JP4825830B2 - Flexible printed circuit board with metal reinforcement - Google Patents

Description

  The present invention relates to a flexible printed wiring board (FPC) provided with a metal reinforcing plate, and improves the connection structure between the FPC and the metal reinforcing plate.

Conventionally, a thin metal reinforcing plate made of stainless steel or the like is attached to the FPC on the opposite side on the component mounting side, the terminal side where the connector is connected, or the like.
Usually, an adhesive sheet having electrical insulation is used to attach the metal reinforcing plate to the FPC. However, when the metal reinforcing plate is electrically connected to the FPC ground circuit, the metal reinforcing plate is shielded against electromagnetic noise. The effect can be demonstrated. Therefore, JP 2007-189091 A (Patent Document 1) proposes connecting a metal reinforcing plate and an electrode of a flexible printed circuit board through a conductive adhesive.

  In the circuit board of Patent Document 1, as shown in FIG. 3, the surface of the electrode 102 made of copper or the like exposed from the opening of the insulating layer 101 of the circuit board main body 100 is electrically conductive made of a binder containing conductive particles 103. A conductive adhesive sheet 105 provided with the conductive adhesive layer 104 is attached, and then a metal reinforcing plate 106 is attached to the surface of the conductive adhesive layer 104, and the electrode 102 and the metal reinforcement are provided via the conductive adhesive layer 104. The plate 106 is electrically connected and bonded.

JP 2007-189091 A

  In Patent Document 1, since it is only necessary to attach the conductive adhesive sheet 105 provided with the conductive adhesive layer 104 made of a binder containing the conductive particles 103, the manufacturing can be simplified. However, in order to increase the adhesive force of the conductive adhesive layer 103 and firmly fix the metal reinforcing plate 106 to the surface of the circuit board main body 100, the binder having adhesiveness by reducing the blending ratio of the conductive particles 103 is used. It is necessary to increase the amount of resin. On the other hand, if the concentration of the conductive particles is lowered to improve the adhesion, the conductivity is lowered, the resistance between the metal reinforcing plate and the ground circuit is increased, and a potential difference is generated. When the potential difference occurs, the electromagnetic shielding effect is reduced and the transmission reliability of the FPC is lowered.

Furthermore, in Patent Document 1, since the electrode 102 and the metal reinforcing plate 106 are bonded only by attaching the conductive adhesive sheet 105, it is assumed that the electrode 102 is positioned near the opening surface of the insulating layer 101. Under this precondition, it is necessary to push the conductive adhesive sheet 105 into the opening of the insulating layer 101.
Usually, in FPC, the thickness of the insulating coating layer provided on the surface of the ground circuit is 20 to 30 μm, and in order to expose the ground circuit facing the opening, it is necessary to cut out the insulating coating layer having a thickness of 20 to 30 μm. is there. When the conductive adhesive sheet 10 of Patent Document 1 is pushed into the opening having a depth of 20 to 30 μm to bond the ground circuit and the metal reinforcing plate, the connection distance between the ground circuit and the metal reinforcing plate is increased, and the resistance is increased. Since it becomes higher, the potential difference between the metal reinforcing plate and the ground circuit becomes large, and there is a problem that the shielding effect by the metal reinforcing plate is reduced.

  The present invention has been made in view of the above problems, and while ensuring the adhesive force between the FPC and the metal reinforcing plate, the potential difference between the metal reinforcing plate and the ground circuit is eliminated or reduced to a negligible level. It is an object to bond and fully exert the electromagnetic shielding function on the metal reinforcing plate.

In order to solve the above problems, the present invention provides an opening in an insulating layer at a position sandwiched between a ground circuit of a flexible printed wiring board and a metal reinforcing plate attached to the surface of the flexible printed wiring board. Filled with conductive adhesive, the ground circuit exposed on the bottom surface of the opening and the metal reinforcing plate are electrically connected and fixed,
The conductive adhesive uses a highly conductive highly conductive adhesive having a conductive particle concentration as high as 45% by volume to 70% by volume on the side bonded to the ground circuit. A high-adhesive adhesive having a conductive particle concentration as low as 0.01% by volume to 20% by volume is interposed between the surface and the metal reinforcing plate, and a potential difference is generated between the ground circuit and the metal reinforcing plate. Provided is a flexible printed wiring board provided with a metal reinforcing plate, which is bonded without reducing or generating.

