HOLE-DRILLED SINTERED FERRITE SHEET, ANTENNA ISOLATOR,

AND ANTENNA MODULE

FIELD OF THE INVENTION

This invention relates to a ferrite sheet, an antenna isolator, and an antenna module. More specifically, the invention relates to a hole-drilled ferrite sheet, a sandwich-structured antenna isolator wherein the hole-drilled ferrite sheet is a middle layer of this isolator, and an antenna module comprising the antenna isolator.

BACKGROUND OF THE INVENTION

Near Field Communication (NFC) technology has recently become more popular for use in cellular phones in the background of the rapid growth of the Radio Frequency Identification (RFID) market. This technology opens up many new possibilities for cellular phones, for example, enabling the cellular phones to have the function of electronic keys, an ID card and an electronic wallet, and also enabling the exchange of phone numbers with other people to be done in a quick manner via wireless channels.

NFC is based on a 13.56 MHz RFID system which uses a magnetic field as carrier waves. However, the designed communication range may not be attained when a loop antenna is close to a metal case, shielded case, ground surface of a circuit board, or sheet surfaces such as a battery casing. This attenuation of carrier waves occurs because eddy current induced on the metal surface creates a magnetic field in the reverse direction to the carrier wave. Consequently, materials, such as Ni-Zn ferrites (with the formula: Ni _a Zn ₍ i. _a) Fe ₂ 0 ₄ ), with high permeability that can shield the carrier wave from the metal surface are desired.

Japanese Patent JP2005015293 discloses a ferrite sheet with a protective film on its top surface and an adhesive tape on its bottom surface. The ferrite sheet has continuous U- shaped or V-shaped grooves therein and these U-shaped or V-shaped grooves are intersected so that division of the sheet along the grooves is possible. And the sheet can be attached on a flat or curved surface. Japanese Patent JP2009182062 describes a manufacturing method of breaking a complex ferrite sheet. The ferrite sheet comprises a covering layer and a double-sided adhesion layer. The ferrite sheet has thickness of about 300 micrometers or less and a plurality of grooves is formed on one surface of the sheet.

While the grooved slots can increase the ease of die-cutting, they can decrease the permeability of the ferrite sheet. Thus, it is desired in the art to obtain a ferrite sheet having improved permeability and being die-cut easily.

SUMMARY OF THE INVENTION

Thus, it is an objective of the invention to provide a ferrite sheet having improved permeability, the ability to be die-cut easily, and optionally, the capability to provide a regular breaking pattern when there is an applied external force.

At least part of the above objective can be solved by a sintered ferrite sheet having an array of drilled, small holes in the ferrite sheet (hereinafter sometimes referred to as "hole-drilled ferrite sheet").

In one aspect, the invention provides a sintered ferrite sheet having a thickness of about 0.01 mm to about 0.5 mm, wherein the sheet has a plurality of holes therein.

In another aspect, the invention provides an antenna isolator including the sintered ferrite sheet disclosed above, a protective film provided on a first side of the sintered ferrite sheet, an adhesive layer provided on a second side of the sintered ferrite sheet, and alternatively a liner provided on the adhesive layer.

In a further aspect, the invention provides an antenna module suitable for use in a radio communication medium or a radio communication medium processing device. The antenna module includes the antenna isolator disclosed above, a conductive loop antenna provided on a first side of the antenna isolator, and a conductive layer provided on a second side of the antenna isolator.

In the present invention, the sintered ferrite sheet having a plurality of holes therein, particularly when the plurality of holes are arranged in a pattern, can not only maintain the high permeability of the antenna isolator, but can also make the brittle ferrite sheet easier to die-cut. The patterns of these holes can also act as a guide for a regular breaking pattern when there is an external force applied to the sheet. The sintered ferrite sheet is applicable in Near Field Communication (NFC) by effectively providing a flux path between an antenna circuit and a metal case. This specialty can reduce the eddy current loss when the NFC antennas approach or attach on the metal case.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the present invention, and not to be limited thereby, the following drawings are included herein wherein:

Fig. 1(a) is a schematic diagram of a hole-drilled ferrite sheet of the present invention;

Fig. 1(b) is a schematic diagram of a perforated grooved ferrite sheet as a comparative example;

Fig. 2(a) is a schematic diagram showing the magnetic resistance of the hole-drilled ferrite sheet of Fig. 1(a);

Fig. 2(b) is a schematic diagram showing the corresponding simulant magnetic resistance circuit of the hole-drilled ferrite sheet

