RFID tag and manufacturing method thereof

An RFID tag includes an inlay which has a base, an antenna formed on the base, and an IC chip. The IC chip is enclosed in a surface mount package and soldered to the antenna and carries out radio communication through the antenna. The RFID tag further includes underfill that fills a gap between the base and the surface mount package, and a sheath protecting material enclosing the entire inlay.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an RFID (Radio Frequency IDentification) tag through which information is exchanged with external equipment in a non-contact manner. The “RFID tag” is called “radio IC tag” in some cases.

2. Description of the Related Art

In recent years, there are proposed various RFID tags through which information is exchanged with external equipment such as a reader/writer in a non-contact manner (see Japanese Patent Application Publication Nos. 2000-311176, 2000-200332 and 2001-351082, for example).

FIG. 1is a schematic sectional view showing a structure of an ordinary RFID tag.

According to the RFID tag10shown inFIG. 1, an antenna pattern12is formed on a substrate11made of PET, a circuit which carries out radio communication through the antenna pattern12is incorporated in an IC chip13, the IC chip13in its bare state is soldered to the antenna pattern12with solder14, a filler15is poured into a space under a lower surface of the IC chip13and is solidified, thereby forming an RFID tag inlay to which the IC chip13is strongly secured, an adhesion layer16is placed on the RFID tag inlay and is sealed with a label17.

In the case of the RFID tag10having the structure shown inFIG. 1, however, the following problems are found:

since thermal expansion coefficients of silicon which is a base body of the IC chip13and the flexible substrate11made of PET are large, long term bonding reliability is poor; and

water or moisture is prone to enter from an end edge of the adhesion layer16between the label17and the inlay, and the IC chip13cannot be applied to a high moisture environment, outside or the like.

Japanese Patent Application Publication No. 2005-275802 discloses an idea to enhance the mounting reliability of the IC chip, but this idea relates to reliability when the chip is mounted, and it is unclear as to the long term stability.

In addition, in the Japanese Patent Application Publication No. 2005-275802, the IC chip is bare, and waterproofing is not provided.

Further, Japanese Patent Application Publication No. 2005-354661 discloses an idea to avoid influence of radio communication on an electronic circuit substrate in a device such as a cellular phone having a radio antenna section, a non-contact communication device which carries out radio communication between the non-contact communication device and external equipment through the radio antenna section, and an electronic circuit substrate, but there is no relation to the long term reliability in respect to influence of temperature, moisture and the like.

SUMMARY OF THE INVENTION

In view of the above circumstances, the present invention to provides an RFID tag having excellent long term reliability.

An RFID tag according to the invention includes:

an inlay which has a base, an antenna formed on the base, an IC chip which is enclosed in a surface mount package and soldered to the antenna and which carries out radio communication through the antenna, and underfill which fills a gap between the base and the surface mount package; and

a sheath protecting material enclosing the entire inlay.

According to the RFID tag of the present invention, the IC chip is first enclosed in the surface mount package, the surface mount package is mounted on the base and connected to the antenna, the surface mount package is strongly secured by underfill, and the entire inlay thus manufactured is enclosed in the sheath protecting material. Therefore, there is no margin of entrance of moisture, and an RFID tag having excellent long term stability is realized.

In the RFID tag of the present invention, it is preferable that where

a thermal expansion coefficient of the surface mount package is defined as α1,

a thermal expansion coefficient of the underfill is defined as α2,

a thermal expansion coefficient of the base is defined as α3, and

a thermal expansion coefficient of the sheath protecting material is defined as α4,

relations α1<α2 and α4<α3 are satisfied.

If underfill having an intermediate thermal expansion coefficient α2 between a thermal expansion coefficient α1 of the surface mount package and a thermal expansion coefficient α3 of the base is employed as the underfill of the present invention, a soldering bond between the antenna on the base and the surface mount package is reliably maintained for a long term. Further, by employing a material having the thermal expansion coefficient α4 close to the average thermal expansion coefficient, i.e., a material having the relation of α1<α4<α3 for the sheath protecting material, the internal structure is stably maintained.

In the RFID tag of the present invention, it is preferable that the surface mount package is rectangular in shape as viewed from above, the surface mount package is provided with leads projecting from sides of the rectangular shape, the antenna and the IC chip enclosed in the surface mount package are connected with each other through any of the leads except those which are adjacent to corners at which two sides of the rectangular shape are in contact.