In the present invention, as described above, two types of conductive adhesives, that is, a highly conductive adhesive and a highly adhesive adhesive prepared by changing the concentration of conductive particles between the ground circuit and the metal reinforcing plate, are used. It is characterized by being connected using.
The conductive adhesive is blended by dispersing conductive particles in a binder resin, and the highly conductive adhesive increases the concentration by increasing the blending amount of the conductive particles in order to increase conductivity. Therefore, since the compounding quantity of binder resin falls, adhesive force becomes low. On the other hand, the high-adhesive adhesive reduces the concentration by reducing the blending amount of the conductive particles and increases the blending amount of the binder resin, so that the adhesive force increases but the conductivity decreases.
The present invention provides a metal reinforcing plate while ensuring the adhesion between the metal reinforcing plate and the FPC by using the high conductive adhesive separately on the ground circuit side and using the high adhesive adhesive on the metal reinforcing plate side. Therefore, the metal reinforcing plate can sufficiently exhibit the electromagnetic noise shielding function, and the influence of electromagnetic noise on the circuit in the FPC can be eliminated.

As noted above, highly conductive adhesive conductive particles concentration of 45 vol% to 70 vol%, the high adhesion adhesive that a silver particle concentration are 0.01 vol% to 20 vol%.
The silver particle concentration of the highly conductive adhesive is 45% by volume to 70% by volume. If it is less than 45% by volume, the adhesiveness is good, but sufficient conductivity cannot be obtained. A potential difference is generated between the ground circuits. On the other hand, if it exceeds 70% by volume, the conductive particles are not uniformly dispersed in the binder resin, and it becomes difficult to actually use the conductive adhesive as a conductive adhesive.
The silver particle concentration of the high-adhesive adhesive is 0.01 vol% to 20 vol%. If it is less than 0.01 vol%, the adhesiveness is good but sufficient conductivity cannot be obtained. On the other hand, if it exceeds 20% by volume, the conductivity becomes good, but sufficient adhesion cannot be obtained.

  In the highly conductive adhesive and the highly adhesive adhesive, conductive particles made of silver, copper, nickel, silver-coated copper, silver-coated nickel, or the like can be used as the conductive particles. Silver having a high conductivity and silver-coated copper are preferably used as the conductive particles to be blended with the highly conductive adhesive, while nickel is suitably used as the conductive particles to be blended with the highly adhesive adhesive. Note that the conductive particles of the highly conductive adhesive and the highly adhesive adhesive may be different as described above, or the same kind of conductive particles may be used.

Specifically, when silver or silver-coated copper is used as the conductive particles, any of particles (A) having an average particle size of 0.5 μm to 20 μm and particles (B) having an average particle size of primary particles of 50 nm or less. When one or both are used and both are mixed, it is preferable to use the mixture ratio (A) :( B) = 99: 1 to 80:20.
The shape of the conductive particles is not particularly limited, but a spherical shape, a scale shape, or the like can be used. In consideration of conductivity, it is preferable to use scaly particles.
The particle diameter is the maximum diameter of individual particles, and the average value is the average particle diameter. Measurement is performed using a scanning electron microscope (SEM) or the like. In addition, the number conversion content% is expressed as a percentage by measuring all the particle diameters and particle numbers on the screen of the SEM or the like, and dividing the number of particles having the corresponding diameter by the total number of particles.

  The binder resin is not limited as long as it has high adhesiveness, and epoxy resin, polyimide resin, polyester resin, phenol resin, polyurethane resin, acrylic resin, melamine resin, polyamideimide resin, and the like can be used. In consideration of heat resistance, it is preferable to use a thermosetting resin, and in consideration of use in flexible printed wiring boards, epoxy resins and polyimide resins are preferably used.

