Conformity Magazine - September 2007 - (Page 50)

Charged Device Model (CDM). Assembly Magazine, an industry journal covering manufacturing and assembly, defines CDM as follows: “a failure model where the part in question holds an electrostatic charge and rapidly discharges to another object when they are brought into contact. The discharge could cause a device failure.” An expanded technical explanation of CDM can be found at this ESD Association link: www.esda.org/esdbasics5.htm The CDM model is not a newly recognized failure model. B. Unger et. al. from Bell Labs identified CDM problems in the early 1980s. In an ESD Symposium paper, they identified at least two scenarios where a CDM event could occur: 1) when a charged device is placed on a work surface that is too conductive; and 2) when a charged device is placed in highly conductive packaging. This early work is the reason companies today use static dissipative and not conductive table covering. One of the best examples of the CDM threat is on automated placement equipment. If a circuit board or individual device is picked up and placed down by a nonconductive fixture, a static charge and discharge may occur. This is a particularly risky problem in a high volume factory, as every part is subjected to the same threat, and the extent of the damage cannot be determined until final test. If the electronic parts have low CDM thresholds, the fallout could be catastrophic to a company’s bottom line or reputation. ANSI/ESD S.20.20 does not mention CDM failures or offer prescriptions for reducing them. CDM failures are discussed, however, in the Association’s ESD Technology Roadmap (available at www.esda.org). In 2005, as shown on their graph (Figure 2), minimum device sensitivity was just below 500 volts; by 2010 the value is expected to drop to just above zero. This increased sensitivity to CDM failure presents a major problem for any program manager dealing with new technology in an electronics manufacturing, test, or repair application that is not equipped to handle an HBM threshold below 100 volts. (Note that a common misunderstanding of the Bell Labs study led to the false belief that dissipative flooring is better than conductive flooring. In fact, flooring should not be evaluated in the same manner as table covering. The recognized standard for ESD flooring is a flooring-footwear system that will not exceed 35 megohms.) 3rd Consideration: Understand Induction and Voltage Suppression While electrostatic physics can become quite complicated when the problems involve electrets, the effects of corona treating or the theory behind why certain materials become charged in the first place, there are two basic principles that every ESD program manager should understand. They are electrostatic induction and voltage suppression. Electrostatic Induction Electrostatic induction occurs when one object picks up a charge from the electrostatic field of a second object. When a grounded conductor is placed in the vicinity of an electrostatic field, because the conductor is grounded, its voltage is zero. When it breaks from ground while still in the presence of the electrostatic field, induction occurs—i.e., a charge opposite that of the electrostatic field is trapped on the conductive object. Induction occurs when an aircraft is parked directly below a thunder cloud and its ground connection is removed. If the aircraft fuel intake were to reunite with ground while still charged, a CDM event would occur, potentially causing an explosion. In a factory, an ESDS device touches a grounded table mat; lying on or near the mat is a charged insulative object such as plastic bag or roll of tape. When the ESDS device is lifted from the mat, it picks up or loses electrons from the electrostatic field around the bag or tape. If the charged device now touches a grounded person’s finger, a circuit board or any other conductor, the ensuing discharge could destroy the device. It is a common misconception that the electrostatic field around a charged insulator will cause the failure of staticsensitive devices. Although this is Figure 2: CDB and HBM thresholds will decrease to Class 0 levels by 2010. (Copyright 2007, Danglemayer Associates) 50 Conformity September 2007 http://www.esda.org/esdbasics5.htm http://www.esda.org

Table of Contents Feed for the Digital Edition of Conformity Magazine - September 2007

Contents Editor’s Note FCC Extends Disability Access Requirements to VOIP Commission Fines Importer $3 Million Over Non-Compliant TVs FCC Gets Tough on Retailers Over Analog TV Tuner Labeling Analog Cellular Service Sunset Date Reaffirmed Commission Adopts Recommendations of Katrina Panel Free-Space Antenna Factors Through the Use of Time-Domain Signal Processing Editorial Correction ESD Open Forum An Update on Revisions to IEC 61000-4-2, the Basic Standard on ESD Practical Advice for Meeting the DFS Testing Requirements of the FCC The EMC Impact of Embedded Software ESD Design Buyer's Guide Tate Renominated to the FCC EPA Releases Report on U.S. Electronic Waste Additional Standards For The EU’s Toy Directive Medical Device Manufacturer Enters Consent Order Updated Guidance Document on Laser Products Issued FDA Offers Advice on Defective Heating Pads CPSC Issues China Product Safety Program Report Other CPSC Actions In The News IEC Standards Update Product Reviews Product News UL Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - September 2007

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