RFID tags with shielding structure for incorporation into microwavable food packaging

RFID tags are provided for incorporation into the packaging of a microwavable food item, with the RFID tag being configured to be safely microwaved. The RFID tag includes an antenna defining a gap and configured to operate at a first frequency. An RFID chip is electrically coupled to the antenna across the gap. A shielding structure is electrically coupled to the antenna across the gap and overlays the RFID chip. The shielding structure includes a shield conductor and a shield dielectric at least partially positioned between the shield conductor and the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a second frequency that is greater than first frequency. In additional embodiments, RFID tags are provided for incorporation into the packaging of a microwavable food item, with the RFID tag being configured to be safely microwaved. The RFID tag includes an RFID chip and an antenna electrically coupled to the RFID chip. The antenna may have a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms, optionally with an optical density in the range of approximately 0.18 to approximately 0.29. Alternatively, or additionally, the antenna may be configured to fracture into multiple pieces upon being subjected to heating in a microwave oven. Alternatively, or additionally, the RFID tag may be incorporated in an RFID label that is secured to the package by a joinder material with a greater resistance than that of the antenna, such as a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms.

BACKGROUND

Field of the Disclosure

The present subject matter relates to packaging for microwavable food items. More particularly, the present subject matter relates to radio frequency identification (“RFID”) tags incorporated into packaging for microwavable food items.

Description of Related Art

It is known for packages for microwavable food items to include cooking aids that are to be placed into the microwave oven with the food item for cooking/heating the food item. For example, foods having crusts, such as frozen pies or stuffed bread, may benefit from “crisping sleeves,” which are paper items that at least partially surround the food item during microwaving. Typically, a “crisping sleeve” has a paper substrate, with a susceptor incorporated into the inner surface of the “crisping sleeve,” facing and preferably in contact with the food item. The susceptor, which may be a metallized film, absorbs microwave energy and converts it into heat, which crisps and/or browns the crust or surface of the food item, thus improving the look and texture of the food item. Due to the absorbing nature of the film used as the susceptor, relatively low levels of energy are reflected by it, such that it does not strike an arc due to generating high differential voltages between adjacent parts of the film, which could otherwise cause the packaging to catch fire.

It is also known to incorporate RFID technology, such as an RFID tag, into product packaging for various purposes, including inventory management and theft prevention.FIG. 1shows an RFID tag T according to conventional design, which may be secured to or otherwise associated with an enclosure like that of enclosure13ofFIG. 1A(typically, a paper or cardboard sleeve or box) of the package9for a microwavable food item in respect toFIG. 1A. The entirety of the packaging9ofFIG. 1Ais not intended to be microwaved, but rather the food item (and, optionally, a “crisping sleeve” or the like) is removed from the enclosure13ofFIG. 1Aand inserted into the microwave oven for heating/cooking.

The RFID tag T ofFIG. 1includes an RFID chip C, with an associated dipole antenna A for transmitting information to and/or receiving information from an RFID reader (not illustrated). The RFID chip C is electrically coupled to the antenna A across a gap G defined by the antenna A between two conductor pad areas P.

RFID tags inherently must, at some point, have a gap across which the RFID chip is placed that has a voltage at the intended frequency of operation when in the field of a reader device. The power required incident on the RFID chip C may be as low as 10 microwatts, whereas a microwave oven may typically operate at a power level in excess of 800 watts, which can generate very high voltages across the gap G and the associated RFID chip C. The antenna A is designed to operate at a first frequency F1, for example in the range of approximately 860 MHz to 930 MHz, with the antenna A taking incident power at the first frequency F1 from an RFID reader and converting it to a voltage across the RFID chip C to allow it to operate.

