JP2013193253A - Electromagnetic shielding coverlay film, flexible wiring board and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flexible wiring board excellent in transmission characteristics of an electric signal with high frequency, etc., and also excellent in flexing characteristics.SOLUTION: An electromagnetic shielding coverlay film is composed by laminating a cover film-like base material 11 made of insulating resin, a conductive layer 12, and an insulating adhesive layer 13 containing elastomer, in this order. Alternatively, a release material layer 14 is further pasted on a rear surface of the insulating adhesive layer 13. The cover film-like base material 11 and the conductive layer 12 are laminated by film formation such as laminating through an adhesive, thermocompression bonding, a PVD method, a CVD method, and a plating method. The insulating adhesive layer 13 is laminated on the conductive layer 12 by a casting method, thermocompression bonding, sticking, etc. Here, the insulating adhesive layer 13 can be suitably made of synthetic resin containing oligophenylene ether and styrene butadiene elastomer.

Description

  The present invention relates to an electromagnetic shielding coverlay film having excellent bending characteristics, a flexible wiring board using the electromagnetic shielding coverlay film, and a method for producing the same.

  BACKGROUND OF THE INVENTION Flexible printed wiring boards (hereinafter referred to as FPC: Flexible Printed Circuit or simply flexible wiring boards) are widely used in electronic devices such as portable devices, network devices, servers, and testers. And this FPC is expanding its application also as a composite part provided with a circuit, a cable, a connector function, etc. to an electronic device. In addition, FPC makes use of its flexibility, for example, in movable parts wiring of consumer / industrial equipment such as OA equipment, various computers, automobiles, or transmission wiring such as coaxial cables and wire harnesses. Used as an alternative to

  In recent years, electronic devices have become smaller and data processing functions have become faster and more multifunctional, and electromagnetic noise generated from FPCs, electronic parts, etc. has an effect on other nearby electronic circuits. This is likely to cause malfunctions. Therefore, in the FPC, it is essential to provide an electromagnetic wave shielding function for shielding electromagnetic wave noise. Various FPCs having this electromagnetic wave shielding function have been proposed so far (see, for example, Patent Documents 1, 2, and 3). These FPCs have a structure in which a conductive layer that shields electromagnetic waves is formed on the surface thereof.

  Further, in an electronic device that is miniaturized and mounted with high density, the space allowed for the FPC is narrowed, and the bending angle is sharp. Therefore, it is necessary to improve the folding resistance. Furthermore, the FPC attached as a movable part of an electronic device needs to have excellent bending resistance with high resistance against repeated bending.

  As a technique for improving the bending characteristics of the FPC consisting of the above-mentioned bending resistance and folding resistance, the thinning of the FPC is generally known. Therefore, the conductive layer is formed so that its thickness is reduced within a range in which the electromagnetic wave shielding effect can be sufficiently secured. For example, Patent Document 1 proposes that a conductive layer made of a metal material such as aluminum has a thickness of 1 μm or less. And in patent document 3, it is supposed that the thickness becomes 0.2 micrometers or less as a vapor deposition film used as a conductive layer. In Patent Document 2, an example using a 0.1 μm silver thin film as a conductive layer is shown together with its bending resistance.

Japanese Patent No. 4201548 JP 2010-238870 A JP 2011-071397 A

  By the way, in an electronic device in which an electronic component such as a semiconductor device that speeds up data processing is mounted, the operating frequency increases with the miniaturization. Therefore, the FPC is required to have excellent electrical signal transmission characteristics due to stable impedance and low transmission loss in high-frequency signals. For example, when a high-speed digital signal having an electric signal frequency of several GHz to several tens of GHz is used, high-speed transmission is required without impairing the frequency characteristics. Therefore, for example, in the case of an FPC used in an electronic device that transmits an image with high definition, stabilization of the specific impedance or differential impedance is strongly required.

  Therefore, for example, in the FPC having a structure having the electromagnetic wave shielding function described above, stabilization of the ground potential of the conductive layer becomes important. In order to ensure a stable ground potential in FPC, the conductive layer depends on the use of FPC, but it is necessary to increase the thickness to 1 μm or more with a low-resistance metal material such as copper. The stabilization of the ground potential in the conductive layer also serves to reduce electromagnetic field coupling between circuit wirings of the FPC and to suppress so-called crosstalk between wirings to prevent malfunction of electronic equipment. The insulating resin layer formed between the conductive layer and the circuit wiring of the FPC has stable and excellent dielectric characteristics (for example, low relative dielectric constant (ε), low dielectric loss tangent (tan δ)) in the high frequency band. The material it has becomes important.

  However, in the conventional FPC having an electromagnetic wave shielding function, the conductive layer is focused on shielding electromagnetic waves and thinned to improve the bending characteristics. For this reason, it is not easy to stabilize the ground potential in the FPC, and it is difficult to realize excellent transmission characteristics such as high-frequency signals or to prevent crosstalk between circuit wirings.

