface resistance 106-109Ω; Friction voltage <100V; Ground resistance 106-108Ω.

  (5) The working chair faces the caster resistance 106-108Ω.

  (6) Work clothes, caps, gloves, friction voltage <300V; shoe friction voltage <100V.

  (7) Wrist strap connection cable resistance 1MΩ; system resistance 1-1OMΩ when wearing wrist strap. The heel strap (shoes) system resistance is 0.5 x 105-108 Ω.

  (8) The logistics vehicle platform faces the wheel system resistance 106-109Ω.

  (9) The surface resistance of the material transfer box such as material box, turnover shelf, and PCB frame is 103-108Ω; the friction voltage is less than 100V.

  (10) Packing box friction voltage <100V.

  (11) The total body resistance is 106-108Ω.

6. Anti-static measures in the manufacture of electronic products and general requirements for electrostatic operation areas (spots)

  The SMT production equipment must be well grounded. The Pick&Place machine should use a three-phase wireless system grounding method and be independently grounded. Floors, countertop mats, seats, etc. at the production site should meet the requirements for anti-static electricity. Maintain a constant temperature and humidity environment in the workshop. Should be equipped with anti-static material boxes, turnover shelves, PCB racks, logistics vehicles, anti-static packaging, anti-static wrist strap, anti-static iron and tools and other facilities.

    (1) Set anti-static area according to anti-static requirements, and have obvious anti-static warning signs. According to the electrostatic sensitivity of the devices used in the work area, it is divided into 1, 2, 3 levels, and different protective measures are formulated according to different levels.

  Level 1 electrostatic sensitivity range: 0-1999V

  Level 2 electrostatic sensitivity range: 2000-3999V

  Level 3 electrostatic sensitivity range: 4000-15999V

  More than 16000V is a non-static sensitive product.

  (2) The room temperature of the electrostatic safety zone (dot) is 23±3°C and the relative humidity is 45-70%RH. It is forbidden to operate SSDs (electrostatic sensitive devices) in less than 30% of the environment.

  (3) Regularly measure the surface resistance values of the floor, desktop, and turnover shelf etc.

  (4) Non-products such as cutlery, tea sets, bags, wool fabrics, newspapers, and rubber gloves are prohibited from being placed on the worktable in the electrostatic safety zone (point).

  (5) Workers enter the anti-static area and need to be discharged. Operators must wear overalls and anti-static shoes and socks when operating. Each time you go to work, you must do an electrostatic protection safety check. Only after passing the inspection can you produce.

  (6) Wear an anti-static wrist strap during operation and measure whether the wrist strap is effective daily.

   (7) When testing the SSD, take a piece from the box, tube, and tray. Measure one piece, place one piece, and do not stack on the table. Unqualified devices that have been tested should be returned.

   (8) Power-on test must follow the sequence of power-on and power-off: low voltage → high voltage → signal voltage sequence. The order of power-off is the opposite of this. Also note that the polarity of the power supply must not be reversed and the supply voltage must not exceed the rated value.

   (9) Inspectors should be familiar with the SSD model, type, test knowledge, and understand the basic knowledge of electrostatic protection.

 (1) SSDs must not be dropped on the ground during transportation and must not be detached from packaging.

 (2) Relative humidity of warehouses storing SSDs: 30-40% RH.

 (3) Keep the original packaging during storage of the SSD. If it is necessary to replace the packaging, use a container with antistatic properties.

 (4) In the warehouse, special anti-static labels should be placed on the SSD devices.

 (5) Apply visual inspection methods when dispensing SSD devices and count the quantity in the original package of SSD devices.

 (6) When writing, erasing, and protecting the information of the EPROM, the writer/editor should be fully grounded, and an anti-static bracelet should be provided.

 (7) Operators of assembly, welding, repairing, debugging, etc. must strictly follow the electrostatic protection requirements.

 (8) The tested and inspected printed circuit boards should be sprayed one time with the ion spray gun before encapsulation to eliminate the static charge that may accumulate.