A second frequency applied by the microwave oven, identified inFIG. 1at F2, typically on the order of approximately 2,450 MHz, may also be incident on the antenna A when the RFID tag T is placed into the microwave oven. The antenna A is not designed to operate at the second frequency F2, as the very high power levels incident at second frequency F2 will generate high voltages on the antenna A. These high voltages can appear at a number of places on the antenna A; however, by methods such as introducing large gaps L between antenna elements and controlled radii (identified generally at R inFIG. 1), a voltage across said elements that would generate a high voltage breakdown and, hence, arc can be avoided. However, the gap G bridged by the RFID chip C is necessarily relatively small and, hence, a high voltage arises at the second frequency F2, which high voltage may cause a breakdown and generate an arc. Similarly depicted inFIG. 1A; the dipole antenna17can receive microwave energy (identified inFIG. 1Aat M) and reflect the microwave energy (represented inFIG. 1Aat R) into the microwave source. There is the possibility that an arc may be created between adjacent sections of the dipole antenna17(which location may be between the two conductive elements of the dipole antenna17, as identified inFIG. 1Aat19). Additionally, referring toFIG. 1Athe dipole antenna17of the conventional RFID tag11is formed of relatively thick, low resistance conductor, which has different properties than the metallic film used to define a typical susceptor. For example, common susceptors are made from metal-coated films with optical densities ranging from 0.18 to 0.29, corresponding to a sheet resistance of 100 ohms to 230 ohms, whereas a material of less than 1 ohm per square is commonly used to form the antenna18of the RFID tag11. On account of the characteristics of the dipole antenna17, the RFID tag11can cause issues if it is not dissociated from the food item prior to microwaving the food item (i.e., if the entire package9ofFIG. 1Ais placed into the microwave oven).

To avoid problems of this nature, the RFID tag T and11ofFIGS. 1 and 1Arespectively, are typically configured to be readily removable or otherwise dissociable from the food item, such as by securing it to the enclosure of the package, which may include instructions to not microwave the enclosure. However, it is possible that a user failing to take proper care could place the entire package (including the RFID tag T and11shown inFIGS. 1 and 1Arespectively) into the microwave oven with the food item, thereby failing to dissociate the RFID tag T or11from the food item. Accordingly, it would be advantageous to provide an RFID tag that may be microwaved without resulting in the problems associated with microwaving a conventional RFID tag T or11.

SUMMARY

In one aspect, an RFID tag includes an antenna defining a gap and configured to operate at a first frequency. An RFID chip and an antenna electrically coupled to the antenna across the gap. A shielding structure is electrically coupled to the antenna across the gap and overlays the RFID chip. The shielding structure includes a shield conductor and a shield dielectric at least partially positioned between the shield conductor and the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a second frequency that is greater than the first frequency.

In another aspect, packaging is provided for a microwavable food item. The packaging includes an enclosure and an RFID tag secured to the enclosure. The RFID tag includes an antenna defining a gap and configured to operate at a first frequency. An RFID chip is electrically coupled to the antenna across the gap. A shielding structure is electrically coupled to the antenna across the gap and overlays the RFID chip. The shielding structure includes a shield conductor and a shield dielectric at least partially positioned between the shield conductor and the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a second frequency that is greater than the first frequency.

In a further aspect, an RFID tag includes an antenna defining a gap and configured to operate at a first frequency. An RFID chip is electrically coupled to the antenna across the gap. A shielding structure is electrically coupled to the antenna across the gap and overlays the RFID chip. The shielding structure includes a shield conductor and a shield dielectric at least partially positioned between the shield conductor and the RFID chip. A second shielding structure is electrically coupled to the antenna across the gap, underlying the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a second frequency that is greater than the first frequency.

In another aspect, packaging is provided for a microwavable food item. The packaging includes an enclosure and an RFID tag secured to the enclosure. The RFID tag includes an antenna defining a gap and configured to operate at a first frequency. An RFID chip is electrically coupled to the antenna across the gap. A shielding structure is electrically coupled to the antenna across the gap and overlays the RFID chip. The shielding structure includes a shield conductor and a shield dielectric at least partially positioned between the shield conductor and the RFID chip. A second shielding structure is electrically coupled to the antenna across the gap, underlying the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a second frequency that is greater than the first frequency.

In another aspect the antenna comprised of an antenna with a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms. In another aspect, an RFID tag includes an RFID chip and an antenna electrically coupled to the RFID chip. The antenna is comprised of a conductor formed of a base material and a second material with different coefficients of thermal expansion configured to cause the antenna to fracture into multiple pieces upon being subjected to heating.

