If you are an ESD control engineer, the inevitable buildup of static charges in your electronic assembly areas places a constant burden on you. And it is getting worse.
Arnold Steinman, chief technology officer at Ion Systems, underscored this situation. “Smaller feature sizes in semiconductors, next-generation magnetorestrictive (MR) and giant magnetorestrictive (GMR) heads in storage devices, and larger flat-panel displays make electrostatics management even more critical than before. These components are especially susceptible to ESD because the critical parts themselves are insulators,” he said.
While you are not likely to eliminate all charges in your production area, most problems with static buildup on conductors can be minimized. The first steps in ESD control are fairly well defined. Personnel can be grounded by the proper use of wrist and heel straps and antistatic outer clothing. Worksurfaces can be protected by dissipative tabletops and mats. Conductive floors help, too. And materials and devices can be transported in protective containers.
However, none of these procedures can dissipate static charges on insulators or on isolated conductors. Many of the materials used in a typical production area, such as plastic, quartz, ceramics, glass, and silicon, are good insulators and can become charged to very high potentials, sometimes greater than 20,000 V .
You can’t neutralize the charges on these insulators by grounding them because there is no electrical path for a ground connection. The only practical way to cancel them is to lower the resistance of air, the common element in your production area, so that it will dissipate the charges.
Typically, air in an electronics manufacturing area is not very conductive. It has a relatively small number of ions and a resistivity far too low to neutralize a static buildup. But if you can artificially increase the number of ions in the work area, it is possible to reduce the resistivity of the air by as much as 10,000:1. Static charges on insulators can be cancelled very effectively by this technique.
What Is an Ion?
An ion is an invisible molecule of air that has gained an electron to become negatively charged or lost one to become positively charged. A low concentration of natural ions is present in normal outdoor air, some formed by the decay of nature’s radioactive elements. Energy transfer in weather phenomena such as lightning and wind increases the count.
It would help if these natural ions were in the air around each workstation. However, we are not so fortunate. Most are stripped out when air is filtered, dehumidified, and conditioned in the typical industrial environment.
Generating More Ions
You can increase the number of ions in your production area by generating millions of them with an ionizer and forcing them to the locations where the threats exist. Some of the unwanted charges on those insulators are positive and some are negative, so your ionizer must generate both positive and negative ions to overcome the buildup. Positively charged objects will repel the
positive ions in the air but will attract and be neutralized by the negative ions and vice versa.
Electrical Ionizers
The most widely used ionizers are electrical, and most use the phenomenon called corona discharge to generate ions. In this technique, a high voltage is applied across sharp points.
In the AC ionizer, voltage is applied across several closely spaced emitter points which cycle at the power-line frequency. Positive half-cycles produce positive ions, and negative half-cycles put out negative ions. This ionizer must have a high level of air flow to blow ions away from the emitter points. This unit is used most often in air guns and benchtop units.
The steady-state DC ionization technique uses separate emitter points for positive and negative ion generation. This equipment will work with either low or high airflow.
The newest type in this category is the pulsed DC ionizer. It has positive and negative points that are alternately turned on and off. This unit is very versatile because the cycle timing can be adjusted to regulate the positive/negative ion balance.
Nuclear-Based Ionizers
A nuclear-based ionizer generates ions through the decay of a bar of radioactive polonium 210. Ions created by this means have an inherently equal distribution of positive and negative polarities so they are not likely to damage sensitive components in modern storage devices. Also, since this technique does not use electricity for ion generation, it can be used in environments where explosives exist.
The half-life of polonium is just 238 days, so you must have the bar replaced regularly. Radioactive materials are controlled by the Nuclear Regulatory Commission, so the bar is available for lease by the ionizer manufacturer.
Selecting an Ionizer
The first step in ionizer selection is defining the work environment and mounting location. You may need a room ionizer mounted overhead or a portable device located on the benchtop. Possibly you will want a small, hand-held air gun.
Choose an ionizer that is suitable for your workplace environment and application, self-balancing or self-monitoring or both, and easy to maintain. Also, it should have heated airflow if this is a factor in employee comfort; be priced commensurate with its quality and reliability; and not present any training, documentation, or maintenance problems. Finally, it must pass trial testing.1
Reliable operation with low maintenance costs is important according to Richard Rodrigo, the engineering manager at SIMCO Static Control and Cleanroom Products. Since all corona-discharge ionizers produce ammonium nitrate which forms a crystal structure on the points, automatic point-cleaning is a virtual necessity. Without routine cleaning, the formation grows and causes a system to go out of balance. Also, there is a risk that the buildup will break off and damage the products being manufactured.
EMC compatibility and a good ion polarity balance are other features listed by Mr. Rodrigo. Balance produces maximum help in neutralizing electrostatic charges, and severely unbalanced ion output polarities can damage sensitive equipment.
