JP2011228480A - Conductive paste, flexible printed wiring board and electronic apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a conductive paste, a flexible printed wiring board having jumper wiring formed of the conductive paste, and an electronic apparatus having the flexible printed wiring board, capable of coping with high-density conductive wiring with a small circuit pitch, securing high connection reliability, and enabling the formation of the jumper wiring by one time coating process to achieve a thin formation of the flexible printed wiring board.SOLUTION: A flexible printed wiring board 1 comprises conductive particles, a binder resin to distribute the conductive particles, a solvent to solve the binder resin and a hardening agent to harden the binder resin having the distributed conductive particles. Jumper wiring 5 is formed by use of the conductive paste having viscosity of 30 to 200 Pa s.

Description

  The present invention relates to a conductive paste, a flexible printed wiring board provided with a jumper wiring formed by the conductive paste, and an electronic device provided with the flexible printed wiring board.

In the electronic equipment field, in recent years, electronic equipment has been rapidly reduced in size and weight. Accordingly, flexible printed wiring boards built in electronic devices are required to have higher conductor wiring density and higher connection reliability.
In order to satisfy such a requirement, a so-called double-sided flexible printed wiring board in which a plurality of high-density conductor wirings are formed by using a double-sided copper-clad laminate is conventionally used.
However, in such a double-sided flexible printed wiring board, an insulating layer covering the conductor wiring is required on both sides of the substrate, and there is a problem that it is difficult to reduce the thickness.
In order to solve such a problem, a single-sided copper-clad laminate is used, and a plurality of conductor wirings are electrically connected by jumper wirings. The jumper wiring is a circuit that is formed of a conductive paste and electrically connects the specific conductor wirings among a plurality of conductor wirings formed on the flexible printed wiring board.
Such a jumper wiring is formed by a coating process in which a conductive paste is filled into a hole and applied to the surface of the insulating layer with respect to the insulating layer having a bottomed hole having a conductor wiring as a bottom surface. It has a configuration.
Therefore, as described above, with recent downsizing and weight reduction of electronic devices, flexible printed wiring boards with jumper wiring have a fine circuit pitch that can handle high-density conductor wiring and high connection reliability. There is a demand for a conductive paste that enables the formation of a jumper wiring that can secure the property.
A conventional technique for forming a jumper wiring by filling a hole with a conductive paste and applying it to the surface of the insulating layer for an insulating layer having a bottomed hole with a conductor wiring as a bottom surface For example, there is the following Patent Document 1.

Japanese Patent Laid-Open No. 61-224396

The above-mentioned patent document 1 is an invention relating to a method of forming a jumper circuit, and by forming a carbon powder-containing conductive resin layer between conductive circuits and conductive connection portions of the jumper circuit, formation of a local battery and electrolytic corrosion resulting therefrom There is an advantage that a highly reliable conductive connection can be made possible.
However, it is not intended to form a jumper wiring with a small circuit pitch that can handle high-density conductor wiring, and there is a problem that such description and suggestion are not made.

  Therefore, the present invention solves the above-mentioned problems in the prior art, can handle high-density conductor wiring, has a fine circuit pitch, can ensure high connection reliability, and can realize a thin flexible printed wiring board. It is an object of the present invention to provide a conductive paste capable of forming a jumper wiring in a single coating process, a flexible printed wiring board including a jumper wiring formed by the conductive paste, and an electronic device including the flexible printed wiring board.

  The conductive paste of the present invention cures conductive particles, a binder resin for dispersing the conductive particles, a solvent for dissolving the binder resin, and a binder resin in which the conductive particles are dispersed. The first feature is that the conductive paste comprises a curing agent having a viscosity of 30 to 200 Pa · s.

According to the first aspect of the present invention, conductive particles, a binder resin for dispersing the conductive particles, a solvent for dissolving the binder resin, and a binder resin in which the conductive particles are dispersed. Is a conductive paste comprising a curing agent for curing the conductive paste, and since the viscosity of the conductive paste is 30 to 200 Pa · s, the conductivity is excellent in conductivity, fluidity, curability, and adhesiveness. Can be made into a paste. Therefore, in the coating process when forming the jumper wiring, the conductive paste can be accurately filled into the holes formed in the insulating layer, and at the same time, the circuit pitch of the jumper wiring formed on the surface of the insulating layer is made minute. be able to.
Therefore, it is possible to obtain a conductive paste that can form a jumper wiring that can deal with high-density conductor wiring in a single coating process, has a small circuit pitch, and can secure connection reliability.

