Electro Static Discharge: Controlling static electricity

May 1, 2002
Wrist straps are one solution to control electrostatic discharge (ESD)

Technicians can test to make sure of proper grounding.

Controlling static electricityBy Jim Sparks

Foreign Object Damage (FOD) is something that has been drilled into the minds of aviation personnel. We are all aware of the result when a foreign object enters the inlet of a turbine engine or when the wing of a moving aircraft comes in contact with a stationary rigid object like the hangar wall.

One form of FOD that has been all but overlooked is electrostatic discharge or ESD. Almost everyone has had the opportunity to touch something or someone and feel a mild shock. This is a form of ESD. Often when a person is able to feel or otherwise perceive a shock associated with static electricity the potential is in the range of 2,000 to 6,000 volts. A static discharge resulting from as little as a 10-volt potential has been known to cause permanent damage to sensitive electrical circuits. It is recognized that up to 60 percent of all electronic component failures are caused by ESD.

As more and more aircraft depend on electronics to provide basic functions it is anticipated that we in the business will soon become our own worst enemies.

To understand is to control

Can formation of electrostatic charges be realistically eliminated? Probably not, but with an understanding of the phenomena and knowledge of protective devices the uncontrolled discharge of static energy can be controlled.

Static electricity is often referred to as "triboelectricity." Tribo is a Greek word meaning, "to rub". Therefore triboelectricity is a form of electricity produced by rubbing. Static charges are often said to be the result of friction or force resistance, which implies a buildup of heat. In reality it is the contact or touching of materials that causes the buildup of electrical charges and not the effect of heat. A contact potential will occur almost anytime two solid objects make contact. This voltage is generally quite low but varies with material types. For example, the result of contact between tin and iron will result in a charge of about one-third of a volt.

To understand static charges we have to first have a grasp of the atom. The hydrogen atom is the simplest as well as the lightest. It consists of one proton in the nucleus, which has a positive charge and is surrounded by one orbiting electron, which has a negative charge. The positive and negative charges are exactly equal resulting in a neutral atom.

Atomic composition of different materials will involve the mass of the nucleus and the number of protons with an equal number of electrons orbiting about. Electrons in the outermost orbit may be either firmly or weakly attached. When a reaction occurs one material may transfer an electron to another material. This results in a change in the state of charge. The material that looses the electron becomes a positive ion while the material gaining the electron will become negative.

Conductors and insulators

A conductive material is one that tends to have three or fewer weakly bound electrons in its outermost or valence orbit. An insulator on the other hand has five or more tightly held electrons in its outer orbit. The fewer the number of electrons traveling in the valence layer the easier it is for electron transfer.

Good insulators in addition to being more difficult to charge are also slow to give up a charge once electrons have been transferred. This effect can often be demonstrated by observing someone attempting to clean the acrylic windshield of an aircraft shortly after a flight. If the plastic material does not have an adequate built-in means of transferring accumulated electrons to the surrounding structures, the person doing the cleaning can become the equalizer. Please do not attempt this at home.

The effect on aircraft

Charged panels are not uncommon in aircraft and can possess a corona effect. This is where electrons transfer from a surface to the air. The result is a ghostly glow visible often in low light conditions.

Saint Elmo's Fire is the common name for this corona discharge effect. Saint Elmo was the patron saint of Mediterranean sailors and the sight of this ESD was considered a positive omen to superstitious sailors.

Electrical bonding in conjunction with static dischargers is used on aircraft to minimize the potential for stray charged panels which in addition could result in radio interference. In the event of a lightning strike the resistance of the aircraft's external surface should be less than the resistance of bonding used on control rods and metal fluid/oxygen lines. Aircraft electrical racks also have a very low resistance path to the aircraft structure.

Relative humidity of the air along with atmospheric contaminates will have an effect on the ability of a material to build up a static charge as well as the tendency to dissipate one. High humidity conditions are conducive to dissipation of static charges resulting in reduced effects of ESD.

Damage to components

Special shoes, disposable heel grounders and ESD protective chairs are some of the products available to manage static electricity.

ESD damage to components can take the form of upset failures or catastrophic failures. Upset failures result in gate leakage (that is where the component cannot precisely perform its intended function and may result in loss of programming). Catastrophic failures occur in two forms, direct and latent.

Direct catastrophic failures occur when a component is damaged to the point where it fails and won't 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 wounds the component to the point where it still functions properly during testing, but over time the wounded component will cause poor system performance and could result in failure.