The surface mount package is usually rectangular in shape, and the corner portions at which two sides of the rectangle shape come into contact are most varied by the thermal expansion (or thermal contraction). Hence, the IC chip enclosed in the surface mount package and the antenna on the base are connected to each other through the leads except those adjacent to the corner portions. With this, the influence of thermal expansion and thermal contraction of the surface mount package is small, and the long term reliability is further enhanced.

In the RFID tag of the present invention, it is preferable that the surface mount package is rectangular in shape as viewed from above, connection terminals are arranged on a bottom surface of the surface mount package, the antenna and the IC chip enclosed in the surface mount package are connected with each other through any of the connection terminals except those which are adjacent to corners at which two sides of the rectangular shape are in contact.

For the same reason as above, the IC chip enclosed in the surface mount package and the antenna on the base are connected to each other through the connection terminals except those which are adjacent to the corner portions. With this, resistance to the influence of thermal expansion and thermal contraction of the surface mount package is achieved, and the long term reliability is further enhanced.

Further, the present invention provides a method of manufacturing an RFID tag having an inlay which includes a base, an antenna formed on the base, an IC chip which is enclosed in a surface mount package and soldered to the antenna and which carries out radio communication through the antenna, and underfill which fills a gap between the base and the surface mount package, and a sheath protecting material enclosing the entire inlay, the method including the steps of:

printing a soldering paste on a portion of the base on which the antenna is formed, the portion being where the surface mount package is to be soldered;

aligning and mounting the surface mount package on the base;

connecting the surface mount package to the antenna on the base by soldering reflow;

injecting underfill into a gap between the base and the surface mount package;

hardening the underfill, thereby completing the inlay;

disposing a resin lower member which has a recess into which the inlay is fitted and which corresponds to a lower portion of the sheath protecting material, together with the inlay fitted into the recess of the lower member, into a mold having a gap corresponding to an outer shape of the sheath protecting material; and

molding the sheath protecting material by flowing resin corresponding to an upper portion of the sheath protecting material into the gap in the mold, thereby molding the upper portion and fusing the upper portion and the lower member together.

The RFID tag of the present invention is manufactured through the above manufacturing method.

According to the present invention, an RFID tag having excellent long term stability is realized.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described.

FIG. 2is a diagram showing a structure of an inlay constituting a first embodiment of an RFID tag of the present invention.

The inlay20shown inFIG. 2includes a base21formed of a glass epoxy substrate, an antenna22formed by copper etching on the base21, a surface mount package23which encloses an IC chip carrying out radio communication through the antenna22and which has a lead connected to the antenna22with solder, and an underfill24filling a gap between the base21and the surface mount package23.

FIG. 3is a diagram showing the RFID tag of the embodiment, andFIG. 4is a sectional view thereof.

The entire inlay20having a structure shown inFIG. 2is enclosed in a sheath protecting material31by molding, thermocompression bonding or the like. The sheath protecting material31can be made of resin such as PTFE, PEEK and PPS.

According to the RFID tag30of the embodiment, the IC chip is enclosed in the surface mount package23, the surface mount package23is strongly secured with the underfill24, and the entire inlay20is enclosed in the sheath protecting material31. Therefore, the RFID tag has long term reliability.

Materials are selected such that relations α1<α2 and α4<α3 are satisfied where:

a thermal expansion coefficient of the surface mount package23is defined as α1,

a thermal expansion coefficient of the underfill24is defined as α2,

a thermal expansion coefficient of the base21is defined as α3, and

a thermal expansion coefficient of the sheath protecting material31is defined as α4.

Thus, the underfill24reduces stress caused by thermal expansion and thermal contraction between the surface mount package23and the base21. The sheath protecting material31also has intermediate thermal expansion coefficient among members constituting the inlay20, and this further enhances the long term stability of the RFID tag30.

FIG. 5is a plan view showing a connecting portion between a surface mount package and an antenna according to a second embodiment that is different from the first embodiment shown inFIGS. 2 to 4. The second embodiment is different from the first embodiment only in the connecting portion of the surface mount package and the antenna.

The surface mount package23A is rectangular in shape as viewed from above, and leads231for soldering connection project from both left and right sides of the rectangular shape. Of the leads231, certain leads except leads231awhich are adjacent to corner portions at which two sides of the rectangular shape are in contact, here, the leads231bwhich are as close to the center as possible are connected to the antenna22. The other leads that are not connected to the antenna22are soldered to connection pads221formed on the base21. The connection pads221are made of the same material as that of the antenna22. When the surface mount package23is thermally expanded or contracted due to variation in temperature, influence on a lead close to the center of the side is smaller than that of a lead close to the end (corner portion). Therefore, by using the leads close to the central portion of the side for connecting with the antenna22, the long term stability is enhanced.

