The present invention relates to a substrate for an RFID tag containing a paper substrate having formed thereon an antenna circuit, and an RFID tag using the same.
RFID (radio frequency identification) is being widely applied to such purposes as logistics management and the like. An RFID tag attached to an item is constituted by a substrate having an antenna circuit, and an IC chip mounted on the substrate. The IC chip used is frequently a type that performs conduction to the antenna circuit on the substrate through a metal member in the form of a protrusion, which is referred to as a bump.
PTL 1: JP-A-2013-127913
As the base for disposing an antenna circuit of an RFID tag (i.e., a member corresponding to 1 in
In such purposes as logistics management of clothing items and pulp products, and authenticity determination of alcoholic beverages and the like, there is a demand of an RFID tag using paper as a substrate. Paper also has such nature as easy breakability, and it is expected in the future that there is an increasing demand of an RFID tag using a paper substrate in a purpose where easy breakability is important. As described above, a low-temperature baking type conductive paint containing silver nanoparticles has been developed in recent years. By using a printing technique using, for example, the conductive paint, an antenna circuit with good flex resistance can be drawn on a surface of a paper substrate, and industrial mass production of an easily breakable RFID tag using a paper substrate can be performed.
However, there is an emerging problem occurring in promotion of spread of an RFID tag containing a paper substrate having an antenna circuit formed thereon. Specifically, it has been found that in the case where an IC tag having a noble metal plated type bump as shown in
The object can be achieved by an RFID tag substrate containing a paper substrate having an eluted chloride ion amount per unit mass (1 g) according to the following item (A) of 0.100 mg or less, having formed on a surface thereof a conduction circuit:
(A) a specimen of the paper substrate having an area corresponding to an A4 size (210×297 mm) determined in ISO 216 is broken into small pieces each of 100 mm2 or less; the small pieces are entirely placed in a polypropylene vessel; 50 mL of water having an electric conductivity of 0.2 mS/m or less is added thereto to immerse the small pieces entirely into the water; after allowing to stand at 23° C.±2° C. for 1 hour, the water is filtered with a membrane filter to recover a filtrate; and the filtrate is analyzed by an ion chromatography method to obtain a chloride ion (Cl−) concentration in the filtrate, from which a total chloride ion amount eluted into 50 mL of the water is obtained, and is divided by the mass (g) of the specimen of the paper substrate to provide a value, which is designated as the eluted chloride ion amount per unit mass.
The “paper” herein means one that is produced by agglutinating vegetable fibers or other fibers, as defined in JIS P0001:1998, “Terms of paper, paper board, and pulp”, No. 4004, and includes synthetic paper produced by using a synthetic polymer substance as a raw material, and one containing a fibrous inorganic material. One having been subjected to a surface treatment with a resin or the like is also included.
On the RFID tag substrate, an IC chip having a metallic bump coated with noble metal plating is mounted. The noble metal referred herein includes gold, silver, and platinum group elements (e.g., platinum, palladium, rhodium, iridium, ruthenium, and osmium). The metal coated with noble metal plating, i.e., the internal metal constituting the bump, is a metal having a standard electrode potential that is more negative than the noble metal of the plated layer, and examples thereof include nickel. The conduction circuit on the surface of the paper substrate is formed by printing, and is preferably formed, for example, of a silver conductive film.
The invention also provides an RFID tag containing an RFID tag substrate containing a paper substrate having an eluted chloride ion amount per unit mass according to the item (A) of 0.100 mg or less, having formed on a surface thereof a conduction circuit, and bonded thereto an IC chip having a metallic bump coated with noble metal plating, the conduction circuit and the metallic bump of the IC chip being electrically connected to each other.
According to the invention, in an RFID tag containing a paper substrate having an antenna circuit formed on the surface thereof, the weather resistance in the case where an IC chip having a noble metal plated type bump is mounted can be stably improved. Accordingly, the invention contributes to the spread of a low-cost RFID tag utilizing the easy breakability of a paper substrate.