The highly conductive and highly adhesive adhesive contains a curing agent, a solvent and the like in addition to the conductive particles and the binder resin.
The solvent is not limited as long as it can dissolve the binder resin, and examples thereof include ester-based, ether-based, ketone-based, ether-ester-based, alcohol-based, hydrocarbon-based, and amine-based organic solvents. When printing the conductive adhesive, a high boiling point solvent having good printability is preferable, and specifically, carbitol acetate, butyl carbitol acetate, and the like are particularly preferable. Further, several kinds of these solvents may be used in combination.
Examples of the curing agent include amine-based curing agents, polyaminoamide-based curing agents, acid and acid anhydride-based curing agents, basic active hydrogen compounds, tertiary aminos, imidazoles, and other various conventionally known curing agents. Therefore, those suitable for the binder resin to be assembled are used.
Furthermore, additives such as thickeners and leveling agents may be added to improve printing workability.
Furthermore, you may add inorganic fillers, such as carbon and a silica, in the range which does not impair adhesiveness. These components are mixed and dispersed by, for example, a three-roll, rotary stirring and defoaming machine to make a uniform state, thereby creating a conductive adhesive.

  The ratio of the thickness (T1: T2) of the thickness (T1) of the highly conductive adhesive and the thickness (T2) of the highly adhesive adhesive is 10: 1 to 2: 1. The thickness of the agent is increased, and sufficient conductivity is obtained so as not to generate a potential difference between the metal reinforcing plate and the ground circuit.

Specifically, the opening formed by cutting out the insulating layer with the highly conductive adhesive is filled up to the tip of the opening, and the surface of the highly conductive adhesive on the opening surface continuous with the surface of the insulating coating layer is filled. The highly adhesive adhesive is applied or pasted.
The FPC itself has a known configuration, and a conductor pattern including a ground circuit is provided on a base film made of a polyimide film or the like, and the surface provided with the conductor pattern is covered with an insulating resin film via an insulating adhesive. An insulating layer made of a laminate of the insulating adhesive and the insulating film is called a coverlay.
The insulating adhesive has a thickness of 20 to 30 μm, an insulating resin film made of polyimide film or the like has a thickness of 10 to 25 μm, and an insulating layer (coverlay) has a thickness of about 30 to 55 μm. Therefore, the depth of the opening that cuts the insulating layer and exposes the ground circuit to the bottom surface is 30 to 55 μm, and the thickness (T1) of the highly conductive adhesive that fills the opening is 30 to 55 μm. 55 μm.
On the other hand, the high-adhesive adhesive that is applied or pasted to the surface of the highly conductive adhesive need only have a function of adhering a metal reinforcing plate, and in order to maintain flexibility, the thinner one is better, The thickness (T2) may be about 5 to 15 μm. Therefore, as described above, (T1: T2) is 10: 1 to 2: 1. In this way, even if the opening depth is large, the high conductive adhesive is filled so that the potential difference can be ignored by reducing or eliminating the connection resistance between the ground circuit and the metal reinforcing plate. The potential difference can be reduced or the potential difference can be eliminated.

  Since the area of the opening is smaller than the area of the metal reinforcing plate, the surface of the insulating layer at the periphery of the opening also includes the high area in a region corresponding to the entire adhesion side of the metal reinforcing plate including the opening area. It is preferable that an adhesive adhesive is applied or pasted as a sheet to fix the entire adhesion side of the metal reinforcing plate.

In many band-shaped FPCs, conductor patterns are provided at a fine pitch in the width direction, and a part of the conductor pattern is a ground circuit in many cases.
On the other hand, the metal reinforcing plate to be attached to the opposite surface of the FPC component mounting portion or the FPC terminal side on which the connector is mounted is often attached over the entire surface in the width direction.
Since the opening provided in the FPC is provided only in a portion sandwiched between the metal reinforcing plate and at least one ground circuit, that is, in an insulating layer covering the ground circuit, the opening area extends to the FPC of the metal reinforcing plate. Smaller than the contact area.
Therefore, as described above, a highly adhesive adhesive is applied or pasted as a sheet on the surface of the insulating layer at the periphery of the opening, and the entire adhesion side of the metal reinforcing plate is fixed.
In addition, when there are a plurality of ground circuits at a position facing the arrangement portion of the metal reinforcing plate, the opening may be provided in an insulating layer between the plurality of ground circuits and the metal reinforcing plate. When the ground circuit is wide, one large opening communicating with the wide ground circuit may be provided.

