Electro Static Discharge: Controlling static electricity

Wrist straps are one solution to control electrostatic discharge (ESD) Technicians can test to make sure of proper grounding. Controlling static electricity By Jim Sparks Foreign Object Damage (FOD) is something that has been drilled...

Disposable Heel Grounders
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?

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