FIG. 6is a sectional view of the connecting portion between the surface mount package and the antenna shown inFIG. 5.

A lead231projecting from the surface mount package23A is connected to the antenna22formed on the base21or to the connection pad221(seeFIG. 5) with solder25, and the gap between the surface mount package23A and the base21is filled with the underfill24. The underfill24is made of material having an intermediate thermal expansion coefficient α2 between a thermal expansion coefficient α1 of the surface mount package23A and a thermal expansion coefficient α3 of the base21.

FIG. 7is a plan view showing a connecting portion between a surface mount package and an antenna of a third embodiment.

The connecting portion of an antenna21with respect to the surface mount package23A is adjacent only to one side of the surface mount package23A.

In this case also, leads which are adjacent to corner portions of the surface mount package23A are not used for connecting with the antenna21, and leads as close to the center of the side as possible are used.

The embodiment shown inFIG. 7is the same as the second embodiment shown inFIGS. 5 and 6except the disposition of the surface mount package23and the antenna as described above.

FIG. 8is a plan view showing a connecting portion between a surface mount package and an antenna in an embodiment in which connection terminals are arranged on a bottom surface of the surface mount package. The surface mount package is rectangular in shape as viewed from above.FIG. 9is a sectional view of the part shown inFIG. 8.

A bottom surface of the surface mount package23B is formed with a large number of connection terminals232for soldering connection. Connection pads222having the same arrangement as that of the connection terminals232are formed on the base in addition to the antenna22. Each connection pad222and corresponding connection terminal232are connected with solder25.

To reduce the influence caused by the thermal expansion and thermal contraction of the surface mount package23B as much as possible, connection terminals232aof the surface mount package23B which are adjacent to corner portions at which two sides are in contact are not used for connection with the antenna22, and connection terminals232bclose to a central portion of the bottom surface of the surface mount package23B are used for connection with the antenna22. The connection pads on the base21connected to the connection terminals232bwith solder are connected to the base21through the antenna22and lead wires223formed in the same manner as the connection pad222.

In such connection, the gap between the surface mount package23B and the base21is also filled with the underfill24.

FIG. 10is a diagram showing a surface mount package having a bottom surface on which a large number of connection terminals are arranged like the surface mount package shown inFIG. 8. The difference between the embodiment shown inFIG. 10and the embodiment shown inFIGS. 8 and 9will be explained. In this embodiment, connection terminals232bclose to a central portion of one side of the surface mount package23B among the connection terminals disposed along the one side of the surface mount package23B, except connection terminals232awhich are adjacent to corner portions, are used for connection with the antenna22.

In this case, the influence of the thermal expansion or thermal contraction caused by temperature variation of the surface mount package23B is also reduced as compared with a case in which a connection terminal which is adjacent to a corner portion is used for connection with the antenna.

FIG. 11is a diagram of steps showing one example of a manufacturing method of the RFID tag as an embodiment of the present invention.

Here, a soldering paste251is printed on a portion of the base21formed with the antenna22to which a surface mount package is to be connected with solder (step A).

Next, the surface mount package23incorporating an IC chip is aligned and mounted onto the base21by a bonding tool51(step B). The soldering paste251is heated and solder connection is carried out by soldering reflow (step C). Next, liquid underfill241is injected into underneath the surface mount package23, i.e., a gap between the base21and the surface mount package23by a dispenser52(step D). The injected underfill241is hardened by heating the same to form an underfill24, thereby completing the inlay20(step E).

A lower member311provided with a recess311ahaving a size corresponding to the base21is previously molded (step F), and the completed inlay20is set in the recess (step G).

Next, the lower member311with the inlay20set therein is set in a mold53having a cavity53acorresponding to the outer shape of a sheath protecting material31(step H). Resin corresponding to the upper portion of the sheath protecting material31is injected into the cavity53aof the mold53, and the upper portion of the sheath protecting material31is molded. The upper portion and the lower member311are integrally formed by heat seal. Then the entire inlay20is enclosed in the sheath protecting material31, and the RFID tag30is completed.

An RFID tag of the present invention is manufactured through the above steps, for example.