The build-up of charge on surfaces near semiconductor devices can be very damaging if discharge occurs when the device is not at ground potential. Since it is not always easy to have semiconductor devices at zero-charge, device damage occurs.
In many environments such as semiconductor production and PCB manufacturing, maintaining a continuously effective ground can be very difficult. Automated techniques for device assembly, testing and board manufacturing can create hazardous ESD conditions that are best combated by air ionization methods.
Air ionizers may use high voltage to produce negative and positive ions injected into a static-damaging area. The ions combine on the material surface, where the material returns to zero-charge potential.
The simplest and oldest ionization technique uses an alternating-current (AC) single-emitter system. Simplistic in design, this system consists of a step-up transformer connected to a single emitter. To be effective, it requires rapid airflow past the emitter.
Airflow movement created by a strong fan sometimes has disadvantages. The single-emitter units produce undesirable ozone that may be corrosive. With the variation in supply voltage between cycles, you cannot optimize the ion output and minimize ozone generation.
The dual-emitter direct-current (DC) system is an overhead room ionization unit that controls the output voltage level or duty cycle of the voltage to the emitters. This system maximizes ionization and limits ozone production.
The dual-emitter system also has disadvantages: uneven emitter wear and corrosion. It requires frequent calibration since the positive emitter degrades quicker than the negative, causing charge imbalances.
Self-balancing systems must be properly maintained to operate effectively. Unipolar voltages cause an attraction of charged dust particles to the emitters that decreases efficiency and degrades the system.
Bipolar pulse DC room ionization systems have many advantages, such as ease of setup, control and maintenance. This system does not need rapid airflow; natural flow is sufficient. In fact, this system incorporates the best features of the AC single-emitter and the dual-emitter DC system with few negative effects.
Let’s explore some of the differences between portable and room ionizers. An ionization system that covers the entire room sounds appealing, but the room must be designed for the system. For example, a large piece of equipment could impede the flow of ions. If you have the space for it, a portable ionizer works well in a small area
that needs direct ionization.
Ionization is not a cure-all. But when used strategically, it is a defensive tool that neutralizes charge as it forms. If used as an offensive tool, ionizers can help create neutralized charge environments.
About the Authors
Michael R. Hoogstra is a senior electronic technician for the Advanced Power Group in the Analog Integrated Circuits Division at Motorola. He has a degree in electrical and electronic technology. Motorola, Advanced Power and Computer Peripherals Operation, Analog IC Division, 2100 E. Elliot Rd., Tempe, AZ 85284, (602) 413-3957.
Don Jones is a test/product engineer for the Commercial Plus Technologies Operation of Motorola’s semiconductor products sector. He holds a B.S.E.E. degree from Rochester Institute of Technology and an M.B.A. degree from Western International University. Motorola, 2100 E. Elliot Rd., MD EL377, Tempe, AZ 85224, (602) 413-3457.
Copyright 1996 Nelson Publishing Inc.
November 1996
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