ESD Guide

This is a basic guide to electrostatics for non technical folk without the electrons or protons.

Before ESD there are conductors and insulators:

We always start with conductors and insulators because we have found this area causes the most confusion. Without a basic understanding of the difference between the two properties electrostatics is always a mystery.

That conductors will conduct is obvious. But the concept that insulators can carry a charge and not conduct is more difficult. Insulators are used to stop electrical current flowing. A good example is an electrical cable: the copper wire carries the current and the plastic insulation makes sure the current cannot go anywhere else.

Whenever you look at an electrostatic problem the first question to ask is:

Is the charged material a conductor or an insulator?

A charged conductor has a much higher potential for damage than a charged insulator.

Triboelectric charging:

This fancy term simply means a charge is generated when two materials are separated.

Some everyday examples:

The school trick of running a comb through your hair and using the charged comb to pick up a piece of paper.
Static cling when removing a nylon sweater.
A static shock after getting out of a car.

In each case two materials separated and each developed a charge.

The comb separating from your hair.
The sweater separating from your body.
Your clothing separating from the seat of the car.

The effect is that both materials are charged but in opposite polarities.

This charge can now do work.

The comb can attract a small piece of paper.
The charged sweater can make a noise.
Your body has enough potential to give you a shock as it discharges to the car.

An electrostatic charge: an ability to do work and cause problems:

Like most properties a charge can be helpful or cause problems.

In general electrostatics has a bad reputation for causing problems but it can also be very handy in industry.

A microfiber cloth like the Swiffer uses electrostatic charge to attract dust.
Photocopying/laser printers works by transferring powder from an image onto paper using a controlled electrostatic charge.
Dust is removed from smoke by first charging it and then attracted to an opposite polarity plates.

Problems, problems:

For all its positive uses electrostatic charging has caused problems since it started causing gunpowder explosions in the 14th centuary.

There are a number of problem areas:

Damage to semiconductor product during manufacture and assembly.
Damage to electronic products during assembly, test and repair.
Irregularities caused by static in printing, heat sealing, silk screening and lamination.
Sticking, jamming and tearing of plastics, paper or textiles in high speed processing.
Glitches to electronic equipment causing latchups, data errors, memory loss and system failures.
Ignition of combustible vapors, dust or solvents, causing fires or explosions.
The unwanted attraction and retention of dust, dirt and bacteria to surfaces.

Factors that affect level of charge:

Size and surface contact area:

The size and contact area between two materials has a significant affect on charge geneartion. Two large sheets of plastic will obviously generate more charge than two small sheets.

Humidity:

Relative humidity(RH) measures the amount of moisture in air at a specific temperature (e.g. 70F ). RH has a major effect on charge generation. Walking across a vinyl floor at 80% RH (very moist) generates 250V. At 20% RH (very dry) the same walk will generate 12,000 Volts. Moisture causes lower static in most cases simply by the extra conductivity of the water layer on surfaces.

Materials:

Materials and the mixture of materials can have a major effect on charge generation. Some materials generate a high charge with relatively little work. Low grade plastics tend to be great generators. Wood and natural materials tend to be neutral. To confuse the process different combinations of materials can combine to generate high levels of charge. This is why a plastic comb can lift a piece of paper. If you want further information just google Triboelectric series.

Special dissipative materials have been developed to safely discharge static.

Speed of separation:

How fast materials is another factor in the level of charge generated: the faster the separation the more charge.

Adhesion:

How well the two materials are stuck together will influence the level of charge generated. Highly adhesive materials like tapes will generate more charge than materials with no adhesion.

Battery effect:

While each individual item may have a relatively small charge where they are combined or stacked the total charge can be significant. For instance individual sheets of plastic with relatively low surface charges when stacked together can carry extremely high voltages.

Repetition:

Where an action is repeated charge can accumulate. When a person walks across a floor each individual step may only generate a small charge (~150V) but this can become significant (12,000) with repetition. Anyone who has walked down a long hotel corridor and got a shock when placing the key near the door knows the effect.

Options to reduce static generation:

For conductors the reduction of static is relatively simple. All conductors should be bonded and grounded. Employees are grounded using heal grounders or static safe shoes. Conductors in equipment are normally grounded in any event for electrical safety reasons.

Ionizers are the only practical way of reducing the static on insulators. They produce a steam of positive and negative ions which neutralize the charge on insulators. Positive ions neutralize negative charge on the object since opposite charges attract.

Changing the type and size of materials is not often a feasible option in most environments.

The most obvious answer is to increase humidity. However this is normally costly and can lead to corrosion and lamination problems.

In most industrial environments increasing speed is the normal goal for improved productivity. Suggestions of reducing speed to reduce static is not usually well received.

For liquids and granules additives can used to reduce the static effect. However in many environments regulatory approval is required which makes this option difficult to implement.

Measuring Static:

Static can be measured using a static locator or field meter. Measuring static accurately is not a simple process. Meters do not measure static directly they detect the static field around the charged object. The field can be easily suppressed when adjacent to conductors. Meters readings are normally measured at specific point (1″) since the static field is inversely proportional to distance.

The most critical point is to understand that a charge on an insulator is different to a charge on a conductor. A relatively small charge on a conductor can be a much higher risk than a large charge on an insulator.