  The opening provided in the insulating layer in the portion facing the ground circuit may be either circular or rectangular. The opening may be provided in advance on an insulating layer (cover lay) and covered with a base film provided with a circuit, or the opening may be formed after coating. The opening is preferably formed with high accuracy using a laser.

In the method for forming an FPC with the metal reinforcing plate of the present invention attached, first, an opening is provided in an insulating layer, and the opening is filled with the highly conductive adhesive using screen printing or a dispenser.
The high conductive adhesive is heated at a temperature at which it does not cure, and the solvent in the adhesive is removed to dry.
Next, a highly adhesive adhesive is applied or applied in the form of a sheet so as to correspond to the entire surface of the opening of the highly conductive adhesive and the adhesion surface of the metal reinforcing plate at the periphery of the opening.
Next, after a metal reinforcing plate is affixed on the highly adhesive adhesive material, the metal reinforcing plate is heated and pressurized and fixed to the surface of the FPC.

  As described above, in the present invention, the metal reinforcing plate to be attached to the surface of the FPC is a ground circuit in the FPC using two types of conductive adhesives composed of a high conductive adhesive and a high adhesive. And metal reinforcing plate are bonded. The high conductive adhesive prevents a potential difference from occurring between the metal reinforcing plate and the ground circuit, or has a negligible potential difference, so that the metal reinforcing plate can sufficiently exhibit an electromagnetic wave shielding function. The adhesive strength between the metal reinforcing plate and the FPC can be increased by the high adhesive agent.

Hereinafter, an embodiment of an FPC provided with a metal reinforcing plate of the present invention will be described with reference to the drawings.
1 and 2, the surface S of the FPC 1 is a thin plate stainless steel having a thickness of 50 μm to 100 μm on the opposite side of the part on which the component 2 such as a resistor or a capacitor is mounted and on the terminal side on which the connector 3 is mounted. A metal reinforcing plate 5 made of a plate (indicated by cross diagonal lines in FIG. 2) is pasted over substantially the entire surface of the FPC 1 in the width direction.

  The FPC 1 itself has a well-known configuration, and a conductor pattern 6 is provided at a fine pitch on a base film 4 made of polyimide. The surface of the base film 4 provided with the conductor pattern 6 is covered with an insulating layer (hereinafter referred to as a cover lay) 7. If the conductor pattern 6 or a part of the FPC 1 is an FPC for ground connection, the entire conductor pattern 6 becomes the ground circuit 8. In the FPC of this embodiment, one circuit among the conductor patterns 6 arranged in parallel in the width direction at a fine pitch is the ground circuit 8. As will be described later, the ground circuit 8 is conductively bonded to the metal reinforcing plate 5 via a conductive adhesive.

  The coverlay 7 has a configuration in which an insulating film 10 made of polyimide or the like is bonded via an insulating adhesive 9. The insulating adhesive 9 has a thickness of 20 to 30 μm, and the insulating film 10 has a thickness of 12 to 25 μm. is there. The cover lay 7 has a thickness of 32 to 55 μm, which is the total thickness of the insulating adhesive 9 and the insulating resin film 10.

In the cover lay 7, an opening 12 is provided in a portion facing the ground circuit 8, and the ground circuit 8 is exposed on the bottom surface of the opening 12. Therefore, the depth of the opening 12 is 32 to 55 μm, which is the depth of the cover lay 7.
The opening 12 may be formed in advance by forming the cover lay 7 and laser processing on the cover lay 7, or may be formed by laser processing after the base film 4 is covered with the cover lay 7. The opening 12 does not have to be provided in the entire length of the ground circuit 8 at a position facing the metal reinforcing plate 5, and may be any opening that exposes a part of the ground circuit 8, and may be circular or square in cross section.

