Circuits Assembly - March 2008 - (Page 32)

Controlling Static ESD risk via automated movement ESD risk via human interface Board Loading Screen Printer Chipshooter Pick/ Place Reflow Oven PTH Assembly Wave Solder ICT Functional Inspector Function Board Test Flipper Figure 2. No aspect of automated manufacturing is immune to the effects of ESD. Most static-related problems involving production equipment occur while devices are being transported in their carriers. ment 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. 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 32 Circuits Assembly MARCH 2008 circuitsassembly.com http://www.pkaymetal.com http://www.pkaymetal.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - March 2008

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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