General Application Guide
ELECTROSTATIC CHARGE-DISCHARGE
Static Electricity
To understand electrostatic discharge, let’s start by explaining electrostatic charge or static
electricity… Static electricity is a natural phenomenon which, in some cases, can have devastating
effects. One of the most impressive manifestations of this phenomenon is lightning but many other
far more discrete manifestations can be just as dangerous. Static electricity is most commonly
created by friction and separation of two materials (triboelectric charging).
For example, if you walk across a nylon carpet, you generate static electricity as your shoe soles
contact then separate from the floor. In fact, as contact and separation take place, electrons are
transferred from the surface of one material to the surface of the other material. This creates an
imbalance of electrons, causing an electrical charge known as static electricity. This imbalance of
electrons produces an electrical field, which can influence other objects at a distance.
Figure 1: Triboelectric Charge: Materials Make
Intimate Contact
(Courtesy of the ESD Association)
Figure 2: Triboelectric Charge: Materials
Separate
(Courtesy of the ESD Association)
The amount of static electricity generated depends on the materials in presence, on the amount of friction
but also on other factors such as the relative humidity of the atmosphere. To give an idea, here are some
typical voltage levels encountered in electrostatic charge generation.
Examples of Static Generation
Typical Voltage Levels
Means of Generation
10-25% RH
65-90% RH
Walking across carpet
35,000V
1,500V
Walking across vinyl tile
12,000V
250V
Worker at bench
6,000V
100V
Poly bag picked up from bench
20,000V
1,200V
18,000V
1,500V
Chair with urethane foam
Figure 3: Typical Voltage Levels
(Courtesy of the ESD Association)
ElectroStatic Discharge (ESD)
Electrostatic discharge is defined as the transfer of charge between bodies at different electrical potentials.
Sometimes, after walking across a carpet, we feel a shock when touching the elevator button. This
corresponds to an electrostatic discharge. Such a shock gives an unpleasant feeling, but is not life
threatening. In general, a human being can feel electrostatic discharges of 3,000 Volts or more. Looking at
Figure 3, we now understand why we are far more likely to get a shock when closing the car door in winter,
when the relative humidity of the air is low, than in summer. In the world of electronics, charges of 1,000
Volts or less can suffice to weaken or permanently damage a component. In sensitive environments like
(agro)-chemical factories, gas stations or mines, the slightest spark can lead to a catastrophe.
2. How can I control ESD?
It is impossible to avoid completely the generation of static electricity. The whole idea of ESD control is to
safely dissipate any electrostatic charge, avoiding excessive accumulation. We must provide paths that lead
the charge to earth. For this, conductive materials play a major role. We must also prevent discharges that
do occur from reaching sensitive parts. Conductive packaging helps in this field. In today's world, more and
more items are made of plastics because they offer many advantages over traditional materials such as
metal:
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no corrosion
light weight
recyclable
ease of creating complex geometries
often the most economical solution
Unfortunately, virgin polymers are insulative. They can however be made conductive by various means. One
very cost effective way of obtaining permanent ESD control is to add conductive carbon black to the polymer.
3. How does carbon black make plastics conductive?
The volume resistivity of a polymer is 1014 to 1016 ohm.cm. A virgin polymer is therefore a good insulator.
The volume resistivity of dry carbon black is 10-2 to 10-3 ohm.cm. Dry carbon black is therefore a good
conductor. When carbon black is compounded in plastics, conductivity is imparted by the carbon black. The
level of conductivity obtained is a function of the carbon black loading but the relationship is not linear.
Figure 4: Percolation curve
The graph presenting resistivity in function of carbon black loading is called percolation curve. At low
loadings, the carbon black particles are not close enough to each other to form a path for the passage of
electrical charge. Around a certain loading, called percolation threshold, the system passes rapidly from
insulative to fully conductive as the particles are now sufficiently numerous to form a network to lead the
current. At higher loadings, conductivity will improve further but not dramatically.
4. What are the different types of potential ESD damage in electronics?
In the world of electronics, there are different types of ESD damage, such as:
Catastrophic failure: If an electronic component is submitted to an ESD event, permanent and immediate
damage such as junction breakdown, oxide failure or metal melt can occur. In this case, the component no
longer functions. If the part is tested in the factory after the ESD event, the defect will be easily detected. If
not, the final customer will be disappointed when he tries to turn on his new acquisition.
