Patent Description:
A known acoustic diaphragm for use with an audio device such as a speaker or a sonar sensor includes a laminate in which a metal foil and a thermoplastic resin are laminated.

Patent Literature <NUM> discloses an acoustic diaphragm that is obtained by laminating and thermocompression-bonding an aluminum foil and a cast unoriented thermoplastic resin film. Examples of the cast unoriented thermoplastic resin film used in Patent Literature <NUM> include a polyurethane-based thermoplastic resin film, a polyamide-based thermoplastic resin film, and a polyester-based thermoplastic resin film.

The acoustic diaphragm of Patent Literature <NUM> is manufactured through a lamination process in which an aluminum foil and a cast unoriented thermoplastic resin film placed one upon the other are heated to a temperature near the melting temperature of the cast unoriented thermoplastic resin film, and the cast unoriented thermoplastic resin film is pressure-bonded to the aluminum foil. The aluminum foil and the cast unoriented thermoplastic resin film of the acoustic diaphragm have different coefficients of thermal expansion. Thus, the acoustic diaphragm obtained through the lamination process will warp greatly. Warping of the acoustic diaphragm adversely affects workability when shaping the acoustic diaphragm into the form of a speaker or the like.

Accordingly, one objective of the present invention is to provide an acoustic diaphragm that resists warping.

An acoustic diaphragm that solves the above problem includes a metal foil and a thermoplastic resin film laminated on the metal foil. The thermoplastic resin film has a ratio of a coefficient of linear thermal expansion in a thickness-wise direction to a smaller one of a coefficient of linear thermal expansion in an MD-direction and a coefficient of linear thermal expansion in a TD-direction that is between <NUM> and <NUM>. A total weight per unit area of the metal foil and the thermoplastic resin film is between <NUM>/m<NUM> and <NUM>/m<NUM>.

In some embodiments, the metal foil may have a specific gravity of between <NUM> and <NUM>.

In some embodiments, a difference between a coefficient of linear thermal expansion of the metal foil and the smaller one of the coefficient of linear thermal expansion in the MD-direction and the coefficient of linear thermal expansion in the TD-direction of the thermoplastic resin film may be between <NUM> ppm/K and <NUM> ppm/K.

In some embodiments, the coefficient of linear thermal expansion of the metal foil may be between <NUM> ppm/K and <NUM> ppm/K.

In some embodiments, the smaller one of the coefficient of linear thermal expansion in the MD-direction and the coefficient of linear thermal expansion in the TD-direction may be between <NUM> ppm/K and <NUM> ppm/K.

In some embodiments, the thermoplastic resin film may include at least one polyimide film adjoining the metal foil.

A method for manufacturing an acoustic diaphragm that solves the above problem includes a lamination process that thermocompression-bonds the metal foil and the thermoplastic resin film. Advantageous Effects of Invention.

The acoustic diaphragm in accordance with the present invention resists warping.

An embodiment of the present invention will now be described.

As shown in <FIG>, an acoustic diaphragm <NUM> is a laminate of a sheet-like metal foil <NUM> and a thermoplastic resin film <NUM> that is disposed on one side of the sheet-like metal foil <NUM>. The acoustic diaphragm <NUM> is used in an audio device as a transducer for acoustic oscillation. The acoustic diaphragm <NUM> is used in an audio device such as a speaker, a sonar sensor, and a microphone.

Examples of the metal forming the metal foil <NUM> include aluminum, titanium, magnesium, copper, and an alloy combining two or more of the above metals. In these metals, a metal having a specific gravity of between <NUM> and <NUM> is preferred, and a metal having a specific gravity of between <NUM> and <NUM> is further preferred. This improves the sound quality when the acoustic diaphragm <NUM> is applied to a speaker.

Preferably, the metal foil <NUM> has a coefficient of linear thermal expansion CTEM of, for example, between <NUM> ppm/K and <NUM> ppm/K, more preferably between <NUM> ppm/K and <NUM> ppm/K, and further preferably between <NUM> ppm/K and <NUM> ppm/K. When the coefficient of linear thermal expansion CTEM of the metal foil <NUM> is set within the above-described ranges, the difference is decreased between the coefficient of linear thermal expansion CTEM of the metal foil <NUM> and the coefficient of linear thermal expansion of the thermoplastic resin film <NUM>. This effectively limits warping in the acoustic diaphragm <NUM> caused by the difference in coefficient of linear thermal expansion.