In yet another aspect, a package is provided for a microwavable food item. The package includes an enclosure, an RFID label, and a joinder material sandwiched between the RFID label and the enclosure. The RFID label includes a substrate and an RFID tag associated with the substrate. The RFID tag includes an RFID chip and an antenna electrically coupled to the RFID chip. The joinder material has a greater resistance than the antenna.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIGS. 2A and 2Bshow an RFID tag10according to the present disclosure, whileFIG. 2Bshows the RFID tag, generally designated at10, secured to the enclosure12(e.g., a paper box) of packaging, generally designated at14, for a microwavable food item. The packaging14may include other items, such as a “crisping sleeve” configured to be microwaved with the food item. The RFID tag10may be incorporated into the packaging14by any suitable approach and, while the RFID tag10is secured to the enclosure12in the embodiment ofFIG. 2B, the RFID tag10may be associated with another portion of the packaging14(e.g., a “crisping sleeve” housed within the enclosure12) in other embodiments. Further, while RFID tags are described herein as being incorporated into the packaging of a microwavable food item, it should be understood that RFID tags according to the present disclosure may be useful in any of a number of possible applications, particularly when it is contemplated that they may be exposed to frequencies (referred to herein as a “second frequency”) that are significantly higher than the frequency (referred to herein as a “first frequency”) at which an antenna of the RFID tag is intended to operate.

The RFID tag10includes an antenna16with an RFID chip18electrically coupled thereto. The antenna16is provided as a dipole antenna, which is formed of a conductor defining a gap20between two conductor pad areas22(FIG. 2A), which is bridged by the RFID chip18. The antenna16and RFID chip18may be provided generally according to conventional design (e.g., as described above with respect to the embodiment ofFIG. 1), with the antenna16being designed to operate at a first frequency, which may be in the range of approximately 860 MHz to 930 MHz. As in the conventional RFID tag T, the antenna16takes incident power at the first frequency and converts it to a voltage across the RFID chip18to allow it to operate.

The RFID chip18may take any of a number of forms (including those of the type commonly referred to as a “chip” or a “strap” by one of ordinary skill in the art), including any of a number of possible components and being configured to perform any of a number of possible functions. For example, in one embodiment, the RFID chip18includes an integrated circuit for controlling RF communication and other functions of the RFID tag10.

The RFID tag10further includes a shielding structure, generally designated at24, which is comprised of a shield conductor26and a shield dielectric28. The shield conductor26is formed of a material having conductive properties and, as will be described in greater detail, may be variously configured without departing from the scope of the present disclosure. The shield dielectric28is formed of a material having dielectric properties and, as will be described in greater detail, may be variously configured without departing from the scope of the present disclosure. For example, in the embodiment ofFIGS. 2A and 2B, the shield conductor26and shield dielectric28are generally flat or planar, substantially identically shaped, and oriented with the perimeter of the shield conductor26coinciding with the perimeter of the shield dielectric28. In other embodiments, the shield conductor and shield dielectric may be differently configured and/or oriented at least partially out of alignment (i.e., with a portion of the shield conductor extending beyond the perimeter of the shield dielectric and/or a portion of the shield dielectric extending beyond the perimeter of the shield conductor).

The shielding structure24is electrically coupled to the antenna16across the gap20, being coupled by capacitance to the conductor pad areas22on either side of the gap20(FIG. 2A). As shown inFIG. 2B, the shielding structure24overlays the RFID chip18, with the shield dielectric28at least partially positioned between the RFID chip18and the shield conductor26. The shielding structure24may overlay or cover all (as inFIGS. 2A and 2B) or only a portion of the gap20.

As described above, it is possible for the RFID tag10to be exposed to signals operating at first or second frequencies. When the RFID tag10is exposed to the first frequency, the shielding structure24forms a partial short circuit across the gap20. However, the antenna16is configured so as to compensate for the presence of the partial short circuit, thereby allowing the RFID tag10to operate properly.

As described above, when a conventional RFID tag T is exposed to the second frequency F2, a large voltage arises across the gap G, which risks the creation of an arc. If the voltage and power at the second frequency F2 are limited sufficiently, the RFID chip C may survive, but the main objective is to prevent an arc that could ignite the RFID tag T or the packaging14into which it is incorporated. The shielding structure24ofFIGS. 2A and 2Bprovides this function by “shorting” the high voltage generated across the gap20(and, hence, the RFID chip18) when the RFID tag10is exposed to the second frequency, thereby reducing the voltage below the level that can cause a breakdown and possible arc, which prevents ignition. Accordingly, the RFID tag10may be placed into a microwave and exposed to the attendant high-frequency signals (which may be on the order of approximately 2,450 MHz) without the risk of ignition, unlike a conventional RFID tag T.

