Principle and method of electrostatic protection

Principle and method of electrostatic protection

 

Publication date: April 7, 2015 article editor: Shanghai Jiabai page views: 176

 

In the manufacture of electronic products, it is impossible not to generate static electricity. The generation of static electricity is not the harm, but the harm lies in the accumulation of static electricity and the resulting electrostatic discharge. The core of electrostatic protection is "meditation elimination".

 

1. Electrostatic protection principle:

 

(1) Where static electricity may be generated, static electricity shall be prevented from accumulating. Take measures within the safety range.

 

(2) The existing static electricity accumulation shall be eliminated quickly and released immediately.

 

2. Electrostatic protection method:

 

(1) Use anti-static materials: metal is a conductor. Due to the large leakage current of the conductor, the device will be damaged. In addition, since insulating materials are easy to generate friction electrification, metal and insulating materials cannot be used as anti-static materials. Instead, the surface resistance L is used × The so-called electrostatic conductor below 105 Ω· cm and the surface resistance 1 × 105-1 × An electrostatic sub conductor of 108 Ω· cm is used as an antistatic material. For example, the commonly used electrostatic protective material is realized by mixing conductive carbon black into rubber, and the surface resistance is controlled at 1 × Below 106 Ω· cm.

 

(2) Leakage and grounding: the parts that may or have generated static electricity shall be grounded to provide static electricity release channels. The "independent" ground wire is established by embedding the ground wire. Make the resistance between the ground wire and the earth < 10 Ω. (see gbjl79 or SJ / t10694-1996)

 

Grounding method of electrostatic protective materials: connect the electrostatic protective materials (such as table mat, ground mat, anti-static wrist strap, etc.) to the conductor leading to the independent grounding wire through a resistance of 1m Ω (see SJ / t10630-1995). 1m Ω resistor is connected in series to ensure the discharge of less than 5mA current to the ground, which is called soft grounding. The equipment shell and electrostatic shield are usually directly grounded, which is called hard grounding.

 

The grounding method of anti-static workbench recommended in ipc-a-610c standard.

 

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

 

UT=U0L1/RC

 

Where, the voltage (V) U0 at ut-t time together with the starting voltage (V) r-equivalent resistance (Ω) c-conductor equivalent capacitance (PF)

 

Generally, it is required to leak static electricity within 1 second. That is, the safety zone where the voltage is reduced to below 1oov within 1 second. This can prevent damage to SSD caused by too fast leakage speed and too large leakage current. If U0 = 500V and C = 200pf, if you want to make ut reach 100V within 1s, r = 1.28 is required × 109Ω。 Therefore, 1m Ω current limiting resistor is commonly used in electrostatic protection system to limit the discharge current below 5mA. This is designed for operational safety. If the operator accidentally touches 220V industrial voltage in the electrostatic protection system, it will not bring danger.

 

(4) Elimination of static electricity in non-conductor: for static electricity on insulator, because charge cannot flow on insulator, static electricity cannot be eliminated by grounding. The following measures can be taken:

 

(a) Use ion fan - ion fan generates positive and negative ions, which can neutralize the static electricity of static power supply. It can be set near the space and the pasting head of the pasting machine.

 

(b) Use Static Eliminators - Static Eliminators are surfactants. The instrument and object surface can be wiped and washed with electrostatic eliminator, which can quickly eliminate the static electricity on the object surface.

 

(c) Control environmental humidity - increasing humidity can improve the surface conductivity of non-conductive materials and make it difficult for static electricity to accumulate on the surface of objects. For example, humidification and ventilation measures can be taken in the dry environment in the north.

 

(d) Use electrostatic shielding - for equipment prone to static electricity, use shielding cover (cage) and effectively ground the shielding cover (cage).

 

(5) Process control method: in order to minimize the generation of static electricity in the manufacturing of electronic products, control the accumulation of static charge, quickly eliminate the existing static electricity accumulation and release it immediately, effective measures should be taken from the aspects of plant design, equipment installation, operation and management system.