Conformity Magazine - January 2009 - (Page 22) Testing to Bridge the Gap How do we provide a solution to bridge the gap between these two worlds? The answer is a susceptibility test that can be done by both the system manufacturer and the device manufacturer. Here are the key features of such a test: • It’s a test that can be done by both the system and device manufacturer, and the results are meaningful to both; • As a preventive measure during design or device selection, it provides the engineer with a tool to see potential problems before a product is put into production; • For resolving EMC susceptibility problems in a sub-assembly or finished product, it provides a method to quickly see those areas likely to be the root cause of upset or malfunction; • For the device manufacturer it’s an invaluable tool for determining the sensitive of a device, something rarely done in industry today. the probe needs to be quite small and the stimulus level kept low. A 0.5mm H field probe has been found to be adequate for most boards and many devices.5 Scanning can be done either manually or with an automated system. An automated system has the advantage of being able to precisely locate the probes, increase and decrease stimulus levels, plot relative sensitivities and perform analysis. The manual method can be used to quickly locate a sensitive area, but it is more cumbersome when it comes to doing analysis. Scanning itself isn’t new. There are commercial EMI (electromagnetic interference) scanners for detecting near field radiation analyzing the radiation from a board or system, and manual EMC immunity scanning has been done for some time to localize sensitive circuits.6 But what is new is the ability to do susceptibility scanning in a controlled, quantifiable and repeatable way at both the system and device level. Figure 1: H field, E field and direct injection probes used for susceptibility scanning (courtesy of Amber Precision Instruments) Susceptibility scanning not only Figure 2: Results of a susceptibility scan. Susceptibility testing doesn’t replace identifies sensitive areas of a circuit Dark browns and reds show areas of greatest system level testing to compliance or device but, with the proper sensitivity (photo courtesy of Amber Precision standards or device level testing for Instruments) software, can also plot the relative voltage withstand during handling. sensitivities in three dimensions. In What it will do is provide a tool the example illustrated in Figure 2, that can be used by both the system level manufacturer and the most sensitive areas of a device and its associated circuitry the semiconductor manufacturer to identify susceptibility are easily identified. Replacing the device in this example problems at every level, that is device, board, sub-assembly with an identical device from a different manufacturer lot will and system. quickly show if there is a difference in susceptibility. In addition to identifying a sensitive component or circuit, testing must be able to quantify susceptibility levels and ideally correlate these results with the system level test results. Simply doing a system level test on a device is useless unless susceptibility levels can be determined and quantified. What is Susceptibility Scanning? Susceptibility scanning is essentially a method of stimulating a circuit or device either directly at a pin or trace, or via a small electrical field probes. In order to localize the sensitive area, 5 An example of an automated EMC immunity scanner showing the necessary components is shown in Figure 3. In this case, a TLP (transmission line pulser) is used as the stimulus source to an H field probe (hidden below the robotic cross-arms). Other sources, such as an electrical fast transient or IEC ESD waveform generator, could be used, but experience has show the H field generated by a TLP tester works extremely well. Data collection units monitor the equipment under test (EUT), looking for expected upset or malfunction. Giorgi Muchaidze, Jayong Koo, Quing Cai, Tun Li, Lijun Han, Andrew Martwick, Kai Wang, Jin Min, James L. Drewniak, David Pommerenke, “Susceptibility Scanning as a Failure analysis Tool for System-Level Electrostatic Discharge (ESD) Problems, IEEE Transactions on Electromagnetic Compatibility, Vol 50, No.2, May 2008. Doug Smith (2007), “Using Current Probes to Inject Pulses for Troubleshooting (Current Probes, More Useful Than You Think – Part 2) “ Technical Tidbits, High Frequency Measurements Web Page, www.emcesd.org. 6 22 Conformity JAnUAry 2009 http://www.emcesd.org
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