  The present invention has been made in view of the above circumstances, and in a flexible wiring board having an electromagnetic shielding function, an FPC having excellent bending characteristics can be provided even when the conductive layer for electromagnetic shielding is made thicker. The main purpose is to do. Then, the ground potential in the FPC can be simply stabilized, the transmission characteristics of electric signals such as high frequencies are excellent, and crosstalk between signal circuit wirings can be easily prevented. Alternatively, such an FPC can be manufactured at low cost.

  In order to achieve the above object, an electromagnetic shielding coverlay film according to the present invention comprises a film-like base material made of an insulating resin, a conductive layer, and an insulating adhesive layer containing an elastomer laminated in this order. It is characterized by. Here, aluminum metal is suitable for the conductive layer. Further, a synthetic resin containing oligophenylene ether and a styrene butadiene elastomer is suitable as the insulating adhesive layer.

  In the flexible wiring board according to the present invention, circuit wiring is formed on the main surface of a base film made of an insulating resin, and the above-described electromagnetic shielding coverlay film is insulated against the base film and the circuit wiring. And a predetermined circuit wiring of the circuit wiring and the conductive layer are electrically connected by a conductive member.

  In the method for manufacturing a flexible wiring board according to the present invention, a solder ball is placed on a predetermined circuit wiring among circuit wirings formed on a main surface of a base film made of an insulating resin, and the circuit wiring and the solder are arranged. The insulating adhesive layer of the electromagnetic shielding coverlay film described above is overlaid on the ball and subjected to heat and pressure treatment, and the solder ball is inserted into the insulating adhesive layer by the heat and pressure treatment. The electromagnetic shielding coverlay film is brought into electrical contact with the conductive layer of the electromagnetic shielding coverlay film, and the electromagnetic shielding coverlay film is integrally joined to the circuit wiring and the base film via the insulating adhesive layer. .

  According to the present invention, in a flexible wiring board having an electromagnetic shielding function, the conductive layer for electromagnetic shielding can be thickened and the ground potential in the FPC can be stabilized. In addition, it is possible to provide an FPC that is excellent in transmission characteristics of electric signals such as high frequencies and that has high bending resistance and folding resistance. Further, such an FPC can be manufactured at a low cost.

Sectional drawing which showed two examples of the electromagnetic shielding coverlay film concerning embodiment of this invention. The partial expanded sectional view which shows an example of the flexible wiring board concerning embodiment of this invention. Sectional drawing according to manufacturing process which shows an example of the manufacturing method of a flexible wiring board same as the above. Sectional drawing according to manufacturing process following the process of FIG. The partially expanded sectional view which shows the other examples of the flexible wiring board concerning embodiment of this invention. The table | surface which shows the evaluation result of the bending characteristic of the flexible wiring board in the Example of this invention.

  A preferred embodiment of the present invention will be described below with reference to FIGS. Here, the drawings are schematic, and ratios of dimensions and the like are different from actual ones. In these drawings, the same or similar parts are denoted by the same reference numerals, and a duplicate description is partially omitted.

  As shown in FIG. 1 (a) or (b), the electromagnetic shielding coverlay film of the present embodiment includes a cover film-like substrate 11 made of an insulating resin, a conductive layer 12, and an insulating material containing an elastomer. The adhesive adhesive layer 13 is laminated in this order.

  Here, when the conductive layer 12 is a metal foil, the cover film-like base material 11 and the conductive layer 12 are laminated through, for example, a hot melt adhesive. Or it joins directly by the thermocompression bonding between the cover film-like base material 11 and the conductive layer 12. Further, the conductive layer 12 may have a structure in which a film is formed and laminated on the main surface of the cover film-shaped substrate 11 by a PVD method such as vacuum deposition or sputtering, a CVD method, a plating method, or the like.

  And the insulating contact bonding layer 13 is laminated | stacked by the casting method, thermocompression bonding, sticking, etc. on the other surface facing the cover film-like base material 11 in the conductive layer 12. Here, when the insulating adhesive layer 13 is made of a thermosetting resin, the insulating adhesive layer 13 is in an uncured state or a semi-cured state.

  In FIG. 1B, a release material layer 14 is further attached to the back surface of the insulating adhesive layer 13.

  The cover film-like substrate 11 may be either a thermosetting resin or a thermoplastic resin. For example, a liquid crystal polymer, a polyimide (PI) resin, a polyethylene naphthalate (PEN) resin, or a composite resin thereof can be used as a suitable resin material. Moreover, an epoxy resin can also be used.

  A liquid crystal polymer is preferable because it exhibits excellent high-frequency transmission characteristics and flexibility. Here, the liquid crystal polymer is, for example, a multiaxially oriented thermoplastic polymer represented by xidar (trade name, manufactured by Dartco) or Vectra (trade name, manufactured by Clanese). Further, it may be modified by adding and blending another insulating resin. Examples thereof include Bexter FA type (melting point 285 ° C.), Bexter CT-X type (melting point 280 ° C. to 335 ° C.), BIAC film (melting point 335 ° C.), and the like. In addition, the glass transition point Tg of these liquid crystal polymers exhibits a high temperature of 205 ° C to 300 ° C.

