November_EDFA_Digital

edfas.org 1 7 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 4 In terms of removal ofmetal plates after baking, several countermeasures have been successfully implemented. The simplest approach is to short-circuit the plates by using a cable. However, the cablewill start to disassemble itself rather quickly due to frequent use. In addition, it limits the free space available for removing the plates. An alternative method is to conduct the separation in a bath of small aluminumor steel balls, which short-circuits the plates automatically, as shown in Fig. 9. In the case of one manufacturer, the whole separation process is now performed under water, which also prevents charging. But due to the penetration of water through plastics, this method is not generally recommended without previous careful evaluation. ESD AND EMI CHALLENGES IN THE FIELD Because passive RFID devices are powered via their antennas, they are supplied with high-frequency AC voltage. To use this power to start the chip operation, the antenna pads are linked to a chip-internal DC recti- fier bridge. This bridge can be seen at the RFID charac- teristics, which normally shows as a point-symmetric voltage-current characteristic at very lowcurrent (Fig. 10). In between 2 and 5 V, the symmetric current slightly increases in a bipolar fashion with microampere scaling. RFID devices are typically lowpower units that rely on the small amount of power the antenna can provide. If for any reason, strong transient pulses or even ESD is applied to the chip, its characteristicsmay change toward a leaky rectifier with a current increase significantly Fig. 7 Carbon discharge brushes mounted on a rail in an adjustable manner. Fig. 8 Discharge brush (left side) at the cutting section. Fig. 9 Suppression of charging/discharging during metal plate removal by using a bath of small metal balls (at ground potential). Fig. 10 Symmetric I-V characteristics of a good RFID device.