Radio frequency identification (RFID) technology has come a long way since widespread implementation by US organisations, the Department of Defense, and Walmart in 2003. So much so, in fact, that according to a September 2008 RFID World article, ‘Walgreens Gets RFID’, the drugstore chain will ship up to 80,000 cases to 700 stores in the south-east when the programme is at full capacity.
However, conflicting schools of thought exist regarding the use of RFID-labelled packaging, giving rise to questions such as, is there risk to RFID tags from electrostatic discharge (ESD)-generated electromagnetic interference (EMI) during the materials handling process?
According to industry sources, the cost of an active tag can be as little as $1 while passive RFIDs are now affordable at $0.05 to $0.10 each. But RFID tags constitute an ESD-sensitive device. During air shipment where the relative humidity can easily drop to <6% to 9% relative humidity (RH), triboelectrification of the packaging and RFID labels can take place causing unanticipated device failures.
Now retired, Dr Ray Gompf, who worked for Nasa and the Kennedy Space Center, has measured more than 10,000V on kraft paperboard (corrugated or cardboard) at 12% RH, and during shipment, a charged package making intimate contact with a metal surface can produce an ESD event, causing RFID tag failure.
Static charge: causes and prevention
In use, it is not always possible to control static charge in locations where RFID tags are employed. Conveyor belts that use metal rollers (Figure 1) represent one such case. If all rollers had a path to common point ground, ESD events would be minimised.
In reality, the shafts of rollers may not be electrically connected to the roller or the shafts may not be electrically connected to the side rails. This could happen if the central shafts of rollers sit in painted (insulative) ‘V’
grooves in the side rails.
In conditions of low RH when packages traverse conveyor systems, the rollers act as isolated conductors and become charged. Charge build-up occurs until a breakdown voltage of the paint is reached; consequently, a high voltage discharge is generated between the roller shaft and side rails. This type of ESD event may be more prevalent in newer conveyor belts as the paint in the ‘V’ grooves in the side rails has not yet worn off from years of vibration. A resistance to ground measurement (as
shown in Figure 1) is used to validate continuity to ground.
During the RFID application process, tags can be incorrectly placed. Often, corrugated containers can travel on a conveyor in an upside down position, meaning an RFID tag ends up on the bottom of a box. Then the tag passes between two rollers at the instant when one of the rollers generates an ESD event (between its shaft and the side rail).
Consequently, the extreme electric field between rollers can induce currents in the RFID tag of such a magnitude to damage the chip on the RFID tag. The possible electric field strength generated by the ESD event can easily be in the range of many thousands of volts per metre.
Another instance often overlooked by packaging engineers and device designers is paperboard’s ability to charge upwards of several thousand volts during the material flow process. If an RFID label becomes charged during placement in low RH, the tag touching a metal object could produce a discharge.
In similar fashion, a corrugated container will charge up rapidly at less than 25% RH. During air or truck transport, friction between two surfaces (tribocharge generation or triboelectrification) can facilitate hundreds or thousands of ESD events. In one instance, ESD events have been observed at peaks of -10kV to 6kV for a package charged at more than 5kV in low RH and during vibration testing. The reader should also not discount the taping process of box sealing, which will generate voltages of more than +/-40kV during automatic top and bottom (reel-to-reel) sealing.
How then can one measure the stress on the chip so as to design adequate protection into the RFID tag? The voltage and current delivered to the chip will be a function of the magnitude of the ESD event and the size of the RFID tag, including its layout and design.
One method of measuring stress on an RFID chip is to use a variable high-voltage source (such as an ESD simulator or other high-voltage low-current power supply) applied to a roller that is isolated from the side rail. The voltage on the roller is incrementally increased until a discharge between the roller and side rail occurs. An RFID tag can be mounted onto the end of a passive differential probe to measure the electrical stress applied to the chip as the tag is held in close proximity to simulate bridging of the gap between rollers until an ESD event occurs, demonstrated in Figures 2 and 3.
First, remove the chip on the RFID tag (Figure 3). Then fasten two short wires on the pads of the displaced chip using solder or conductive glue (depending on the RFID tag). These short wires are then inserted and seated onto the tip of a passive differential probe of adequate bandwidth (Figure 2, main image).
The oscilloscope screen (Figure 2, upper right) shows the results of such measurement of the open circuit voltage at the chip position. The chip was removed and the RFID tag (different from Figure 3) is positioned between two rollers. The vertical scale is 5V/div (each vertical division represents 5 volts) and the waveform has a peak value of approximately 20V.
The horizontal scale is 1ns/div showing very fast sub-nanosecond rising edge rates in the waveform. The passive differential probe (Figure 3) has a bandwidth of almost 2GHz, a high input impedance and a high common mode voltage range without saturation.
Active probes cannot be used for this purpose as they have acceptable common mode voltage ranges on the order of only a few volts. Two conditions should be measured:
- open circuit with just the probe bridging the chip pads on the tag with the chip removed
- with 50 Ohms across the chip pad on the tag.
These two measurements give the open circuit voltage across the chip and a measure of the source impedance behind the voltage by the amount the measured voltage is reduced by a 50-Ohm load. Useful information is also be generated by measuring the voltage across the chip itself during the ESD stress.
With the measured data in hand, the design of the RFID tag and chip can be modified to add appropriate protection. There are a number of relatively new ESD protection devices on the market with very low capacitance that could be utilised for this application.
The shipment of medical device ESD-sensitive circuit cards requires EMI/RFI shielding packaging. One cannot rely on a vacuum former’s claims that an antistatic tray provides adequate attenuation when the tray can become charged in flight when RH drops below 9%, rendering the humidity-dependent thermoformed carrier ineffective and charge generating.