  Examples of the PI resin include polyimide film “Kapton” (trade name, manufactured by Toray DuPont) and Aurum (trade name, manufactured by Mitsui Chemicals). And as a polyethylene naphthalate type-resin, the thermoplastic resin of Teonex (brand name. Product made by Teijin DuPont) is illustrated as a suitable thing, for example.

  Here, the film thickness of the cover film-like base material 11 is appropriately determined according to the FPC and the material to which it is applied, and is set to, for example, about 5 μm to 50 μm.

  The conductive layer 12 can include aluminum, copper, silver, and gold as a metal material. Here, in the FPC to which the electromagnetic shielding coverlay film is applied, in order to stabilize the ground potential of the conductive layer 12, for example, when a thick film exceeding 1 μm is required, aluminum metal is particularly preferable. Among these, an aluminum foil capable of making the electromagnetic shielding coverlay film and FPC inexpensive is suitable. For example, a thick aluminum foil can be formed at low cost by rolling a metal plate, electrolytic plating, or the like. The film thickness of the conductive layer 12 may be 1 μm or less depending on circumstances.

  The insulating adhesive layer 13 is made of a thermosetting resin or a thermoplastic resin containing an elastomer component. Examples of the thermosetting resin include a PI resin and an epoxy resin. Examples of the thermoplastic resin include a liquid crystal polymer and a polyethylene naphthalate resin. Here, a resin excellent in its low water absorption or low moisture absorption is preferred. With such an insulating resin, the change over time in its dielectric characteristics is small and stable. Moreover, the adhesiveness with the base film and circuit wiring in FPC becomes stable and becomes excellent. And in the insulating contact bonding layer 13, the elasticity modulus after solidifying by thermosetting etc. can be suitably adjusted with the amount of elastomer components.

  The insulating adhesive layer 13 preferably exhibits low dielectric properties after being solidified by heat curing or the like. In this low dielectric property, it is preferable that the relative dielectric constant at a frequency of 1 MHz is 3.5 or less. Such a relative permittivity facilitates high-speed transmission of high-speed digital signals in the FPC, for example, several GHz to several tens GHz band. Further, in the low dielectric property, the dielectric loss tangent in the high frequency band is preferably 0.005 or less. With such a dielectric loss tangent, transmission of FPC in a high-speed digital signal in a high frequency band of several GHz to several tens GHz has a small dielectric loss. Then, high quality signal transmission is facilitated without impairing the high frequency characteristics of the electrical signal.

  Examples of the elastomer include urethane rubber, silicone rubber, and unsaturated polyester elastomer as thermosetting materials. Examples of the thermoplastic material include polyester-based, polyamide-based, polyolefin-based, and polystyrene-based elastomers. Among them, a resin having a small dielectric loss tangent such as polyester resin, nitrile rubber (NBR), styrene butadiene rubber (SBR) is suitable as the elastomer component. For example, the tensile elastic modulus of the insulating adhesive layer 13 after solidification is set to be in a range of about 100 MPa to 1 GPa.

  The film thickness of the insulating adhesive layer 13 is appropriately determined in accordance with the material of the cover film substrate 11, the material of the base film in the FPC, the thickness of the circuit wiring formed on the base film, etc. It is set to about 100 μm.

  Among these, a particularly preferable insulating adhesive layer 13 is a thermosetting synthetic resin containing an oligophenylene ether and a styrene butadiene elastomer. For example, as an uncured film, an ADFLEMA OPE (Product name: NAMICS, Inc.) is exemplified. Alternatively, an adhesive prepared by appropriately dissolving or dispersing a component containing an oligophenylene ether or styrene butadiene elastomer in an organic solvent such as toluene can also be used.

  The above-described oligophenylene ether is also called, for example, OPE (bifunctional polyphenylene ether oligomer), and has a polymer structure in which polyphenylene ether is added to both ends of the bifunctional core. Here, the average molecular weight of OPE is about 500 to 5000, preferably 1000 to 3000. Examples of such commercially available products include OPE2St-1200 and OPE2St-2200 manufactured by Mitsubishi Gas Chemical Company, Inc. The OPE is produced by using, for example, a vinyl compound described in JP-A Nos. 2009-161725 and 2011-68713 as a composition. Here, it may be a derivative of OPE, such as an epoxy derivative or a styrene derivative.

  The insulating adhesive layer 13 made of oligophenylene ether may contain other components. For example, in order to adjust the curing temperature of the insulating adhesive layer 13, a maleimide-based curing agent, a phenol-based curing agent, an amine-based curing agent, and the like are appropriately mixed. Or in order to adjust adhesiveness, an epoxy resin and its curing catalyst are added, for example. Here, as the curing catalyst, for example, an amine-based curing catalyst or an imidazole-based curing catalyst for shortening heat curing is used.

  In any case, the insulating adhesive layer 13 is made of a resin whose curing temperature or glass transition point Tg is lower than the glass transition point Tg or melting point Tm of the cover film substrate 11 made of a thermoplastic resin. Here, the curing temperature is a temperature at which the uncured resin film or adhesive layer is cured by polymerization or crosslinking. The resin of the cover film-like substrate 11 may have a glass transition point and may not show a clear Tg. However, when the resin does not show a clear Tg, the heat curing temperature or Tg of the insulating adhesive layer 13 is a thermoplastic resin. The melting point is lower than Tm.