Fig. 2(c) is a sectional schematic diagram of the hole-drilled ferrite sheet, in which the holes are partly perforated along a thickness of the sheet;

Fig. 2(d) is a schematic diagram showing the simulant magnetic resistance circuit of the hole-drilled ferrite sheet shown in Fig. 2(c);

Fig. 3(a) is a schematic diagram showing the magnetic resistance in the perforated grooved ferrite sheet of Fig. 1(b);

Fig. 3(b) is a schematic diagram showing the simulant magnetic resistance circuit of the perforated grooved ferrite sheet of Fig. 3(a);

Fig. 3(c) is a schematic diagram of the perforated grooved ferrite sheet, in which the grooves are partly perforated along a thickness of the sheet;

Fig. 3(d) is a schematic diagram showing the simulant magnetic resistance circuit of the perforated grooved ferrite sheet shown in Fig. 3(c);

Fig. 4(a) is a picture showing the die-cutting performance of samples of the hole- drilled ferrite sheet of the present invention;

Fig. 4(b) is an enlarged picture showing the die-cutting performance of one sample shown in Fig. 4(a); Fig. 4(c) is a picture showing the die-cutting performance of samples of the ferrite sheet without holes therein as comparative examples;

Fig. 4(d) is an enlarged picture showing the die-cutting performance of one sample shown in Fig. 4(c); and

Fig. 5(a) is a schematic diagram showing the effective permeability of the completely perforated hole-drilled ferrite sheet and the completely perforated grooved ferrite sheet as a function of different areal densities of the holes and the grooves;

Fig. 5(b) is a schematic diagram showing the effective permeability of an 80% perforated hole-drilled ferrite sheet and an 80% perforated grooved ferrite sheet as a function of different areal densities of the holes and the grooves;

Fig. 5(c) is a schematic diagram showing the effective permeability of a 60% perforated hole-drilled ferrite sheet and a 60% perforated grooved ferrite sheet as a function of different areal densities of the holes and the grooves;

Fig. 5(d) is a schematic diagram showing the effective permeability of a 40% perforated hole-drilled ferrite sheet and a 40% perforated grooved ferrite sheet as a function of different areal densities of the holes and the grooves;

Fig. 6 is a schematic diagram comparing the effective permeability of the completely perforated hole-drilled ferrite sheet with the 40% perforated, 60% perforated, 80% perforated and completely perforated grooved ferrite sheet as a function of different areal densities of the holes and the grooves;

Fig. 7 is a schematic diagram of an antenna isolator of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The term "about" is used throughout the specification and means an approximation to an exact value under a reasonable tolerance in the art.

Exemplary embodiments of a hole-drilled sintered ferrite sheet, an antenna isolator and an antenna module of the present invention will be described respectively hereinafter.

A. The hole-drilled sintered ferrite sheet

In one aspect, this invention discloses a sintered ferrite sheet having a thickness of about 0.01 mm to about 0.5 mm, wherein the sheet has a plurality of holes therein. In one embodiment, as shown in Fig. l (a) and Fig.7, a sintered ferrite sheet 10 is provided and a plurality of holes 13 is positioned in the sheet 10. These holes 13 are drilled through one surface 11 (referred to as "top surface") of the sheet. Alternatively, these holes may also be drilled through another surface 12 opposite the top surface 11 (referred to as "bottom surface"). The sintered ferrite sheet 10 may be fully perforated through the top surface 11 and the bottom surface 12, i.e., through the thickness of the sheet.

Commonly, a ferrite sheet having permeability of larger than about 80 is preferable when they are used to make an antenna isolator capable of being used in NFC. Easy die- cutting of the ferrite sheets is desirable so that the sheets can be divided into particular shapes and sizes to meet various consequent processes. In the art, it has been difficult to attain these two needs at the same time. Fortunately, the sintered ferrite sheet proposed in the present invention can meet the two needs by providing holes with certain parameters as described below.

The permeability of the hole-drilled ferrite sheets is largely determined by the materials of the sheet, the areal density of the holes and the depth of the holes. While, the die-cutting performance of the sheet is largely determined by each hole size and the distance of the two neighboring holes. The shape of the hole will affect the formation of the holes in the sheet. Based on these factors, more details about the holes are given below to optimize the hole-drilled ferrite sheet with respect to the permeability and die-cutting performance.

