JP2016115725A - Electromagnetic shield sheet and printed wiring board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic wave shield sheet, the step followability and conductivity of which are compatible when it is thermo-compression bonded to a wiring board having a step.SOLUTION: In an electromagnetic wave shield sheet including a conductive layer containing flake-like conductive fine grains and binder resin, and an isolating layer, average aspect ratio of the flake-like conductive fine grains on the cut surface of the conductive layer is 7-15, the area occupied by the components other than the conductive fine grains is 55-80 when the cross-sectional area of the conductive layer before thermo-compression bonding is 100, and the difference of area occupied by the components other than the conductive fine grains is 5-25 before and after the electromagnetic wave shield sheet is subjected to thermo-compression bonding for 30 minutes at 150°C and 2 MPa.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an electromagnetic wave shielding sheet that shields electromagnetic waves.

  Among the printed boards used in electronic devices, flexible printed boards (hereinafter referred to as FPCs) utilize the functions that can be bent, connecting wiring between printed boards and connecting wiring between liquid crystal panels and printed boards, etc. It is used for mobile devices such as mobile phones, smartphones, tablet terminals, etc. Since electromagnetic waves generated from the outside and internal parts of the electronic device cause malfunction of the electronic device, it is common to form an electromagnetic wave shielding layer on the FPC. This electromagnetic wave shielding layer is generally formed by heat-pressing an electromagnetic wave shielding sheet to an FPC.

Mobile devices are increasing the density of printed wiring boards in order to realize multiple functions such as camera functions and GPS functions. To further increase the number of functions, it is necessary to further increase the density of wiring circuits. There is. However, since it is not realistic to increase the size of the mobile device without limitation, it is necessary to realize a high density by designing the printed wiring board to be thin. The pursuit of this thinness is no exception to the electromagnetic shielding sheet used as the electromagnetic shielding layer.
However, in general, when the thickness of the electromagnetic wave shielding sheet is reduced, when thermocompression bonding is applied to an FPC having a step, the conductive layer extends excessively at the stepped portion, the thickness is reduced, and the electric resistance value is increased. In some cases, the property could not be obtained, or the electromagnetic wave shield sheet itself was broken.

  Therefore, Patent Document 1 discloses an electromagnetic wave shielding sheet using a solvent-soluble polyimide for the insulating layer.

JP 2013-65675 A

  However, the conventional electromagnetic wave shielding sheet has good step followability of the insulating layer at the time of thermocompression bonding, but the conductive layer is insufficient in step followability of the conductive layer, so the conductive layer is excessively elongated at the step and becomes too thin. There was a problem that it was difficult to obtain a uniform shielding property over the entire electromagnetic wave shielding layer due to breakage.

  An object of the present invention is to provide an electromagnetic wave shielding sheet that achieves both step followability and conductivity when heat-pressed on a wiring board having a step.

The electromagnetic wave shielding sheet of the present invention is an electromagnetic wave shielding sheet comprising a conductive layer containing flaky conductive fine particles and a binder resin, and an insulating layer,
The average aspect ratio of the flaky conductive fine particles at the cut surface of the conductive layer is 7 to 15,
The area occupied by components other than the conductive fine particles when the cross-sectional area of the conductive layer before thermocompression bonding is 100 is 55 to 80,
The electromagnetic wave shielding sheet is characterized in that the difference in area occupied by components other than the conductive fine particles before and after thermocompression bonding is 5 to 25 under the conditions of 150 ° C., 2 MPa, and 30 minutes.

  According to the present invention, in the cross-section cut in the thickness direction of the electromagnetic wave shielding sheet, the flaky conductive fine particles having a specific aspect ratio are overlapped in the conductive layer, and the flaky shape is formed before thermocompression bonding. Components other than the flaky conductive fine particles (mainly thermosetting resin) that existed in a state of low density between the conductive fine particles are compressed into flakes when the electromagnetic wave shield sheet is thermocompression bonded under predetermined conditions. A conductive layer in which the interval between the fine conductive particles is reduced and the fine particles filled with the flaky conductive fine particles are densely packed is obtained. For example, when an electromagnetic wave shielding sheet having a conductive layer having such characteristics is laminated on a printed wiring board having a step, the conductive flakes are overlapped when the conductive layer extends at the corner of the step. A portion where the overlap of the flaky conductive fine particles follows the elongation is hardly generated, so that the resistance value is hardly deteriorated at the step portion, and the shielding property is hardly lowered. In addition, since the components other than the conductive fine particles (mainly binder resin) are also filled at high density between the flaky conductive fine particles, the conductive layer can be formed by the binder resin following the corners of the step well during thermocompression bonding. The effect that it was hard to break was obtained.

According to the present invention, it is possible to provide an electromagnetic wave shield sheet that has both step following ability and conductivity when thermocompression bonded to a wiring board having a step.