  In addition, a glass transition point is normally calculated | required by two methods, TMA method and DMA method, by the glass transition temperature measuring method (it applies to JISC6493). Here, in the TMA method, the test piece is heated from room temperature at a rate of 10 ° C./min, the amount of thermal expansion in the thickness direction is measured with a thermal analyzer, and a tangent line is drawn on the curves before and after the glass transition point. Tg is obtained from the intersection of the tangent lines. In the DMA method (tensile method), the test piece is heated from room temperature at a rate of 2 ° C./min, the dynamic viscoelasticity and loss tangent of the test piece are measured with a viscoelasticity measuring device, and the peak temperature of the loss tangent is measured. Obtain Tg. As the elastic modulus, the tensile elastic modulus or bending elastic modulus of the resin film is used. The elastic modulus is measured according to JIS K 7127 or ASTM D882.

  The release material layer 14 is not particularly limited as long as it can be peeled without impairing the shape of the insulating adhesive layer 13. Specific examples of the release material constituting the material include polypropylene (PP) film, polyethylene (PE) film, PP film with silicone release material, and PE film. Further, for example, a release paper having a plastic layer for preventing curling due to moisture absorption / release on both surfaces of a base paper such as high-quality paper and kraft paper. Examples of such a material include PP resin-coated paper and PE resin-coated paper. The release material layer 13 may have any thickness, but for example, the resin film has a thickness of 10 μm to 50 μm, and the release paper has a thickness of 50 μm to 100 μm, for example.

  Next, a flexible wiring board using the electromagnetic shielding coverlay film of the present embodiment and a manufacturing method thereof will be described with reference to FIGS. Here, an FPC in which transmission wiring for transmitting a high-frequency signal is formed will be described. As shown in FIG. 2, in the FPC as an example, an inner layer signal wiring 16 and a ground wiring 17 which are a plurality of strip lines are arranged in a wiring pattern on one main surface of a base film 15 made of resin. Here, the signal wirings 16 are shown as two pairs of transmission wirings that can cope with LVDS (Low Voltage Differential Signaling).

  On the surface layer of the FPC, the electromagnetic shielding coverlay film as described above is thermocompression bonded to the signal wiring 16, the ground wiring 17, and the base film 15 through the insulating adhesive layer 13 and integrally formed. . Here, a conductive member 18 provided in a predetermined region of the ground wiring 17 penetrates the insulating adhesive layer 13 and is electrically connected to the conductive layer 12 of the electromagnetic shielding coverlay film.

  Here, when the conductive layer 12 is formed to a thickness exceeding 1 μm, such as an aluminum foil or a copper foil, the ground potential of the conductive layer 12 and the ground wiring 17 in the FPC becomes stable. And the crosstalk between the signal wirings 16 is stably prevented. Further, the specific impedance, differential impedance, etc. of the signal wiring 16 become stable. The conductive layer 12 also functions as an electromagnetic wave shield.

  The insulating adhesive layer 13 has a low elastic modulus and is excellent in stretchability as well as strong adhesiveness at the interface with the conductive layer 12. The insulating adhesive layer 13 strongly adheres to circuit wiring such as the signal wiring 16 or ground wiring 17 of the FPC, and has excellent stretchability even in the interface region. For this reason, it absorbs the stress from the conductive layer 12 caused by the bending of the FPC and has a high buffering function against the stress from the conductive layer 12. The stress from the conductive layer 12 increases as the thickness increases, and easily causes disconnection of the circuit wiring of the FPC. This disconnection becomes conspicuous as the circuit wiring becomes finer. The above-described high stress absorption capability of the insulating adhesive layer 13 serves to prevent disconnection of the circuit wiring in the FPC. The FPC has excellent bending characteristics.

  In the FPC, it is preferable to use the above-described liquid crystal polymer for the cover film-like substrate 11 and the base film 15 and use a synthetic resin containing oligophenylene ether and a styrene-butadiene elastomer for the insulating adhesive layer 13. In this combination, the relative dielectric constants of the cover film-like substrate 11, the insulating adhesive layer 13, and the base film 15 constituting the electromagnetic shielding coverlay film are all 3 or less. Further, their dielectric loss tangent is 0.003 or less. The signal wiring 16 in this FPC exhibits extremely excellent transmission characteristics of high-speed digital signals in the range of several GHz to several tens of GHz.

  In the FPC, as the base film 15, in addition to the liquid crystal polymer, a PI resin, a PEN resin, or a composite resin such as these is preferably used. Alternatively, other thermoplastic resins or thermosetting resins having low dielectric properties can be used. For example, there are polyethylene terephthalate (PET), polyamideimide and the like.

