Mastering ESD Control in Automated Handling Systems Print E-mail
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Written by Bob Taplett   
Friday, 29 February 2008 19:00

Most damage comes not from personnel, but from charged equipment or components.

As device geometries get smaller and processing speeds grow faster, their ESD sensitivity increases. Designers face the challenge of fitting more active component features into smaller chip territory, often at the expense of on-chip protection devices. The tradeoff: greater risk for ESD damage.

Mastering ESD control has always been critical to high production yields, but not to the degree it will be in the next few years. While the industry has a solid understanding of ESD safety in manual operations involving personnel, there is room for improvement in automated applications. To be effective, ESD control programs must ensure automated handling equipment is capable of handling highly sensitive devices.

ESD impacts productivity and product reliability in virtually every aspect of the electronics environment. ESD costs the electronics industry billions of dollars every year. Industry experts attribute an estimated 8 to 33% of all product losses are caused by ESD.1 However, ESD damage results in more than just device loss. It affects production yields, manufacturing costs, product quality and reliability, customer relationships, and ultimately, profitability.

For automated facilities, conventional methods of ESD control must be re-examined and new methods applied. Automated assembly equipment is capable of processing 4,000 to 20,000 components an hour and up.2 At these speeds, poorly designed equipment permitted to charge devices can damage large amounts of components in a very short amount of time. Perhaps even more important, an ESD event can in turn damage the automated equipment.

ESD generates a significant amount of electromagnetic interference (EMI). EMI resulting from an ESD event is often powerful enough to interrupt the operation of the production equipment. Equipment controlled by microprocessors is especially susceptible to damage, as they operate in the same frequency range as the EMI from ESD events. Often mistaken for a software error or glitch in the system, EMI can cause a variety of equipment operating problems, such as stoppages, software errors, testing and calibration inaccuracies, as well as mishandling (Figure 1). All can cause significant physical component damage and affect production yields. The effects of EMI tend to be random in nature and can affect equipment across the room, but leave the equipment where the ESD event occurred untouched. This can make the location of the ESD event difficult to locate.

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ESD, simply stated, is the rapid transfer of an electrostatic charge between two objects. ESD happens when two objects of different potentials come into direct contact with each other. Charging results when one object’s surface gains electrons to become negatively charged and another object loses electrons from its surface to become positively charged. Triboelectric charging occurs when an electron transfer results from two objects coming into contact with each other and then separating.

One of three events is usually the cause of ESD damage to devices: direct electrostatic discharge to the device, electrostatic discharge from the device or field-induced discharges. Several models are used to characterize how devices are damaged: the human body model (HBM), the machine model (MM), the charged device model (CDM), and the effect of electric fields on devices. In an automated assembly facility, the last three models or modes are the largest cause of concern.

MM damage happens when a machine component discharges through a device. Automated assembly equipment uses a variety of methods such as conveyors to move and guide devices through the assembly process. Poor equipment design can cause the handling systems to accumulate significant charges that will eventually discharge through the devices.

CDM damage occurs when the device discharges to another material. When a charge builds in a device, it will dissipate through a conductor on the device when the device is placed in contact with a surface with a lesser charge.

Influence of electric fields (e-fields), or the space surrounding an electrical charge, can cause a charged device to polarize. Polarization creates a difference of potential, which may cause the device to discharge to an opposite charge, causing two discharges or equalization events.

Identifying ESD


While a great deal of attention is spent on preventing ESD caused by the HBM, recent studies have indicated that less than 0.10% of all documented damage actually resulted from ungrounded personnel touching ESD-sensitive (ESDS) products. The studies concluded that 99.9% of ESD damage originated from the other models, specifically CDM.3

ESD control embedded into machinery is essential but problematic. To effectively control static buildup, both MM and CDM ESD events must be prevented. The first step in establishing an ESD control program is to identify exactly where ESD events occur or are likely to occur. A good place to start is to ask two primary questions: Is the equipment properly grounded, and does it handle devices in such a way that they do not generate static charge above an acceptable level? Studies indicate the majority of static-related problems involving production equipment occur while devices are being transported in their carriers, or transferred in and out of them by robots (Figure 2).4 Additional areas of concern might include IC handlers and other methods of transporting devices.5


IC Handlers. ICs can become highly charged as they pass through the equipment and are subsequently discharged as a part of normal operation.