The opening 12 is filled by screen printing, intaglio printing, lithographic printing, or a dispenser so as to be filled with the highly conductive adhesive 20. Note that screen printing is most preferably used because it is applied to the opening 12 of the FPC 1.
Highly adhesive adhesion having conductivity to the surface of the opening 12 serving as the surface of the highly conductive adhesive 20 and the surface portion 7a of the cover lay 7 at the periphery of the opening 12 corresponding to the entire position of the metal reinforcing plate 5 The agent 21 is applied using the printing method or dispenser, or is pasted as a sheet. The thickness of the high adhesive 21 is about 10 μm. Therefore, the thickness (T1) of the highly conductive adhesive 20 and the thickness (T2) of the highly adhesive adhesive 21 are set to T1: T2 = 10: 1 to 2: 1.

Placing the metal reinforcing plate 5 on the high adhesion adhesive 21, at a heating temperature of 0.99 ° C. to 200 DEG ° ^C., pressurized with ^{10kg / cm 2 ~15kg / cm 2} , the surface of the metal reinforcing plates 5 FPC1 Affixed to and fixed.

In the high-conductive adhesive 20 and the high-adhesive adhesive 21, the same type or different types of conductive particles A are dispersed in the binder resin B. The highly conductive adhesive 20 is a conductive adhesive having a high concentration by increasing the blending amount of the conductive particles A, and the highly adhesive adhesive 21 is the blending amount of the conductive particles A of the highly conductive adhesive 20. The conductive adhesive has a lower concentration than the blending amount.
Specifically, in the highly conductive adhesive 20, the conductive particles are blended at a ratio of 45 volume% to 70 volume%, and the high adhesive adhesive 21 contains 0.01 volume% to 20 volume% of conductive particles. Formulated in proportions.

  The conductive particles A of the high conductive adhesive 20 use silver or silver-coated copper, while the high adhesive 21 uses nickel. The highly adhesive adhesive 21 may also use silver, silver-coated copper, or copper as the conductive particles A.

Below, the formation method of FPC1 which affixed the metal reinforcement board 5 is demonstrated.
First, a cover lay 7 with an insulating adhesive 9 attached is provided on one surface of the insulating resin film 10, and the cover lay 7 is laser processed to provide an opening 12. In addition, after bonding the cover lay 7 to the base film 4, the cover lay 7 may be laser processed to provide the opening 12.

Next, the highly conductive adhesive 20 is filled in the opening 12 by screen printing, and the highly conductive filler 20 is positioned up to the surface of the opening 12.
Next, it is left to stand in a thermostat at about 70 ° C. for about 30 minutes, and the solvent contained in the highly conductive adhesive 20 is removed and dried.
Next, on the surface of the opening 12 and the surface of the cover lay 7 at the periphery of the opening 12, the high adhesive 21 is applied to the entire surface of the metal reinforcing plate 5 with a dispenser or attached in a sheet form.
Next, the metal reinforcing plate 5 is placed on the surface of the highly adhesive material 21, and the metal reinforcing plate is heated at the heating temperature to soften or melt the high adhesive material 21 and pressurize with the pressure. ,
A metal reinforcing plate 5 is fixed to the surface of the FPC 1.