The shielding structure may be variously configured without departing from the scope of the present disclosure, as noted above. For example,FIGS. 3A and 3Bshow an embodiment of an RFID tag, generally designated at10a, (and associated packaging, generally designated at14a, inFIG. 3B) in which the shielding structure24aincludes a differently configured shield dielectric28a(FIG. 3B). In the embodiment ofFIGS. 3A and 3B, the shield dielectric28ais incorporated into an over-lamination layer, which overlays the RFID chip18, at least a portion of the gap20, and at least a portion of the conductor pad areas22of the antenna16(FIG. 3A). The shield conductor26amay comprise a patterned conductor to provide the desired bridging and shielding effects. As best seen inFIG. 3B, the shield conductor26aand shield dielectric28amay be differently sized and shaped, with the shield conductor26abeing smaller than the over-lamination layer into which the shield dielectric28ais incorporated.

FIGS. 4A and 4Billustrate another embodiment of an RFID tag, generally designated at10b, according to the present disclosure. In the embodiment ofFIGS. 4A and 4B, the shielding structure, generally designated at24b, is incorporated into an RFID strap comprised of a strap conductor30and strap substrate32(along with the RFID chip18), which is electrically coupled to the antenna16, across the gap20. The shielding structure24bmay be comprised of a shield conductor26bapplied to the strap substrate32, which serves as the shield dielectric28b. The strap substrate32(and any of the other shield dielectrics described herein) may be formed of any of a variety of materials, such as polyethylene terephthalate.

FIG. 5illustrates another embodiment of an RFID tag, generally designated at10c, with a differently configured shielding structure24c. In the embodiment ofFIG. 5, the shield conductor26cincludes an extended area34, which may increase the size of the shield conductor26cbeyond that of the associated shield dielectric (which is not visible inFIG. 5). In contrast to other embodiments, in which the shielding structure is primarily configured and oriented to overlay or cover the gap20, the extended area34of the shield conductor26cis oriented so as not to overlay the gap20(or the antenna16), but rather is positioned laterally of the antenna16and the gap20, extending away from the antenna16. The extended area34of the shield conductor26cmay be variously sized and configured without departing from the scope of the present disclosure, being approximately the same size as the shield conductor26ofFIGS. 2A and 2Bin one embodiment, larger than the shield conductor26ofFIGS. 2A and 2Bin another embodiment, and smaller than the shield conductor26ofFIGS. 2A and 2Bin yet another embodiment.

Regardless of the particular size and configuration of the extended area34of the shield conductor26c, the extended area34assists in dissipating heat generated across the gap20. This effect is enhanced by increasing the size of the extended area34, so it may be advantageous for the extended area34to be relatively large for improved heat dissipation. The extended area34(along with the remainder of the shield conductor26c, as well as any of the other shield conductors described herein) may be formed of a non-flammable material, such as but not limited to, an aluminum material, heat resistant, flame resistant paper (Flex Dura HR, http://www.flexlinkllc.com/heat-resistant-paper.html), and non-flammable adhesive (Eclectic E6000 Adhesive, http://eclecticproducts.com/products/e6000.html) to provide a barrier to any arc that may be generated across the gap20to prevent a fire from spreading.

FIGS. 6A and 6Billustrate yet another embodiment of an RFID tag, generally designated at10d, (and associated packaging, generally designated at14d, inFIG. 6B) with a differently configured shielding structure24d. In the embodiment ofFIGS. 6A and 6B, the shield dielectric28dis formed of a material which undergoes reversible or non-reversible dielectric breakdown at high voltages of the type induced by a high-power microwave field. By such a configuration, the shorting effect provided by the shielding structure24din the presence of a second frequency (e.g., in a microwave field) may be enhanced. In this embodiment (as well as in other embodiments described herein), the shield conductor26dmay be formed by printing a conductive material (which becomes and defines the shield conductor26d) onto the shield dielectric28d, such as an over-lamination.

A single RFID tag may include more than one shielding structure, as shown in the embodiment ofFIGS. 7A and 7B. InFIG. 7A, the RFID tag, generally designated at10e, is provided with a first shielding structure, generally designated at24e, in general accordance with the preceding description of the embodiment ofFIGS. 3A and 3B. Rather than the antenna16of the RFID tag10ebeing free for direct connection to the enclosure of packaging (as inFIG. 3B), a second shielding structure, generally designated at24f, (FIG. 7B) is associated with an underside of the antenna16, with the second shielding structure24funderlying the RFID chip18(i.e., with the shielding structures24eand24felectrically coupled to opposing faces of the antenna16). The shield dielectric28fof the second shielding structure24fcontacts the underside of the antenna16, while the associated shield conductor26fis free to be secured or otherwise associated to the enclosure of a package for microwavable food or the like.

