Basic knowledge of ESD in SMT assembly
Several common ESD phenomena in life: combing hair, peeling off plastic, walking, running, taking off a sweater in winter, touching a doorknob, lightning, etc.
(different ESD phenomena)
What is static electricity?
Static electricity generally refers to static, relatively stable electric charges. That is the excess or lack of electrons on the surface of an object. If the surface has insufficient electrons, it is positively charged, and if it has excessive electrons, it is negatively charged. The total amount of insufficient or excessive electricity determines the surface charge.
Causes of static electricity
When two different objects come into contact with each other, one object will lose some of its charge, such as electrons transferred to another object, making it positively charged, while the other object will gain some remaining electrons and become negatively charged. If the charges are difficult to neutralize during the separation process, the charges will accumulate and cause the object to carry static electricity.
（cause of electrical static）
There are many ways to generate static electricity, such as contact, friction, rushing, freezing, electrolysis, piezoelectricity, temperature difference, etc., but there are mainly two forms, namely static electricity generated by friction and static electricity generated by induction. Here are some common ways of generating static electricity:
1) Contact charging: static electricity is easy to occur when two objects are in contact with each other and then separated.
2) Friction charging: static electricity is generated by heat generated when two objects rub against each other.
3) Transfer charging: parts packaged in plastic bags are easy to generate static electricity.
4) Peeling charging: static electricity is easy to occur when the pasted label or plastic is peeled off, and static electricity is also easy to generate when the shaft and belt are rolled and separated.
5) Jet charging: static electricity is generated by friction between the jet particles and the holes when liquid or gas is sprayed.
6) Thermionic emission charging: electrons are emitted when the object is heated.
7) Photoelectron charging: light reacts with the surface of the material and leaves a positive charge. Ultraviolet rays (sun rays) irradiating electronic products for too long are easy to generate static electricity… and so on.
What objects can generate static electricity in an SMT factory?
Any object can generate static electricity!
Everything in our work area can generate static electricity!
* Carts, chairs, manipulators, adhesive tape, containers, and other material handling items
* Human bodies, gloves, cleanroom rags, cleanroom space suits, regular clothing
* Work surfaces, CRT computer monitors, hand tools, paper, notebooks, folders, pens
* Walls, floors, alcohol bottles,
* ESD packaging, non-ESD packaging, etc. can all generate static electricity.
What is ESD?
ESD is the abbreviation of Electrical Static Discharge. It refers to the transfer of static charge between two objects with different electrostatic potentials.
Electrostatic sensitivity: The electrostatic discharge voltage value that a component can withstand is called electrostatic sensitivity. It is related to factors such as the structural size, material properties, and power consumption of the component.
Electrostatic discharge sensitive devices (ESDS): Electrostatic discharge sensitive devices refer to chips, discrete devices, integrated circuits and components, modules, complete equipment, etc. that are easily damaged by electrostatic fields or electrostatic discharge during daily operation, testing, packaging, transportation, etc.
（Electrostatic discharge sensitive devices）
How does ESD damage products in the SMT process?
A large amount of static charge is quickly transferred between an object (person or machine) with an electric potential difference and the product, generating a large current in a short period of time (far exceeding the current that the product can withstand). In severe cases, the circuit is damaged, rendering the product unusable; in mild cases, the circuit is overheated and damaged, affecting the life and reliability of the product.
（ESD Damage）
Most electronic components are electrostatically sensitive devices. ESD has the following adverse effects on electrostatically sensitive electronic components:
1) The components absorb dust and change the impedance between the circuits, affecting the function and life of the components
2) The electric field or current destroys the insulation or conductor of the component, making the component unable to work (completely unable to work)
3) The heat generated by the instantaneous electric field or current damages the component, which can still work but shortens its life.
There are two forms of damage to electronic components: complete loss of function and intermittent loss of function.
What kind of material is anti-static material?
（anti-static materials）
Places where ESD protection is required
In SMT assembly companies, the following places need to pay attention to electrostatic protection:
- Incoming material inspection
- Warehouse and material receiving area
- Components insertion area (automatic and manual)
- box building
- Packaging and transportation
- On-site maintenance and repair are the main causes of ESD damage
How to control ESD in the SMT assembly process?
There are three fundamental principles to prevent ESD in SMT assembly:
— Ground/Bond all conductors ( Note: People are also conductors!）
— Control Charges on Nonconductors
— Use protective packaging for transit and storage
ESD control = Man + Machine + Material + Method + Environment
Man
–Anti-static wrist strap, ESD shoes, ESD chair & clothes …
Machine
— Grounding to control static current
— Eliminate/shield electromagnetic interference and AC electric fields
Material
— Use conductive materials
— Use static dissipative materials as much as possible
— Use ESD floors and ESD table mats
— Do not use or use as little insulating materials as possible
Method
— Prevention (create an ESD-safe environment)
— Protection (protect ESD-sensitive products or components from metal-to-metal low-resistance contact)
— Shielding (perform Faraday or distance shielding on non-ESD-safe items)
— Elimination (eliminate static electricity generated through grounding and ionization)
Environment
— Temperature/humidity control
— Local use of ion blowers
— Employee ESD training
Detailed ESD control methods in SMT
#1. All Operators should wear anti-static work clothes, zip them up, button them up, dress neatly, and keep the cuffs in close contact with the skin.
