Controlling ESD through Polymer Technology

Neil T. Hardwick

Using static-safe polymers to control ESD can reduce the risk of damage to sensitive electronic components.

Static electricity cannot be avoided. Hundreds of times each day, electrostatic discharge (ESD) events occur without notice. Many ESD events are well below the human sensitivity threshold of 3000 V.

Unfortunately, an increasing number of electronic components are susceptible to damage from increasingly lower voltage levels. This trend will continue as consumers demand more-compact products, with increasing circuit density and decreasing component size. Event levels as low as 20 V can damage some of the more sensitive components.

As the use of polymers and other insulating materials continues to increase, the number of ESD events will likely increase. Because it is impossible to eliminate polymers from the electronics industry, designers must learn to use polymer chemistry to minimize the risk of ESD damage.

Static Electricity

Static electricity occurs when an object has an imbalance in its electric charge. Objects with excessive electrons carry a negative charge, whereas objects lacking electrons carry a positive charge. Charged objects want to be neutralized to remedy this unstable energy state. Consequently, electrons take the path of least resistance to ground, sometimes jumping through the atmosphere from a charged object to a ground. This discharge of electrons is called ESD. ESD occurs when the resistance provided by the air gap is less than that of other available paths to ground.

The timing of an ESD event can be quite unpredictable, making it difficult to control component failures. The timing of a discharge depends on the type of material that is discharging. Conductors discharge very quickly. Dissipative materials discharge within a couple of seconds. Insulators, on the other hand, may not discharge for several minutes or hours.

Generation of Static Electricity

Either tribocharging or induction can supply the energy needed for an object to achieve an imbalance in electrons. Tribocharging occurs when contact and separation leads to the transfer of electrons. This includes simple movements like walking through an assembly area, removing a piece of tape from a bag, sliding an integrated circuit (IC) out of a dual inline package (DIP) tube, or picking a wafer carrier up from a table.

The amount of static electricity generated by tribocharging depends on the atomic makeup of the objects involved. Objects made of materials that are far apart in the triboelectric series will tribocharge more than materials that are close together in the series. Other factors affecting the tribocharge level include relative humidity, contact time, and contact force.

Induction occurs when an object is placed in a strong electric or magnetic field. The presence of this field charges the objects. The resulting charged objects present an ESD risk because the unbalanced charge seeks the path of least resistance to ground. Computer screens and large machinery often generate strong electric fields that can induce static charge.

Dangers of Static Electricity

ESD events do not always cause catastrophic component failures. An undetected ESD event can cause a latent defect in a component that has already passed quality-inspection tests. The reliability of such components is jeopardized by the ESD damage to internal circuitry. These components are more likely to fail in the field, which can clearly lead to customer dissatisfaction.

ESD in the Electronics Industry

ESD events affect the electronics packaging industry at all levels of production. Examples of ESD problems include the following:

As the use of polymers grows in the electronics industry, even more situations will arise in which generated charges will put electronic components at risk. Polymers continue to replace metals in manufacturing areas because of their versatility, flexibility, and cost-effectiveness. However, because polymers are natural insulators, additional precautions must be taken to minimize ESD risk.

ESD Control

A few straightforward guidelines can minimize ESD risk.

Maintain a High Relative Humidity. In sensitive areas, select a humidity level that minimizes charge generation and accumulation while avoiding metal corrosion.

Avoid the Use of Insulative Materials. Use permanently static-dissipative polymers (surface resistivity ≈ 10E5–10E11 W/square) rather than traditional insulative polymers (surface resistivity ≥10E12 W/square).

Use Local Ionization Systems. Ionizers provide both negative and positive charges that effectively neutralize charged objects in the airstream of the ionizer. However, ionizers are effective only in a limited, targeted area.

Static-Dissipative Polymers

Replacing all insulative polymers in ESD-sensitive areas with permanently static-dissipative polymers effectively minimizes ESD risk. Static-dissipative polymers discharge in a controlled, predictable fashion, and most of them discharge in less than a fraction of a second (<2 seconds by definition). The different approaches to transform a traditional polymer into a static-dissipative polymer are summarized in Tables I, II, and III.

Antistats. Low-molecular-weight chemicals such as ethoxylated amines or ethoxylated esters are commonly referred to as antistats or antistatic surfactants. These antistats can be either mixed into a polymer compound or topically coated onto a sheet, a tray, or a tube to act as a surfactant (see Figure 1).

Figure 1. Antistat surfactants.