Measuring the Effectiveness
The electrical ionizer needs frequent attention, and a charged-plate monitor is the commonly used instrument to measure its performance. This instrument, defined by ANSI/EOS/ESD Standard 3.1-1991, has a 6″ × 6″ conductive plate with 20-pF capacitance and 1012 W to ground. The plate is charged at each polarity to perhaps 1,000 V and monitored to see how quickly that charge dissipates.
The rate of neutralization, or discharge time, for each polarity can be defined by how long it takes the plate’s voltage to decay from its initial value to 10% of that value. In normal air with 60% RH, the charge dissipates naturally, but the discharge time will be about 12 h. With a bench ionizer, it can be less than 20 s. And with an ionizing gun directed at the plate, it can be 1 s to 2 s.
The New ESD Standard
Regarding the new ANSI/ESD S20.20-1999 Standard for the Development of an Electrostatic Discharge Program, there seems to be unanimous agreement that wherever it is implemented, the ionizer user benefits. Mr. Steinman of Ion Systems described it as “…the first standard that establishes a baseline for developing a facility-wide electrostatic management program. Air ionization products are only one component of a total ESD management solution.”
Curt Maynes, technical service engineer at the 3M Electronic Handling and Protection Division, agreed. “Although many companies already have ESD-control programs in place, S20.20 reduces the ESD differences among manufacturing suppliers and their customers so the industries adopting S20.20 will realize enhanced quality, profitability, and performance.”
Reference
1. Dangelmayer, G. T., ESD Program Management, Van Nostrand Reinhold, 1990, p.204.
Ionizers
Overhead Ionizer
The 60478-60482 Critical Environment Overhead Ionizers use steady-state DC ionization. They are balanced within ±10 V at setup, and closed-loop feedback maintains the setting. Each output fan’s zone is adjustable. The 100% tungsten emitters are housed in removable cassettes for cleaning or replacement. The 24″ unit has two fans, and the 48″ unit has four. Each size accommodates 120 VAC or 240 VAC operation. Starts at $710. Desco Charleswater, (909) 627-8178.
Air Gun
The Ion Air Gun™ is an air jet with an electrical emitter point. Compressed air from a separate source is injected into the chamber, inducing a higher volume of surrounding air to be forced up to 15 ft. The emitter, powered by a separate 7901 (115 VAC) or 7907 (230 VAC) Supply, ionizes the entire stream. With a 60 psig air supply, the unit will dissipate a 5,000-V charge 6″ away in 0.24 s. $275. EXAIR, (800) 903-9247.
Corona or Alpha Ionizer
The 5810 Critical Environment Ionizer comes in two versions: corona discharge or the alpha model with polonium 210, a low-level radioisotope. The ionizers provide a zero-balanced ion output for ESD-sensitive applications such as MR/MGR disk head manufacturing. Each ionizer has a 3″ × 6″ cross section and is 32″ or 44″ long. The corona ionizer’s emitter points are field-replaceable, while the alpha ionizer’s cartridge must be replaced annually by the supplier. Contact company for price. Ion Systems, (510) 548-3640.
Benchtop Ionizer
The ZVI5000™ Zero Volt Ionizer generates ions using steady-state DC. The typical balance is 0 ±5 V with automatic sensing and feedback to compensate for emitter contamination or other drift. Failure to reach a balance prompts audible and visible alarms plus a message to the central computer calling for maintenance. Up to 512 ionizers can be connected to the central computer. The air velocity is 35, 55, or 70 ft/min. $445. Semtronics, (800) 247-4863.
Portable Ionizer
The EndStat® Portable Ionizer (EPI) occupies 8 in.2 on the workbench, or a swivel bracket makes it possible to mount it overhead or on a wall. The output produced by steady-state DC ionization reduces a 1,000-V charge to 100 V in <6 s from a distance of 1 ft. The EPI operates from 115 VAC or 230 VAC and delivers 35 cfm. The 230-VAC unit has the CE Marking. $330. SIMCO Static Control and Cleanroom Products, (215) 997-0590.Benchtop Ionizer
The 963 Benchtop Air Ionizer has a two-speed fan and a static-dissipative plastic housing for use in ESD-controlled areas. It maintains an even balance of positive and negative ions and reduces a 1,000-V static charge to 100 V in <1 s at 1 ft or in <5 s at 3 ft with high fan speed. The unit operates from 115 V or 230 V power and carries the CE Marking. Air velocity is 200 ft/min or 370 ft/min. It is 6.0" × 7.4" × 3.0". $383.50. 3M Electronic Handling and Protection Division, (800) 814-8709.Small Ionizer
The Eliminator Junior Ionizer is compact for use in small work areas. It has auto-balancing with a clean-me indicator that flashes when the emitters need attention. The cassette-based emitter may be removed and cleaned or replaced. The airflow is 60 cfm to 77 cfm. The unit operates on 120 VAC or 220 VAC power. The 120-V unit can be supplied with a heater. $441. ESD Systems, (508) 485-7390.
Copyright 2000 Nelson Publishing Inc.
April 2000