Cross-sectional photograph of electromagnetic shielding sheet before and after thermocompression bonding Cross-sectional view illustrating the layer structure of the electromagnetic shielding sheet Sectional drawing explaining a level | step difference test. Sectional drawing explaining how to create a sample for measurement of connection resistance and bending test

  Before describing the present invention, terms will be defined. First, an electromagnetic wave shielding sheet forms an electromagnetic wave shielding layer by heat-pressing on the other party, such as a printed wiring board. Sheet is synonymous with film and tape. The plane direction of the electromagnetic wave shielding layer is a direction that is not the thickness direction. The aspect ratio is a numerical value obtained by dividing the major axis length by the minor axis length when the major axis length and the minor axis length exist in the particle. The flake shape is a concept including leaf shape, scale shape, plate shape, disk shape and the like. In the description of the drawings, the upward direction is upward and the downward direction is downward.

The electromagnetic wave shielding sheet of the present invention includes a conductive layer containing flaky conductive fine particles and a binder resin, and an insulating layer.
The average aspect ratio of the flaky conductive fine particles at the cut surface of the conductive layer is 7 to 15,
The area occupied by components other than the conductive fine particles when the cross-sectional area of the conductive layer before thermocompression bonding is 100 is 55-80,
The electromagnetic wave shielding sheet is characterized in that the difference in area occupied by components other than the conductive fine particles before and after thermocompression bonding under conditions of 150 ° C. and 2 MPa for 30 minutes is 5 to 25.
That is, in the electromagnetic wave shielding sheet of the present invention, the step following property is improved by thinning the conductive layer within an appropriate range after thermocompression bonding. The electromagnetic wave shielding sheet becomes an electromagnetic wave shielding layer after thermocompression bonding.

Specifically, the area occupied by components other than the conductive fine particles when the cross-sectional area of the conductive layer before thermocompression bonding is 100 is 55 to 80, and the area occupied by components other than the conductive fine particles before and after thermocompression bonding is The difference is 5-25.
When the electromagnetic wave shielding sheet of the present invention is heat-pressed, the cross-sectional area 55-80 occupied by components other than the conductive fine particles of the conductive layer is reduced by 5-25. In order to realize such a reduction in the cross-sectional area, the average aspect ratio of the cross section of the flaky conductive fine particles needs to be 7 to 15. This reduction in cross-sectional area has an average aspect ratio of 7 to 15 in the conductive layer before thermocompression bonding. That is, the overlapping of the flake-shaped conductive fine particles having a small thickness and a long shape in the horizontal direction is dense, and the binder resin is filled between the flake-shaped conductive fine particles. Next, when the electromagnetic wave shielding sheet is heat-pressed to a printed wiring board or the like having a step, the conductive layer is thinned at the corners of the step, and the conductive fine particles tend to be interrupted. However, since the present invention includes flaky conductive fine particles that are thin and long in the horizontal direction, it is easy to maintain the overlap between the fine particles, and voids are not easily generated in the conductive layer. Furthermore, since the binder resin can follow the elongation of the electromagnetic wave shielding sheet well, the conductive layer is hardly broken and the conductivity can be maintained. Thereby, the effect that the shielding property of the electromagnetic wave shielding layer is hardly lowered was obtained. The difference between the cross-sectional areas occupied by components other than the conductive fine particles before and after thermocompression bonding is also referred to as “cross-sectional area difference”.
An example of the change of the conductive layer before and after thermocompression bonding of the electromagnetic wave shielding sheet of the present invention will be described with reference to FIG. FIG. 1 (1) is a laminate of an electromagnetic shielding sheet provided with an insulating layer 1 and a conductive layer 2 and a peelable sheet 3 in an electron micrograph of the electromagnetic shielding sheet before thermocompression bonding. In FIG. 1 (2), the laminate is heated and pressure-bonded to fill the gaps between the flaky conductive fine particles of the conductive layer 2 and the fine particle arrangement becomes dense, so that components other than the flaky conductive fine particles are present. It can be seen that the occupied area is reduced as compared with that before the thermocompression bonding. Needless to say, the present invention is not limited to the embodiment shown in FIG.

<Conductive layer>
The conductive layer can be formed using a conductive resin composition. The conductive resin composition contains components other than flaky conductive fine particles and flaky conductive fine particles.

  The flaky conductive fine particles have a cross-sectional average aspect ratio (vertical / horizontal) of 7 to 15, and more preferably 10 to 14. When the aspect ratio of the flaky conductive fine particles is 7 to 15, both the elongation of the conductive layer and the conductivity during thermocompression bonding can be achieved.

As the material for the flaky conductive fine particles, for example, conductive metals such as gold, platinum, silver, copper and nickel, alloys thereof, and conductive polymer fine particles are preferable. The flaky conductive fine particles use not only fine particles of a single composition but also composite fine particles in which a metal or a resin is used as a core, and a coating layer covering the surface of the core is formed of a material having higher conductivity than the core. It is also preferable. This facilitates cost reduction.
The core is preferably selected from nickel, silica, copper, resin, and the like, more preferably a conductive metal and an alloy thereof.
The covering layer may be a material having excellent conductivity, and is preferably a conductive metal or a conductive polymer. Examples of the conductive metal include gold, platinum, silver, tin, manganese, indium, and the like, and alloys thereof. Examples of the conductive polymer include polyaniline and polyacetylene. Among these, silver is preferable from the viewpoint of conductivity.

