Electrostatic Sensetive Devices (ESD) Handling & Care

ESD (Electrostatic Sensetive Devices)

Advancements in technology and the development of modern avionics for new generation aircraft have brought new challenges for those involved with the operation and maintenance of modern passenger aircraft. The management of semiconductor devices that are subject to harm from stray electric charges is one of the most difficult tasks. This is a problem that can affect a wide range of electronic equipment installed in an aircraft and have a wide range of consequences, including complete failure of the LRU (Line Replaceable Unit) with no apparent symptoms of damage.

Electrostatic Sensitive Devices (ESD) are electronic devices that are very susceptible to the effects of unwanted static electric charges. This issue is most widespread in modern LSI (Large Scale Integration) and VLSI (Very Large Scale Integration) devices, but it also impacts other components, including MOS (metal oxide semiconductor) transistors, microwave diodes, displays, and many other modern electronic components.

Mishandling and unsuitable methods of storage and transit can cause extensive (and irreversible) damage to static sensitive devices.

Static electricity

We should all be familiar with static electricity, especially in its most dramatic form, lightning. An electric shock received when stepping out of a car is another example of static electricity that we have encountered. The synthetic materials used in clothing and the inside of the vehicle are capable of producing significant levels of static charge, which is only released when the driver or passenger steps on the ground. When two dissimilar, initially uncharged nonconducting materials are rubbed together, friction helps to transmit charge from one to the other, boosting the electric potential between them.

Triboelectric Series

The triboelectric series divides materials into groups based on how well they generate static electricity when rubbed together. The contents in the series are arranged on a scale of from positive to increasingly negative.

The following materials give up electrons and become positive when charged with rubbing against other materials:

Air (Most Positive Charge)
Dry Human Skin
Leather
Rabbit fur
Glass
Human hair
Nylon
Wool
Lead
Cat fur
Silk
Aluminium
Paper (Least Positive Charge).

The following materials are neutral on the triboelectric scale as they that do not tend to attract or give up electrons when brought in contact or rubbed with other materials:

Cotton
Steel

The following materials, when rubbed against other materials, attract electrons and become negative charged, making them negative on the triboelectric scale:

Wood (Least Negative Charge)
Amber
Hard Rubber
Nickel, Copper, Brass and Dilver
Gold & Platinum
Polyester
Polystyrene
Saran
Polyurethane
Polyethylene
Polypropylene
Poly Vinyl Chloride (PVC)
Silicon
Teflon (Most Negative Charge).

The highest levels of induced charge will be produced when materials at opposite extremes of the triboelectric scale are rubbed together. PVC rubbing against glass, for example, or polyester rubbing against dry human skin. It's worth noting that people who work in a dry atmosphere frequently complain about sparks when they touch metal objects. This is due to their dry skin, which may readily become highly positive in charge, especially when their garments are made of man-made materials (such as polyester), which can easily become negatively charged. In a humid atmosphere, where stray charges can escape harmlessly into the atmosphere, the effect is much less pronounced. Wearing 100% cotton clothing is recommended for people who build up static charges owing to dry skin (cotton is neutral on the triboelectric scale). Additionally, damp skin dissipates charges more quickly.

When human hair is combed, it takes on a positive charge. On its surface, a plastic comb will gather negative charges. The charge hair strands will push away from each other because identical charges repel each other, especially if the hair is very dry. The comb (which is negatively charged) will attract positive-charged things (like hair). It will also attract non-charged materials like little bits of paper.

When materials with the same triboelectric polarity rub together, an electric charge can be generated. Rubbing a silk cloth over a glass rod, for example, will charge the glass with positive charges. The silk does not hold charges for a long time. When both materials are on the positive side of the triboelectric scale (as they are in this case), the material with the greatest ability to create charge becomes positive in charge. When two negative triboelectric materials are rubbed together, the one with the greatest potential to attract charge becomes negative in charge.

Static Sensitive Devices

Stray electric charges can destroy any modern microelectronic component, although some are more vulnerable than others. Field effect technology, rather than bipolar junction technology, is used in the devices that are most vulnerable to damage. CMOS logic devices (such as logic gates and MSI logic), MOSFET logic devices (such as transistors), NMOS and PMOS VLSI circuits are all examples (used for dynamic memory devices, microprocessors, etc). Some optoelectronic and display devices, as well as microwave transistors and diodes (because of their small size and junction area), are extremely static sensitive.

The moral of the storey is to treat any semiconductor equipment with extreme caution and to avoid circumstances where stray static charges could come into contact with it.

Electrostatic discharge can also cause damage to printed circuit board assemblies. Printed circuit board mounted components, in general, are less vulnerable than individual semiconductors. This is because the conductive channels in a printed circuit can often help dissipate excessive static charges that could otherwise injure unmounted semiconductor devices (there are no static electricity dissipative paths when a transistor, diode or integrated circuit is handled on its own without precautions).

ESD warnings

ESD Warning notices are always placed on static sensitive components (such as printed circuit board cards, circuit modules, and plug-in devices). Typically, black text is written on a yellow background. ESD handling and transportation When handling, transporting, fitting, and removing ESD, certain measures must be followed.

ESD Warnings

ESD Protection Mats & Wrist Strap

Handling and Transporting ESD

Special precautions must be taken when handling, transporting, fitting and removing ESD (Electrostatic Sensetive Devices).

ESD Protection Gears & Tools

ESD Precautions

These precautions include the following:

When working with ESD, wrist straps must be used. These are conductive bands with a short wire lead connecting them to an effective ground point. The lead is normally equipped with an inbuilt 1 M resistor, which helps to reduce the risk of shock to the wearer (the series resistor serves to limit the current travelling through the user in the event of contact with a live wire). Wrist straps are typically stowed in strategic locations throughout the aircraft or carried by maintenance personnel.
The use of heel straps that work similarly to wrist straps
Floor and bench mats with static dissipative properties
Avoiding extremely dry environments (or at least the need to take additional precautions when the relative humidity is low)
Ground jacks should be readily available.
Test equipment should be grounded
Use of anti-static soldering stations and low-voltage soldering equipment (low-voltage soldering irons with grounded bits)
Integrated circuit insertion and removal tools with anti-static properties
Avoiding high-voltage sources in the vicinity (fluorescent light units)
Anti-static packing should be used to store the components, (static sensitive components and printed circuit boards should be stored in their original anti-static packaging until such time as they are required for use).

It's worth noting that there are three types of materials used to safeguard static-sensitive gadgets. The three categories of materials are conductive (metal foils and carbon impregnated synthetic materials), static dissipative (a less expensive variant of conductive material), and anti-static materials that are neutral on the triboelectric scale which includes cardboard, cotton, and wood). Anti-static materials provide the least protection, whereas conductive materials provide the most.