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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 4 18 different fromthe reference characteristic as shown in Fig. 11a-d. Frequently, this doesn’t degrade it to be completely nonfunctional, but it may significantly reduce the reading distance due to the reduced power supply, where the leakage current (often within one of the rectifier diodes) converts to heat instead of powering the chip. In his Ph.D. workof 1995, JoachimReiner has shown that ESDdefects can recrystallize themselves and thus may also recover by themselves, but remain a latent damage source. [3] Due to their symmetric low-current in- put characteristics, RFID devices are ideal monitoring components to verify Reiner’s work. I did this experimentally in 2003, us- ing a very unconventional setup, which I call the “Marconi Test.” A low-frequency RFID chip is contacted at the two coil pads, each by a manual probe needle; the needle is connected by an unshielded laboratory cable (length about 1.5 m) to a curve tracer, displaying the characteristics from-5 to +5 V. At a distance of about 1 m from these two cables, a small Ruhmkorff induction machine is set up with a spark-gap width of about 1-2 cm (equivalent to about 15 kV pulsed DC) and a cable about 1 m in length as an antenna (Fig. 12). No direct or indirect electric con- nection exists between the RFIDand the Ruhmkorff setup. After starting the Ruhmkorff inductor for a few seconds, one can see at the curve tracer how the RFID characteris- tic becomes steeper in one polarity direction at the first operation. After further short Ruhmkorff operations, the curve sometimes becomes even steeper—also in the other polarity—but sometimes flatter or even nearly complete- ly recovered. In subsequent failure analysis, rectifier damage was found. Considering their function, degradation of their reading distance operability had been observed before, but never total failures. Thus, the most exposed circuitry withinanRFIDchip suffering fromfieldESDor EMI damage is usually the rectifier unit, which more or less connects directly to the antenna. Thus, the ESD sensitivity of such a device depends on the accessibility of the antenna to ESD imports fromoutside,whichalsodependson theapplication. An important aspect of smart cards and small elec- tronic gadgets is the fact that most operate within a float- ing environment, which means there is no reference to ground potential. The lack of a defined ground reference makes internal ESDprotectiondifficult but not impossible. Risks are taken inmost cases by careless handling, e.g., if someone is carryingRFID tags at his neck and stretches his woolly pullover, or if a car key (with embedded RFID as an anti-theft device) is used for ESD discharging a person to ground. If the device is close to the reader at the moment of an ESD strike (i.e., under operational conditions), irre- versibly blown EOS signatures may be the consequence, as sometimes seen after the Marconi Test as well. (continued on page 20) Fig. 11a-d Various levels of rectifier degradation in I-V characteristics of an RFID chip. If only one of the rectifier diodes becomes leaky, asymmetric leakage results (a, b). As long as not completely shorted, intermediate recovery may happen, e.g., from (b) to (a) several times when the chip is stressed by ESD or EMI pulses. Fig. 12 Setup of the Marconi Test. (a) (c) (b) (d)