In the illustrated embodiment, the second shielding structure24fis substantially identical to the first shielding structure24e, but it is within the scope of the present disclosure for the shield conductor26fand/or the shield dielectric28fof the second shielding structure24fto be differently configured from the shield conductor26eand shield dielectric28eof the first shielding structure24e. Regardless of the particular configurations of the two shielding structures24eand24f, by providing them on both faces of the antenna16, additional shielding is provided. This additional shielding involves additional “shorting,” as there are now two partial short circuits across the gap20. However, in accordance with the preceding description of the embodiment ofFIGS. 2A and 2B, the antenna16is configured so as to compensate for the presence of the partial short circuits, thereby allowing the RFID tag10eto operate properly when exposed to the first frequency.

FIG. 8is a basic equivalent circuit representing the basic components of an RFID tag10according to the present disclosure. InFIG. 8, the gap20defined by the antenna16is bridged by an RFID chip18(represented by a resistor Rp and a capacitor Cp) and a shielding structure24comprising a shield conductor26and a shield dielectric28(represented by two identical capacitors CBin series). The total capacitance of the shield dielectric28is half of the capacitance of the individual capacitors CBused to represent the shield dielectric28inFIG. 8. This is calculated using the standard formula in which the total capacitance of a series of capacitors is the inverse of the sum of all inverse capacitances.

The impedance of the shield dielectric28is equal to the inverse of the product of 2×π×F× total capacitance, in which F is the frequency at which the RFID tag10is powered. Thus, if the first frequency is on the order of approximately 800 MHz and the second frequency is on the order of approximately 2,400 MHz, then impedance drops by a factor of approximately three between the first and second frequencies, which enhances the “shorting” and, hence, shielding effect at the second frequency.

Additionally, there is the possibility that an arc may be created between adjacent sections namely gap G and associated RFID chip C. This is in part due to adjacent sections being surrounded by a material (i.e. air or other elements) having a dielectric strength lower than that of the electric field achieved by said differential voltages across said adjacent sections. Also an arc may be created and exacerbated in part due to materials surrounding said sections that reach a temperature, due to RF current flowing along/through said adjacent sections gap G and chip C, that lowers dielectric strength of the surrounding material as well as creates flammable/combustible volatiles. This arc can be avoided without the use of a shield by surrounding said sections with a material having the properties such as; a dielectric strength that can withstand the electric field at said sections, along with having heat resistant, flame resistant and non-flammable properties i.e. heat resistant and flame resistant paper and non-flammable adhesive(s).

Furthermore, within the same scope of the invention additional embodiments are disclosed. In the illustrated embodiment ofFIG. 9, the enclosure23is associated with the RFID tag25includes an RFID chip27with an antenna29electrically coupled thereto. The antenna29is formed of a conductor31having a resistance that is greater than the resistance of the antenna18of a conventional RFID tag11, which allows the package21(including the RFID tag25) to be safely microwaved. For example, the conductor31may have a sheet resistance that is comparable to that of the sheet resistance of a susceptor (i.e., in the range of approximately 100 ohms to approximately 230 ohms). The conductor31may also have an optical density in the range of approximately 0.18 to 0.29, similar to a susceptor. By such a configuration, when the RFID tag25is microwaved, it acts in the way that a susceptor does when being microwaved, by absorbing microwave energy M and heating up and reflecting minimal energy R′, rather than reflecting high levels of energy to the microwave source or creating an arc.

The higher sheet resistance of the conductor31may affect the efficiency of the antenna29compared to the dipole antenna17of a typical RFID tag11. While the sheet resistance of the material (measured in ohms per square at a given thickness) is a fixed value, the resistance experienced by an RF current flowing through the conductor31may be effectively decreased by increasing the area of the conductor31(e.g., by increasing its thickness). This is particularly effective in reducing the resistance for an RF current, as skin depth is more of a factor than for a DC current, due to the tendency of an RF current to flow in the outer surface of the conductor31(i.e., as conductor thickness is reduced with respect to the skin depth, RF resistance becomes higher than DC resistance would be). Accordingly, it may be advantageous for the antenna29to have a relatively large area or thickness to decrease the RF resistance.