As described above, in the case where an IC tag having a noble metal plated type bump as shown in
Even an IC chip that causes no problem in an RFID tag using a resin substrate often undergoes bimetallic corrosion when the IC chip is applied to an RFID tag using a paper substrate. It is considered therefrom that the paper substrate, which is liable to contain water as compared to the resin substrate, becomes a cause of bimetallic corrosion of the bump. On the other hand, the progress of bimetallic corrosion is also largely influenced by the factor of the corrosion environment, i.e., the amount of the ion species (electrolytes) that facilitate the progress of corrosion contained in the aqueous solution in contact with both the metals. There is a high possibility that the ion species are supplied from the paper substrate containing water.
Under the circumstances, the inventors have made accumulated investigations for finding the relationship among the kind and the amount of the ion source substances contained in the paper substrate and the corrosion of the bump. As a result, it has been found that the amount of a chloride ion (Cl−) supplied from the paper substrate largely influences the bimetallic corrosion of the bump. Specifically, it has been found that the bimetallic corrosion of the bump can be significantly prevented by mounting an IC chip having a noble metal plated type bump on a paper substrate that has an eluted chloride ion amount per unit mass according to the item (A) of 0.100 mg or less. Accordingly, the weather resistance of the RFID tag containing an IC chip having a noble metal plated type bump mounted on a paper substrate antenna can be significantly improved. The use of the paper substrate that has an eluted chloride ion amount per unit mass according to the item (A) of 0.060 mg or less is more effective, and the use of the paper substrate that has an eluted chloride ion amount of 0.050 mg or less is further preferred.
The operation of breaking the paper substrate specimen into small pieces each of 100 mm2 or less according to the item (A) is preferably performed by fingers wearing gloves for clean room operations or the like, for preventing contamination. In the case where the paper is broken into small pieces each of 100 mm2 or less by fingers wearing gloves of this type, the sizes of the small pieces may be generally in a range of from 25 to 100 mm2. When the sizes of the small pieces broken by fingers are in the range, the influence of the sizes of the paper pieces on fluctuation of the analysis values can be ignored.
The influence of a sulfate ion (SO42-) among the ion species supplied from the paper substrate has also been investigated. As a result, it has been found that the significant improvement effect of the weather resistance can be basically obtained when the eluted chloride ion amount per unit mass of the paper substrate is sufficiently suppressed, and the influence of a sulfate ion is small. From the standpoint of achieving higher reliability, the eluted sulfate ion amount per unit mass according to the following item (B) is preferably 0.800 mg or less.
(B) A specimen of the paper substrate having an area corresponding to an A4 size (210×297 mm) determined in ISO 216 is broken into small pieces each of 100 mm2 or less; the small pieces are entirely placed in a polypropylene vessel; 50 mL of water having an electric conductivity of 0.2 mS/m or less is added thereto to immerse the small pieces entirely into the water; after allowing to stand at 23° C.±2° C. for 1 hour, the water is filtered with a membrane filter to recover a filtrate; and the filtrate is analyzed by an ion chromatography method to obtain a sulfate ion (SO42-) concentration in the filtrate, from which a total sulfate ion amount eluted into 50 mL of the water is obtained, and is divided by the mass (g) of the specimen of the paper substrate to provide a value, which is designated as the eluted sulfate ion amount per unit mass.
In this case, when the sizes of the small pieces broken by fingers are in the aforementioned range, the influence of the sizes of the paper pieces on fluctuation of the analysis values can be ignored, as similar to the case of the item (A).
The substrates in the form of a sheet shown in Table 1 were prepared. The substrate No. 9 is PET (polyethylene terephthalate), and the others are paper. The paper substrates include products of plural manufacturers.