Antistats migrate to the material surface to react with environmental humidity. This reaction creates a dissipative material surface (usually around 10E11 W/square). Due to the low molecular weight and migratory nature of these antistats, they are easily rubbed off the surface and have only a short window of effectiveness. In addition, these chemical antistats may contain contaminants that can affect sensitive electronic components (particularly wafers prior to die attach). These contaminants include chemicals (toluene, stryrene, etc.) that can offgas onto a surface, and ions (Cl^–, Na⁺, SO₃–, PO₄–, NO₃–, etc.) that may accelerate the corrosion of sensitive surfaces and device leads. Finally, polymers treated with surfactants may require specialized recycling—an issue that is gaining importance in many regions of the globe.

Conductive Fillers. Another way to convert an insulat- ing polymer into a static-dissipative polymer is to fill it with conductive particles such as carbon black, carbon fibers, or stainless-steel fibers. This approach relies on creating a network of interconnecting particles within the polymer compound, which allows electric charges to conduct through the insulating polymer (see Figure 2).

Figure 2. Conductor-filled polymer.

The difficulty with this approach is getting consistent electrical performance from the filled polymer. Conductive fillers have very steep loading curves (see Figure 3). This means that any slight adjustment in filler loading or in the distribution of the filler within the polymer can result in an insulative pocket instead of a conductive package. When insulative pockets occur within a conductive package, tribocharging can create trapped charges that cannot dissipate as intended. Trapped charges can discharge in an uncontrolled and unpredictable fashion.

Figure 3. The effect of filler loading on surface resistivity.

Small-sized conductive fillers such as carbon black often particulate from a filled polymer onto a component lead or wafer surface, whereas larger conductive fillers such as carbon fiber are less likely to contaminate contact surfaces in this way. An added advantage of carbon-fiber fillers is that they dramatically increase the flexural modulus of the molded component. This increase in modulus results in better structural support of sensitive components.

Coated Sheets. For polymer sheets or thermoformed component packages, conductive coatings containing carbon or some other conductor are sometimes used to provide a static discharge path on the surface (see Figure 4). These approaches improve on conductive-filler or antistat-surfactant approaches by placing the conductive filler directly at the sheet surface. However, this approach is useless for injection-molded components such as JEDEC trays or wafer carriers.

Figure 4. Cross section of a conductor-coated polymer.

In application, coated-sheet technology can produce inconsistent ESD protection. The inconsistent ESD protection arises during the thermoforming process when the sheet (along with the coating) is stretched into its desired shape. Because these coatings are only a few hundreths of a millimeter thick, the conductive surface will break apart as it stretches. As the coating breaks apart, islands of insulation are created in the package. These islands have no means of transporting static electricity to ground. Consequently, tribocharged or field-induced voltages remain trapped in the package as a hot spot, with the potential to discharge whenever a sensitive component is brought near the package. This hot-spot phenomenon is simulated in Figure 5. Finally, the cleanliness (i.e., the ionic content and offgassing) of some of these coatings is unacceptable for contact with some electronic components.

Figure 5. Simulated red and yellow areas in the coated sheet show disruptions in the static-dissipation path.

Inherently Dissipative Polymer Alloys. A limited number of inherently dissipative polymers (IDPs) and inherently conductive polymers (ICPs) are currently on the market. Because of their nonrobust mechanical properties, IDPs and ICPs may not be feasible on their own as packaging materials. However, when they are alloyed with traditional packaging polymers, such as polyethylene terephthalate glycol (PETG) or polyvinyl chloride (PVC), the result is a system that combines the desirable mechanical properties of the host polymer with the electrical properties of the IDP (see Figure 6).

Figure 6. Scanning electron microscope photograph of an IDP alloy.

This alloying approach provides a polymer that can be injection molded, extruded, or thermoformed without deteriorating either the electrical or the mechanical properties. Moreover, these alloys can sometimes be designed to be clear instead of black.

As environmental concerns begin to factor into an increasing number of business decisions, polymer alloys that can be reground, reused, or recycled may become a good option. In addition, the alloy approach results in components or thermoformed trays that may allow charges to flow through their entire volume instead of only at the surface. This means that the chance of creating hot spots is eliminated. Finally, this approach introduces no particulate contaminants to the polymer and typically contains only trace amounts of anions, cations, or offgassing materials.

Summary

As electronic components become increasingly sensitive to static discharge, it is increasingly dangerous to use insulative materials such as polymers near sensitive components. Therefore, it is critical for design engineers to understand their options for making static-safe polymers from insulative polymers.

Neil T. Hardwick is a marketing manager at Noveon Static Control (Cleveland, OH). He can be reached at neil.hardwick @noveoninc.com.