Compared to a dipole antenna, the conductor of a slot-loop hybrid antenna typically has a greater area, such that it may be advantageous for the antenna29to be provided as a slot-loop hybrid antenna (sometimes referred to as a “sloop” antenna), as inFIG. 9. Such a slot-loop hybrid antenna29may be formed of a conductor31comprising a conductor sheet which, in the illustrated embodiment, is generally rectangular, with a slot33defined therein and positioned at an edge or end35of the conductor sheet31. As shown, the slot33may extend between a closed end37and an open end39associated with the end or edge35of the conductor sheet31. While there are various advantages to the antenna29being configured as a slot-loop hybrid antenna, it is within the scope of the present disclosure for the antenna29to be variously configured.

Further observing the RFID chip27, it may take any of a number of forms (including those of the type commonly referred to as a “chip” or a “strap” by one of ordinary skill in the art), including any of a number of possible components and configured to perform any of a number of possible functions. For example, in one embodiment, the RFID chip27includes an integrated circuit for controlling RF communication and other functions of the RFID tag25. In the illustrated embodiment, two ends or points of the RFID chip27are connected to the conductor sheet31at opposite sides of the slot33, adjacent to the open end39of the slot33, which serves to electrically couple the RFID chip27to the conductor sheet31.

According to another aspect of the present disclosure, which may be incorporated into the antenna29of the RFID tag25ofFIG. 9or may be separately practiced, an RFID tag41(FIGS. 10 and 10A) that is suitable for incorporation into a package for a microwavable food item may be configured to fracture into multiple pieces or otherwise dissociate upon being subjected to heating in a microwave oven. By fracturing, interaction with the microwave field is reduced, thereby avoiding the potential problems of excessive reflected microwave energy and/or the creation of an arc when the RFID tag41is heated in a microwave oven. Such a configuration allows for the resistance of the conductor43of the antenna45of the RFID tag41to be lower than in the embodiment ofFIG. 9(e.g., a sheet resistance of less than 100 ohms), if desired.

The RFID tag41shown inFIG. 10is provided in accordance with the foregoing description of the RFID tag25ofFIG. 9, with an RFID chip47electrically coupled to the conductor sheet43of a slot-loop hybrid antenna45, although the antenna45may be differently configured without departing from the scope of the present disclosure.

Regardless of the particular configuration of the antenna45, its conductor sheet43is preferably formed of at least two materials (a base material and a secondary material, which may be provided in a lesser quantity than the base material) having different coefficients of thermal expansion. By such a configuration, the materials expand at different rates when heated (e.g., in a microwave oven) until the conductor sheet43fractures into multiple pieces or otherwise dissociates. The magnitude of the difference in the coefficients of thermal expansion of the materials may vary without departing from the scope of the present disclosure, although a relatively large difference may be advantageous to more quickly cause the conductor sheet43to fracture or otherwise dissociate upon heating.

In one exemplary embodiment, the conductor sheet43may be formed of a base material, such as a plastic material, and a second material, such as a metallic material or conductive ink, which have different coefficients of thermal expansion. More particularly, the base material may be polyethylene terephthalate (which has a coefficient of thermal expansion of approximately 60 m/(m K)), while the secondary material is aluminum (which has a coefficient of thermal expansion of approximately 22 m/(m K)). When bonded together and heated, the aluminum will eventually break, thus rendering the RFID tag41inoperative or at least causing the RFID tag41to operate at a lower level, which reduces the interaction between the RFID tag41and the microwave field. While the base material has a greater coefficient of thermal expansion than the secondary material in this example, it is within the scope of the present disclosure for the secondary material to have a greater coefficient of thermal expansion. Furthermore, in one embodiment, this breakage may be promoted by including one or more points or lines of weakness (which are evident inFIG. 10A), such as scored or thinned areas of decreased thickness, which encourages the conductor sheet43to break at that particular location or locations.

If it is desired to employ an RFID tag11according to conventional design, the manner in which it is incorporated into the package49of a microwavable food item may be modified.FIG. 11illustrates a package49incorporating an RFID tag11according to conventional design (as inFIG. 1A), although it is also within the scope of the present disclosure for the RFID tag11to be configured as inFIG. 9 or 10.

