Circuits Assembly - March 2008 - (Page 34)

Controlling 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. 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 voltmePreventing ESD Buildup ters and electrostatic fieldmeters with small To prevent or reduce MM damage, equipprobes. Static sensors incorporate high input ment must be properly grounded while in impedance circuitry and can be mounted motion. All equipment parts that come into inside automated equipment. This permits contact with the static-sensitive devices must them to measure the field generated by a have a sufficient grounding path to dissipate charged part as it moves through the process. accumulated charge. Proper grounding of Ideally, the sensor should be mounted as close conductive and dissipative surfaces prevents Figure 3. Voltmeters can be customto the part as possible. Because it does not the buildup of static charge on machine com- ized for on-board use in automated require the nullification of existing fields, it is ponents and eliminates them as a source of equipment. ideal for measuring charges on parts moving charge-creating ESD events. through high throughput machines.6 Grounding alone, however, will not prevent all CDM ESD events. Component charging is a much Electrostatic voltmeters (Figure 3) and electrostatic fieldmemore challenging problem to solve, primarily because most ters with small probes offer an alternative option for monitoring electronics components contain insulators as part of their inside equipment. The probes are small enough to be placed in design. Insulating materials naturally accumulate a charge, and critical locations to measure the charge on components as they grounding the materials does not remove or reduce the static pass by. However, care must be taken when mounting them to charge. When the charge cannot be removed or avoided, air ensure they take accurate measurements and do not interfere ionization is often the most effective method of neutralizing with equipment operation. Several factors can affect the accuracy the charge on insulators or isolated conductors. In the case of their measurements, including the charged surface’s orientaof automated equipment, air ionizers can be mounted inside tion with respect to the probe, and the size, speed and distance the process chambers. Creating mini environments by enclosof the part from the probe. ing specific machines and mounting ionizers inside is another SEDs are tiny sensors small enough to fit on a PCB. They are option. designed to measure the current pulse in an ESD event and can Once ESD countermeasures are in place, it is important be monitored optically as they pass through operating equipto verify they work properly. Continuous process monitoring ment. SEDs are ideal for verifying whether the equipment is is recommended over periodic audits of the ESD program generating dangerous static-charge levels. Several different types because ESD countermeasures will eventually fail. For this reaare available, each with varying features. Many, however, must be son, if and when failure does occur, it should be identified as removed from the device and placed into separate instrumentasoon as possible to prevent ESD damage. tion to ascertain whether an ESD event actually occurred. 34 Circuits Assembly MARCH 2008 circuitsassembly.com http://circuitsassembly.com

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Circuits Assembly - March 2008 Caveat Lector Letters Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing What Drives the Crowd? Mastering ESD Control in Automated Handling Systems Beating the RoHS Heat Trade Secrets True to Its Roots The Road Abroad - Strategic Alliance or Greenfield Facility? Tech Tips Wave Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Materials World Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - March 2008

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