Every year, millions of dollars are lost unnecessarily by the semiconductor, disk drive, and flat panel display (SDF) industries. The cause of these losses? Static-charge problems, the two most damaging being electrostatic discharge (ESD) and electrostatic attraction (ESA).1
ESD is capable of vaporizing metal lines and silicon and punching through the oxide layers in SDF products. ESD can shorten the lifetime of a static-sensitive part in electronic assemblies or installed equipment.2,3 ESD events also can introduce defects that weaken products, allowing them to pass initial testing but causing premature failures once installed in equipment.
In addition to directly damaging products, ESD is the source of electrical noise and electromagnetic interference that can interrupt the operation of production equipment. The results are unscheduled downtime, reduced equipment availability, and increased maintenance. This is particularly true of equipment dependent upon microprocessors for control.
ESA happens even in the highest quality clean-room environments. While filtration systems prevent exterior particles from entering the clean room, people, equipment, and some parts of the manufacturing process still generate particles.
If critical product surfaces become charged, ESA will increase the deposition of particles onto these surfaces, resulting in more defects. One single contaminating particle larger than about 0.3 microns has the capacity to destroy an entire flat panel display, and even smaller particles will damage the coating on hard-disk media.
Grounding conductive and static-dissipative materials is of primary importance in designing and using static-safe work areas. Providing a path for the static charge to flow to ground ensures that the charge will dissipate before it has a chance to cause a problem.
Every static-control program includes methods for grounding the objects in a work area. There are grounding devices for personnel, equipment, worksurfaces, and other objects, such as tools and parts trays that may come into contact with sensitive products. In some cases, it may be appropriate to ground the product itself. In critical applications, it is important to monitor the integrity of the grounding system because a failure can cause significant losses.
If it were possible to ground everything and eliminate insulators from the work area, there would be far fewer problems caused by static charge. Unfortunately, this is not possible. Many electronic devices contain insulators or require insulators for their manufacture. Almost all electronics assembly occurs on some type of insulated circuit board using components with insulated packages.
Insulators are easily charged when they contact other materials, even if the other materials are grounded conductors. This is called triboelectric charging. Insulators retain their charge for long periods of time because charge cannot move through them. Grounding the insulators has no effect. And charged insulators can induce charges on nearby conductors, such as the leads of an isolated IC package, causing ESD events when the leads subsequently touch ground. Obviously, any static-control program that does not deal with charge on insulators is incomplete at best.
Ionizers for Static Control
Generally, ionization is the only acceptable method for eliminating charges on insulators in clean-room applications since chemical antistats cause contamination, and high humidities are incompatible with manufacturing processes. Opportunities to use antistats and high humidity in electronics assembly also are limited. Fortunately, outside of clean-room environments, there are fewer limitations in the use of static-dissipative materials to replace insulators.
Many applications rely on air ionizers to deal with static charges on insulators. Ionizers use a variety of techniques to charge the molecules of the gases in the air, which then are air ions.
Able to produce both positive and negative air ions, ionizers include corona discharge, radioactive sources, and soft X-ray generators. Flooding the air of the work environment with balanced quantities of both negative and positive air ions allows ionizers to neutralize whatever charges occur in most work areas.
There is no single type of ionizer for effective static-charge control in all applications. Clean rooms, mini environments, and laminar flow hoods with high levels of airflow use ceiling or bar-type ionizers. Fan-powered ionizers or compressed gas blow-off ionizing guns are best used in workbench assembly areas that lack a source of airflow. Specialized point-of-use ionizers, X-ray ionizers, and nuclear ionizers are used in production tools where much smaller dimensions often are needed.
Discerning the best way to use ionization in a static-charge control program requires two important choices:
Determine the area in which the static problems have the greatest impact on production yields.
Select the type of ionizer that is appropriate for that work area.
Widely used in clean rooms in the SDF industry, air ionization meets the exacting requirements of Class 1 semiconductor clean rooms and mini environments. Air ionizers also are appropriate for Class 10 areas where flat panel displays and disk media are manufactured. The problems presented by these environments are both ESD and ESA related.
With the current emphasis on increasing factory throughput, frequently ESD problems command the greatest attention. Ceiling ionization blankets the production areas and bar-type ionizers work efficiently over equipment and in mini environments. Figure 1 shows applications of both types of ionizers in a clean room.
The disk-drive industry now uses magnetoresistive (MR) heads for high-density data storage. These MR heads are extremely sensitive to ESD damage before they are assembled into the disk drive, requiring fast neutralization of charge on any nearby insulating materials.
Even though the assembly occurs in clean rooms, optimal results only can be achieved by using ionizing blowers. Figure 2 shows a typical application of ionizing blowers in disk drive assembly.4
Electronic assembly operations require the movement of small parts from carrier trays or tape-and-reel feeders to circuit boards. The high speeds involved in this transfer make the problem of static charging difficult to solve.
Charging occurs as the insulated component packages are removed from their carriers or tape by vacuum pickups, resulting in ESD events when parts are placed in their circuit-board locations. To prevent damage, small ionizing blowers can be mounted on the pick-and-place mechanisms, or ionizing blow-out devices can be directed into the assembly equipment (Figure 3).5
Insulators always will be used in electronics manufacturing. Most often, they are found in products highly susceptible to static charge. Current trends in technology include higher device speeds, smaller device geometries, better display resolution, and denser data storage—each of which is likely to increase static-charge problems.
Grounding and the use of static-dissipative materials in static-safe work areas cannot eliminate the static-charge problems presented by insulators. The laws of physics are the same everywhere: If insulators are used, they will become charged, inevitably resulting in ESD and particle attraction. Ionization is an essential tool for any effective static-control program to neutralize static charge on insulators and on conductors that cannot be grounded.
1. Steinman, A., “Static-Charge—The Invisible Contaminant,” Cleanroom Management Forum, Microcontamination, October 1992.
2. Cooper, et al, “Deposition of Submicron Aerosol Particles During Integrated Circuit Manufacturing: Experiments,” Journal of Environmental Sciences, Volume 32, No. 1, 1989.
3. Rush, J., et al, “Reducing Static-Related Defects and Controller Problems in Semiconductor Production Automation Equipment,” Proceedings of the SEMI Ultraclean Manufacturing Symposium, October, 1994.
4. Steinman, A. “Electrostatic Discharge: MR Heads Beware!,” Data Storage, July/August 1996.
5. Tan, W., “Minimizing ESD Hazards in IC Test Handlers and Automatic Trim/Form Machines,” EOS/ESD Symposium Proceedings, September 1993.
About the Author
Arnold Steinman is the chief technology officer at Ion Systems. Before joining the company in 1983, he was affiliated with Lawrence Livermore and Lawrence Berkeley Laboratories and later was an independent consultant. Mr. Steinman is a member of the ESD Association Standards Committee and several other standards work groups and leader of the SEMI ESD Task Force. He graduated from the Polytechnic Institute of Brooklyn with B.S.E.E. and M.S.E.E. degrees. Ion Systems, 1005 Parker St., Berkeley, CA 94710, (510) 548-3640, e-mail: [email protected].
Copyright 1997 Nelson Publishing Inc.