Gel packs. If proper ESD control methods are not in place, IC chips can become highly charged as they are lifted off the sticky bottom liner and then immediately discharged by the collets removing them.

PCBs mounted in plastic panels. The plastic panels regularly used for housing PCBs can routinely charge to very high levels when handled, subsequently charging the PCBs themselves. The assemblies are subsequently discharged during normal operator handling.

To be fully prepared for handling devices of the future, equipment should be capable of handling components with an ESD tolerance as little as 50V.

Preventing ESD Buildup


To prevent or reduce MM damage, equipment must be properly grounded while in motion. All equipment parts that come into contact with the static-sensitive devices must have a sufficient grounding path to dissipate accumulated charge. Proper grounding of conductive and dissipative surfaces prevents the buildup of static charge on machine components and eliminates them as a source of charge-creating ESD events.

Grounding alone, however, will not prevent all CDM ESD events. Component charging is a much more challenging problem to solve, primarily because most electronics components contain insulators as part of their design. Insulating materials naturally accumulate a charge, and grounding the materials does not remove or reduce the static charge. When the charge cannot be removed or avoided, air ionization is often the most effective method of neutralizing the charge on insulators or isolated conductors. In the case of automated equipment, air ionizers can be mounted inside the process chambers. Creating mini environments by enclosing specific machines and mounting ionizers inside is another option.

Once ESD countermeasures are in place, it is important to verify they work properly. Continuous process monitoring is recommended over periodic audits of the ESD program because ESD countermeasures will eventually fail. For this reason, if and when failure does occur, it should be identified as soon as possible to prevent ESD damage.

Several test methods exist to validate the integrity of the ground path to equipment parts and measure whether machines are charging devices. When selecting the best measurement instruments, consider the safe charge level to be measured and select an instrument that can measure within that range. Note the size of the area to be measured and whether the spacing is fixed between the surface of the object to be measured and the instrument.

Identifying and measuring static charge inside automated equipment presents specific challenges. The problem with most conventional methods is that they are not particularly suited to automated equipment. Most require direct contact with the charged object or require the device to be removed from the object, making it necessary to take the equipment offline to perform the testing. To avoid lost production time, alternative solutions are necessary for measuring charges inside the equipment.

To measure static charge without disrupting equipment operation, assemblers can mount sensors or probes inside the equipment or mount static event detectors (SED) on the devices themselves. Two options for mounting instruments inside equipment include static sensors and special electrostatic voltmeters and electrostatic fieldmeters with small probes. Static sensors incorporate high input impedance circuitry and can be mounted inside automated equipment. This permits them to measure the field generated by a charged part as it moves through the process. Ideally, the sensor should be mounted as close to the part as possible. Because it does not require the nullification of existing fields, it is ideal for measuring charges on parts moving through high throughput machines.6

Electrostatic voltmeters (Figure 3) and electrostatic fieldmeters with small probes offer an alternative option for monitoring inside equipment. The probes are small enough to be placed in critical locations to measure the charge on components as they pass by. However, care must be taken when mounting them to ensure they take accurate measurements and do not interfere with equipment operation. Several factors can affect the accuracy of their measurements, including the charged surface’s orientation with respect to the probe, and the size, speed and distance of the part from the probe.

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SEDs are tiny sensors small enough to fit on a PCB. They are designed to measure the current pulse in an ESD event and can be monitored optically as they pass through operating equipment. SEDs are ideal for verifying whether the equipment is generating dangerous static-charge levels. Several different types are available, each with varying features. Many, however, must be removed from the device and placed into separate instrumentation to ascertain whether an ESD event actually occurred.

