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ESD Ionizers
December 31, 1969 |Estimated reading time: 2 minutes
By Jay Skolnik, Skolnik Technical Training
With an impetus on meeting RoHS requirements, companies should not forget another hazardous phenomenon electrostatic discharge (ESD). Electronic parts are becoming smaller, silicon layer geometry is shrinking, pin pitches are reducing, and speeds are increasing, making ESD problems more prevalent than ever. ESD findings vary widely in deficiencies; however, ionizers are, by and large, the culprit of confusion.
IonizersIt appears that there is a general lack of understanding to the function and purpose of an ionizer. Manufacturers often do not know how ionizers work exactly; therefore, they are unable to make well-informed, educated purchase decisions about them. For example, some audited facilities had inherited ionizers from previous buyouts or auctions, but had not turned them on. There also were many ESD-safe workstations that did not have ionizers installed in the areas. Understanding of the basic principles of ionizers appears to be absent.
One company installed an ionizer at each workstation, although the operators were instructed to turn them on only when the relative humidity (RH) dropped below 30%. RH that is above or at 30% is not a substitute for ionizers; thus, if the company has ionizers available, they should be used whenever the operators are handling electrostatic discharge-sensitive (ESDS) products. On the other hand, since the RH was about 58% at one company, operators had the bench-top ionizer running at full speed on the ESD-safe workbench. The only problem was that the operators did not aim the ionizers at the work surface, the process-essential insulators, or the products where they were supposed to be. Instead, the ionizers were aimed directly toward the operators' bodies to keep them cool. Also, for cost savings, some companies chose to purchase less-expensive ionizers; however, these were too small to be adequate for proper ESD control. They would install one small ionizer on the right or left side of the workbench and expect its output to protect the entire bench effectively. In reality, however, the air flow did not cover the complete workbench, especially for products that were located farthest away from the ionizer. Those products had certainly suffered from ion starvation. At another lab, other products inadvertently blocked the ionizer's air flow.
In clean rooms, overhead ionizers typically are used. Because the laminar air flow starts at the ceiling and exhausts through vents in the wall near the floor, this air flow will suck the ionized air into its stream. Overhead ionizers sometimes are set to alternate between positive and negative ions. In one lab, a charge-plate monitor was charged to +150 V, and then discharged to -150 V (oscillating about every 15 seconds). The only problem with this arrangement is that the customer's ESDS level on devices was 75 V maximum, which means that the ESD protective equipment (the ionizer) was actually inducing a damaging, destructive charge on the ESDS products every 15 seconds.
ConclusionJust as with most equipment, preventive maintenance is important for ionizers. Without periodic maintenance, ionizer probes will collect dust and inherently reduce the corona effect. This, in turn, results in an output that is unbalanced and can actually create a harmful static charge on the product. Therefore, to ensure that your facility will pass an ESD audit, make certain that you do not fall into any of these deficiencies. SMT
Jay Skolnik, PE, Skolnik Technical Training, is NARTE certified in ESD control and an international ESD auditor. Contact him via e-mail: jay@skolnik-tech.com.