(anti-static work clothes)
#2. Work shoes that are normally used during the production process should be tested with a work shoe tester after each time they are worn again.
（ESD shoe tester）
#3 Wear an anti-static wrist strap. If the user finds any defect, stop using it immediately. It is forbidden to use untested and defective products. During use, if the wrist strap is damaged, or broken, or the wrist plug shrapnel is deformed and loses elasticity (it is very loose when inserted into the wrist jack), it needs to be replaced in time. Anti-static wrist strap testing should be carried out before starting work in the morning and afternoon every day. After replacing the wrist strap, it must be tested. During the production process, if the operator leaves the post, there is no need to retest after returning. Put on the wrist strap again, and the wrist strap bracelet cannot slide freely on the wrist.
（Anti Static Wrist Strap）
When the operator leaves the workstation, he should take off the wristband and hang it on a grounded metal hook on the workstation to prevent false alarms. He can only work after returning to the post and putting on the wristband.
（grounded metal hook on the workstation）
#4 Use anti-static packaging products, such as anti-static shielding bags, anti-static packaging bags, anti-static sponges, anti-static IC packaging tubes, anti-static component boxes (boxes), anti-static bubble films, and anti-static transport vehicles. In addition to the static conductors used for static shielding, most of these packaging products are made of static dissipative materials, and some are made of anti-static materials. The purpose is to protect the installed circuits or devices and printed circuits from static electricity.
（different anti-static packaging materials)
#5 Antistatic floor and table mat are also indispensable in the electrostatic protection project. There are many kinds of antistatic floor, which can be divided into permanent and temporary according to timeliness; conductive rubber, PVC, conductive ceramic, etc. according to materials; and direct ground laying and overhead raised floor according to laying methods. It can be determined according to actual needs and costs.
#6 Use anti-static soldering irons, anti-static chairs, chair covers, anti-static repair kits, etc. Among them, anti-static soldering irons are very commonly used in post-processing and maintenance. Ordinary soldering irons need to be unplugged when soldering static-sensitive components, while anti-static soldering irons use DC temperature-voltage power supplies, and the heating elements are mostly made of materials with constant temperature characteristics and small static capacitance, which can greatly reduce various interference signals. In addition, the soldering iron can also be used as an electrostatic grounding, which can further eliminate various signals on the soldering iron tip. Therefore, there is no need to unplug the power head when soldering.
Summary of measures to prevent ESD damages
| Target | Static Electricity Prevention Measures |
| Operator | 1. Wear cotton clothes and anti-static work clothes.
2. Operators must wear an anti-static wrist strap and must be grounded through 1Mohm at this time, and the metal part should be in close contact with the skin. 3. Fingers cannot directly touch the IC’s legs but should pinch the body of the IC. 4. When taking the PCB board, be careful: do not touch the PCB’s surface, multimeter pen, CONN, etc. 5. The iron chain hanging from the chair where the operator sits is grounded to the conductive pad on the ground. The iron chain can conduct static electricity. |
| Machines | 1. Fixtures, measuring instruments, and machinery should be directly grounded to metal parts.
2. It is reasonable to use conductors for the driving parts of the equipment. 3. Static electricity will be generated during the processing of the processed object, so it should not be directly in contact with the metal parts of the machine. |
| Workbench and floor mats | 1. Cover the workbench with a conductive mat and ground it with a 1Mohm resistor.
2. Cover the floor with a conductive mat and ground it with a 1Mohm resistor. 3. Do not place ICs, PCBs, etc. directly on the workbench or floor without taking anti-static measures. 4. It is effective to place conductive component boxes on a workbench covered with a conductive mat and grounded with a 1Mohm resistor. |
| Transportation and storage | 1. It is best to use the original packaging container of the IC manufacturer for transportation and storage.
2. Wear an anti-static ring when taking out the IC. 3. The taken out IC should be stored in a conductive FORM or conductive SAST. 4. Use containers with conductive materials with resistance when storing and transporting IC, PCB, etc. Do not use plastic packaging, foam, plexiglass, etc., and the resistance value of metal containers is very small, which is easy to discharge rapidly and must not be used. 5. To prevent the occurrence of static electricity during transportation, try to avoid mechanical vibration and friction. 6. Use anti-static storage boxes, storage racks, trolleys, etc. Cover the storage rack with a conductive pad and add a 1Mohm resistor to ground it, and place the conductive storage box on it. The trolley should also be made according to the above requirements. |
