As is known, static electricity refers to relatively-static charges accumulated on object surface, which is the imbalance of positive and negative charges within local scope. ESD, the abbreviated form of Electrostatic Discharge, refers to static charges transfer between objects or surfaces with different electrostatic potential. ESD can be classified into contact discharge and electric field breakdown discharge. Furthermore, ESD damage refers to the phenomenon that performance degradation or failure suffered by electronic components due to the effect of ESD. As one of the most popular assembly technologies currently, SMT (Surface Mount Technology) has been widely applied in products serving different fields. Therefore, to guarantee optimal reliability and performance of electronic products, ESD damage should be prevented during SMT assembly process and this article will tell how.
Generation of ESD
Static electricity is actually generated through a series of physical processes including substance contact and separation, electrostatic induction, dielectric polarization and charged corpuscle attachment. Due to partial imbalance of electric charges on substance surface, static electricity exists on substance surface as high electric energy. Where there is movement, there is static electricity. In terms of SMT assembly process, static electricity can be met in the following situations:
a. ESD by Friction
As a leading source of discharging static electricity, friction has been a main cause for static electricity in SMT assembly process. After all, as long as any two substances of different materials are separated after a contact in daily life, static electricity will be generated. When it comes to SMT assembly process, ESD can be usually met after friction, contact and separation between human body and clothes, shoes, socks etc. Generally speaking, the more insulating material maintains, the more static electricity will be generated through friction. Based on concerning study, the static electricity due to human’s movement features a voltage in the range from 100V to 3,500V, which is regarded as the leading cause for hard damage or soft breakdown on electronic components.
b. ESD by Induction
In the process of electronic manufacturing, when electrostatic substances storing huge energy come close to electronic parts, those electronic parts will become electrostatic sources as well. If conducting material is placed in the field, positive and negative charges will be transferred with static electricity generated. If, however, two substances have a tight contact, electron migration will be generated at the joint. Furthermore, ESD can be also induced on equipment and tools used for electronic manufacturing, such as SMT assembly equipment, computer and monitor, electric soldering iron, reflow soldering oven, wave soldering oven and inspection instrument.
c. ESD by Conduction
It’s commonly seen that electronic components hold metal leads or pins and once they have contact with objects with static electricity, charges will be quickly transferred from charged body to metal body, making electronic components electrostatic. As soon as electrostatic components have contact with ground, ESD damage will be possibly aroused.
Attributes of ESD
Static electricity features high potential and low power and its voltage can be as high as hundreds to thousands volt, sometimes even higher. Static electricity usually works for a couple of microseconds and humidity is so closely related with static electricity that the improvement of humidity helps reducing ESD damage. Other attributes static electricity holds include:
Static electricity usually fails to be physically sensed by human and humans can’t surely have a sense of electric shock since only ESD with a voltage of 2kV higher can be sensed by human, which is electrostatic attribute of disguise. The failures or performance degradation caused by ESD damage mostly cannot be exposed unless inspections are implemented or components are practically applied.
The power of ESD belongs to space charge which features limited energy storage. As instantaneous high voltage pulse, it is only capable of providing short-time electric shock energy to components. In spite of low electric energy static electricity holds, it’s not that easy to release it safely and it’s complex to control it. ESSD (Electrostatic Sensitive Device) features such fine circuit that failure analysis is difficult to be implemented after electronic components suffer from ESD damage.
The potential failure owing to ESD just arouses partial parameter changes. As long as they stay within a reasonable domain, the components suffering from ESD damage will smoothly pass inspections, which will be the root for early failure. The defects resulting from ESD damage can hardly be overcome in later stages. What’s worse, it fails to be exposed during inspection phase.
As far as electronics industry is concerned, electronics products tend to become miniaturized and multi-functionalized, integrity of some components maintains rising up. In addition, the internal insulating layer becomes increasingly thinner, interconnecting wire finer, the capacity to withstand applicable voltage reduced. A number of electrostatic sensitive SMDs (Surface Mount Devices) feature shock voltage that is lower than that of electrostatic voltage people are able to sense. However, the electrostatic voltage generated in the process of manufacturing, transportation and storage is far higher than shock voltage, which usually leads components to suffer from hard shock or soft shock. Finally, the SMDs will suffer from failure or their reliability will be dramatically reduced.