  In addition to the first feature of the present invention, the conductive paste of the present invention is composed of silver powder, the average particle diameter is 1 to 10 μm, and the content is 91 to 94% by weight. This is the second feature.

  According to the second feature of the present invention, in addition to the function and effect of the first feature of the present invention, the conductive particles are made of silver powder and have an average particle diameter of 1 to 10 μm and a content of Is 91 to 94% by weight, the circuit pitch in the jumper wiring can be made much finer, and a conductive paste that can form a jumper wiring that can ensure high connection reliability can be obtained. it can.

  Further, in the conductive paste of the present invention, in addition to the first or second feature of the present invention, the binder resin is made of an epoxy resin, has a molecular weight of 10,000 to 100,000, and a content of 6.5 to 7 The third characteristic is that it is 5% by weight.

  According to the third feature of the present invention, in addition to the function and effect of the first or second feature of the present invention, the binder resin is made of an epoxy resin and has a molecular weight of 10,000 to 100,000. Is 6.5 to 7.5% by weight, so that the heat resistance is excellent and the adhesion (cohesive force) with the conductive particles can be improved, so that the coating property and adhesiveness are further improved. Can be made into a paste.

  Moreover, the flexible printed wiring board of this invention is the electrically conductive paste in any one of Claims 1-3 in which the jumper wiring which bridge | crosslinks and electrically connects between specific conductor wiring among several conductor wirings. The fourth feature is that it is used.

According to the fourth aspect of the present invention, in the flexible printed wiring board, jumper wiring that bridges and electrically connects specific conductor wirings among the plurality of conductor wirings is any one of claims 1 to 3. Since the conductive paste described in the section is used, a jumper wiring that can handle high-density conductor wiring, has a fine circuit pitch, and can secure high connection reliability in a single coating process. A flexible printed wiring board that can be formed can be obtained. Therefore, it is possible to obtain a flexible printed wiring board with high density and high connection reliability and high manufacturing efficiency.
Moreover, it can be set as the flexible printed wiring board which is excellent in heat resistance, and can make thickness reduction and cost reduction compatible.

  According to a fifth aspect of the present invention, there is provided an electronic apparatus comprising the flexible printed wiring board according to the fourth aspect.

  According to the fifth aspect of the present invention, since the electronic device includes the flexible printed wiring board according to claim 4, it is highly functional, compact, lightweight, thin, improved in production efficiency, cost. The electronic device can be reduced.

According to the conductive paste of the present invention, a jumper wiring that is compatible with high-density conductor wiring, has a small circuit pitch, can secure high connection reliability, and has excellent heat resistance can be formed by a single coating process. A possible conductive paste can be obtained.
Further, according to the flexible printed wiring board of the present invention, a flexible printed circuit capable of forming a jumper wiring that can handle high-density conductor wiring, has a small circuit pitch, and can ensure high connection reliability in a single coating process. It can be a wiring board. Therefore, it is possible to obtain a flexible printed wiring board with high density and high connection reliability and high manufacturing efficiency. Moreover, it can be set as the flexible printed wiring board which is excellent in heat resistance and can be reduced in thickness.
Further, according to the electronic device of the present invention, it is possible to provide an electronic device that can be enhanced in function, reduced in size, reduced in weight, reduced in thickness, improved in production efficiency, and reduced in cost.

It is a perspective view explaining the flexible printed wiring board concerning the embodiment of the present invention. It is a figure which shows the principal part of sectional drawing in the AA line direction of FIG. It is a figure which shows the principal part of sectional drawing explaining the manufacturing method of the flexible printed wiring board which concerns on embodiment of this invention. It is a figure which shows the principal part of sectional drawing explaining the manufacturing method of the flexible printed wiring board which concerns on embodiment of this invention. It is a perspective view which simplifies and demonstrates the internal structure of the electronic device which concerns on embodiment of this invention.

  With reference to the following drawings, an embodiment of a conductive paste according to the present invention, a flexible printed wiring board provided with a jumper wiring formed by the conductive paste, and an electronic device provided with the flexible printed wiring board will be described. For the understanding of the present invention. However, the following description is an embodiment of the present invention, and does not limit the contents described in the claims.