  Next, an example of the manufacturing method of a flexible wiring board is demonstrated. As shown in FIG. 3A, for example, a single-sided copper-clad laminate in which a copper foil 19 is adhered to the surface of a base film 15 made of a liquid crystal polymer having a thickness of about 15 μm to 50 μm is prepared. Here, the thickness of the copper foil 19 is about 3 μm to 35 μm. The single-sided copper-clad laminate may be one obtained by thermocompression bonding a copper foil on the surface of a resin film, or one obtained by electrolytic plating. Then, as shown in FIG. 3B, the copper foil 19 is formed into a wiring pattern as a strip line by known etching. Then, for example, a signal wiring 16 having a line width of about 20 μm to 100 μm and a ground wiring 17 having a line width of about 200 μm to 500 μm are formed.

  Next, as shown in FIG. 3C, for example, a solder ball 21 to which a non-cleaning type flux 20 is applied by contact is placed in a predetermined region of the ground wiring 17. In placing the solder ball 21, for example, the suction nozzle of the suction jig is sucked under reduced pressure, and the flux 20 is applied to the lower surface of the solder ball 21 by contact. Then, the suction nozzle for sucking the solder ball 21 is aligned with a predetermined region of the ground wiring 17 and the suction is released and mounted.

  The size of the solder ball 21 is appropriately determined depending on the line width of the ground wiring 17 or the thickness of the insulating adhesive layer 13. The solder ball 21 is a lead-free eutectic solder, for example, Sn—Zn solder, Sn—Bi solder, Sn—Cu solder, or Sn—Ag solder. Here, the solder ball 21 is selected so that its melting point becomes higher than the softening or flow temperature of the insulating adhesive layer 13 described later and lower than the curing temperature or the melting point Tm of the insulating adhesive layer 13. When the insulating adhesive layer 13 is a thermosetting synthetic resin containing an oligophenylene ether and a styrene butadiene elastomer, the solder ball 21 is preferably Sn-92Zn solder.

  Next, for example, a single-sided aluminum metal-clad laminate in which an aluminum foil conductive layer 12 is stuck to the surface of a cover film-like substrate 11 made of a PI resin having a thickness of about 5 μm to 50 μm is prepared. Then, as shown in FIG. 3D, for example, an uncured film-like insulating adhesive layer 13 having a thickness of about 15 μm to 100 μm and the above-described single-sided aluminum metal-clad laminate are sequentially stacked from above the base film 15. Set up together. Here, the insulating adhesive layer 13 is a thermosetting synthetic resin containing an oligophenylene ether and a styrene butadiene elastomer. The single-sided aluminum metal-clad laminate is overlaid so that the insulating adhesive layer 13 is in contact with the other surface of the conductive layer 12 facing the cover film-like substrate 11.

  In addition, instead of the above-mentioned single-sided aluminum metal-clad laminate and the uncured film-like insulating adhesive layer 13, an electromagnetic shield comprising the cover film-like substrate 11, the conductive layer 12, and the insulating adhesive layer 13 shown in FIG. A protective coverlay film may be stacked.

Then, as shown in FIGS. 4A and 4B, the laminated cushion sheet 22 is stacked on the cover film-like base material 11, and the insulating adhesive layer 13 is obtained by a known heat and pressure treatment (heat press). The cover film-like substrate 11 and the conductive layer 12 are bonded to the base film 15 via In this hot press, the atmospheric gas is in a reduced pressure state, and the heating temperature is, for example, about 160 ° C. to 200 ° C. And the pressurization is about 10-40 kgf / cm < ² >, for example.

  Here, when the solder ball 21 is made of Sn-92Zn eutectic solder having a melting point of about 198 ° C., the insulating adhesive layer 13 that softens initially as shown in FIG. And then comes into contact with the conductive layer 12. Then, when the heating temperature rises and the insulating adhesive layer 13 begins to harden, the solder ball 21 melts and becomes a conductive member 18 as shown in FIG. 4B and is electrically connected to the conductive layer 12. In this way, the cover film substrate 11 having the conductive layer 12 is joined and integrated with the base film 15, the signal wiring 16 and the ground wiring 17 through the insulating adhesive layer 13.

  Then, as shown in FIG. 4C, the laminated cushion sheet 22 is peeled off. In the subsequent steps, heat treatment for a predetermined time may be performed at a predetermined temperature without applying pressure. Further, although not shown, a predetermined portion of the electromagnetic shielding coverlay film or the predetermined portion of the base film 15 consisting of the cover film-like substrate 11, the conductive layer 12 and the insulating adhesive layer 13 is selectively removed by etching. The signal wiring 16 and the ground wiring 17 exposed by this removal are plated with Ni, Au or the like to form a terminal portion. In this way, the flexible wiring board having the cross-sectional structure shown in FIG. 2 is produced.

  The insulating adhesive layer 13 has an elastic modulus after thermosetting that is smaller than the elastic modulus of the cover film-like base material 11 or the base film 15, and is superior to the conductive layer 12, the base film 15, the signal wiring 16, and the ground wiring 17. Can be joined. This is because the thermal stress generated when the temperature is lowered from the heated state to room temperature is absorbed and relaxed by the insulating adhesive layer 13.

