Conformity Magazine - December 2007 - (Page 35) T his is the second in a series of articles on system level ESD testing. In the first article, the current waveform for system level ESD testing, such as IEC 61000-4-2 and ISO 10605, were compared with the current waveforms for Human Body Model (HBM), Machine Model (MM) and Charged Device Model (CDM) for ESD testing of integrated circuits. It was shown that the current levels for a system level stress are much higher than those for integrated circuits, and combine the fast rise time and peak current characteristics of CDM with the longer duration stress of HBM. Systems need the higher levels of stress during test because few systems such as cell phones or computers are intended for use only in an ESD-controlled environment such as those used for the handling of electrical components during electronics manufacture. The specification of the stress waveform is only the first step in defining system ESD testing. Many elements go into designing an ESD stress test that will give useful and repeatable results. This article will outline the essential features of a repeatable ESD test on a full system, based on the most widely used ESD test for systems, IEC 61000-4-2. Aspects of System Level ESD Testing The goal of system ESD testing is to ensure that ESD events that occur during normal day-to-day use of a finished product do not result in an unhappy user. Physical damage is not required for a system to fail an ESD test. The system must also perform its intended function during and after the ESD event. This results in two important features of a system level test. The first is that the system under test must be a powered and functioning during the test. The second is that the response of the system to the stress is not a simple pass/fail, but has at least 4 levels of possible system response as outlined in Table 1. (The different levels of system response to ESD stress will be discussed in more detail in the section on “Operating State and Failure Definition.”) Test Environment The next step in defining a reproducible ESD test for a system is the definition of a well-defined test environment. We will discuss the environment specified by IEC 61000-4-2 for table top or small systems. There is a separate, but similar, setup for floor standing equipment. The basic setup is shown in Figure 1. On the laboratory floor is a metallic Ground Plane (GP) that is electrically tied to the building ground of the laboratory. A wood table is placed on the Ground Plane. On top of the table is a metallic Horizontal Coupling Plane (HCP) covered by a 0.5mm thick insulator. The HCP is connected to the GP through a pair of 470kΩ resistors in series, one close to the GP and one close to the HCP. To prevent interference from the surrounding environment, the test setup must be kept away from other equipment and objects such as metal walls. The Equipment Under Test (EUT) is placed on the insulator away from the edges of the insulator and HCP. If the unit is mains powered, it is plugged into the building power in a situation close to use condition. If it is battery–powered, there is no explicit ground connection. Since air discharge is affected by atmospheric conditions there are limits specified in the IEC standard for temperature, humidity and atmospheric pressure. Response Class 1 2 Description of Failure System performs within specification during ESD stress System upset by ESD stress but recovers without user intervention System upset by ESD stress; user intervention needed to correct problem System physically damaged requiring repair or replacement 3 4 Table 1: Different levels of system response to ESD stress. (The response class numbers are not defined in the standard but are being used for clarity in this article.) DECEMBER 2007 CONFORMITY 35 http://www.monroe-electronics.com http://www.monroe-electronics.com
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