In an embodiment of the invention, the areal density of the holes is from about 0.01% to about 60%). If all of the holes on the sheet are fully perforated through the thickness of the sheet, the range of the areal density of the holes is particularly about 0.01%> to about 15%), and more particularly about 0.01%> to about 6%>. As used in the specification, the areal density of the holes means a ratio of the area of all holes in the sheet to the area of the ferrite sheet; the term "area" means the sectional area of the hole or the sheet in the direction parallel to the top surface of the sheet. The area of the top surface of the sheet can be deemed as to be the area of the ferrite sheet.

In one embodiment of the invention, a sectional shape of each of the holes in the direction parallel to the top surface of the sheet can be selected from the group including, but not limited to: round, ring, diamond, triangle, cross, and rectangular. In one embodiment, a round cross-section may be preferable as holes having such shape are more easily e drilled in the sheet. In one embodiment of the invention, at least a portion of the holes are positioned in a beeline or a curved pattern so that the sintered ferrite sheet can be divided along the beeline or the curve when an external force is exerted on the sheet.

In one embodiment of the invention, the span of the two neighboring holes, that is, the center to center distance between two neighboring holes, is between about 0.5 mm and about 4.0 mm, for example, the span is about 2.0 mm. If the span is too small, it is not easy to make holes based on existing drill techniques. If the span is too large, there may be die- cutting issues.

In one embodiment of the invention, the sectional area of each hole can be from about 100 μπι ² to about 9.6 mm ² , particularly about 100 μιη ² to about 3.7 mm ² , and more particularly about 100 μιη ² to about 0.9 mm ² . In particular, the area is less than about 0.01 mm ² .

In one embodiment of the invention, the depth of each of the holes in the sheet is more than about 50% of the thickness of the sintered ferrite sheet. Alternatively, the depth of each of the holes in the sheet can be equal to the thickness of the sintered ferrite sheet. In one embodiment, all of the holes have the same depth. In another embodiment, each hole has a different depth from that of other holes, or a portion of the holes have the same depth. If the depth of the holes is equal to the thickness of the sintered ferrite sheet, such holes are referred to as "completely perforated holes" or defined by the term "completely perforated" in this specification. Such holes may also be defined by the term "perforated", unless otherwise stated.

In some embodiments of the invention, the holes can be arranged in an array. In one embodiment of the invention, the array can be a rectangle array or a diamond array. In another embodiment of the invention, at least a portion of the holes in the sheet are positioned in a beeline pattern so that division of the ferrite sheet becomes easier along this line. If there is a requirement that the division of the ferrite sheet is to be along a curve, at least a portion of the holes can be also positioned in a curved pattern to meet this

requirement.

In one embodiment of the invention, the sintered ferrite sheet can be formed of oxides of Fe which is doped by at least one metal element selected from the group including, but not limited to: Ni, Zn, Cu, Co, Ni Li, Mg and Mn. For example, the ferrite can be selected from the group including, but not limited to Ni-Zn-Cu ferrite, Mn-Zn-Cu ferrite, and Mn-Mg-Cu ferrite.

All the above parameters are based on minimizing the magnetic flux leakage while keeping the ease of die-cutting of the sintered ferrite sheet.

Below, more details are given on how to make a hole-drilled sintered ferrite sheet of the present invention. It is well known in the art on how to make a sintered ferrite sheet. Therefore, the steps described as follows are exemplary and should not limit the scope of the present invention.

(1) Ferrite Powder Composition

The main composition of the ferrite powder can be (Ni ₀ . ₂ Zno. ₅ Cuo. ₁₅ )(Fe ₂ 0 ₄ )o.97.

Furthermore, some additives including Bi ₂ 0 ₃ 0.3 wt% of the (Ni ₀ . ₂ Zno. ₅ Cuo. ₁₅ )(Fe ₂ 0 ₄ )o.97, Co ₃ 0 ₄ 1 wt% of the (Nio. ₂ Zno. ₅ Cuo. i5)(Fe ₂ 0 ₄ )o.97, Cr ₂ 0 ₃ 0.3 wt% of the

(Ni ₀ . ₂ Zno. ₅ Cuo. ₁₅ )(Fe ₂ 0 ₄ )o.97 can be added into the ( i ₀ . ₂ Zno. ₅ Cuo. ₁₅ )(Fe ₂ 0 ₄ )o.97 to optimize the magnetic performance. These additives are optional depending on different needs in practice.

The mean particle size distribution of the ferrite powder is 0.53 μιη to 2.38 μιη based on the DIO and D90.

(2) Ferrite slurry composition

To make the ferrite slurry, the binder resin, solvent and plasticizer are added as shown in Table 1.