According to statistics, among all the causes for electronics failures, ESD accounts for 8% to 33% and the caused damage reach billions US dollars. In the process of high-tech SMT assembly, therefore, the effective control of ESD is capable of increasing manufacturing efficiency, improving products’ quality and gaining profits. So, it’s of great significance to carry out effective measures to prevent ESD.
ESD will bring forward sudden failures or latent failures to sensitive components. Sudden failure, also called hard damage, may lead components to lose overall functions, making components internal part suffer from permanent failure or opens. Latent failure, also called soft damage, may lead components to be degraded in terms of performance parameters, making them unstably run or partial functions to be degenerated or lost. When it comes to the overall failures caused by ESD, latent failures account for 60% to 90% while sudden failures for 10%, which means most failures belong to latent failure. The nature of latent failure is that the defects of components can be hardly exposed through inspection and it’s difficult to find out the real cause for failure. Plus, rework and processing feature high cost and components’ shelf life will be also shortened.
ESD Protections in SMT Assembly Process
The essential purpose of ESD protection during SMT assembly process is to stop ESSD (Electrostatic Sensitive Device) from being harmed by static electricity. The basic principle is that electrostatic accumulation should be effectively controlled in places where ESD may be generated, electrostatic field should be prevented from being formed, and electrostatic sources should be rigorously controlled. Additionally, the number of electrostatic charges should be finally reduced and real-time monitoring, inspection and maintenance should be implemented on all electrostatic-resistant equipment. ESD protections are mainly carried out in terms of workshop environment, operation staff, grounding, and electrostatic charge neutralization.
•ESD Protections in Workshop Environment
a. Static-Free Ground Application
Manufacturing shop is such an important ESD protection area that all its parts should be free from static electricity, including ground, wall, ceiling, door, window, operation table, manufacturing tools etc. As a key electrostatic source, ESD protections should be well made. The commonly-used measures include anti-static PVC floor application, anti-static floor paint and rubber application.
To make better use of anti-static floor and prolong its shelf life, some measures should be made in daily life. For example, floor should be cleaned and free from sharp objects that may cause scratches. Dirt should be avoided away from workshop and greasy dirt on the floor should be cleaned off in time.
b. Relative Humidity Control
RH of manufacturing factory plays a significant role in influencing ESD so high ESD will be generated when it fails to be well controlled, leading to higher possibility of manufacturing accidents due to static electricity. Thus, RH can be properly increased in assembly workshop without causing harm to products. Additionally, materials with high static electricity should be far from manufacturing line.
c. Static Protective Material Usage
Static protective material refers to a type of material that is capable of stopping static electricity from being generated, dissipating electrostatic charges or preventing ESD. Thus, the material performance is essential to the whole ESD protection system.
•Electrostatic Grounding System of Human Body
Body ESD protection system consists of ESD protection uniform, wrist band, shoes, socks, hats, gloves or fingerstall, apron, foot strap etc. The whole system features functions of leaking, neutralizing and shielding static electricity. Electrostatic grounding system of human body is actually a complete grounding circuit to stop static electricity from being accumulated, consisting of band wrist, foot strap, shoes, pad, floor, mat, grounding branch and public branch.
The essential way to get rid of static electricity in SMT assembly process is to quickly dissipate static electricity generated in the manufacturing process and operation process. Grounding refers to the process that electric devices can be connected with the object that is capable of supplying or receiving a huge number of charges. Grounding can be classified into hard grounding and soft grounding. The former refers to ground connection through low impedance while the latter refers to ground connection through high impedance.
•Electrostatic Charge Neutralization
When it comes to conductors or dissipating semiconductor materials, ESD protections can be achieved through grounding but it doesn’t work on insulators. Therefore, ion neutralization should be used to eliminate electrostatic charges.
In a word, ESD protections should be applied in the whole process of SMT assembly manufacturing. As electronics technology makes constant progress today, ESD protections should be improved as well. As a complicated process, different measures should be made in order to be compatible with different requirement so that effective ESD protections can be made on devices.