  The composite fine particles preferably have a coating layer at a ratio of 1 to 40 parts by weight, more preferably 3 to 30 parts by weight, with respect to 100 parts by weight of the nucleus. Covering with 1 to 40 parts by weight can further reduce the cost while maintaining conductivity.

  It is preferable that the conductive fine particles composed of the nucleus and the coating layer are completely covered with the nucleus. However, in practice, a part of the nucleus may be exposed. Even in such a case, if the conductive material covers 70% or more of the core surface area, the conductivity is easily maintained.

  The shape of the flaky conductive fine particles is a flaky shape, but may be a flaky shape as a whole of the fine particles, and may be elliptical, circular or notched around the fine particles.

The flaky conductive fine particles preferably have an average particle diameter of 1 to 100 μm, more preferably 3 to 50 μm. The electroconductivity and thinness of a conductive layer improve more because an average particle diameter is 1-100 micrometers. The average particle diameter is a D50 average particle obtained by measuring conductive fine particles with a tornado dry powder sample module using a laser diffraction / scattering particle size distribution analyzer LS 13320 (manufactured by Beckman Coulter, Inc.). It is a particle diameter, and the cumulative value in the particle diameter cumulative distribution is a particle diameter of 50%. In the measurement, the refractive index of the flaky conductive fine particles was set to 1.6.
The average particle diameter of the conductive fine particles can also be obtained from a numerical value obtained by averaging about 20 fine particles randomly selected from an enlarged image (about 1,000 to 10,000 times) of an electron microscope. In this case, the average particle size is also preferably 1 to 100 μm, and more preferably 3 to 50 μm.

  If it is in the range which can solve the subject of this invention in the conductive resin composition, in addition to the flaky conductive fine particles, conductive fine particles of other shapes can be blended. Adhesive strength is further improved by blending conductive fine particles of other shapes.

  Examples of the conductive fine particles having other shapes include dendritic and spherical conductive fine particles. The composition other than the shape of the conductive fine particles having other shapes is the same as that of the flaky conductive fine particles. When the flaky conductive fine particles and other shapes of conductive fine particles are used in combination, the area occupied by components other than the conductive fine particles when the cross-sectional area of the conductive layer before thermocompression bonding is 100 is about 55 to 80. Components other than the conductive fine particles indicate components other than the flaky conductive fine particles and the conductive fine particles of other shapes. In addition, regarding the electromagnetic shielding sheet at 150 ° C., 2 MPa, for 30 minutes, the difference in area occupied by components other than the conductive fine particles before and after thermocompression bonding is 5 to 25. Components other than fine particles and other shapes of conductive fine particles are shown.

  Components other than the flaky conductive fine particles include a binder resin. The binder resin preferably has a glass transition temperature (hereinafter referred to as Tg) of -20 to 100 ° C, more preferably 0 to 80 ° C. Binder resins can be used alone or in combination of two or more. In addition, when using 2 or more types together, it is preferable that Tg is -20-100 degreeC binder resin as a main component.

  The binder resin may be selected as long as the problem of the present invention can be solved, and can be appropriately selected from a thermoplastic resin and a thermosetting resin. Further, the binder resin is preferably a thermoplastic resin in applications where there is no heating process such as a reflow process in the post-process of thermocompression bonding. On the other hand, when there is a heating process such as a reflow process in the subsequent process of thermocompression bonding, a thermosetting resin is preferable. As the thermosetting resin, a self-crosslinking type and a curing agent reaction type can be used. As the curing agent reaction type binder resin, a thermosetting resin having a reactive functional group capable of reacting with the curing agent is suitable.

  Examples of the thermosetting resin include epoxy, acrylic, urethane, polystyrene, polycarbonate, polyamide, polyamideimide, polyesteramide, polyether ester, urethane urea, addition-type polyester, condensation-type polyester, and polyimide. The thermosetting resin usually has a functional group capable of self-crosslinking or a functional group capable of reacting with a curing agent. Among these, for example, considering the temperature conditions in the reflow process, the thermosetting resin may contain at least one of epoxy, addition type polyester, condensation type polyester, urethane, urethane urea, and polyamide. preferable. Moreover, if it is the range which can endure a reflow process, a thermosetting resin and a thermoplastic resin can be used together.

  The curing agent has a plurality of functional groups capable of reacting with the reactive functional group of the thermosetting resin. The curing agent is preferably an epoxy compound, an acid anhydride group-containing compound, an isocyanate compound, an aziridine compound, a dicyandiamidoamine compound, a phenol compound, or the like.

  It is preferable that 1-50 mass parts is included with respect to 100 mass parts of thermosetting resins, as for a hardening | curing agent, 3-30 weight part is more preferable, and 3-20 weight part is further more preferable.

  Examples of the thermoplastic resin include polyester, acrylic, polyether, urethane, styrene elastomer, polycarbonate, butadiene, polyamide, ester amide, isoprene, and cellulose.

  The flaky conductive fine particles are preferably blended in an amount of 50 to 1500 parts by weight, more preferably 100 to 600 parts by weight with respect to 100 parts by weight of the binder resin. By mix | blending 50-1500 weight part, it becomes easy to make electroconductivity and adhesiveness compatible more.