Table 1

- Binder resin: Poly vinyl butyral (CAS Number: 63148-65-2, for example, bought from Supplier : KURARAY CO., LTD )

- Plasticizer: Bis(2-ethylhexyl) Phthalate (CAS Number: 117-81-7, for example, bought from Supplier: LG CHEMICAL CO., LTD) - Solvent: Toluene (CAS Number: 108-88-3), Ethanol (CAS Number: 64-17-5, for example, bought from Supplier: DAE- JUNG CHAMICAL CO., LTD)

- Dispersion agent: Alkyl ammonium salt of a poly-carboxylic acid (for example, bought from Supplier: HUNG SAN HWA SUNG CO., LTD)

(3) Preparing ferrite slurry

Mixing is conducted using a two step process with the conditions described in Table 2, below. Once the first mixing process is complete, the components of Batch B are added into Batch A and the second mixing process is conducted. The mixing is conducted via a ball milling mixer having a 6 liter volume, and the steel ball diameter φ(ρΐιί) of the mixer is about 10mm.

Table 2

(4) Tape casting to make ferrite green sheet

The ferrite slurry is applied to a silicon coated PET film by using tape caster and dried to obtain a green sheet having a thickness of 100 μπι. The rate of applying the slurry is about 2m/min, the drying temperature is about 60°C to 80°C, and the drying duration is about 5 minutes.

(5) Ferrite sintering

After detaching the green sheet from the silicone coated PET film, the green sheet is sintered in a furnace for binder burnout and densification of the ferrite particles to obtain the ferrite sheet. The sintering temperature is about 900 ^° C for 5 hours in the air condition.

The thickness of the sintered ferrite sheet can be about 0.1 mm or other value. If the sintered ferrite sheet is to be used in a radio communication medium or a radio

communication medium processing device, commonly its thickness is between 0.01 mm to 0.5 mm.

(6) Hole-drilling A laser can be used to drill an array of holes in the sintered ferrite sheet. For example, the Firestar t-100 laser from Synrad Inc. is an option and its parameters can be set as follows: Frequency, 10 kHz; Energy Level, 100%; Scanner speed, 400 mm/s; Defocus, +/- 1mm; Trips, 4.

Alternatively, the holes can be punched in the green ferrite sheet, which is an intermediate product for producing the sintered ferrite sheet, or be drilled in the sintered ferrite sheet by other tools, or may be produced by the other methods that are suitable for making holes in the ferrite or green ferrite sheet.

As shown in Fig. 1(a), for example, the sectional shape of the hole is round. The span L ₀ of each two neighboring holes, that is the center to center distance of each two neighboring holes, is about 2.0 mm and the Li (i.e. diameter of round hole) of each hole is about 115 μιη. In practice, the specific value of the span L ₀ and the diameter Li are directly depended on the areal density of the holes and the sectional area of each hole, and finally depended on the required permeability of the sintered ferrite sheet.

(7). Permeability calculation for the hole-drilled ferrite sheet of the present invention and for the grooved ferrite sheet as comparative examples.

According to the designed patterns in Fig. 1(a), a mathematic model was developed (shown in Figs. 2(a)-(d)) according to magnetic theory, where L ₀ is the span of the two neighboring holes, and Li is the width of the hole. Ri, R ₂ and R ₃ represent the magnetic resistances illustrated in Fig. 2(a), which shows the holes on the sheet are completely perforated along the thickness of the sheet. Refr- oiei represents the effective magnetic resistance of the part of ferrite sheet with holes.

Then, the following equations are established:

R (1)

Where d is the thickness of the ferrite sheet.

Because the Reff- oiei is parallel and series connected by Ri, R ₂ and R ₃ , the equivalent magnetic circuit is shown in Fig. 2(b), the Reff-hoiei can be written as:

Then, μ _ε ίτ-ΐιοΐεΐ can be written as: Take as the areal density of the drilled holes, then

Shale ^{n ' L} l ² (L L ₀ ) ²

hole

S total n - (L _{1 +} L ₀ ) ² (1 + 4 /Jo )

(7)

Wherein, S oie represents the sum of the sectional area of all holes on the sheet, S _to tai represents the sectional area of the sheet. The Heff-hoiei is affected by the areal density η as: ueff-hoM = , M> I ^' A · ¾T / + u - ( V1 ¹ - Vn 'th ^~ ole )/

> ole ) + Ml ^' ole (g)

If the hole is not compeletly perforated as shown in Fig. 2(c), there is a ferrite plate parallel connected with the Reff-hoiei · The equivalent magnetic resistance circuit is shown in Fig. 2(d). The effective permeability μ _ε ίτ-ΐιοΐε of the incompletely perforated hole-drilled ferrite sheet a hole depth ratio, K ₀ i _e , can be written as:

