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Induction and ESD Failures

Owen J. McAteer
Advisory Engineer
Westinghouse Electronic Systems Gp.
Baltimore, Maryland

The preceding columns on the basics of electrostatics have discussed charge and dielectric constant, triboelectric generation, and conductivity and resistivity. These topics provide a good foundation from which to proceed into the subject of induction.

Induction Misconception

Electrostatic induction is commonly misunderstood even though the principles involved are straightforward. Sometimes otherwise-knowledgeable individuals seem to disregard inductive effects when analyzing circumstances involving conductors in the presence of an electric field.

A primary obstacle appears to be a misconception about equi-potential surfaces and the behavior of charges on conductors. An equi-potential surface has the same voltage everywhere on the surface. Do you think that a conductor is always at the same potential at all surface locations? What must the potential be at one end (A) of a conductor if its opposite end (B) is hard-wired to ground? Consider these questions with the conductor in the presence of an electric field.

Potential Gradients

From early observations of the attractive or repulsive forces between items after rubbing, it became evident that forces were associated with charged items. These forces are commonly represented as lines constituting an electric field.

All electric charges in a field are under the influence of field lines which emanate from positively charged items, or are directed toward negatively charged items. Mobile charges will be influenced and moved accordingly as a conductor is brought into the field.

A volt is the unit of work required to move a unit charge one centimeter in a field of 1 dyne/coulomb. Thus the quantities of charges moved, as well as the distance traversed, will result in a voltage or potential gradient across the conductor.

Simple Induction

Figure 1: Simple Induction with an initially uncharged conductor under the influence of a charged item.

 

Figure 1 shoes a conductor in the presence of an electric filed emanating from a positively charged item. The resultant separation of positive and negative charges is defined as simple induction. Upon removal from the field influence there is a tendency toward charge recombination resulting from the attractive forces of separated mobile charges of opposite polarities culminating in neutralization. Whether upon entry or removal from a field, the rapid charge movement can lead to ESD failure.

 

 

Compound Induction

Figure 2a-d: Compound Induction.

Simple induction is shown in Figure 2.a. In this case the source of the field is a negatively charged item. In Figure 2.b the conductor side farthest away from the field source is grounded while still under the field influence. This provides the repelled electrons with a path to ground, whereby they can get even further from the repulsive positive field source.
Note that because of the field forces of attraction the positive charge has its greatest concentration nearest the negative filed source. Thus a potential gradient exists even when the conductor is grounded.

After disconnecting the ground a net positive charge remains on the conductor, still attracted to the negative source, as shown in Figure 2.c. When the field is removed, or the conductor is taken from the field, the charges equalize resulting in a positive equi-potential across the conductor surface as shown in Figure 2.d. This entire sequence is called compound induction.

Compound induction in the factory can present double jeopardy to static susceptible items. A conductor with an induced potential gradient between its two extreme edges can deliver a damaging potential to a susceptible item. This electrostatic discharge (ESD) event can be analogous to grounding the conductor as shown in Figure 2.b.

Thus after the first ESD event and removal of the field source, the conductor may be left with a possibly damaging voltage level of the opposite polarity awaiting a second ESD failure upon contact with an ESD susceptible item.

Sources of Induction

One of the primary ESD control measures is to purge static generative materials from areas where susceptible items are processed. Notorious static generators include common insulative materials such as teflon, acetate, common plastics, polyethylene, styrofoam, wool, silk, and nylon. These materials tend not only to generate high potentials but are likely to retain them for considerable time, i.e. minutes to hours.

Since the charges on insulators are relatively immobile, a charged insulator presents virtually no threat as a result of contact discharge to a susceptible item. However, charged insulators can cause failures through means of the induction phenomenon.

Induction-caused failures can occur from induced potentials on a sensitive item itself. One example could result from a printed circuit board being placed on a styrofoam cushion. A charge on the cushion would induce a charge on the board. A person touching the board could bring about an ESD failure, even if the person were grounded.

A similar example could occur with a insulative transparent template placed over the board to check for proper part placement. A charge on the template would induce a charge on the board resulting in static discharge when the board is touched or grounded. If the template were then removed from the board, a charge of opposite polarity would remain by the process of compound induction. The charged board now presents a second opportunity for failure upon contact.

A failure can also result form contact of a static-sensitive item to an ungrounded conductor that has been charged by induction. Do you think a grounded conductor would increase, decrease, or not alter the induction threat?

Owen J. McAteer is an advisory engineer for the Westinghouse Electronic Systems GP, Baltimore, Maryland. A past president of the EOS/ESD Association, he chairs the Association's Professional Development Committee and is author of he recently published "Electrostatic Discharge Control." Extended treatments of many of the themes to be treated here may be found in this book.