Preferably, the metal foil <NUM> has a thickness of, for example, between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>.

Preferably, the weight per unit area of the metal foil <NUM> is, for example, between <NUM>/m<NUM> and <NUM>/m<NUM>, and more preferably between <NUM>/m<NUM> and <NUM>/m<NUM>.

Specific examples of the thermoplastic resin film <NUM> include a polyimide film such as a multilayer aromatic polyimide film or a single-layer polyimide film, a polyetherimide film, a polyester film (including liquid crystal film), a polyamide film (including aramid film), a vinyl ester film, a thermoplastic fluorine resin film, a polyetherketone film (including polyetheretherketone film), a polyphenylsulfone film, and the like. A multilayer aromatic polyimide film includes a layer of polyimide that has a thermocompression bonding property disposed on both sides of an aromatic polyimide film that does not have a pressure-bonding property. For example, a commercially available product such as UPILEX VT™ manufactured by UBE INDUSTRIES, LTD. can be used. <CIT> discloses examples of such multilayer aromatic polyimide film. Among them, a polyimide film is particularly preferably used as the thermoplastic resin film <NUM>.

The thermoplastic resin film <NUM> may include another component such as an additive.

The thermoplastic resin film <NUM> may be a resin having voids such as a foamed body.

The thermoplastic resin film <NUM> may have a structure combined with a non-thermoplastic resin film as long as the thermoplastic resin film <NUM> is adherable to the metal foil <NUM> without impairing the audio characteristics and the effects of the invention. For example, the thermoplastic resin film <NUM> may have a multilayer structure in which the thermoplastic resin film <NUM> is adhered to one side or both sides of the non-thermoplastic resin film. Alternatively, the thermoplastic resin film <NUM> may have a seaisland structure in which the thermoplastic resin film <NUM> forms a sea component and the non-thermoplastic resin film forms an island component.

The thermoplastic resin film <NUM> has a ratio CTEZ/CTEX of between <NUM> and <NUM>. The ratio CTEZ/CTEX is a ratio of a coefficient of linear thermal expansion CTEZ in a thickness-wise direction to a coefficient of linear thermal expansion CTEX of the smaller one of the coefficient of linear thermal expansion in an MD-direction and the coefficient of linear thermal expansion in a TD-direction. Further, it is preferred that the ratio CTEZ/CTEX be between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>.

When the ratio CTEZ/CTEX is <NUM> or greater, molecules in the thermoplastic resin film <NUM> are oriented in a planar direction at a specific level or greater. This limits warping of the acoustic diaphragm <NUM>. Also, when the ratio CTEZ/CTEX is <NUM> or less, the thermoplastic resin film <NUM> will be stretchable in the planar direction without lowering resistance against shearing in the planar direction. This improves the workability of the acoustic diaphragm <NUM>. For example, the acoustic diaphragm <NUM> can easily be drawn into a predetermined shape such as the shape of a dome.

Preferably, the coefficient of linear thermal expansion CTEX of the thermoplastic resin film <NUM> is, for example, between <NUM> ppm/K and <NUM> ppm/K, more preferably between <NUM> ppm/K and <NUM> ppm/K, and further preferably between <NUM> ppm/K and <NUM> ppm/K. When the coefficient of linear thermal expansion CTEX is set within the above-described ranges, the thermoplastic resin film <NUM> is stretchable in the planar direction so that the workability of the acoustic diaphragm <NUM> is improved.

Preferably, the thermoplastic resin film <NUM> has a thickness of, for example, between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>.

Preferably, the thermoplastic resin film <NUM> has a weight per unit area of, for example, between <NUM>/m<NUM> and <NUM>/m<NUM>, and more preferably between <NUM> /m<NUM> and <NUM>/m<NUM>.

Preferably, in the acoustic diaphragm <NUM>, the difference CTEX-M (absolute difference) between the coefficient of linear thermal expansion CTEX of the thermoplastic resin film <NUM> and the coefficient of linear thermal expansion CTEM of the metal foil <NUM> is between <NUM> ppm/K and <NUM> ppm/K, and more preferably between <NUM> ppm/K and <NUM> ppm/K. When the difference CTEX-M is set within the above-described ranges, warping of the acoustic diaphragm <NUM> is effectively limited.

