Conformity Magazine - December 2007 - (Page 38) gun to be positioned very precisely, and can even be used to scrape away any thin insulating material on the surface. In air discharge, a rounded tip is used, similar in radius to a finger. The round tip produces a more sever air discharge stress than a pointed tip will produce. A pointed tip produces a larger electric field around the tip of the gun which results in an arc forming at a greater distance form the EUT. Longer arcs have higher resistance and greater inductance, resulting in slower rise times and lower peak currents. The obvious question is, which test should be done, air discharge or contact discharge? It is the nature of the EUT that determines this. Contact discharge is done to conducting surfaces such as metal cases, metal fittings associated with knobs or switches, and to the metal shells of connectors. Note that IEC 61000-4-2 specifically states that connector pins are not to be stressed with contact discharge. (Testing of connectors will be discussed in more detail below.) Insulating surfaces must be tested using air discharge. Developing the Test Plan The development of a test plan for direct discharge is the next issue to address. The IEC standard calls for 10 discharges at each test point for both positive and negative stresses. Where to apply the stress is important. As in any test plan, it is important to balance doing a thorough test and spending more time doing the test than necessary. The nature of the EUT determines the test plan guided by the question “how is this product used in a day to day basis?” The first step is where not to test. The IEC standard excludes any non-user accessible parts of the equipment. Emphasis needs to be on areas that are touched most frequently and where an ESD event is most likely to cause an upset. Obvious targets are data entry keys, vent holes in a case, seams in a plastic case and the area around connectors. It makes little sense, however, to blast away at the center of an unbroken plastic case. As mentioned above, connectors deserve special discussion, and the IEC standard devotes considerable attention to the testing of connectors. For connectors with metal shells, contact discharge is to be done only to the metal shell and never directly to the pins. For connectors with a plastic body, only air discharge is to be used. An additional issue is whether the test should be conducted with the cable in place or not in place. If the cable to the connector is always in place during use, then the test should be performed with the cable in place. If the cable may or may not be present during normal operations, it may be prudent to do the testing both with and without the cable attached. When cables are attached during the test, their opposite end should be terminated either with a functioning unit or at least to a termination that matches that used in normal operation. ESD events involve a large burst of current, in the range of several to 10s of amps, in a 1 to 10s of ns time period. This results in considerable electromagnetic disturbances that can upset a systems function. This explains why an ESD discharge in the near proximity of a functioning system can cause system upset. Indirect discharge tests look for this kind of susceptibility in the EUT. Figure 2: Schematic of an ESD gun Response Class 1 2 Description of Failure Performs within specification Upset; recovery without intervention Symptoms Computer continues to function Increased error rate on wireless but error correction works Screen flickers but recovers System lockup requiring re-boot Error in calculation LAN connector no longer functions even after re-boot Indirect discharge is done with the ESD gun in the contact discharge mode to the HCP and the Vertical Coupling Plane (VCP) as shown in Figure 1. The VCP is attached to the GP with a pair of 470kW resistors in a similar manner as the HCP. The ESD gun is held in the plane of the coupling plane and perpendicular to the edge and discharged into the edge of the plane. For the VCP, the unit is placed 0.1m away from the VCP, and tests should be conducted with the coupling plane adjacent to all 4 sides of the EUT. Operating State and Failure Definition Before testing can begin it is necessary to define the operating state for the EUT, and to define the failure criteria. As always, these decisions are a compromise, and will vary from product to product. Ideally, “all operating states” of a system should be tested. This is, however, clearly impractical for most units. A mode of operation that exercises a large fraction of the internal circuits of a product is obviously a plus. 3 Upset; user intervention needed Physically damaged requiring repair 4 Table 2: Sample failure symptoms for different classes of failure 38 Conformity DeCember 2007
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