  Moreover, the insulating adhesive layer 13 after thermosetting has a low hygroscopic property, and exhibits stable values of a relative dielectric constant of 2.4 to 3.0 and a dielectric loss tangent of 0.0015 to 0.003. Moreover, the glass transition point shows a high value with 180 degreeC-230 degreeC, for example.

  Further, in the bonding with the base film 15, the signal wiring 16 and the ground wiring 17, the amount of the oozing of the insulating adhesive layer 13 is greatly reduced as compared with the conventional adhesive layer. For this reason, the workability | operativity in joining integration using a hot press method is excellent.

  In the FPC manufacturing method, as the insulating adhesive layer 13, a PI resin or an epoxy resin, which is another thermosetting resin, can be used as a base material. However, as described above, the solder ball 21 is selected so that its melting point is higher than the softening or flow temperature of the insulating adhesive layer 13 and lower than the curing temperature of the insulating adhesive layer 13. For example, in the case of the insulating adhesive layer 13 having a thermosetting temperature of about 160 ° C. to 170 ° C., the solder ball 21 is preferably a low melting point solder such as Sn-58Bi solder.

  When a thermoplastic resin is used as the insulating adhesive layer 13, its melting point Tm is lower than the glass transition point Tg of the cover film-like substrate 11 or the base film 15 and higher than the melting point of the solder ball 21. Selected as

  In the FPC manufacturing method, the conductive member 18 may be formed of a known conductive bump instead of the solder ball 21. In this case, in the step of FIG. 3C, a conical conductive paste is bumped to a predetermined region of the ground wiring 17 using, for example, a stainless steel screen plate instead of the solder ball 21. Here, the conductive paste is, for example, a mixture of metal particles such as silver, gold, copper, tin, lead, and carbon and an epoxy resin, a phenol resin, an acrylic resin, and the like. Then, when hot pressing is performed as described above, the conical conductive paste undergoes a plastic deformation in which its head is crushed, resulting in a change in the composition, resulting in a conductive bump. Then, the conductive bump penetrates the insulating adhesive layer 13 and is connected to the conductive layer 12 as a conductive member 18.

  When the above-described ground wiring 17 is electrically connected to the conductive layer 12, a large number of conducting members 18 are not necessary and may be in an appropriate number. For this reason, in the case of the FPC, the FPC manufacturing method using the solder balls 21 for forming the conductive member 18 facilitates cost reduction. This is because the work of placing the solder balls 21 is very simple compared to the case of forming conductive bumps using screen printing, which is effective when a large number of conductive members 18 are formed.

  Next, another example of the flexible wiring board will be described with reference to FIG. This is the case of an FPC in which circuit wiring having a two-layer structure is provided. Here, an FPC in which circuit wiring for transmitting a high-speed digital signal is formed will be described. As described with reference to FIG. 2, the signal wiring 16 and the ground wiring 17 are disposed on one main surface of the base film 15. The ground wiring 17 is electrically connected to the conductive layer 12 by the conductive member 18, and one main surface of the base film 15 has a first electromagnetic shielding property including the cover film-like base material 11, the conductive layer 12, and the insulating adhesive layer 13. Covered with a coverlay film 23.

  Similarly, a plurality of signal lines 16a and ground lines 17a are arranged on another main surface of the base film 15 in another wiring pattern of, for example, copper foil. The ground wiring 17a is electrically connected to the conductive layer 12a by a conductive member 18a. And the other main surface of the base film 15 is coat | covered with the 2nd electromagnetic shielding coverlay film 23a. The 2nd electromagnetic shielding coverlay film 23a consists of the cover film-like base material 11a, the conductive layer 12a, and the insulating adhesive layer 13a. Here, the second electromagnetic shielding coverlay film 23 a is made of the same or different material as the first electromagnetic shielding coverlay film 23.

  In the FPC having the two-layer structure, it is preferable to use a synthetic resin containing oligophenylene ether and a styrene-butadiene elastomer for the insulating adhesive layers 13 and 13a. A liquid crystal polymer is preferably used for the base film 15. In this combination, as in the case of the FPC having the single layer structure shown in FIG. 2, the signal wirings 16 and 16a in this FPC exhibit extremely excellent transmission characteristics of high-speed digital signals in the several GHz to several tens GHz band. Then, for example, an FPC having a wiring circuit corresponding to a high-speed digital signal reaching the GHz band like a CPU clock of a computer is easily provided.

  Note that, in the base film 15 in the two-layer structure FPC, in addition to the liquid crystal polymer, a PI resin, a PEN resin, or a composite resin such as these is preferably used. Alternatively, other thermoplastic resins or thermosetting resins having low dielectric properties can be used.

  In the above embodiment, the case of an FPC in which a wiring having a single layer structure or a two layer structure is formed has been described. Even in the case of a multilayer wiring board having three or more layers of wiring, the surface of the FPC is similarly applied. An electromagnetic shielding coverlay film can be formed.