Preferably, the acoustic diaphragm <NUM> has a thickness of, for example, between <NUM> and <NUM>, and more preferably between <NUM> and <NUM>.

Preferably, the weight per unit area of the acoustic diaphragm <NUM>, that is, the total weight per unit area of the metal foil <NUM> and the thermoplastic resin film <NUM>, is between <NUM>/m<NUM> and <NUM>/m<NUM>, and preferably between <NUM>/m<NUM> and <NUM>/m<NUM>. When the weight per unit area of the acoustic diaphragm <NUM> is set within the above-described ranges, warping of the acoustic diaphragm <NUM> is limited. Also, when the weight per unit area of the acoustic diaphragm <NUM> is <NUM>/m<NUM> or less, the sound pressure will not be reduced by the weight. When the weight per unit area of the acoustic diaphragm <NUM> is <NUM>/m<NUM> or greater, the acoustic diaphragm <NUM> becomes more rigid so that the acoustic diaphragm <NUM> can easily obtain a self-supporting property even when used in an audio device such as a large speaker.

Preferably, the acoustic diaphragm <NUM> has a resin ratio, or the volume percent of the thermoplastic resin film <NUM> to the total volume of the metal foil <NUM> and the thermoplastic resin film <NUM>, that is <NUM>% or less, and more preferably <NUM>% or less. When the thermoplastic resin film <NUM> is set within the above-described ranges of the resin ratio, warping of the acoustic diaphragm <NUM> is effectively limited. Also, when the acoustic diaphragm <NUM> is applied to a speaker, the acoustic diaphragm <NUM> can achieve both the reduction of warping and the improvement of the sound quality at a high level. The lower limit of the resin ratio of the thermoplastic resin film <NUM> is, for example, <NUM>%.

Preferably, the adhesion strength between the metal foil <NUM> and the thermoplastic resin film <NUM> in the acoustic diaphragm <NUM> is, for example, <NUM> N/mm or greater. This avoids delamination of the acoustic diaphragm <NUM> when shaped into a predetermined form.

Preferably, internal loss tanδ of the acoustic diaphragm <NUM> is between <NUM> and <NUM>. This improves the sound quality in high-frequency and low-frequency ranges when the acoustic diaphragm <NUM> is applied to a speaker.

The acoustic diaphragm <NUM> is shaped into a predetermined form such as a flat plate shape or a dome-like shape depending on the intended use and used in an audio device.

The acoustic diaphragm <NUM> is manufactured through, for example, a lamination process in which the metal foil <NUM> and the thermoplastic resin film <NUM> are placed one upon the other and thermocompression-bonded. The method used for the thermocompression-bonding in the lamination process is not limited and may be, for example, a known method using a roller-type lamination apparatus, a double belt press machine, or the like.

The present embodiment has the following advantages.

This method allows for manufacture of the acoustic diaphragm <NUM> that resists warping.

The present embodiment may be modified as follows. The present embodiment and the following modifications can be combined as long as the combined modifications remain technically consistent with each other.

The number of layers of the metal foil <NUM> forming the acoustic diaphragm <NUM> is not limited to one. Alternatively, the acoustic diaphragm <NUM> may include two or more layers of the metal foil <NUM>.

<FIG> shows an example in which the acoustic diaphragm <NUM> includes a first metal foil 11a, the thermoplastic resin film <NUM>, and a second metal foil 11b that are laminated in order from one side in a lamination direction. That is, the thermoplastic resin film <NUM> is located between the first metal foil 11a and the second metal foil 11b. In this case, warping in the acoustic diaphragm <NUM> is significantly limited.

When the acoustic diaphragm <NUM> includes two or more metal foils <NUM>, the acoustic diaphragm <NUM> may have a part where the metal foils <NUM> are successively laminated without an intervening layer in the lamination direction. The metal foils <NUM> may all be the same or different.

The number of layers of the thermoplastic resin film <NUM> forming the acoustic diaphragm <NUM> is not limited to one. Alternatively, the acoustic diaphragm <NUM> may include two or more layers of the thermoplastic resin film <NUM>.

<FIG> shows an example in which the acoustic diaphragm <NUM> includes a first thermoplastic resin film 12a, the metal foil <NUM>, and a second thermoplastic resin film 12b that are laminated in order from one side in a lamination direction. That is, the first thermoplastic resin film 12a and the second thermoplastic resin film 12b are laminated on the two sides of the metal foil <NUM>, respectively. In this case, warping in the acoustic diaphragm <NUM> is significantly limited.

