The tidal wave of excitement for Radio Frequency Identification, "RFID," has reached the converting industry, and the surge of implementation is significant as suppliers strive to become one of the first in their industry to offer RFID solutions. The applications are unlimited; unfortunately with all opportunity, there is often a challenge associated with change.

  Most tag and label converters are still asking, "What are RFID tags, anyway?" RFID is a method of remotely storing and retrieving data, using devices called RFID tags. These tags contain antennae to enable them to receive and respond to radio-frequency queries from an RFID transceiver. There are active tags, which have an internal battery, and passive tags, which do not. The battery units send a stronger signal for a greater distance, but active tags are also larger and more expensive.

  This is where the "shock" comes in. RFID tags are electronic devices that are not designed to withstand the rigors of converting environments. I'm not certain how familiar most converters are with electronic components, but I can assure you that converting factory environments are no place for an unprotected chip. Yes, as mentioned above, there are different types of chips, and some are less susceptible to static electricity than others; however there have been many documented instances where an electrostatic discharge (ESD) event has caused damage to an RFID device. The damage can happen in an instant causing poor production yields, and worse, unhappy customers when they find that their tags aren't working.

  Problem

  Static electricity is an age-old problem in packaging/converting operations such as slitting, printing, coating.

  The static charges are generated by the contact and separation of the material as it passes through the various production and handling stages. The result of the friction (triboelectrification) between materials as they contact and separate is a surface charge, or static electricity. By definition, static electricity is an electrical charge at rest caused by an imbalance of electrons. The charges usually occur on insulative materials, such as films or coated papers, and may occur on conductive surfaces that are isolated from ground. This is an important point because many materials suppliers are claiming that their new "anti-static" materials will protect RFID devices from the effects of static electricity. Unfortunately, this may not always be correct because static charges can be inductively transferred to a conductive object that is isolated from ground. The next time the conductive object (such as an RFID device) comes into proximity of ground, the transfer of electrons could cause damage.

  Contact and Separations of Surfaces

  When two surfaces contact, an alignment of electrons takes place between both surfaces. When the surfaces separate an exchange of electrons takes place. One surface will give off its electrons and be left in a deficient state, or positively charged, while the other surface has an overabundance of electrons or is negatively charged. The materials involved as well as the intimacy and pressure of the contact and separation affects the magnitude of the charge. To understand the charge characteristics of materials, you must consider the material's ranking in the Triboelectric Series. The relative position of the material on the chart determines the magnitude and polarity of the charge that results when the materials contact and separate. The further apart materials are in the series, the greater the magnitude of the charge. Also, materials located at the top of the chart will have a propensity to acquire a positive charge when they are in contact with materials ranked lower on the chart.

  This is further complicated by the fact that static charges are cumulative; charge potentials can continue to increase each time the material contacts another surface. This is evident in processes where the material may come into contact with several surfaces, like idler rollers in a web-converting application. What is important to recognize is that these charges need to be controlled to safe levels as part of your plan to protect RFID devices during manufacturing. Typical manufacturing areas that tend to generate significant charges include: Web-transport systems-unwinds, nip rollers, accumulators, idlers with insulative sleeves, corona treaters, lay-on rolls, rewinds-automated wrapping operations, individual package or pallet wrappers.

  Electronic Devices

  The involvement of RFID devices in packaging/converting materials changes everything because these tiny circuits cannot withstand exposure to stray voltages. They can experience damage from different sources, the most damaging are:

  * Damage from a direct electrostatic discharge (ESD). When a charged object or individual touches a device, some of the stored energy is transferred or discharged either to the device or through the device to ground. The charge is transferred to the device with sufficient energy to cause damage to the circuit. This transfer of energy manifests itself in the form of heat, which will cause melting of one or more of the layers within the device.

  * Static charges can also be induced onto conductive objects, which are isolated from ground, when the device is exposed to an electrostatic field. In these instances, the isolated conductor (RFID device) becomes polarized in the charged field, and if the device is momentarily grounded during this condition, a flow of electrons will occur as the device attempts to gain equilibrium in relation to the field. The result is that when the field is removed, a charge of the opposite polarity has been induced onto the device leaving the device in a charged condition so that the next contact with ground will result in an ESD event.