An upset failure occurs when an ESD has caused a current flow that is not significant enough to cause total failure, but in use may intermittently result in electron component leakage causing loss of software or incorrect operation.

Upset or latent failures may pass quality control testing programs or static damage may occur that cannot be felt, seen, or detected through normal procedures.

Awareness and precaution key

How do we deal with this invisible and silent foe? Awareness is the true key. Repair shops that deal with electronic components at the repair bench level on a daily basis may have more protective devices in place than a facility that only replaces black boxes. However periodic dealings do not eradicate the need for precautions. Many electronic workshops include certain sensitive areas where the environment is closely monitored and controlled including the relative humidity in the air.

Besides environmental control, other precautionary measures can be used that will greatly reduce the possibility of ESD. Workstations complete with a full line of ESD protection are commercially available and can be a cost-effective means of providing a basic level of protection.

Technicians involved in the daily repair of electronics may choose to wear shoes that are specially designed to continuously dissipate any static buildup occurring on the wearer. Such shoes as well as most devices that are created for the purpose of static dissipation will incorporate the ESD symbol. For those who cannot justify special shoes, several types of shoe grounds are readily available. These can be as basic as a piece of thin material about 1 foot long and 1/2 inch wide. Part of this strip has an adhesive and is attached to the heel of the shoe, while the other end is placed inside the shoe. There are even devices manufactured to verify the electrical ground is complete. They work on the same principle as a high resistance ohmmeter. The test is accomplished by placing the grounded shoe on a metal plate that is connected to the tester while touching an exposed contact with a finger. If the circuit is complete and resistance to ground is adequately low the machine will advise. ESD protective floor mats are also a basic low-cost means of reducing electrical FOD.

Wrist straps also counter unwanted static discharge. Like other means of ESD protection their function should be verified and not taken for granted. Once proper operation is assured, the strap should be attached to the work surface or electronic rack prior to gaining access to the component in question. Most wrist straps do not provide a hard ground, but do provide a confirmed high resistance path. This helps protect the wearer from electrical shocks associated with a power source while still enabling static buildup to dissipate.

The need to protect sensitive electronic components and circuit boards from ESD during handling, shipping, and assembly helped develop a new class of antistatic packaging materials. Key developments in polymers, especially conductive polyethylene and sophisticated laminates with very thin metallized films, have made successful transportation of sensitive equipment a reality.

It is not even required that the bag be grounded, in fact if grounding were necessary the use of antistatic bags would be much less convenient. Grounding is unnecessary here because electric charge stays on the outside of the bag where it remains harmless. But at some point the component will have to be removed from the bag. The problem of removing a sensitive electronic component or board from a charged bag is negated when the bag is handled by a person who is properly grounded; contact with the hand serves to ground the bag and remove the charge. If the person handling the bag wears insulating gloves or is otherwise not capable of discharging an electrical buildup, then the component may draw a strong electrical spark as it is withdrawn from the protective casing and may be damaged.

Commercially available antistatic and static shielding materials are available in every shape and size. Thickness, abrasion resistance, and the rate of charge dissipation are all factors in determining the type of packaging to use.

Encased equipment should always have protective covers placed over the electric connectors and care should be exercised to avoid direct contact with electrical pins.

Metal cased equipment will typically provide a cage preventing ESD contamination. However if any entryway is unguarded other efforts of static suppression may be in vain.

In the event no protective devices are available and a circuit card or other electronic device must be replaced, first touch the grounded box or rack supporting the unit to be replaced for several seconds. This will reduce any potential shock.

ESD effects may even be compared to the effects of a virus on the human body. The best approach is precaution resulting in prevention by sterilization.

Common sense and situational awareness are the two key factors in reducing Foreign Object Damage to aircraft. Why should there be any difference in reducing electrical FOD?

Basic guidelines •Wear an antistatic wrist guard. Make sure the wrist guard directly touches the skin. To ensure against ground faults, use a wrist guard that has a 1M (1 meg-ohm) resistor. •Always store modules in their original packaging. •Never operate the system with exposed power modules or slots. •Handle the modules only by their edges. Never directly touch components on the modules. •Never remove a module from its protective packaging or from a chassis until the chassis and the work surface are properly grounded. If the work surface is metallic, ground it by attaching a wire from the surface to the electric ground in the building. If the work surface is not metallic, use a ground-conductive rubber mat as the work surface.