  In the present embodiment, the insulating adhesive layer 13 of the electromagnetic shielding coverlay film is adjusted to have a small elastic modulus and is particularly rich in elasticity in the tensile direction. And it absorbs the stress from the conductive layer 12 generated by bending the FPC and has a high buffering function. For this reason, when the FPC is bent, the circuit wiring of the FPC is not easily subjected to stress from the conductive layer 12 and is difficult to be disconnected. The stress from the conductive layer 12 generated in the circuit wiring of the FPC increases as the thickness increases, and the disconnection becomes remarkable as the circuit wiring becomes finer. In this case, the above-described high stress absorbing ability of the insulating adhesive layer 13 works remarkably with respect to FPC bending. The FPC has excellent bending characteristics.

  For this reason, the conductive layer 12 can be formed as a copper foil with a thickness exceeding 1 μm, for example, while maintaining the excellent bending characteristics of the FPC. In the FPC, the ground potential is stabilized, and the specific impedance and differential impedance of the circuit wiring become extremely stable. In addition, crosstalk between circuit wirings can be stably prevented. At the same time, the FPC is shielded against electromagnetic waves.

  Moreover, the electromagnetic shielding coverlay film and the FPC using the same can be made inexpensive by making the conductive layer 12 an aluminum metal such as an aluminum foil. Further, in the FPC, by forming the conductive member 18 that electrically connects the circuit wiring and the conductive layer 12 using a solder ball, the cost of manufacturing the FPC can be easily reduced.

  In the case of the electromagnetic shielding coverlay film of the present embodiment, an adhesive that increases the relative dielectric constant and dielectric loss tangent as in the prior art is used in joining and integrating the base film 15 and the circuit wiring with the electromagnetic shielding coverlay film. You don't have to. Furthermore, as described above, stable bonding can be achieved by combining the base film 15 and the electromagnetic shielding coverlay film. Then, it becomes easy to reduce the relative dielectric constant of the substrate material and the dielectric loss tangent thereof in the FPC manufactured by joining and integrating them. Therefore, such an FPC can easily increase the speed of high-frequency signals and reduce the dielectric loss, and can be a high-performance one having excellent transmission characteristics.

  Furthermore, the glass transition point of the insulating adhesive layer can be 180 ° C. or higher, and an FPC having high heat resistance and high bending resistance becomes possible. And it becomes possible to meet the demand for in-vehicle devices such as automobiles. In this case, the cover film-like substrate 11 and the base film 15 are preferably made of a liquid crystal polymer, a PI resin, or a PEN resin.

  In addition, the insulating adhesive layer used in the present embodiment can have a glass transition point of 230 ° C. or higher by adjusting its composition. For this reason, in mounting components such as semiconductor elements on the FPC, it is possible to realize a high heat resistance FPC that can withstand even a severe environment in which lead-free solder reflow at about 230 ° C. is performed.

  Next, the effects of the present invention will be described in detail with reference to examples, but the present invention is not limited to the following examples.

(Example 1, Comparative Example 1)
An FPC sample (test coupon) for the MIT bending test was produced by the method for manufacturing a flexible wiring board described in the embodiment. The test coupon of Example 1 is a flat cable having a width of 15 mm and a length of 220 mm. Here, the cover film-like substrate 11 is made of a polyimide resin having a thickness of 25 μm, the conductive layer 12 is made of an aluminum foil having a thickness of 9 μm, and the insulating adhesive layer 13 is an ADFEMA OPE system having a thickness of 50 μm (trade name, manufactured by NAMICS). ) Resin. The base film 15 is made of a liquid crystal polymer having a thickness of 25 μm, and a signal wiring for a disconnection test having a thickness of 20 μm and a line width of 40 μm is disposed on the main surface. In Comparative Example 1, the structure is the same as that of Example 1 except that the conductive layer 12 is a copper foil having a thickness of 9 μm.

  And the bending characteristic of FPC of Example 1 and Comparative Example 1 was evaluated. Here, the evaluation method of a bending characteristic is a bending resistance test and a bending resistance test according to JIS-C5016. These results are summarized in FIG. 6 as an average value when six test coupons are tested.

  The bending resistance test was a slide test, and the bending outer diameter was 6 mm (folding radius 3 mm × 2). The slide conditions were such that the temperature was 85 ° C., the stroke was 30 mm, and the number of reciprocations was 30 times / minute. Then, the number of times the resistance of the signal wiring for the disconnection test becomes twice the initial value is set to the number of times that the circuit wiring is disconnected.

  For the folding test, an MIT bending tester manufactured by Toyo Seiki Co., Ltd. was used. Here, under the conditions of a load of 500 g, a refraction angle of 135 °, a refraction cycle of 175 cpm, a bending portion curvature radius of 3 mm, and a room temperature (25 ° C.), the number of times until the disconnection test signal wiring was turned off was measured. .

(Evaluation results)
As shown in FIG. 6, in Example 1, the resistance value of the signal wiring for the disconnection test was not changed in all the samples even when the bending resistance test was 250,000 times. Therefore, the further slide test was canceled and exceeded 250,000 times. On the other hand, in the comparative example 1, the average is about 2,000 times from 1,700 times to 2,200 times. In the case of the bending resistance test, the bending property of Example 1 is improved at least 100 times that of Comparative Example 1.