First, a flexible printed wiring board 1 according to the present invention will be described with reference to FIG.
The flexible printed wiring board 1 according to the present invention is a so-called single-sided flexible printed wiring board disposed inside an electronic device (not shown) in a state where the electronic component 6 shown in FIG. 5 is mounted.
As shown in FIG. 1, the flexible printed wiring board 1 includes a substrate 2, a conductor wiring 3, a coverlay 4, and a jumper wiring 5.
Although not shown in FIG. 1, in a cover lay 4a described later, an opening is formed in a portion corresponding to an electrode electrically connected to the electronic component 6, and the electrode is exposed.

The substrate 2 serves as a base for the flexible printed wiring board 1.
In this embodiment, it is formed of a so-called single-sided copper-clad laminate in which a copper foil is laminated on the surface of a resin film with a heat-resistant adhesive resin.
As a resin film, what consists of a resin material excellent in the softness | flexibility is used. For example, any film may be used as long as it is normally used as a resin film for forming a flexible printed wiring board, such as a polyimide film or a polyester film.
In particular, it is desirable to have high heat resistance in addition to flexibility. For example, polyamide resin films, polyimide resin films such as polyimide and polyamideimide, and polyethylene naphthalate can be preferably used.
The heat-resistant resin may be any resin as long as it is normally used as a heat-resistant resin for forming a single-sided copper-clad laminate, such as a polyimide resin or an epoxy resin.

The conductor wiring 3 is a circuit pattern made of a metal foil formed on the surface of the substrate 2.
The conductor wiring 3 is not shown in detail in FIG. 1, but is electrically connected to the conductor wirings 3a-3e and 3b-3f by the jumper wiring 5, and the conductor wiring 3c not electrically connected by the jumper wiring 5. 3d and 3d.
In consideration of conductivity and durability, for example, copper or an alloy containing copper as a main component can be suitably used as the metal foil. For example, copper thin film, tin-containing copper alloy thin film, chromium-containing copper alloy thin film, zinc-containing A copper alloy thin film, a zirconium containing copper alloy thin film, etc. can be used.

In addition, about the conductor wiring 3a, 3b, 3e, 3f electrically connected by the jumper wiring 5, it is good also as a structure which forms a surface treatment layer by performing surface treatment to the connection part with the jumper wiring 5. FIG.
By setting it as such a structure, it can be set as the flexible printed wiring board 1 with much higher connection reliability.
The surface treatment layer can be formed by gold plating, nickel gold plating, solder plating, or the like.
The number, shape, arrangement position and the like of the conductor wiring 3 are not limited to those of the present embodiment, and can be changed as appropriate.

The cover lay 4 constitutes an insulating layer of the flexible printed wiring board 1.
As shown in FIGS. 1 and 2, the cover lay 4 includes a first cover lay 4 a that covers the conductor wiring 3 and a second cover lay 4 b that covers the jumper wiring 5.
As shown in FIG. 2, the first coverlay 4a is provided with a hole L for forming the jumper wiring 5.
As the coverlay 4, a polyimide film with an adhesive, a photosensitive resist, a liquid resist, or the like can be used.

The jumper wiring 5 is a circuit that bridges and electrically connects specific conductor wirings among the plurality of conductor wirings 3 formed on the flexible printed wiring board 1. In the present embodiment, the conductor wiring 3a-3e and the conductor wiring 3b-3f are electrically connected by the jumper wiring 5.
By adopting the configuration using the jumper wiring 5 in this way, the flexible printed wiring board 1 having a plurality of conductive wiring layers can be formed with a single-sided flexible printed wiring board without using a double-sided flexible printed wiring board. Therefore, the flexible printed wiring board 1 that can achieve both reduction in thickness and cost reduction can be obtained.
As shown in FIG. 1 and FIG. 2, even when there are conductor wirings 3c and 3d that cannot be bypassed on the plane between the conductor wirings 3a to 3e and between the conductor wirings 3b to 3f that are desired to be electrically connected to each other, A connection circuit can be easily formed in a separate layer via one coverlay 4a.

