Conformity Magazine - December 2007 - (Page 40) Complex, software-based systems such as computers present special challenges and opportunities for devising good test plans. A computer sitting idle with a static web page on the screen would likely leave much of its functionality untested. A better plan would be to devise a test state with active storage and retrieval of data from the hard drive, some form of CPU activity, such as a repetitive calculation, as well as communicating over its Ethernet or wireless connections, providing a very active and easier to upset test condition. A good design will also be able to automatically capture upset performance. Definition of the failure responses is also important and best determined before testing begins. Examples of possible system responses to stress based on the categories in Table 1 are shown in Table 2, based on the computer example above. Prevention of Charge Buildup During direct ESD testing of a system, considerable charge is injected into the EUT during each stress. If this charge is not removed from the EUT between stresses, subsequent stresses will be significantly different than intended. The IEC standard warns that charge buildup can result in a far more intense stress than intended. Consider an EUT stressed several times without removing the injected charge. After several stresses, the voltage of the unit can approach the test voltage, and an arc can occur either within the EUT or to its surroundings. This arc could be very severe. The total charge in the arc could be much larger than the charge from a single stress from the ESD gun. Furthermore the peak current could be extremely high because the 330Ω resistor in the ESD gun would not be present for charge already stored within the EUT. It is also possible for charge build up to result in understressing of the EUT. As charge builds up, the amount of charge injected into the EUT on subsequent stresses will decrease, reducing the amount of stress during the latter pulses. The buildup of charge on the EUT can be detected with the use of an electrostatic field meter. A less scientific method (and much less pleasant way) is to pick up the EUT after stressing and experience a pronounced shock. Removal of charge can be done several ways. Care must be taken to not drain off the charge too quickly, since discharging a charged device with a low impedance path to ground is an ESD event itself. An EUT with a metal case that is connected to house ground will obviously not build up charge. An EUT with a metal case that is not connected to ground can be connected to ground through a 1 MΩ path that will slowly bleed off the charge between stresses. An EUT with an insulated case provides the biggest challenge. The IEC standard suggests the use of air ionization to speedup the neutralization of the built up charge. The air ionization 40 Conformity DeCember 2007 cannot be on during air discharge stress, since the ionized air will tend to reduce the voltage on the ESD gun as it is moved toward the EUT. Possibly the most useful method of charge removal, or more accurately neutralization, is the use of a dissipative brush connected to ground through a 1MΩ resistor such as a person wearing an ESD wrist strap. This method has several advantages. It removes the charge more quickly than can be done with air ionization and without affecting the effectiveness of air discharge testing. It works on both insulators and conducting surfaces, and it is operationally straight-forward. Summary In the first article in this series, the current waveforms for system level ESD testing were compared to the current waveforms for device level testing. In this second article the basics of system level testing have been reviewed, including the basic test setup, the types of stresses a system is subjected to, how to design an ESD test plan and how to ensure that charge does not build up on the device during testing. Robert Ashton is a senior protection and compliance specialist at ON Semiconductor, and can be reached at Robert.Ashton@onsemi.com. FAST Link www.conformity.com/1260 http://www.conformity.com/1260
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