  In Example 1, the folding resistance test was 51,728 times to 91,348 times, and the average value was 66,071 times. On the other hand, in the comparative example 1, the average is 1,470 times from 963 times to 2,299 times. In the case of the bending resistance test, the bending property of Example 1 is improved to about 50 times that of Comparative Example 1.

  From these results, in the electromagnetic shielding coverlay film, when the conductive layer 12 becomes thicker, the aluminum foil greatly improves the bending characteristics of the FPC than the copper foil, particularly in the case of the thick conductive layer 12. It was confirmed to be suitable for.

  Although the preferred embodiments of the present invention have been described above, the above-described embodiments do not limit the present invention. Those skilled in the art can make various modifications and changes in specific embodiments without departing from the technical idea and technical scope of the present invention.

  For example, the electromagnetic shielding coverlay film described in the present embodiment is applied so as to cover and integrally bond the surface of a rigid wiring board in addition to a flexible wiring board, for example. However, in this case, the excellent bending characteristics of the electromagnetic shielding coverlay film are not utilized much.

  The present invention is not limited to a flexible wiring board that transmits a high-frequency signal as described in the present embodiment. For example, it should be noted that the present invention is similarly effective even in the case of an FPC that transmits a low-frequency electrical signal of less than 1 MHz.

  DESCRIPTION OF SYMBOLS 11, 11a ... Cover film-like base material, 12, 12a ... Conductive layer, 13, 13a ... Insulating adhesive layer, 14 ... Release material layer, 15 ... Base film, 16, 16a ... Signal wiring, 17, 17a ... Ground wiring , 18, 18a ... conductive member, 19 ... copper foil, 20 ... flux, 21 ... solder ball, 22 ... laminated cushion sheet, 23 ... first electromagnetic shielding coverlay film, 23a ... second electromagnetic shielding coverlay film

By the way, in an electronic device in which an electronic component such as a semiconductor device that speeds up data processing is mounted, the operating frequency increases with the miniaturization. Therefore, the FPC is required to have excellent electrical signal transmission characteristics due to stable impedance and low transmission loss in high-frequency signals. For example, when a high-speed digital signal having a frequency of an electrical signal of several GHz to several tens of GHz is used, high-speed transmission is required without impairing the frequency characteristics . Therefore, for example, in the case of an FPC used in an electronic device that transmits an image with high definition, stabilization of the specific impedance or differential impedance is strongly required.

Here, when the conductive layer 12 is formed to a thickness exceeding 1 μm, such as an aluminum foil or a copper foil, the ground potential of the conductive layer 12 and the ground wiring 17 in the FPC becomes stable. And the crosstalk between the signal wirings 16 is stably prevented. Further, the characteristic impedance, differential impedance, etc. of the signal wiring 16 are stabilized. The conductive layer 12 also functions as an electromagnetic wave shield.

For this reason, the conductive layer 12 can be formed as a copper foil with a thickness exceeding 1 μm, for example, while maintaining the excellent bending characteristics of the FPC. In the FPC, the ground potential is stabilized, and the characteristic impedance, differential impedance, and the like of the circuit wiring become extremely stable. In addition, crosstalk between circuit wirings can be stably prevented. At the same time, the FPC is shielded against electromagnetic waves.

Claims (7)

  An electromagnetic shielding coverlay film in which a film-like substrate made of an insulating resin, a conductive layer, and an insulating adhesive layer containing an elastomer are laminated in this order.   The electromagnetic shielding coverlay film according to claim 1, wherein the conductive layer is made of aluminum metal.   The electromagnetic shielding coverlay film according to claim 1, wherein the insulating adhesive layer is made of a synthetic resin containing an oligophenylene ether and a styrene butadiene elastomer.   The electromagnetic shielding coverlay film according to claim 2 or 3, wherein the conductive layer is an aluminum foil, and the base material is a polyimide resin laminated on the conductive layer.   Circuit wiring is formed on a main surface of a base film made of an insulating resin, and the electromagnetic shielding coverlay film according to any one of claims 1 to 4 is insulated from the base film and the circuit wiring. A flexible wiring board characterized in that a predetermined circuit wiring of the circuit wiring and the conductive layer are electrically connected to each other by a conductive member through a conductive adhesive layer.   The flexible wiring board according to claim 5, wherein the conductive member is formed of a solder ball.   5. The solder ball is placed on a predetermined circuit wiring among the circuit wirings formed on the main surface of the base film made of an insulating resin, and the circuit wiring and the solder ball are any one of claims 1 to 4. The insulating adhesive layer of the electromagnetic shielding coverlay film described in the paragraph is overlaid and heated and pressed, and the solder ball is inserted into the insulating adhesive layer by the heating and pressing process, thereby the electromagnetic shield A flexible wiring board characterized in that it is electrically connected to the conductive layer of a conductive cover lay film, and an electromagnetic shielding cover lay film is joined and integrated to the circuit wiring and the base film via the insulating adhesive layer. Manufacturing method.

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