Meff-hole -

Using the same theory and sketch of the perforated grooved ferrite shown in Fig. 1(b), the corresponding model is built (shown in Fig. 3 (a) ~ (d)) for the perforated grooved ferrite. L ₀ represents the span (center to center distance) of the two neighboring grooves, and Li is the width of the groove. The corresponding magnetic resistance of Ri, R ₂ and R ₃ (shown in Fig. 3(a)) can be written as:

n ^■ (Lo ^¬

R ₃

(12)

The of Reff.groovei can be calculated as the series and parallel connected Ri, R ₂ and R ₃ (shown in Fig. 3(b)):

Mejf -groove!

(1 3) The areal density of the grooves can be defined as:

_ ^groove _ (A ⁺ A)) — L _Q _ (J _t / L _Q + 1) — 1

^{8mOVe ~} S _total - (L _{l +} L ₀ f - (l ₊ L L ₀ f (14)

Wherein, Sgroove represents the sum of the sectional area of all grooves on the sheet, Stotai represents the sectional area of the sheet.

Then, the ^ff-groovei can be written as:

Considering the incompletely perforated grooved ferrite sheet as shown in Fig. 3(c), there is a ferrite plate parallel connected with the Reff.groovei . The equivalent magnetic resistance circuit is shown in Fig. 3(d). Then, the effective permeability of the ferrite sheet with certain groove depth ratio, Kgroove, can be written as:

Meff-groovel ^~

Thus, the permeability of a hole-drilled ferrite sheet can be calaulated based on the formulation (9), and the permeability of a grooved ferrite sheet can be calaulated based on the formulation (16), wherein, μι can be taken as 130 for Ni-Zn ferrite sheet which is a typical value for Ni-Zn ferrite at 13.56 MHz, μ ₀ is 1 which is the permeability of air. and rigroove can be fixed by calculating a ratio of the total sectional areal of the holes/ grooves to the sectional areal of the sheet, respectively, rjhoie and can be typically from 0.01% to 30% . K oie and ι _οονε can be also fixed by calculating a ratio of the depth of the hole to the thickness of the sheet, and typically K ₀ i _e and K _groove can be from 60% to 100%.

Based on equations (9) and (16), the permeability of the hole-drilled ferrite sheet and the permeability of the perforated grooved ferrite sheet can be calculated accordingly and a further comparison of the permeability of the hole-drilled ferrite sheet and the perforated grooved ferrite sheet can be made to see their effectiveness.

As shown by the detailed examples below, the plurality of holes on the sintered ferrite sheet can not only maintain higher permeability than grooved patterns, but can also make die-cutting of the sheet easier.

B. The antenna isolator

In another aspect, the invention discloses an antenna isolator, comprising a sintered ferrite sheet as proposed in the present invention, a protective film provided on a first side of the sintered ferrite sheet, and an adhesive layer provided on a second side of the sintered ferrite sheet. In one embodiment, a liner is provided on the adhesive layer to protect the adhesive layer from dirt and debris.

In an embodiment of the invention, the protective film can be a polymeric film. In one embodiment of the invention, the protective film can be selected from the group including, but not limited to: a polyethylene film, a polypropylene film, a polyvinyl chloride film, and a polyethylene terephthalate film. Moreover, in one embodiment of the invention, the protective film can be hard coated such that the hard coated protective film has a hardness of above about 2 H (pencil hardness). The hard coated protective film can be used for protecting the ferrite sheet against scratching and debris.

In one embodiment of the invention, the protective film has a thickness of about 0.002mm to about 0.1mm. In one embodiment of the invention, the adhesive layer is an acrylic or rubber based adhesive layer. In an embodiment in which the adhesive layer is an acrylic based adhesive layer, the acrylic based adhesive layer is an acrylic pressure-sensitive adhesive layer.

Moreover, in one embodiment of the invention, the acrylic pressure-sensitive adhesive layer can be a structured acrylic pressure-sensitive adhesive layer. This structured acrylic pressure-sensitive adhesive layer can be utilized to overcome some of the difficulties associated with the application and repositioning of adhesive articles, such as the trapped air bubbles that are created when the acrylic pressure- sensitive adhesive layer is laminated with the ferrite sheet.

In one embodiment of the invention, the adhesive layer has a peel strength of about 0.05 to about 2N/mm and particularly of about 0.3 to about 1.2 N/mm.