"Example"
An opening 12 having a cross-sectional area of 5 mm × 5 mm and a depth of 33 μm is provided in the cover lay 7 on the arrangement side of the metal reinforcing plate 5 so as to open on the ground circuit, and the cover lay 7 is provided with the conductor pattern 6 of the base film 4. It stuck together on the provided surface side. At the time of bonding, the opening 12 was aligned with the position of the ground circuit 8, and the ground circuit 8 was exposed on the bottom surface of the opening 12.
A highly conductive adhesive prepared in advance by mixing with a solvent comprising silver particles, an epoxy resin, an imidazole curing agent, and butyl carbitol acetone so that the concentration of 3 to 10 μm scale-like silver particles is 50% by volume. 20 was filled into the opening 12 by screen printing.
After filling the opening 12 with the highly conductive adhesive 20, it was left to stand in a thermostat at 70 ° C. for 30 minutes, and the solvent was removed to dry.
A highly adhesive adhesive 21 prepared in advance by mixing with an epoxy resin and a curing agent so that the concentration of nickel particles is 0.5% by volume is prepared in a sheet shape having a thickness of 10 μm. It cut | disconnected to the magnitude | size of the metal reinforcement board 5 of 20 mm, and it aligned and affixed on the surface of the coverlay 11 of the periphery of the opening 12 while covering the said opening 12. FIG.
A metal reinforcing plate 5 made of a stainless steel plate having a thickness of 0.1 mm was bonded to the surface of the sheet made of the high adhesive 21.
Next, the metal reinforcing plate 5 was pressurized at 13 kg / cm ² while being heated to 180 ° C.
Thereafter, the high adhesive 21 and the high conductive adhesive 20 were cured.

The connection resistance of the part where the metal reinforcing plate was attached to the FPC of the above example was measured. The connection resistance was measured by a four-terminal method.
The measured resistance is 1 to 0Ω, and almost no resistance is generated in the adhesive layer composed of the high conductive adhesive 20 and the high adhesive 21, and there is no potential difference between the metal reinforcing plate 5 and the ground circuit 8. The same or potential difference was negligible.
From this measurement result, it was confirmed that the metal reinforcing plate 5 can have sufficient electromagnetic wave shielding characteristics.

It is principal part sectional drawing which shows embodiment of FPC provided with the metal reinforcement board of this invention. It is a top view of FPC of the embodiment. It is sectional drawing which shows a prior art example.

Explanation of symbols

1 FPC
2 Mounting parts 4 Base film 5 Metal reinforcing plate 6 Conductor pattern 7 Insulating layer (cover lay)
8 Ground Circuit 12 Opening 20 Highly Conductive Adhesive 21 Highly Adhesive Adhesive A Conductive Particles B Binder Resin

Claims (4)

An opening is provided in the insulating layer located between the ground circuit of the flexible printed wiring board and the metal reinforcing plate attached to the surface of the flexible printed wiring board, and the opening is filled with a conductive adhesive, and the opening The ground circuit exposed on the bottom surface of the metal and the metal reinforcing plate are electrically connected and fixed,
The conductive adhesive uses a highly conductive highly conductive adhesive having a conductive particle concentration as high as 45% by volume to 70% by volume on the side bonded to the ground circuit. A high-adhesive adhesive having a conductive particle concentration as low as 0.01% by volume to 20% by volume is interposed between the surface and the metal reinforcing plate, and a potential difference is generated between the ground circuit and the metal reinforcing plate. A flexible printed wiring board provided with a metal reinforcing plate, wherein the flexible printed wiring board is bonded without reducing or generating.   The area of the opening is smaller than the area of the metal reinforcing plate, and the surface of the insulating layer on the periphery of the opening has a high adhesive property in a region corresponding to the entire bonding side of the metal reinforcing plate including the opening area. The flexible printed wiring board provided with the metal reinforcing plate according to claim 1, wherein an adhesive is applied or pasted as a sheet, and the entire bonding side of the metal reinforcing plate is fixed. The highly conductive the ratio of the thickness of the thickness of the adhesive and (T1) and the thickness of the high-adhesion adhesive (T2) (T1: T2) is 10: 1 to 2: Claim 1 or 1 A flexible printed wiring board comprising the metal reinforcing plate according to claim 2 . The highly conductive adhesive and the highly adhesive adhesive include conductive particles composed of one kind or two or more kinds of mixed particles selected from silver, silver-coated copper, copper, and nickel, epoxy resin, polyimide resin, and The flexible printed wiring board provided with the metal reinforcement board of any one of Claim 1 thru | or 3 which is disperse | distributed in the binder resin selected from a polyester resin .

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