Static electricity is an electrical charge at rest. Static electricity is most commonly created by friction and separation. Friction causes heat which excites the molecular particles of the material. When two materials are then separated, a transfer of electrons from one material to the other may take place.
As electrons transfer, the absence or surplus of electrons creates an electrical filed known as static electricity. The amount of static electricity generated depends upon the materials subjected to friction or separation, the amount of friction or separation and the relative humidity of the environment.
Materials that easily electrons (or charge) between atoms are called conductors and those ehtat cannot transfer easily are called insulators. Both conductors and insulators may become "charge" with static electricity.
Examples of Static Electricity
Static electricity in some forms can be seen quite easily. Electrical charge can have an effect on the electronic components you handle every day in your work. Unfortunately, the effect is much more hazardous and not as readily apparent.
Typical Electrostatic Voltages
Many of the common activities you perform daily may generate charge on your body that are potentially harmful to components.
Cost Effects of ESD
You may feel electrostatic discharge of 3,000 volts and just feel a static shock but smaller charges can do damage to semi-conductor devices. Many of the components used in your facilities can be damaged by charges of less than 1,000 volts, some as low as 10 volts.
Types of ESD Damage
Static damage to components can take the form of latent failure or catastrophic failures.
1. Latent failure - result in gate leakage
2. Catastrophic failure - occur in two forms, direct and latent
Direct catastrophic failure occur when a component is damaged to the point where it is dead and it will never function again. This is the easiest type of ESD damage to find since it usually can be detected during testing.
Latent failures occur when ESD weakens or wounded the components to the point where, during testing, it will still past but over time the wounded component will cause poor system performance and eventually complete system failure. Because latent failures occur after final inspections or in the hands of your company, the cost for repair is very high. Not only is this type of damage difficult to find, it will severely affects the reputation of your company's product. As upset failure occurs when an electrostatic discharge has caused a current flow that is not significant enough to cause total failure, but in use may intermittently result in gate leakage causing loss of software or incorrect storage information. Upsets or latent failure may pass your company's quality control testing program, for it may not be detected, felt or seen through normal testing procedures.
Just as you would never consider having surgery in a contaminated operating room. You should never handle, assemble or repair electronic assemblies without taking adequate protective measures against ESD.
Method of Control
ESD control methods can be classified under four categories.
Grounding
Isolation
Neutralization
Grounding
Grounding is very important for the elimination of static charges that are generated on conductors. Human beings are conductors and major generators or static electricity. Therefore we must eliminate static charges from building up on any person who will come in contact with ESD sensitive components or assemblies. Preventing static generation on the human body is best accomplished by grounding personnel.
Types of Personnel Grounding Devices Wrist straps are the most common grounding devices used in the industry. The wrist strap will safely and effectively drain static charges from your body. To function properly a wrist strap needs to make proper contact with the skin. A dirty or loose wrist strap may inhibit the draining of static charges and be ineffective in ESD control. Conductive footwear or foot grounders may be used or supplement the wrist strap.
Workstation Grounding Devices Conductive or static dissipative work surface are an integral part of static safe workstation, particularly in areas where hands assembly occurs. As with the wrist strap, it is necessary or the work surfaces to be cleaned and properly grounded to a common ground point. Conductive or dissipative materials can generate static charges but when properly grounded they will effectively drain static charges.
Isolation
The next concept is to isolate components and assemblies during storage or transporation. Isolating them from charged objects, fields and insulators is the best means of preventing or ESD damage from occurring during storage or transport. Since grounding will not drain the static charges or insulators, it is necessary to isolate sensitive components and assemblies from them. Eliminating common plastic and other types of insulators from static works, shipping and transport areas is the best way to isolate product from insulators. Isolation may also be accomplished by restricted entry into complete work areas or workstations.
Lastly, we utilize the fact that static charges cannot penetrate containers that are made of conductive materials or have a conductive layer. This effect is called the Faraday cage. Make sure that any container used during the storage and transport of components or loaded printed circuit boards has appropriate Faraday cage properties, which will isolate them from possible ESD damage.
Faraday Cage The types of Faraday Cages commonly used in controlling ESD are metalized bags, the conductive bags, the conductive tote box with cover. These "Faraday cages" might carry static charges on their exterior and must be removed before opening
Neutralisation
Neutralisation is important because grounding and isolation will not dissipate charges from insulators such as synthetic cloth or common plastic. Neutralization or removal or charges from insulators occurs naturally by a process called ionization. Ions are simply charged particles that are ever present in the air. Ions are created by naural energy events including sunlight, lightning, open flames and radiation.
We can artificially create billions of ions with ionizer. Ionizers employ high voltage to produce a balanced mix charged ions and fans to help the ions flow to the object or area to be nautralized. Ionization