In one embodiment of the invention, the liner can be a PET film plus a silicone release coating.

For example, as shown in Fig. 7, an antenna isolator 100 is provided and comprises the sintered ferrite sheet 10 as described above, a protective film 20, an adhesive layer 30 and a liner 40. The sintered ferrite sheet 10 has a first side 11 (i.e. the top surface) and a second side 12 (i.e. the bottom surface) opposite the first side 11. A plurality of holes 13 is drilled through the first side 11 and the second side 12, that is, the depth of the holes is equal to the thickness of the sheet. The protective film 20 is provided on the first side 11 of the sintered ferrite sheet. The adhesive layer 30 is positioned on the second side 12 of the sintered ferrite sheet 10. The liner 40 is attached on the adhesive layer 30.

The protective film 30 is a black polyethylene film having thickness of about 15 μπι. The adhesive layer 30 includes an acrylic pressure-sensitive adhesive and had a thickness of about 10 μπι. The adhesive layer is used to attach the antenna isolator 100 to a surface. The adhesive layer 30 is usually selected to have an 180° peel strength of the antenna isolator of more than 0.2 N/mm.

As the sintered ferrite sheet having the holes proposed in the present invention has permeability of more than 80 at 13.56 MHz, accordingly, the permeability of the antenna isolator is more than 80 at 13.56 MHz and such isolator can meet the basic requirement of the art. The antenna isolator of the invention can keep the high permeability due to the hole- drilled ferrite sheet while also offering good die-cutting performance when being divided into small pieces due to the plurality of holes on the sintered ferrite sheet.

C. The antenna module

In a further aspect, the invention provides an antenna module, which can be used in a radio communication medium or a radio communication medium processing device, including an antenna isolator as proposed in the present invention, a conductive loop antenna provided on a first side of the antenna isolator, and a conductive layer provided on a second side of the antenna isolator.

The conductive loop antenna can be a copper or aluminum etched antenna with PET substrate. Its shape can be, for example, a ring shape, a rectangular shape or a square shape with the resonant frequency of 13.56 MHz. The size can be from about 80 cm ² to about 0.1 cm ² with a thickness of about 35 μπι to about 10 μπι. The resistance of the conductive loop antenna is below about 5 Ω.

The conductive layer can be an aluminum or copper layer with the maximum thickness of about 80 μπι, and the surface resistance thereof is below about 5 Ω.

EXAMPLES:

The following examples and comparative examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof.

A Fabricated examples

Example 1 Permeability comparison experiment

In order to show the advantage of the hole-drilled ferrite sheet proposed in the present invention in relation to permeability, a comparative experiment was performed.

A completely perforated hole-drilled ferrite sheet was prepared according to the steps given above. The main composition of the ferrite sheet was (Nio.2Zn ₀ .5Cuo.i5)(Fe204). The width of each hole in the completely perforated hole-drilled ferrite sheet was about 0.1 mm, the span of two neighboring holes was about 2 mm. The areal density of the holes was 0.19%.

Meanwhile, a non-drilled ferrite sheet was prepared as a comparative example A according to the steps same to the steps to make the completely perforated hole-drilled ferrite sheet, except that the step of hole-drilling was not needed.

A grooved ferrite sheet FLX-953 bought from Toda ISU Corporation ("Toda") was used as a comparative example B. The width of the grooved ferrite sheet was 0.025 mm, the span of two neighboring grooves was 2 mm. The areal density of the grooves was 2.4%. The depth ratio of the grooves was 20%.

These three sheets had the same thickness.

One the one hand, the Agilent E4991 A RF Impedance/Material Analyzer was used to measure the permeability of the sample of the completely perforated hole-drilled ferrite sheet under the present invention, the permeability of the comparative example A (the non- drilled ferrite sheet) and the permeability of the comparative example B ( the grooved ferrite sheet bought from Toda), at 13.56 MHz, respectively. It is known to a skilled person in the art on how to conduct the measurement, so description on the measurement is omitted here.

On the other hand, the effective permeability of the three sheets was calculated based on the formulations given above. According to equation (8), the effective permeability of the hole-drilled ferrite sheet was calculated to be 125 and the effective permeability of the comparative example A (the non-drilled ferrite sheet) was calculated to be 130. According to equation (16), the effective permeability of the comparative example B (the grooved ferrite sheet bought from Toda ) was calculated to be 114.

The measured data from Agilent E4991 A RF Impedance/Material Analyzer and the calculated data of the effective permeability of the three sheets are summarized in Table 3. Table 3

From the data of Table 3, it can be observed that the permeability of the sheet with holes, as proposed in the invention, was close to the non-drilled sheet without any holes or grooves, while the permeability of the sheet with grooves was significantly different from that of the non-drilled sheet.