When the acoustic diaphragm <NUM> includes two or more thermoplastic resin films <NUM>, the acoustic diaphragm <NUM> may include a part where the thermoplastic resin films <NUM> are successively laminated without an intervening layer in the lamination direction. The thermoplastic resin films <NUM> may all be the same or different.

When two or more thermoplastic resin films <NUM> are included, it is preferred that at least one thermoplastic resin film <NUM> that is in contact with the metal foil <NUM> be a polyimide film. In this case, the above-mentioned advantage (<NUM>) is obtained.

The acoustic diaphragm <NUM> may include another layer, such as a protection layer, other than the metal foil <NUM> and the thermoplastic resin film <NUM>.

Technical concepts that can be understood from the above embodiment and the modified examples will be described hereafter.

The present embodiment will now be described in detail with reference to examples and comparative examples.

Hereinafter, the difference CTEX-M between the coefficient of linear thermal expansion CTEX of the thermoplastic resin film and the coefficient of linear thermal expansion CTEM of the metal foil in the acoustic diaphragm will be referred to as "CTE-difference".

An acoustic diaphragm of Example <NUM> was obtained by laminating and thermocompression-bonding an aluminum foil AL (type: 1N30) having a thickness of <NUM> and a polyimide film PI (UPILEX VT manufactured by UBE INDUSTRIES, LTD. ) having a thickness of <NUM> with a double belt press machine. Table <NUM> shows the specific gravity and the coefficient of linear thermal expansion CTEM of the metal foil and the coefficients of linear thermal expansion CTEX, CTEZ and the weight per unit area of the thermoplastic resin film, which were used in the acoustic diaphragm of Example <NUM>. Table <NUM> shows CTE-difference, weight per unit area, and resin ratio of the acoustic diaphragm of Example <NUM>.

The coefficient of linear thermal expansion CTEX and the coefficient of linear thermal expansion CTEZ of the thermoplastic resin film and the coefficient of linear thermal expansion CTEM of the metal foil were measured as described below.

Samples were cut out from the thermoplastic resin film and preprocessed by heating at <NUM> for thirty minutes. The heat-processed samples were set in a thermal mechanical analysis (TMA) apparatus (TMA-Q400 manufactured by TA Instruments), and the temperature was increased at a rate of <NUM> /min to measure the thermal expansion amount from <NUM> to <NUM> and calculate the coefficient of linear thermal expansion. The samples were collected from two locations on the thermoplastic resin film in MD-direction and TD-direction, and the smaller measurement value of the two samples was defined as the coefficient of linear thermal expansion CTEX.

A sample was cut out from the thermoplastic resin film and set on a thermal dilatometer that uses laser interferometry (laser thermal dilatometer L1X-<NUM> manufactured by ULVAC-RIKO). Preprocessing of the samples was performed by increasing the temperature to <NUM>, holding the temperature for five minutes, and then lowering the temperature to room temperature. Subsequently, the temperature was increased at a rate of <NUM> /min to measure the thermal expansion amount from <NUM> to <NUM> and calculate the coefficient of linear thermal expansion CTEZ.

Samples were cut out from the metal foil and preprocessed by heating at <NUM> for thirty minutes. The heat-treated samples were set on a thermal mechanical analysis (TMA) apparatus (TMA-Q400 manufactured by TA Instruments), and the temperature was increased at a rate of <NUM> /min to measure the thermal expansion amount from <NUM> to <NUM> and calculate the coefficient of linear thermal expansion. The samples were collected from two locations on the metal foil in MD-direction and TD-direction, and the smaller measurement value of the two samples was defined as the coefficient of linear thermal expansion CTEM.

An aluminum foil AL (<NUM>) having a thickness of <NUM> was used as the metal foil. Otherwise, the conditions were the same as Example <NUM>.

A titanium foil having a thickness of <NUM> was used as the metal foil. A polyimide foil PI having a thickness of <NUM> was used as the thermoplastic resin film. Otherwise, the conditions were the same as Example <NUM>.

An aluminum foil AL (<NUM> N30) having a thickness of <NUM> was used as an acoustic diaphragm of Comparative Example <NUM>.

An aluminum foil AL (<NUM>) having a thickness of <NUM> was used as an acoustic diaphragm of Comparative Example <NUM>.

