Basic Concepts in Electrostatic Discharge (ESD)
We have all seen static electricity in the form of lightning or perhaps felt the zap when reaching for a door knob. Similar types of electrical charges can have an effect on the electronic components you handle every day in your work. Unfortunately, their effect is much more hazardous and not as readily apparent.
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 field known as static electricity. The simple separation of two materials, as when tape is pulled off a roll, can also create this same transfer of electrons between materials, generating static electrical fields.
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. Common plastic generally will create the greatest static charge. Low humidity conditions such as those created when air is heated during the winter will also promote the generation of significant static electrical charges.
Materials that easily transfer electrons (or charge) between atoms are called conductors and are said to have "free" electrons. Some examples of conductors are metals, carbon and the human body's sweat layer. Materials that do not easily transfer electrons are called insulators. Some well known insulators are common plastics, glass and air. Both conductors and insulators may become "charged" with static electricity. When a conductor is charged, the free electrons give it the ability to discharge rapidly when it comes close to another conductor with a different potential.
Typical Electrostatic Voltages
Many of the common activities you perform daily may generate charges on your body that are potentially harmful to components.
Some of these activities include:
- Walking across a carpet, 1,500 to 35,000 volts
- Walking over untreated vinyl floor, 250 to 12,000 volts
- Worker at a bench, 700 to 6,000 volts
- Vinyl envelope used for work instructions, 600 to 7,000 volts
- Picking up a common plastic bag from a bench, 1,200 to 20,000 volts
Costly Effects of ESD
When you feel a static shock, you are experiencing a minimum of 3,000 volts of electricity.
This "shock" known as Electro Static Discharge or ESD also may be responsible for damaging many of the rejected electronic components in your company.
While you can feel electrostatic discharges of 3,000 volts, smaller charges are below the threshold of human sensation. Unfortunately, smaller charges can and do damage semiconductor devices. Many of the CMOS technology components used in your facility can be damaged by charges of less than 1,000 volts. Some of the more sophisticated components can be damaged by charges as low as 10 volts. You should be aware of the relative sensitivity to ESD damage of devices you may be working with.
As electronic technology advances, electronic components tend to become smaller and smaller. As the size of the components is reduced, so is the microscopic spacing of insulators and circuits within them, increasing their sensitivity to ESD. As you can predict, the need of proper ESD protection increases everyday.
Types of ESD Damage
Static damage to components can take the form of upset failures or catastrophic failures.
- Upset failures - result in gate leakage
- Catastrophic failures - occur in two forms, Direct and Latent
Direct catastrophic failures occur when a component is damaged to the point where it is DEAD NOW and it will never again function. 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 wounds the component to the point where it will still function properly during testing, but over time the wounded component will cause poor system performance and eventually complete system failure. Because latent failures occur after final inspection or in the hands of your customer, the cost for repair is very high. Not only is this type of damage hard to find, but it severely affects the reputation of your company's product.
An 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 of information.
Upset or latent failures may pass your company's quality control testing program. In other words, static damage may occur that cannot be felt, seen, or detected through normal testing procedures.
Think of Static As Contamination!
Damage caused by invisible and undetectable events can be understood by comparing ESD damage to medical contamination of the human body by viruses or bacteria. Although viruses and bacteria are invisible, they can cause severe damage even before you can detect their presence. A defense against this invisible threat is sterilization.
As an employee, the hidden threat of electrostatic discharge or ESD should be of great concern to you. ESD damage can significantly reduce your company's profitability. This may affect your profit sharing, your company's ability to compete in the market place and even your employment. Everyone likes to take pride in their work, but without proper ESD controls, your best efforts may be destroyed by static electricity that you can neither feel nor see.