  Moreover, it is preferable to mix | blend 5-300 weight part with respect to 100 weight part of binder resin, and, as for the conductive fine particle of another shape, 10-250 weight part is more preferable.

  The conductive resin composition preferably contains an ion catcher agent. When the ion catcher agent is blended, when the flaky conductive fine particles contain a metal, it is possible to further suppress the deterioration of the conductivity and adhesion due to dissociated metal ions over time. Examples of the ion catcher agent include N-salicyloyl-N′-aldehyderazine, N, N-dibenzal (oxal hydrazide), bis (2-phenoxypropionylhydrazine) isophthalic acid, [3- (N-salicyloyl) amino-1 , 2,4-hydroxyphenyl) propionyl] hydrazine and the like. Among these, decamethylene carboxylic acid disalicyloyl hydrazide and N, N′-bis [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionyl] hydrazine are preferable because of their high scavenging effect.

  The ion catcher agent is preferably blended in an amount of 0.5 to 30 parts by weight, more preferably 1 to 20 parts by weight with respect to 100 parts by weight of the flaky conductive fine particles. By blending 0.5 to 30 parts by weight, it is possible to further suppress the deterioration of conductivity and adhesion over time.

  The conductive resin composition preferably contains a thickener. By containing a thickener, the dispersion stability of the flaky conductive fine particles in the conductive resin composition is improved. Examples of the thickener include polycarboxylic acid compounds, polyurethane compounds, urea compounds, polyamide compounds, and the like.

  The conductive resin composition includes silane coupling agents, rust inhibitors, reducing agents, antioxidants, pigments, dyes, tackifying resins, plasticizers, ultraviolet absorbers, antifoaming agents, leveling regulators as other optional components. , Fillers and flame retardants can be blended.

  The conductive resin composition can be obtained by mixing and stirring the flaky conductive fine particles and the binder resin. For the stirring, a known stirring device such as a disper or a homogenizer can be used.

  The conductive layer can be formed by applying a conductive resin composition to a peelable sheet. Alternatively, the conductive layer can be formed into a sheet shape by an extrusion molding machine such as a T-die molding machine or calendar molding.

  Coating, for example, gravure coating method, kiss coating method, die coating method, lip coating method, comma coating method, blade method, roll coating method, knife coating method, spray coating method, bar coating method, spin coating method, dip coating method A known coating method such as can be used. During the coating, a drying process can be performed as necessary. In the drying step, a known drying device such as a hot air dryer or an infrared heater can be used.

  1-100 micrometers is preferable, as for the thickness of a conductive layer, 3-50 micrometers is more preferable, and 4-15 micrometers is further more preferable. It becomes easy to make electroconductivity and other physical properties compatible because thickness exists in the range of 1-100 micrometers.

<Insulating layer>
The insulating layer can be formed using an insulating resin composition. The insulating resin composition includes a binder resin.

  As the binder resin, the binder resin described in the conductive layer can be used. The binder resin used for the insulating layer and the conductive layer may be the same or different.

  Insulating resin compositions include silane coupling agents, antioxidants, pigments, dyes, tackifying resins, plasticizers, ultraviolet absorbers, antifoaming agents, leveling regulators, fillers, inorganic fillers as necessary. A flame retardant or the like may be added. The inorganic filler is different from the flaky conductive fine particles.

  The insulating layer can be formed in the same manner as the conductive layer using the insulating resin composition.

  Although the thickness of an insulating layer can be designed suitably according to a use, 0.5 micrometers-25 micrometers are preferable and 2 micrometers-15 micrometers are more preferable. Heat resistance improves more because the thickness of an insulating layer is 0.5 micrometer-25 micrometers.

<Electromagnetic wave shield sheet>
The electromagnetic wave shielding sheet of the present invention includes a conductive layer and an insulating layer. This electromagnetic wave shielding sheet can be manufactured, for example, by bonding an insulating layer obtained by previously molding an insulating resin composition to a conductive layer. Or it can also manufacture by bonding the insulating layer separately formed on the peelable sheet to the conductive layer previously formed. Or it can also manufacture by forming an insulating layer by coating an insulating resin composition directly on a conductive layer. Another functional layer can be further laminated on the insulating layer. The other functional layer is a layer having functions such as hard coat property, light shielding property, heat dissipation property, water vapor barrier property, oxygen barrier property, low dielectric constant, high dielectric constant property or heat resistance.

  The electromagnetic shielding sheet of the present invention preferably has a configuration in which a conductive layer, a metal thin film layer, and an insulating layer are sequentially laminated. The metal thin film layer is preferably made of gold, platinum, silver, tin, manganese, indium, and the like, and alloys thereof, and copper is preferred in terms of cost and conductivity. Copper is preferably an electrolytic copper foil and a rolled copper foil. When the metal thin film layer is a metal foil, 0.1 to 20 μm is preferable, and 0.5 to 10 μm is more preferable. When the metal thin film layer is a deposited film, 0.001 to 5 μm is preferable, and 0.002 to 2 μm is more preferable. When the metal thin film layer is a sputtered film, 0.001 to 1.0 μm is preferable, and 0.002 to 0.5 μm is more preferable. Moreover, when the metal thin film layer is a film formed of a conductive paste, 0.1 to 20 μm is preferable, and 0.5 to 10 μm is more preferable.