The paper substrates were measured for the eluted chloride ion amount per unit mass (1 g) of the paper substrate by a method according to the item (A). Specifically, for example, the chloride ion concentration in the filtrate by ion chromatography was 1.60 ppm for the substrate No. 1. Thus, 0.0016 mg of chloride ions are present in 1 mL of the filtrate. The total amount of chloride ions eluted in 50 mL of water added is 0.0016×50=0.080 mg. The mass of the A4 size paper substrate No. 1 used in the elution test is 3.018 g, and thus the eluted chloride ion amount per unit mass (1 g) is obtained as 0.080/3.018≈0.0265 mg.
The eluted sulfate ion amount per unit mass of the paper substrate was measured by a method according to the item (B).
The operation of breaking the paper substrate specimen into small pieces was performed by wearing powder-free nitrile gloves for clean room operation (Clean Nol Nitrile Gloves, produced by AS ONE Corporation).
The analysis by the ion chromatography method was performed by using IC 25, produced by Dionex, under the following conditions.
Column: Dionex IonPac AS12A
Column oven temperature: 35° C.
Flow rate of eluent: 1.5 mL/min
Suppressor current: 50 mA
As a conductive paint, a silver ink (Model PFI-700, produced PChem Associates, Inc.) containing 60% by mass of silver particles having a primary average particle diameter of 15 nm and a secondary average particle diameter of 340 nm, 3.0% by mass of a vinyl chloride copolymer latex, 2.0% by mass of a polyurethane thickener, and 2.5% by mass of propylene glycol was prepared. The silver ink was printed on the surfaces of the paper substrates with a sheet feed flexographic printer (produced by Nihon Denshi Seiki Co., Ltd.) and a flexographic plate under condition of an anilox volume of 8 cm3/m2, so as to draw an antenna having the circuit pattern shown in
As an IC chip, Monza4, produced by Impinj, Inc., was prepared. The IC chip has a “noble metal plated type” bump containing nickel having gold plating on the surface thereof. An anisotropic conductive paste (ACP) (TAP0604C, produced by Kyocera Chemical Corporation) containing Au/Ni coated polymer particles was thinly coated on the portion on the RFID tag substrate where the IC chip was to be bonded (i.e., the vicinity of the bump position). The IC chip was disposed on the ACP and then bonded under pressure for 10 seconds by applying a load of 1.0 N at 160° C. with a heat compression bonding machine (TTS300, produced by Muhlbauer AG), thereby mounting the IC chip on the RFID tag substrate, and thus an RFID tag was obtained.
The RFID tags thus produced above were measured for the communication distance (theoretical communication distance forward) in a frequency range of from 800 to 1,100 MHz (according to ISO/IEC 18000-6C) in a radio black box (MY1530, produced by Micronics Japan Co., Ltd.) with a communication distance measuring device (Tagformance, produced by Voyantic, Ltd.). Before the measurement, the environmental setting (setting with the reference tag attached to Tagformance) was performed under the condition.
Subsequently, the RFID tags were subjected to an accelerated weather resistance test by retaining in a thermo-hygrostat chamber under condition of 85° C. and 85% RH for 168 hours, and then measured for the communication distance in the same manner as above.
The communication distance before the accelerated weather resistance test is referred to as an “initial communication distance”, and the communication distance after the accelerated weather resistance test is referred to as a “communication distance after weather resistance test”. Herein, the measured values at 920 MHz were used as the “initial communication distance” and the “communication distance after weather resistance test” of the RFID tags, and the communication distance retention ratio before and after the accelerated weather resistance test was obtained by substituting the values into the following expression (1).
(communication distance retention ratio (%))=((communication distance after weather resistance test (m))/(initial communication distance (m)))×100 (1)
The communication distance retention ratio that is 80% or more can be evaluated to provide practically excellent weather resistance as an RFID tag using a paper substrate. Accordingly, one having a communication distance retention ratio of 80% or more was determined as ◯ (good weather resistance), and the others were determined as x (poor weather resistance).
The results are shown in Table 2.
It is understood from Table 2 and
Number | Date | Country | Kind |
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2014-141382 | Jul 2014 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2015/067538 | 6/18/2015 | WO | 00 |