Furthermore, the above table shows that the difference in permeability of the sheet achieved by measurement and by calculation from the formulations given above is small. Therefore, the permeability attained from the calculation according to the formulations is reliable.

Example 2 Die-cutting properties.

Another comparative experiment was conducted on the die-cutting performance of the hole-drilled ferrite sheets proposed by the present invention with that of the non-drilled ferrite sheets without holes or grooves.

The samples of the completely perforated hole-drilled ferrite sheet and the samples of comparative example A of the non-drilled ferrite sheet prepared under Example 1 were used here.

A ring shaped reamer with an inner diameter 8.0 mm and outer diameter 18.0 mm was used to die-cut the hole-drilled and non-drilled ferrite sheets, respectively. Figs. 4a and 4c show a plurality of small ring pieces die-cut from the hole-drilled sheet and the non- drilled ferrite sheet, respectively. As shown, after die-cutting, the hole-drilled ferrite, as shown in Figs. 4a and 4b, had less cracks on its edges, while the non-drilled ferrite, as shown in Figs. 4c and 4d, had much more cracks.

Therefore, the hole-drilled ferrite sheet of the present invention can not only keep the high permeability close to that of the non-drilled ferrite sheet, but also improves the die- cutting performance. Moreover, the permeability is also higher than conventional grooved sheets.

B Prophetic examples

In order to show the advantage of the hole-drilled ferrite sheet proposed in the present invention in relation to permeability under different areal density and depth, a plurality of hole-drilled sintered ferrite sheets and a plurality of grooved sintered ferrite sheets are modeled and designed, and each sheet has a unique areal density and depth. As the hole size and the distance of the two neighboring holes have limited influence on the permeability of the hole-drilled ferrite sheets, these two factors are ignored in below examples. The permeability of each sheet is calculated based on the formulations given above.

Example 3

A plurality of completely perforated hole-drilled sintered ferrite sheets is designed. For these sheets, the areal density of the holes in each sheet is different from that of the other sheets, as shown in the Table 4 below. The depth of each hole is equal to the thickness of the sheet. However, the parameters and properties of each sheet itself are the same.

Moreover, a plurality of grooved sintered ferrite sheets which are used as

comparative examples are also designed. These grooved sintered ferrite sheets have the same parameters and properties of the completely perforated hole-drilled sintered ferrite sheets, except that grooves, rather than holes, are drilled in the sheets and the areal density of the grooves on each sheet were different from the grooves of the other sheets, as shown in the Table 4 below. The depth of each groove is equal to the thickness of the sheet.

Based on equations (9) and (16), the permeability of the hole-drilled ferrite sheet and the permeability of the grooved ferrite sheet are calculated, wherein μι is taken asl30 which is a typical value for Ni-Zn ferrite at 13.56 MHz, μο is 1 which is the permeability of air, K oie and Kgroove equal 100%, referring to below Table 3, is designed to be 0.01%, 0.25%, 2.5%), 17.5 and so on, respectively, and is designed to be 0.25%, 2.5%, 12.5 and so on, respectively. The calculated results are shown in Table 4 below and in Fig. 5 (a).

In Table 4, Examples 3-1 through 3-9 represent a hole-drilled ferrite sheet with a unique areal density but the same depth in Example 3, respectively. Comparative Examples 3-1 through 3-9 represent a grooved ferrite sheet with a unique areal density but the same depth in Example 3. This explanation is also suitable to other examples below, such as Example 4 and Example 5.

Table 4

Fig. 5 (a) shows the effective permeability of completely perforated hole-drilled ferrite sheets (solid rectangle curve in Fig. 5 (a)) and grooved ferrite sheets (solid circle curve in Fig. 5 (a)). As can be seen in Fig.5(a), the permeability of the fully perforated hole- drilled ferrite is much higher than the perforated grooved ferrite sheet under the same areal density; the difference is shown in the curve depicted by the solid triangles.

A permeability of greater than 80 is preferred. According to this preferred

requirement, the areal density of the holes is below about 15%. More preferably, the areal density of the holes is below about 6% so that permeability of greater than 100 is achieved.

Example 4

Similar to Example 3, another batch of hole-drilled sintered ferrite sheets and another batch of grooved sintered ferrite sheets are designed. For these sheets, the depth of the holes or grooves is about 80% of the total thickness of the sheet, and the areal density of the holes or grooves on each sheet is varied, as shown in Table 5.