A titanium foil having a thickness of <NUM> was used as an acoustic diaphragm of Comparative Example <NUM>.

A magnesium alloy foil (AZ31B) having a thickness of <NUM> was used as an acoustic diaphragm of Comparative Example <NUM>.

A thermoplastic resin film was formed by stacking a first polyimide film PI (UPILEX VT manufactured by UBE INDUSTRIES, LTD. ) having a thickness of <NUM> and a second polyimide film PI (UPILEX VT manufactured by UBE INDUSTRIES, LTD. ) having a thickness of <NUM>, in this order, and thermocompression-bonding them using a double belt press machine. The thermoplastic resin film thus obtained was used as an acoustic diaphragm of Comparative Example <NUM>. In Table <NUM>, the numerical values in the columns headed "Thermoplastic Resin Film" were obtained from the thermoplastic resin films after the thermocompression-bonding. The ratio CTEZ/CTEX for the coefficient of linear thermal expansion CTEZ of the first polyimide film PI was <NUM>, and the ratio CTEZ/CTEX for the coefficient of linear thermal expansion CTEZ of the second polyimide film PI was <NUM>.

A thermoplastic resin film was formed by stacking an aluminum foil AL (1N30) having a thickness of <NUM>, a first polyimide film PI having a thickness of <NUM>, and a second polyimide film PI having a thickness of <NUM>, in this order, and thermocompression-bonding them using a double belt press machine. The thermoplastic resin film thus obtained was used as an acoustic diaphragm of Comparative Example <NUM>.

An aluminum foil AL (<NUM> N30) having a thickness of <NUM> was used as the metal foil, and a polyimide film PI having a thickness of <NUM> was used as the thermoplastic resin film. Otherwise, the conditions were the same as Example <NUM>.

A polyethylene terephthalate film PET having a thickness of <NUM> was used as the thermoplastic resin film. Otherwise, the conditions were the same as Example <NUM>.

Warping in the acoustic diaphragm of each example and comparative example was evaluated.

A sample having a size of <NUM> in length ×<NUM> in width was cut out from the acoustic diaphragm of each example and comparative example and left to rest under the environment of <NUM> and <NUM>%RH for at least twenty-four hours to allow the sample to warp. Then, the warped sample was set on a level bench top such that the inwardly curved surface faces the upward direction. The raised height of the sample was measured at the most raised part of the sample from the bench top to evaluate warping in the acoustic diaphragm using the following indices. The results are shown in Table <NUM>.

Workability of the acoustic diaphragm of each example and comparative example was evaluated.

The acoustic diaphragm of examples and comparative examples each underwent ten operations in which the sheet-like acoustic diaphragm was processed with a die to be dome-shaped. The number of processing defects that occurred during the ten operations was counted to evaluate the workability of the acoustic diaphragm using the following indices. The results are shown in Table <NUM>.

Speakers were produced by attaching a voice coil to the back surface of the acoustic diaphragm of each example and comparative example, which was dome-shaped and had a diameter of <NUM>. Five panelists listened to the sounds output from the produced speakers and evaluated the sound quality of the acoustic diaphragm using the following indices. The results are shown in Table <NUM>. The sound quality evaluation was omitted for the acoustic diaphragm that could not be processed.

Further, a dynamic viscoelasticity measurement apparatus was used to measure the internal loss tanδ of the acoustic diaphragm of each example and comparative example at <NUM> and <NUM>. The results are shown in Table <NUM>.

Adhesiveness in the acoustic diaphragm of each of the examples and comparative examples <NUM> to <NUM> was evaluated.

Strips of sample having the size of <NUM> in width × <NUM> in length were prepared for MD-direction and TD-direction from the acoustic diaphragm of each of the examples and Comparative Examples <NUM> to <NUM>. Then, the adhesiveness was evaluated using the <NUM>°-delamination method as described in JIS C <NUM>. Evaluation was conducted three times in MD-direction and TD-direction, and the smallest value of the results was defined as the adhesiveness in the diaphragm.

Long-term reliability was evaluated with the acoustic diaphragm of each of the examples and Comparative Examples <NUM> to <NUM>.

A heat-cycle test was conducted on the acoustic diaphragm of each of the examples and Comparative Examples <NUM> to <NUM> under a condition of the temperature cycle described below. Subsequently, the adhesiveness was evaluated using the same method as the above adhesiveness evaluation.