Latent defect: In this case, the electronic component exposed to an ESD event is partially degraded but
continues to operate. However, its lifetime can be seriously reduced, leading to breakdown of the part shortly
after the beginning of use. And we all know how annoying it is to have to take our almost new television or
computer back to the shop for repair.
ESD Damage—Facts and Figures: It has been estimated that average product losses in the electronic
industry due to static electricity can be as high as 33%! Per year, the cost of ESD to the industry can be
figured in billions of dollars. As the world of electronics progresses, parts are becoming smaller and smaller.
This means that distances between insulators and circuits are decreasing, thereby increasing the sensitivity
of the device to ESD.
ESD control is, therefore, a flourishing area of activity, with a promising future.
5. ESD Solutions - Conductive Packaging
Electronic components are submitted to handling and transport all along the chain from their production to
their incorporation in a finished article. To avoid accumulation of electrostatic charge which would be
dangerous to the ESD sensitive device, it is advisable to use conductive packaging at all stages. This way,
the sensitive devices are shielded from charge and the generation of charge caused by the movement of the
item within its packaging is reduced.
Thermoplastic based compounds containing conductive carbon black can be transformed by a variety of
techniques to best suit the application. Let’s take a closer look at some examples of conductive packaging for
the electronics industry.
IC Carrier Tapes: The compound used is based on a styrenic polymer or
on polycarbonate. It is extruded into a flat sheet, typically 300 to 500 µm
thick. Pockets are then made by thermoforming. The shapes and sizes of
the pockets are variable, in function of the part to package. When the
pockets have been filled with the electronic component, a transparent
cover tape is applied to close the pocket and the carrier tape is wound on
a reel.
The parts are now ready to be transported to the end user (eg a producer
of computers or mobile phones). When needed, the transparent cover
tape will be peeled off and the parts will then be available for assembly.
IC Thermoformed Trays: The compound used is also based on
styrenic resin. It is extruded into flat sheet, typically 300 to 1000
µm thick. By thermoforming, pockets will be produced to house
ESD sensitive devices.
IC Rigid Trays: IC rigid trays are prepared by injection molding. The
base polymer is variable, depending on whether the tray needs to
withstand baking or not.
ESD Boxes: The compound used is often based on polypropylene. It is
transformed by injection molding to produce crates or boxes, with or
without lids. These are used for the transport of ESD sensitive devices.
ESD Corrugated Board: Compound based on polypropylene is
transformed into corrugated sheet by extrusion. The template of the box
we want to produce is then cut out of the flat sheet. By folding, we obtain
a container of the chosen geometry.
ESD Film: In this case, polyethylene is the preferred base resin. The
compound is transformed by blown film extrusion. The bubble is
subsequently cut and welded to form conductive pouches for the
packaging of ESD sensitive elements.
6. ESD Protected Areas
For optimal ESD control, all electronic components must be produced and assembled in ESD protected areas
(EPA). The idea is to lead all charge safely to ground to avoid discharge into a sensitive part. Conductive
materials are present in a wide variety of applications in EPAs, as we can see from the following list of
examples:
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surfaces: flooring, work benches
equipment for workers: wrist straps, shoe soles
tools: screwdriver handles
furniture: chair wheels
7. Safety-Related Applications
Material in powder form is potentially explosive. We have all heard of dust explosions in chemical factories or
grain silos. Many liquids also present a risk of explosion. Just imagine what a devastating effect an ESD
event would have in a gasoline station. Confined areas such as mines are also high risk zones where the
slightest spark can lead to a catastrophe. In all these areas and situations where an electrostatic discharge
represents a big danger, conductive plastics play a major role.
Blow molded containers: HDPE based compounds can be processed by
blow moulding to produce jerricans or drums for the transport and storage
of dangerous goods, generally flammable liquids or materials in powder
form.
Blown film: blown film is also used for the packaging of liquids or
powders, for example in the form of conductive liners for non-conductive
drums.
Fibers/slit tape: conductive fibers can be woven into the structure of big
bags to form conductive paths, thereby dissipating safely any eletrostatic
charge.
Pipes and profiles: Conductive compounds are used to produce pipes for
mines, chemical or agrochemical factories, gasoline stations or any other
sensitive areas which need ESD protection.
8. Reference
Web site of the ESD Association: http://www.esda.org/