The calculated permeability of each sheet is shown in Table 5 below and in Fig. 5 (b).

Table 5

Example Areal Density (%) Effective permeability

Example 4-1 0.01 129.4

Example 4-2 0.25 125.4

Example 4-3 2.5 114.3

Example 4-4 5 107.5

Example 4-5 7.5 102.3

Example 4-6 10 97.8

Example 4-7 12.5 94.0

Example 4-8 20 90.5

Example 4-9 25 78.7

Comparative Example 4-1 0.01 129.3

Comparative Example 4-2 0.25 115.4

Comparative Example 4-3 2.5 65.1

Comparative Example 4-4 5 49.7

Comparative Example 4-5 7.5 42.8

Comparative Example 4-6 10 38.9

Comparative Example 4-7 12.5 36.4

Comparative Example 4-8 20 32.4

Comparative Example 4-9 25 31

According to a preferred requirement that the permeability is greater than about 80, the areal density of the holes is below about 25%. More preferably, the areal density of the holes is below about 8% so that permeability of greater than 100 is achieved.

Example 5 Similar to Example 3, yet another batch of hole-drilled sintered ferrite sheets and another batch of grooved sintered ferrite sheets are designed. For these sheets, the depth of the holes or grooves is about 60% of the total thickness of the sheet, and the areal density of the holes or grooves on each sheet is varied, as shown in Table 6.

The calculated permeability of each sheet is shown in Table 6 below and in Fig. 5 (c).

Table 6

According to a preferred requirement that the permeability is greater than about 80, the areal density of the holes is below about 42.5%. More preferably, the areal density of the holes is below about 15% so that permeability of greater than 100 is achieved.

Example 6

Similar to Example 3, yet another batch of hole-drilled sintered ferrite sheets and another batch of grooved sintered ferrite sheets are designed. For these sheets, the depth of the holes or grooves is about 50% of the total thickness of the sheet, and the areal density of the holes or grooves on each sheet is varied, as shown in Table 7.

The calculated permeability of each sheet is shown in Table 7 below and Fig. 5 (d).

Table 7

Example Areal Density (%) Effective permeability

Example 6-1 0.01 129.6

Example 6-2 0.25 127.1

Example 6-3 2.5 120.2

Example 6-4 5 115.9

Example 6-5 7.5 112.6

Example 6-6 10 109.9

Example 6-7 40 89.3

Example 6-8 60 80.1

Example 6-9 65 78.0

Comparative Example 6-1 0.01 129.5

Comparative Example 6-2 0.25 120.8

Comparative Example 6-3 2.5 89.4

Comparative Example 6-4 5 79.8

Comparative Example 6-5 7.5 75.5

Comparative Example 6-6 10 73.1

Comparative Example 6-7 40 66.7

Comparative Example 6-8 60 66.0

Comparative Example 6-9 65 65.9

A permeability above about 80 is preferred. The upper limit of the areal density of the holes is 60% in this case. According to equation (7), taking the ordinary size of the ferrite sheet as 1mm ² to 16 mm ² , the upper limit of the area of the hole is 9.6 mm ² .

Figs. 5(a)-(d) show the effective permeability of hole-drilled ferrite sheets and grooved ferrite sheets with multiple depth ratios which were about 100%, 80%, 60% and 50% of the thickness of the sheet, respectively. It can be seen that under same depth ration and same areal density, the hole-drilled ferrite sheets have higher permeability than that of the grooved ferrites.

Example 7 Permeability comparison

Furthermore, in order to compare the permeability of the completely perforated hole- drilled ferrite sheets with the grooved ferrite sheets with certain depth ratios, another drawing was made as shown in Fig. 6 based on the above simulation result in Example 3 through Example 6. It can be seen that under the same areal density the completely perforated hole-drilled ferrite sheet (solid rectangle curve shown in Fig.6) has higher permeability than the grooved ferrites which have depth ratios of 50% (open cross centered circle curve shown in Fig.6), 60% (open circle curve shown in Fig.6), 80% ( half up circle curve shown in Fig.6) and 100% (solid circle curve shown in Fig.6) in the areal density from 0.01% to 23%, respectively.

Thus, for the completely perforated hole-drilled ferrite sheet, if the sectional area of the sheet is between 1 mm ² to 16 mm ² , the S oie can be from 100 μιη ² to 3.7 mm ² according to equation (7).

While the invention has been described above according to its preferred

embodiments, it can be modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using the general principles disclosed herein. Further, the application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the following claims.