The enclosure51of the package49is provided with a joinder material53applied to one or more of its surfaces (illustrated inFIG. 11as an outer surface). The joinder material53may be present as a relatively thin layer or sheet of material with a resistance that is higher than the resistance of the antenna17of the RFID tag11(e.g., a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms). Preferably, the joinder material53has a substantially uniform thickness, although it is within the scope of the present disclosure for the joinder material53to have a non-uniform thickness. It may be advantageous for the joinder material53to have an average thickness that is less than the thickness of the antenna17of the RFID tag15(e.g., the joinder material53may have an average thickness of in the range of approximately 10 nm to approximately 100 nm for joinder material53comprising an aluminum material).

In one embodiment, the joinder material53comprises a metallic film. In another embodiment, the joinder material53comprises an ink of a suitable conductivity. In other embodiments, the joinder material53may be differently configured, provided that it has a suitably high resistance (i.e., a resistance that is at least greater than the resistance of the antenna17of the associated RFID tag11and, more preferably, a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms).

In the embodiment ofFIG. 11, a substrate55of the RFID tag11(to which the RFID chip15and antenna17are mounted) is associated to the enclosure51in a manner that sandwiches or interposes the joinder material53between the RFID tag11and the enclosure51. The joinder material53itself may have adhesive qualities to cause the RFID tag11to be secured with respect to the enclosure51or a separate means may be provided to secure the RFID tag11to the joinder material53(e.g., an adhesive applied to the underside of the substrate55). So separating the manufacturing of the enclosure51with the joinder material53and the RFID tag11allows for greater flexibility in manufacturing. By providing the joinder material53with a relatively high resistance, the effective sheet resistance of the RFID tag11is increased, thereby increasing the tendency to adsorb RF energy and heat up, rather than creating an arc.

The joinder material53may be variously configured without departing from the scope of the present disclosure. For example, the joinder material53may have a perimeter that substantially coincides with the perimeter of the substrate55of the associated RFID tag11, a perimeter that extends beyond the entire perimeter of the substrate55of the associated RFID tag11, a perimeter that is entirely contained within the perimeter of the substrate55of the associated RFID tag11, or a perimeter that extends beyond the perimeter of the substrate55of the associated RFID tag11in at least one location, while being contained within the perimeter of the substrate55of the associated RFID tag11at another location. Additionally, the perimeter of the joinder material53may have the same shape as the perimeter of the substrate55of the associated RFID tag11or a different shape.

In another aspect of the same invention not illustrated, packaging is provided for a microwavable food item. The packaging includes an enclosure and an RFID tag secured to the enclosure. The RFID tag includes an antenna defining a gap and configured to operate at a first frequency. An RFID chip is electrically coupled to the antenna across the gap. A shielding structure is electrically coupled to the antenna across the gap and overlays the RFID chip. The shielding structure includes a shield conductor and a shield dielectric at least partially positioned between the shield conductor and the RFID chip. The shielding structure is configured to limit the voltage across the gap when the antenna is exposed to a second frequency that is greater than the first frequency. The enclosure of the package is provided with the joinder material53previously described, applied to one or more of its surfaces (similarly illustrated inFIG. 11as an outer surface). The joinder material53may be present as a relatively thin layer or sheet of material with a resistance that is higher than the resistance of the antenna17of the RFID tag11(e.g., a sheet resistance in the range of approximately 100 ohms to approximately 230 ohms).

The present invention also contemplates, but is not limited to, the following testing method for the microwaveable RFID set forth herein. The equipment utilized in one method includes an inverter technology over such as a 12000 Watts Oven. For instance, a GE® Model JE 2251SJ02 can be utilized. Additionally, a scale and a plurality of plastic containers to hold samples are used. In one embodiment of the testing method, frozen, ground beef was used as a sample. The steps for the testing method using frozen ground beef are as follows: 1) A sample is prepared. A variety of weights can be utilized. In one instance, a five (5) ounce sample is used. 2) The sample is placed in one half of a container in order to ensure that the sample covers the bottom of the container consistently between different tests. 3) The sample is frozen for approximately twelve (12) hours. 4) At least one RFID label is adhered to the bottom of the container which holds the sample and the sample is placed on a rotational plate within a microwave oven. In one embodiment, the sample is placed in the center of the rotational plate within the microwave oven. 5) The sample is microwaved on a full power setting for two (2) minutes. The present testing method contemplates that several different power settings and times can be utilized in order to test the sample6) A determination is made as to whether there was a “spark” or “arc.”.