  * Equally as dangerous to RFID devices is exposure to electro-magnetic interference. This is often referred to as electrical overstress (EOS) in the electronics industry. It occurs when a device is exposed to the transient energy, or voltage, temporarily exceeding the dielectric capability of the circuit and causing damage.

  Interesting data from failure analysis conducted by the electronics industry shows that static electricity only causes immediate, or catastrophic, damage to a device 10 percent of the time; while 90 percent of these events cause a Latent failure, which will ultimately result in the failure of the device. I relate to Latent device failures much like a crack in a pane of glass. The pane still functions, but each time it is stressed the crack grows larger until ultimately the pane of glass breaks. The problem/question is when does the RFID device fail? Further, there is no way to screen these latent failures out of the manufacturing process; you test the device at your last test station and it works, your customer tests it and it doesn't. Device integrity is the largest problem facing manufacturers at this point. Yield rates are not where they need to be, and the ability to offer assurance to customers of 100-percent device reliability is not there yet.

  Solution

  The most active converting area today, for RFID, is pressure-sensitive label production. Right now the process is very slow, ranging from 50- to 150-fpm web speeds, as the tags are transferred from the carrier reel to the label stock.

  Applications range from manufacturers assembling the RFID devices in-line with their   converting process, to small tag/label operations transferring pre-assembled devices to printed labels. In either instance, the insertion equipment ranges from very sophisticated purchased machines to homemade models, which will often impact the ability to control static electricity.

  To control static electricity in RFID applications you need to use an active (electrical) static eliminator that can provide relatively balanced ionization and has the capability of neutralizing the charges in your specific application (speed, type of material, etc.). The ionizing balance (equality of positive and negative ions) of an eliminator is important because the offset voltage differential could damage some RFID devices. Although there is no industry standard as yet that identifies at what voltage level an RFID device will sustain damage from static electricity, many device manufacturers recommend that charge levels be maintained at around 500 volts when the tags are exposed. As the tags are placed into the p-s label assemblies, as an example, they become more robust, however many insertion-equipment manufacturers recommend controlling static charges to around 1,500 volts once the tag is incorporated into label stock. In either event, balanced ionization from a well-designed ionizer will quickly reduce the static charges to levels that are no longer dangerous to the RFID device.

  Never use passive static elimination (tinsel or conductive string) to protect RFID devices. Although passive eliminators do reduce static charges in some industrial applications, this type of ionizer can actually be dangerous for RFID devices because they will reduce the static charge only to the threshold voltage level where their ionization capability is initiated. The danger is that the threshold voltage levels of passive eliminators will often exceed the voltage range to which most RFID devices should be exposed.

  The active ionization products that have proven the most effective to this point are extended-range static bars that offer mounting capabilities of 2 to 6 in. from the target to be neutralized. These units are generally capable of neutralizing static charges on fast moving webs (2,000 to 2,500 fpm), and the mounting distance will provide a good mix of ions for better ion balance while also allowing a safer distance between the RFID device and the high intensity field that is present close to the emitter electrodes (pins). Static bars have been used successfully to neutralize static charges on the label stock (label material with adhesive and liner), paper or film, as it is being oriented for RFID tag placement and on the device carrier web as it approaches the transfer (insertion) point. The transfer (insertion) point is where the RFID device is exposed, and consequently is the most vulnerable to damage from static electricity. The concept is to maintain a neutral condition in that area at all times.

  The transfer (insertion) operation is critical because it is not uncommon for a static charge to be generated on the RFID device from the contact and separation associated with the process. In this area ionization equipment designed for electronic applications has been used, ionizing air blowers or air-assisted focused area ionizers, because the ion balance characteristics of these units are more conducive to the sensitivity of the devices as well as being capable of delivering ionization into tight machine spaces with mounting restrictions.

  Static bars are also being used downstream of the transfer point, where the RFID tag is now sandwiched in the label stock, to control charges that can be generated by idler rollers or additional process operations such as diecutting. The removal of cut material, after diecutting, is a notorious static-generating area for label converters, and it is not uncommon to see static charges in the 20,000- or 30,000-volt range based on environmental and material conditions in this area. These charges must be controlled to protect the RFID devices and to minimize the charges that can build up in the take-up roll, as the material is rewound. Large static charges in the take-up roll are a potential disaster for the RFID devices now assembled into the label stock, for all the reasons described above.

  Conclusion