Automated Tracking

If an ESD event does occur, the data provided from a device tracking system can help assemblers quickly identify damaged components and contain the impact. In a device tracking system model, a barcode reader is installed at various points throughout the manufacturing process to read the barcodes (or 2-D codes) applied to the devices (Figure 4). Typically, readers scan the barcodes on the device before it enters a station and again after it exits. This documents the type of procedure performed, the equipment that performed it and attaches a time/date stamp for when it occurred.

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While ESD monitoring instruments output all types of data, the barcode reader provides the only link between each device’s serial number and the data supplied from the instrument. For example, when equipment calibration is altered due to EMI from an ESD event, data generated from the device tracking system can help identify specifically which boards were damaged after the equipment’s calibration was altered. Entire lots no longer have to be pulled, scrapped or reworked because of insignificant data.

When selecting a barcode reader, ensure it does not introduce additional risk for ESD events. PCBs, ICs and other electrically sensitive components typically use small, high-density barcodes to conserve space, making it difficult for some readers to scan over long distances. When the scanner is mounted within close proximity of the product, the barcode reader may build up a static charge, depending on whether it is used on a nonconductive surface. If the reader itself has built up a charge and is brought into close proximity with a sensitive component, an ESD event could occur, potentially damaging the component. Some manufacturing environments use a workaround by mounting the scanner after applying a special anti-static spray, which is not without its own risk.

First, the coating must completely cover the area for maximum effectiveness; uncovered areas remain at risk. In addition, anti-static sprays can wear off and require timely replacing. Without an accurate measure of a spray’s efficacy period, companies either waste money by applying too much, or put their components at risk by using them in an unprotected environment.

As an alternative solution, miniature barcode readers are now available with a unique nickel coating and ESD-resistant labels. These units are rated for discharges up to 8kV and feature a surface resistivity of less than 10 * 10-9 V/in2.

Evaluating ESD Handling Capabilities

According to the ESD Association’s (esda.org) Technology Roadmap released in 2005, sensitivity levels to ESD in devices are expected to drop so low assemblers must act quickly to ensure they will be able to handle the new levels.7 Assemblers certified to the ANSI/ESD S20.20, the ESDA standard covering electrostatic discharge programs, already have done much of the work in preparing for the next generation of sensitive devices. For those manufacturers unsure of the voltage capabilities of their automated equipment, the ESD roadmap provides direction in how to get there:

  • Determine the ESD-control capabilities of the facility’s handling processes.
  • Ensure all conductive fixtures or tooling that contact sensitive devices are grounded.
  • Ensure maximum voltage induced on devices is kept below 50V.

Following the requirements outlined in S20.20 will help managers assess component sensitivity levels and identify ESD issues at each stage in the process, from receiving and inventory through assembly, test, rework and shipping. By using the appropriate ESD countermeasures, managers will have the data to articulate their facility’s capabilities by voltage level.

References

  1. ESD Association, “Basics of Electrostatic Discharge Part 1: An Introduction to ESD,” Compliance Engineering, January 2000.

  2. Donn G. Bellmore, “ESD Design Concerns in Automated Assembly Equipment,” July 2004.

  3. Roger J. Pierce, “The Most Common Causes of ESD Damage,” Evaluation Engineering, November 2002.

  4. Semiconductor Equipment and Materials International (SEMI), E78-0706: “Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment,” May 2006.

  5. Roger J. Piece, “The Most Common Causes of ESD Damage,” Evaluation Engineering, November, 2002.

  6. Arnie Steinman, et al, “Detecting ESD Events in Automated Processing Equipment,” Compliance Engineering, Sept./Oct. 2000.

  7. ESD Association, “Electrostatic Discharge (ESD) Technology Roadmap,” 2005.

Bibliography

  • Arnold Steinman and Lawrence B. Levit, Ph.D., “Coping with ESD: Ionization for Production Equipment,” Evaluation Engineering, April 1997.
  • Curtis Maynes, “Technology Roadmap Sees Higher Sensitivity to ESD,” Evaluation Engineering, MARCH 2006.

Bob Taplett
is manager of applications engineering at Microscan Systems (microscan.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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