  The layer structure of the electromagnetic wave shielding sheet of the present invention will be described with reference to FIG. FIG. 2A is a cross-sectional view of a configuration in which an insulating layer 1 and a conductive layer 2 are stacked in this order. FIG. 2B is a cross-sectional view of a configuration in which the insulating layer 1, the metal thin film layer 4, and the conductive layer 2 are laminated in this order. In the electromagnetic wave shielding sheet of the present invention, other layers may be laminated in addition to the three layers.

  The peelable sheet is a sheet obtained by performing a known peeling treatment on at least one of a paper or plastic substrate.

  In general, the electromagnetic wave shielding sheet is stored in a state where a peelable sheet is pasted until just before use in order to facilitate the protection and handling of the conductive layer or the insulating layer.

The electromagnetic wave shielding sheet of the present invention can be used as an electromagnetic wave shielding layer by being attached to a flexible printed circuit board, a rigid printed circuit board, a rigid flexible substrate, etc. and thermocompression bonded. The general conditions for thermocompression bonding can be appropriately selected from a temperature of 150 to 180 ° C., a pressure of 10 to 60 kg / cm ² , and a time of about 3 to 60 minutes.
In the present invention, the area 55 to 80 occupied by components other than the conductive fine particles when the electromagnetic shield sheet is thermocompression bonded under conditions of 150 ° C., 2 MPa, 30 minutes, and the cut surface of the conductive layer before thermocompression bonding is defined as 100. It decreases for 5 to 25 minutes before and after thermocompression bonding. In the electromagnetic wave shielding sheet of the present invention having such a conductive layer, the conductive path between the flaky conductive fine particles is not easily interrupted, and the followability to the extension of the binder resin is good. When the electromagnetic wave shielding layer is formed by thermocompression bonding, a shielding property substantially equivalent to that of the flat portion of the printed wiring board can be obtained even at the step portion.

A method for observing the cut surface of the conductive layer will be described.
First, there are known methods such as a cleaving method, a mechanical polishing method, a microtome method, and a FIB (focused ion beam) method for cutting the conductive layer and exposing the cross section. However, layers containing different materials with different hardness (flaky conductive fine particles and binder resin), such as conductive layers, have structural deformations such as peeling of different interfaces and deformation of voids, so-called artifacts, during cross-section preparation. This may occur and a true cross-sectional structure may not be obtained. On the other hand, the CP (cross section polisher) method is a cross-section preparation method using a broad Ar (argon) ion beam, and even a metal, a semiconductor, a ceramic, and a composite material thereof create a smooth and undistorted sample cross section. be able to. That is, in the present invention, the method of cutting the conductive layer is preferably the CP method.

Next, a method for obtaining an image of components other than the flaky conductive fine particles and the flaky conductive fine particles on the cut surface of the conductive layer will be described.
The cut surface of the conductive layer is observed using a scanning electron microscope (SEM).
When the cut surface is observed by SEM from the vertical direction, when the surface of the cut surface is uniformly deposited by metal, a contrast difference is generated between the resin layer and the metal layer by the atomic number effect, and the shape of the metal can be recognized. Specifically, since the metal is white and the resin layer is color-coded from gray to black, the resin portion and the metal portion can be distinguished and distinguished.
With the image analysis free software “GIMP 2.6.11”, this SEM image can be easily binarized into black and white. At the same time, by counting the number of black and white pixels, the ratio of the area of the flaky conductive fine particles of the conductive layer and the area other than the flaky conductive fine particles (the main component is the binder resin) is calculated from the ratio of the number of pixels. it can.

  The average aspect ratio of the flaky conductive fine particles on the cut surface of the conductive layer is observed by SEM after cutting the conductive layer by the same method as described above. The magnification at the time of observation is arbitrarily set at a magnification at which 30 or more flaky conductive fine particles can be confirmed. Then, using the analysis software of Mac-View Ver.4 (Mounttech), read the SEM image of the conductive fine particles, eliminate the upper limit side 15% and the lower limit side 15% of the fine particles, the central region of the particle diameter Select about 20 in manual recognition mode. When selecting the conductive fine particles on the cut surface, for the fine particles that can confirm the entire shape in which the fine particles do not overlap each other, after extracting about 30 fine particles at an angle at which the plane plate is perpendicular from the observation viewpoint, Select fine particles. The average aspect ratio of the fine particles is calculated as a projected area equivalent circle diameter, and the distribution is calculated as a volume distribution setting.

  The electromagnetic wave shielding sheet of the present invention can be used for various applications where electromagnetic waves need to be shielded. For example, it can be used for rigid printed wiring boards, flexible printed wiring boards, COF, TAB, flexible connectors, liquid crystal displays, touch panels, and the like. It can also be used as a member for shielding electromagnetic waves from cases such as mobile phones, smartphones, tablet terminals and personal computers, building materials such as walls of building materials and window glass, vehicles, ships and aircraft.

