Conformity Magazine - October 2007 - (Page 29)

T he user of an electronic product, whether it is a cell phone, a computer or a game console, is justifiably upset if they touch their device, feel a small ESD discharge and then find their prized possession not working, either temporarily or permanently. The user will not care that they were charged up to many thousands of volts when they touched the product, or that the current into the device was several amps. They want the product to work first time, every time. To prevent these kinds of problems test standards have been developed to test systems for ESD robustness. The best known of these is the IEC 61000-4-2 standard but there are others such as ISO 10605 and SAE J1113-13 for testing of automotive components. Testing with these standards allows one to find and correct vulnerability of products to ESD stress. While testing systems for ESD robustness is done extensively, the nature of the tests are often misunderstood. This is especially true for engineers that are more familiar with the classic device level tests of Human Body Model (HBM), Machine Model (MM) and Charged Device Model (CDM) applied to integrated circuits. This article will explain the intention of ESD tests for both devices and system level, and then discuss the device level and system level stress waveforms. Purpose of Tests The purposes of system level and device level testing are different. System level testing is intended to ensure that finished products can survive normal operation. It is not assumed that the user of the product will take any precautions to lower ESD stress to the product. The purpose of traditional ESD testing of integrated circuits is very different. HBM, MM and CDM tests are intended to ensure that integrated circuits survive the manufacturing process. For example, an integrated circuit needs to survive saw apart from the wafer, packaging, final test, shipment to a board assembly facility, placement on a circuit board with automatic pick and place machinery, and the solder process. All of these processes are done in a controlled ESD environment that limits the level of ESD stress to which the device is exposed. Even when the device is on a circuit board, it is expected that ESD precautions (such as shipment in ESD safe packaging material) will be taken to minimize stress to the board. Once the board is in a system, ESD precautions are no longer assumed. A system level ESD test can therefore be expected to be a much more severe test than that intended for an integrated circuit. The Nature of ESD Tests ESD tests are intended to simulate real world events, with different test standards intended to cover different situations. The HBM device standard is intended to simulate a person becoming charged and discharging from a bare finger to ground through the circuit under test. The MM is intended to simulate a charged machine, such as a pick and place machine, discharging through the device to ground. CDM simulates an integrated circuit becoming charged and discharging to a grounded metal surface. System level tests, such as IEC 61000-4-2 and ISO 10605 replicate a charged person discharging to a system, similar to HBM testing. The only conceptual difference between the IEC 61000-4-2 standard and device level HBM is that for the system test it is assumed that the person is holding a metal object such as a key or screwdriver. As we shall see, this produces a very different waveform. Device Level ESD Waveforms We will now discuss the individual models and the stress waveforms that are associated with each standard. Each of the standards is characterized by a capacitor that is charged and a discharge path. We will start with the HBM standard, since it is the most popular device level test. A person’s capacitance to their surroundings is approximately 100pF, and the HBM standard uses this value to represent a person. The discharge path includes a person’s skin resistance and the resistance of the spark, which has been approximated as 1500Ω. The traditional october 2007 conformity 29 http://www.maruwa-g.com/e/index.htm http://www.maruwa-g.com/e/index.htm

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

Contents Editor’s Note Commission Proposes Changes for 57-64 GHz Spectrum Rules FCC Continues Actions Against Jamming Device Distributors Radio Operator Fined for Failing to Make Equipment Available for Inspection Commission Fines Unsolicited Fax Advertisers Firm Offering Quality Standards Training Also Fined Breaking New Ground with Lithium-Battery Generic Requirements for Use in Telecommunications Facilities ESD Open Forum EMC UK 2007 Conference Set Staying Abreast of U.S. E-Waste Legislation EU Commission Establishes RFID Expert Group Update on China's CCC Compliance Program System Level ESD Testing: The Waveforms An Alternative Approach to Surge Suppression Testing Equipment Buyer's Guide EU Scientific Committee Releases Updated Data on EMF Risks Updated Standards List For The EU’s EMC Directive Updated Standards List For The EU’s Directives on Measuring Instruments Call for Presentations for 2008 International ESD Workshop Announced CPSC Actions In The News IEC Standards Update Product Reviews UL Standards Update New Products From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - October 2007

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