Generation of Static Electricity

There are 3 common ways to generate static electricity, triboelectric generation, field induction, and direct charging. Triboelectric generation (tribocharging) is being the most prevalent method for charge generation is discussed first. The Tribo Series in Figure 1 is a direct result of getting a better understanding this method. Electric fields and field induction is discussed next. Direct charging is a result of applying either a current or a voltage directly to the material but only works with conductors. Tribocharging and field induction works with both insulators and conductors.

(a) Triboelectric Series

A listing of materials, with respect to the polarity and magnitude of charge generated during contact and separation is called the Triboelectric Series. The series is arranged so that those materials at the top of the list lose electrons and charge positively when rubbed with materials below them relative to the Series. Conversely, the materials at the bottom of the list gain electrons and charges negatively. The farther away the materials are from each other within the list, the greater the magnitude of the charge is generation by them under tribocharging conditions.

The series should be considered a guide to both polarity and relative charge magnitude for various materials. Frequently, one finds some slight inconsistencies in magnitude between two materials from what the Series indicates. Sometimes complete reversals of material positions have been observed.

Table 1 TYPICAL ELECTROSTATIC VOLTAGES*
EVENT	RELATIVE HUMIDITY
	10%	40%	55%
Walking across carpet	35,000	15,000	7,500
Walking across vinyl floor	12,000	5,000	3,000
Motions of bench worker	6,000	800	400
Remove DIPs from plastic tubes	2,000	700	400
Remove DIPs from vinyl trays	11,500	4,000	2,000
Remove DIPs from Styrofoam	14,500	5,000	3,500
Remove bubble pack from PCBs	26,000	20,000	7,000
Pack PCBs in foam-lined box	21,000	11,000	5,500

*Source: AT&T ESD Control Handbook-1989

Figure 1

Table 2 TYPICAL CHARGE GENERATORS
ITEM	TYPE
Work Surfaces	FORMICAä Finished Wood Synthetic Mats Ungrounded Metal Glass or Fiberglass
Chairs	Fiberglass Vinyl Other Plastics Ungrounded Metal Finished Wood
Clothing	Clean-Room Garments Finger Cots Gloves Wool Synthetics Shoes and Boots
Floors	Carpet Vinyl Wax
Packaging Materials	Polyethylene Bags Bubble Pack Material Foam Packaging Pellets Plastic Trays and Boxes
Manufacturing Processes	Conveyors Drive Belts Machinery Nylon Scrub Brushes Nonconductive Liquids High Velocity Air Flow Temperature Chambers Environmental Ovens Slides Rails TEFLON

VIII. Electric Fields & Induction

Electric Fields are generated from charge imbalances between two materials or within the same material. An electric field can exist in free space or between some materials. An electric field exerts a force on a unit charge causing it to move (if positive) in the direction of lower potential. This force is derived from the charge being acted upon, the distance the charge is from the field and the field strength. The electric field strength is proportional to the inverse square of the distance R between your test point and the charged surface.

E= 1/(4P e _o) * (Q_T/R²) Equation VII-1

A field meter is used to measure electric fields. An electric field is the collective energy of a multitude of unbalanced surface atoms (QT), i.e., charged surface. An electric field is defined by the force a positive unit charge would undergo or force per unit charge. The units of an electric field are (Newtons/Coulombs) or (Volts/Meter).

The electric field strength (|E|) is proportional to the inverse square of the distance (1/R²) from the charged surface. This is to say that the force an electric field (which is born from a charged surface) would have on a charge (unit test charge in the field) is proportional to the inverse square of the distance of the charge to the source of the electric field (charged surface).

What does it all mean? Let us look at some practical examples that can easily demonstrate the effects that electric fields present within the production floor.

Demonstration of the ESD Training Paddles.

Experiment 1 (Static Electric Fields)

Bring the bottom of both paddles together and rotate the handles to start the tribocharge process on the paddle plates. Separate the plates (causing tribogeneration), placing the insulative acrylic paddle on table and measure the field from the conductive plate (aluminum) with a field meter. Note the polarity of the measurement. Place the conductive paddle on an insulative work surface and pick up the insulative paddle. Measure the insulative plate with your field meter and note both the field strength and polarity. Notice that when you generate a static field, you are causing an imbalance in the charges, one plate will be negative and the other will be positive.

Note: When measuring fields, be careful not to touch the conductive plate so as not to accidentally discharge the paddle.

Experiment 2 (Field Suppression)

Bring the bottom of both paddles together and rotate the handles to tribocharge the paddle plates. Separate the paddles, placing the insulative acrylic paddle on table and measure the field from the conductive plate (aluminum) with a field meter. Now place the conductive paddle face down onto the grounded ESD mat. Wait a few seconds and re-measure the field on the conductive paddle. Note that the field has been reduced or the charge imbalance has been restored.

Now place the insulative paddle face down on the grounded ESD Mat. Take your field meter and measure the charge on the outside of the plate, as it is still face down on the ESD mat. Note that the field has been reduced (to near zero). The free electrons in the conductive mat have balanced the charge imbalance on the insulative paddle nullifying the electric field. Now pick up the paddle and re-measure the field on the acrylic plate. Note that the field is still there. The field is still present because the charge imbalance could not be permanently restored. The material is insulative and restricts the flow of free electrons to allow a neutralized state. The act of hiding the electric field by temporarily balancing it off is calledcharge suppression. Charge suppression can kill sensitive electronic devices and should be controlled or eliminated from your ESD safe areas.

Experiment 3 (Field Induction)

Bring the bottom of both paddles together and rotate the handles to initiate the tribocharge generation of the paddle plates. Separate the paddles, and measure the conductive paddle face with a field meter. Note the field strength and polarity. Repeat field measurement with the insulative paddle. Now place the conductive paddle face down onto the ESD mat. Lift up the conductive paddle and again measure the electric field. Note that the field is gone. Now bring into close proximity (~1 inch) the insulative paddle to the conductive paddle without touching. Set down the insulative paddle on the ESD mat and measure the conductive plate again with the field meter. Note that a field is now present when you’ve already discharged the field earlier on the ESD mat. This field was generated via field induction. As a material passes through or comes in near proximity to another field, a charge imbalance will occur from the presence of the foreign electric field.

Field Induction is a real threat to ESD Sensitive (ESDS) devices at any stage of their handling. Insulators are the biggest cause for charge induced from field induction. When an insulator becomes charged from any method, (tribocharging or field induction), unless the surface is sprayed with an equal balance of air ions (ionization) then the surface will remain charged and become a source for field induced charge generation.

Controlling Charge Generation from Field Induction

There are two main ways to control this phenomena, air ionization and abstinence. Air ionization can be implemented using several types and models, depending on the application. Removing the insulative materials from the work place is the second method, but may not be practical for all applications, therefor air ionization is the universal solution.