In the printed wiring board of the present invention, an electromagnetic wave shielding sheet is heat-pressed to at least one of the cover lay layer or the insulating base material side with respect to the wiring board provided with the cover lay layer, the signal wiring, and the insulating base material. It is preferable that it is used on the coverlay layer side (polyimide film 15) as shown in (2) of FIG. Since the printed wiring board of the present invention uses an electromagnetic wave shielding sheet with good step followability, the decrease in conductivity is suppressed at the corners of the steps.
The signal wiring includes a ground wiring and a wiring circuit that transmits an electric signal to the chip. In addition, as the insulating substrate, glass epoxy is often used for rigid wiring boards, and polyimide is often used for FPC.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the examples do not limit the scope of rights of the present invention. In the examples, “part” represents “part by weight” and “%” represents “% by weight”.

  The conductive fine particles used in the examples are shown in Table 1. The conductive fine particles 1 used were dendritic composite fine particles (the core was made of copper and the coating layer was made of silver. D50 average particle size: 11 μm, manufactured by Fukuda Metal Foil Powder Co., Ltd.).

<Production example of conductive fine particles 2 to 6>
After stirring and mixing the conductive fine particles 1 (100.0 parts) and oleic acid (1.0 parts) so that the raw materials become uniform, the mixture is put into a table ball mill together with zirconia beads, and the dispersion process is performed to obtain flakes. Conductive fine particles were prepared. By sequentially extending the dispersion time, conductive fine particles 2 to 6 having different trans-mean aspect ratios were obtained. Further, from the obtained conductive fine particles, zirconia beads were removed and coarse particles were removed by sieving.
The average aspect ratio of the cut surface was measured by the method described later.

The binder resin used in the examples is shown below.
Urethane resin: Thermosetting urethane resin (Toyochem)
Polyester resin: addition-type polyester resin (manufactured by Toyochem)
Polyamide resin: Thermosetting polyamide resin (Toyochem)

<Example 1>
100 parts of urethane resin, 450 parts of conductive fine particles 4, 15 parts of an epoxy compound (bisphenol A type epoxy resin, epoxy equivalent = 189 g / eq, “JER828”, manufactured by Mitsubishi Chemical Corporation) as a curing agent, and an aziridine compound (Nippon Catalyst) “Chemitite PZ-33” (2.0 parts) was charged into a container, and a mixed solvent of toluene: isopropyl alcohol (weight ratio 2: 1) was added so that the non-volatile content was 40%. The conductive sheet was coated using a bar coater so as to have a dry thickness of 10 μm, and dried in an electric oven at 100 ° C. for 2 minutes to obtain a conductive layer.

  Separately, 100 parts of a urethane resin, 10 parts of an epoxy compound and 10 parts of an aziridine compound as a curing agent were added and stirred with a disper for 10 minutes, and then applied to a peelable sheet using a bar coater so that the dry thickness was 10 μm. And an insulating layer was obtained by drying in an electric oven at 100 ° C. for 2 minutes. Next, an electromagnetic wave shielding sheet was obtained by attaching an insulating layer to the conductive layer.

<Examples 2-11, Comparative Examples 1-3>
An electromagnetic wave shielding sheet was obtained in the same manner as in Example 1 except that the raw materials of Example 1 were changed to the raw materials and blending amounts shown in Table 2.

<Example 12>
100 parts of urethane resin, 200 parts of conductive fine particles 1 and 250 parts of conductive fine particles 4 are charged into a container, and a mixed solvent of toluene: isopropyl alcohol (weight ratio = 2: 1) is added so that the nonvolatile content concentration becomes 40%. And mixed. Next, 15 parts of an epoxy compound (bisphenol A type epoxy resin, epoxy equivalent = 189 g / eq, “JER828”, manufactured by Mitsubishi Chemical Corporation) and 2.0 parts of an aziridine compound (“Chemite PZ-33” manufactured by Nippon Shokubai Co., Ltd.) were added. After stirring for 10 minutes, the peelable sheet was coated using a bar coater so as to have a dry thickness of 10 μm, and dried in an electric oven at 100 ° C. for 2 minutes to obtain a conductive layer.

  Separately, 100 parts of a urethane resin, 10 parts of an epoxy compound and 10 parts of an aziridine compound as a curing agent were added and stirred with a disper for 10 minutes, and then applied to a peelable sheet using a bar coater so that the dry thickness was 10 μm. And an insulating layer was obtained by drying in an electric oven at 100 ° C. for 2 minutes. Next, after bonding the conductive layer described above to one surface of the electrolytic copper foil having a thickness of 3 μm, the insulating layer is bonded to the other surface of the electrolytic copper foil, thereby releasing the sheet / insulating layer / electrolytic copper foil / An electromagnetic wave shielding sheet having a configuration of conductive layer / peelable sheet was obtained.

  Physical properties were measured according to the following evaluation items. The results are shown in Table 2.