In the heat cycle test condition, the temperature was held at -<NUM> for ten minutes and then the temperature was increased to <NUM> in two hours. Then, the temperature was held at <NUM> for ten minutes and then decreased to -<NUM> in two hours. This cycle was repeated <NUM> times.

As shown in Tables <NUM> and <NUM>, warping did not occur in the acoustic diaphragm of Comparative Examples <NUM> to <NUM>, which were formed of only one of a metal foil and a thermoplastic resin film. Warping occurred greatly in the acoustic diaphragm of Comparative Examples <NUM> to <NUM>, which were formed by laminating a metal foil and a thermoplastic resin film. Further, the acoustic diaphragm of Comparative Examples <NUM> to <NUM> had a poor workability or could not be processed.

In the acoustic diaphragm of Examples <NUM> to <NUM>, in which the ratio CTEZ/CTEX of the coefficient of linear thermal expansion CTEZ of the thermoplastic resin film was between <NUM> and <NUM> and the total weight per unit area was between <NUM>/m<NUM> and <NUM>/m<NUM>, significant warping did not occur even though the acoustic diaphragm was formed by laminating the metal foil and the thermoplastic resin film. Further, the acoustic diaphragm of Examples <NUM> to <NUM> were easy to process and did not yield any defects.

The evaluation results of the sound quality, adhesiveness, and long-term reliability indicate that the acoustic diaphragm of Examples <NUM> to <NUM> are applicable as an acoustic diaphragm for a speaker. Although the details are not described, frequency characteristics measured for the acoustic diaphragm of Examples <NUM> to <NUM> indicated that a desirable sound pressure reproduction performance was obtained throughout all frequencies.

As shown in Table <NUM>, the acoustic diaphragms of Examples <NUM> to <NUM> were produced each having different thickness and different arrangement of the metal foil and the thermoplastic resin film. Then, various evaluations were conducted in the same manner as Test <NUM>. The results are shown in Table <NUM>.

A polyimide foil PI having a thickness of <NUM> was used as the thermoplastic resin film. Otherwise, the conditions were the same as Example <NUM>.

An aluminum foil AL (1N30) having a thickness of <NUM> was used as the metal foil, and a polyimide film PI having a thickness of <NUM> was used as the thermoplastic resin film. Otherwise, the conditions were the same as Example <NUM>.

An acoustic diaphragm of Example <NUM> was obtained by laminating and thermocompression-bonding a polyimide film PI having a thickness of <NUM> on both sides of an aluminum foil AL (1N30) having a thickness of <NUM> with a double belt press machine.

An acoustic diaphragm of Example <NUM> was obtained by laminating and thermocompression-bonding an aluminum foil AL (1N30) having a thickness of <NUM> on both sides of a polyimide film PI having a thickness of <NUM> with a double belt press machine.

As shown in Tables <NUM> and <NUM>, the results of Examples <NUM> and <NUM> to <NUM> indicate that when the resin rate is smaller, warping is limited and the sound quality is improved. In particular, when the resin rate is <NUM>% or less, warping is limited and the sound quality is improved highly effectively. When the resin rate is <NUM>% or less, the sound quality is further improved.

The results of Examples <NUM> to <NUM> indicate that when the acoustic diaphragm has a laminated structure in which the metal foil is sandwiched between the thermoplastic resin films or a laminated structure in which the thermoplastic resin film is sandwiched between metal foils, warping is significantly limited.

The present invention is easily processed into a dome-shaped speaker using a die and is thus suitably utilized as a diaphragm for an active speaker or as a support for a voice coil. Also, the audio characteristics of the present invention are satisfactory. Accordingly, the present invention can be desirably utilized as a diaphragm for a flat surface speaker, a headphone, an earphone, and the like.

Claim 1:
An acoustic diaphragm comprising:
a metal foil; and
a thermoplastic resin film laminated on the metal foil,
wherein the thermoplastic resin film has a ratio of a coefficient of linear thermal expansion in a thickness-wise direction to a smaller one of a coefficient of linear thermal expansion in an MD-direction and a coefficient of linear thermal expansion in a TD-direction that is between <NUM> and <NUM>, and
a total weight per unit area of the metal foil and the thermoplastic resin film is between <NUM>/m<NUM> and <NUM>/m<NUM>.