<Step test>
This will be described with reference to FIG. A polyimide film 15 with an adhesive having a thickness of 200 μm, a width of 20 mm, and a length of 30 mm, a stainless plate 16 having a thickness of 200 μm, a width of 50 mm, and a length of 50 mm, and the obtained electromagnetic wave shielding sheet 11 having a width of 40 mm and a length of 40 mm Prepared in size and used as a sample. An adhesive was applied onto the stainless steel plate 16 so that the polyimide film 15 was almost at the center, and was bonded by thermocompression bonding. Next, the peelable film on the conductive layer 14 side from the electromagnetic wave shielding sheet 11 was peeled off, and the electromagnetic wave shielding sheet 11 was placed so that the exposed conductive layer 14 and the polyimide film 15 were in contact with each other. These are pressure-bonded under the conditions of 150 ° C., 2 MPa, 30 min to cure the binder resin of the conductive layer 14 and the insulating layer 13, and then the peelable film 12 on the insulating layer side is peeled off, and the laminate shown in FIG. 17 was obtained.
And the fracture | rupture state of a conductive layer and an insulating layer was evaluated by observing the 50-times enlarged image which used the naked eye and the microscope for the level | step-difference part of the circular broken line 18 of the laminated body 17. FIG.
○: The electromagnetic wave shield seed did not break. Good results.
X: The electromagnetic wave shielding sheet broke. Impractical

<Connection resistance value>
An electromagnetic wave shielding sheet having a width of 20 mm and a length of 50 mm was prepared as Sample 25. 4 (1), the peelable film is peeled off from the sample 25, and the exposed conductive layer 25b is electrically connected to a separately prepared flexible printed wiring board (polyimide film 21 having a thickness of 25 μm). A copper foil circuit 22A and a copper foil circuit 22B with a thickness of 18 μm that are not connected are formed, and a through hole 24 with a thickness of 37.5 μm and a diameter of 1.6 mm is attached on the copper foil circuit 22A. The conductive layer 25b and the insulating layer 25a were cured by pressure bonding to the wiring board on which the cover film 23 was laminated) under the conditions of 150 ° C., 2 MPa, and 30 min. After the pressure bonding, the peelable film on the insulating layer 25a side was removed, and the connection resistance value between 22A and 22B shown in the plan view of FIG. 4 (4) was measured using a BSP probe of “Lorestar GP” manufactured by Mitsubishi Chemical. 4 (2) is a sectional view taken along the line DD ′ of FIG. 4 (1), and FIG. 4 (3) is a sectional view taken along the line CC ′ of FIG. 4 (1). Similarly, FIG. 4 (5) is a DD ′ sectional view of FIG. 4 (4), and FIG. 4 (6) is a CC ′ sectional view of FIG. 4 (4). The evaluation criteria for the connection resistance value are as follows.
A: Less than 300 mΩ Good results.
○: 300 mΩ or more and less than 700 mΩ There is no practical problem.
×: 1000 mΩ or more

<Heat resistance>
An electromagnetic wave shielding sheet was prepared to have a width of 10 mm and a length of 60 mm to prepare a sample. A 50 μm-thick polyimide film (“Kapton 200EN” manufactured by Toray DuPont) is pressure-bonded to the exposed conductive layer by peeling off the peelable sheet on the conductive layer side of the sample at 150 ° C., 2 MPa, 30 min. The insulating layer resin was cured. After curing, the insulating layer side peelable sheet was peeled off to expose the insulating layer. Next, the sample was heat-treated in an electric oven at 180 ° C. for 3 minutes and then in an electric oven at 280 ° C. for 90 seconds. And the external appearance of the sample after heat processing was observed visually, and the presence or absence of external appearance defects, such as foaming, a float, and peeling, was evaluated. The heat resistance was evaluated by the number of samples in which a test was performed with 5 samples each and appearance defects occurred. The heating at 280 ° C. corresponds to the heating temperature in the reflow process.
A: No appearance defect occurred. It is a good result.
○: One appearance defect occurred. There is no problem in practical use.
X: Three or more appearance defects occurred. Not practical.

<Bending test>
The test piece of FIG. 4 (4) was created in the same manner as the “connection resistance value” described above. After that, after the EE ′ of FIG. 4 (4) was folded in a mountain, the operation of returning to flat was performed to make one set. In the bending test, 20 sets were performed. The connection resistance value before and after the test between 22A and 22B shown in the plan view of FIG. 4 (4) was measured using a BSP probe manufactured by Mitsubishi Chemical Corporation “Lorestar GP”, and the rate of change was calculated. The evaluation criteria for the bending test are as follows.
A: Increase in connection resistance value is less than 20%.
○: Increase in connection resistance value is 20% or more and less than 80%.
X: Increase in connection resistance value is 80% or more.

<Adhesive strength>
An electromagnetic wave shielding sheet was prepared with a width of 25 mm and a length of 70 mm as a sample. By peeling the peelable sheet on the conductive layer side from the sample and pressing the polyimide film with a thickness of 50 μm (“Kapton 200EN” manufactured by Toray DuPont) under the conditions of 150 ° C., 2.0 MPa, and 30 min. The resin of the conductive layer and the insulating layer was cured. In order to reinforce the electromagnetic shielding sheet for adhesive strength measurement, the peelable film on the insulating layer side is peeled off, and an adhesive sheet formed from polyurethane polyurea adhesive is used for the exposed insulating layer, polyimide film (Toray DuPont) A laminate having a structure of “polyimide / electromagnetic wave shield sheet / adhesive sheet / polyimide” was obtained by pressure-bonding “Kapton 200EN” manufactured at 150 ° C., 2 MPa, 30 min. About this laminated body, adhesive force (N) is peeled by peeling the interface between the conductive layer and the polyimide film using a tensile tester in an atmosphere of 23 ° C. and 50% RH at a pulling speed of 50 mm / min and a peeling angle of 90 °. / 25 mm) was measured and used as the non-aging adhesive force. Separately, the laminate was left in a thermo-hygrostat set at 85 ° C. and 85% RH for 7 days, then left at 23 ° C. and 50% RH for 1 day, and then the adhesive strength of the laminate was measured in the same atmosphere. Adhesive strength was assumed. The evaluation criteria for adhesive strength are as follows.
A: 9 N / 25 mm or more Good results.
○: 3 N / 25 mm or more and less than 9 N / 25 mm No problem in practical use.
×: Less than 3N / 25mm

<Cross-sectional area difference of conductive layer>
After cutting the electromagnetic shielding sheet into a size of about 5 mm in width and 5 mm in length, 0.05 g of an epoxy resin (Petropoxy 154, manufactured by Marto) was dropped on the slide glass to adhere the electromagnetic shielding sheet. Furthermore, an electromagnetic wave shielding sheet and a polyimide film (“Kapton 200EN” manufactured by Toray DuPont) were bonded with the epoxy resin to obtain a laminate having a configuration of (slide glass / electromagnetic wave shielding sheet / polyimide film). The obtained laminate was cut by ion beam irradiation from the polyimide film side using a cross section polisher (manufactured by JEOL Ltd., SM-09010) to form a cross section of the conductive layer.

After the platinum cross section of the obtained conductive layer was deposited, an enlarged image was observed and stored using a field emission electron microscope (S-4700, manufactured by Hitachi, Ltd.). The observation conditions were acceleration voltage: 5 kV, emission current: 8 mA, and magnification: 2000 to 5000 times.
Use the free software "GIMP 2.6.11" to read the enlarged image obtained, specify the conductive layer range, adjust the threshold automatically to make the conductive fine particles white, and the components other than the conductive fine particles Converted to black. Thereafter, by selecting a black region (0 to 254) in the histogram, the percentage occupied by the components other than the flaky conductive fine particles when the percentage of the number of black pixels, that is, the cross-sectional area of the conductive layer is 100, is referred to as area (a). ) Was calculated. For the area (a), 5 samples were evaluated and the average value was calculated.
In addition, flaky conductive fine particles when the cross-sectional area of the conductive layer after thermocompression bonding is 100 as described above, except that the electromagnetic wave shielding sheet is cured by thermocompression bonding under the conditions of 150 ° C., 2.0 MPa, and 30 min. The area occupied by other components (referred to as area (b)) was calculated. And the cross-sectional area difference (Area (a) -Area (b)) of components other than the flaky conductive fine particles after the thermocompression bonding of the conductive layer after the thermocompression bonding was calculated from these numerical values.

<Aspect ratio>
After the platinum cross-section of the cross section of the conductive layer obtained by the same method as the above “cross-sectional area difference” was used, an enlarged image was observed and stored using a field emission electron microscope (S-4700, manufactured by Hitachi, Ltd.). The observation conditions were an acceleration voltage of 5 kV, an emission current of 8 mA, and a magnification arbitrarily set so that 30 or more flaky conductive fine particles could be observed.
The obtained enlarged image was read using the analysis software of Mac-View Ver.4 (Mounttech), and about 30 flaky conductive fine particles were selected in the manual recognition mode as already described. The upper limit value side 15% and the lower limit value side 15% of the fine particles were excluded, and the aspect ratio was obtained by setting the particle reference data as the projected area equivalent circle diameter and the distribution as the volume distribution.

DESCRIPTION OF SYMBOLS 1 Insulating layer 2 Conductive layer 3 Peelable sheet 4 Metal thin film layer 11 Electromagnetic wave shield sheet 12 Peelable sheet 13 Insulating layer 14 Conductive layer 15 Polyimide film 16 Stainless steel plate 17 Laminate 18 Step test observation part 21 Polyimide film 22A, 22B Copper foil Circuit 23 Cover film 24 Through hole 25 Electromagnetic wave shield sheet

Claims (4)

An electromagnetic wave shielding sheet comprising a conductive layer containing flaky conductive fine particles and a binder resin, and an insulating layer,
The average aspect ratio of the flaky conductive fine particles at the cut surface of the conductive layer is 7 to 15,
The area occupied by components other than the conductive fine particles when the cross-sectional area of the conductive layer before thermocompression bonding is 100 is 55 to 80,
The electromagnetic wave shielding sheet, wherein the electromagnetic wave shielding sheet has a difference in area occupied by components other than the conductive fine particles before and after thermocompression bonding at 150 ° C. and 2 MPa for 30 minutes.   The electromagnetic wave shielding sheet according to claim 1, wherein the binder resin is a thermosetting resin.   The electromagnetic wave shielding sheet according to claim 1, comprising the conductive layer, a metal thin film layer, and the insulating layer.   The printed wiring board provided with the electromagnetic wave shielding sheet of any one of Claims 1-3.