As shown in FIG. 2, the jumper wiring 5 is filled with a conductive paste into the holes L having the conductive wirings 3a and 3e as the bottom by using screen printing or the like, and on the surface of the first cover lay 4a. It forms by the application | coating process which apply | coats an electrically conductive paste.
Of course, the jumper wiring 5 that electrically connects the conductor wirings 3b-3f is also formed in the same process.
The conductor wiring 3 electrically connected by the jumper wiring 5 is not limited to the configuration of the present embodiment, and can be appropriately changed depending on the configuration of the conductor wiring 3 formed on the flexible printed wiring board 1.

  Examples of the conductive paste for forming such jumper wiring 5 include conductive particles, a binder resin for dispersing the conductive particles, a solvent for dissolving the binder resin, and a binder resin in which the conductive particles are dispersed. The conductive paste formed from the hardening | curing agent for hardening is used.

The conductive particles are so-called conductive fillers made of metal powder or the like.
As the metal powder, for example, silver, platinum, gold, copper, nickel, palladium and the like can be used, but it is desirable to use silver powder or silver-coated copper powder. By setting it as such a structure, it can be set as the jumper wiring 5 which can implement | achieve the outstanding electroconductivity and can ensure high connection reliability.
In the present embodiment, silver powder is used as the conductive particles.

Moreover, when using silver powder as electroconductive particle, while setting an average particle diameter to 1-10 micrometers, it is desirable to make content with a paste hardened | cured material into 91-94 weight%. More preferably, the average particle size is 3 to 5 μm.
With such a configuration, the circuit pitch M of the adjacent jumper wirings 5 shown in FIG. 1 can be made much smaller, and the jumper wirings 5 that can ensure high connection reliability can be obtained. .

Here, the “particle diameter” means the maximum diameter of an independent single particle. The “average particle size” means a cumulative 50% particle size (D50) in a dispersion state where the standard deviation of the particle size distribution is σ = 1.5 or less, and Microtrack MT3000II series manufactured by Nikkiso Co., Ltd. was used. It can be measured by laser analysis.
Further, in this and the following description, the “paste cured product” means a heat-cured conductive paste formed from conductive particles, a binder resin, a solvent, and a curing agent. The weight means a value obtained by removing the solvent from the conductive paste.
Further, the “content (%) with respect to the cured paste product” represents the content of each component (conductive particles, binder resin, curing agent) by weight ratio when the total weight of the cured paste product is 100. Shall mean

The binder resin is a resin for dispersing conductive particles.
As the binder resin, for example, epoxy resin, phenol resin, polyester resin, polyurethane resin, acrylic resin, melamine resin, polyimide resin, and polyamideimide resin can be used. It is desirable to use it.
The epoxy resin is not particularly limited. For example, bisphenol A type, F type, S type, AD type, or a copolymer type epoxy resin of bisphenol A type and bisphenol F type, naphthalene type epoxy resin, novolak Type epoxy resin, biphenyl type epoxy resin, dicyclopentadiene type epoxy resin and the like can be used. A phenoxy resin which is a high molecular weight epoxy resin can also be used.
In this embodiment, a bisphenol A type epoxy resin is used as the binder resin.

  When a bisphenol A type epoxy resin is used as the binder resin, the molecular weight is preferably 10,000 to 100,000, and the content with respect to the cured paste is preferably 6 to 8% by weight, and more preferably the molecular weight is 30000 to 80,000, It is desirable to make content with respect to paste hardened | cured material into 6.5 to 7.5 weight%. By adopting such a configuration, the heat resistance and the adhesive force (cohesive force) with the conductive particles can be improved, so that the conductor wiring 3 and the first wiring 1 can be formed during and after the formation of the jumper wiring 5. It can be set as the electrically conductive paste with the applicability | paintability with respect to the coverlay 4a, or adhesiveness.

The solvent is for dissolving the binder resin.
The solvent used in the present invention is not a problem as long as it can dissolve the binder resin described above, but is preferably an ester, ether, ketone, ether ester, alcohol, hydrocarbon, or amine. It is desirable to use the organic solvent. When the conductive paste is used for circuit formation by printing, a high-boiling solvent with good printability is preferable, and specifically, carbitol acetate, butyl carbitol acetate and the like are particularly desirable. It is also possible to use several types of these solvents in combination.
In this embodiment, butyl carbitol acetate is used as a solvent.

The curing agent is for curing the binder resin in which conductive particles are dispersed.
As the curing agent, an isocyanate compound, an epoxy resin, a melamine resin, or the like can be used. In view of storage stability as a conductive paste, reactivity during curing, and the like, it is more preferable to use a blocked isocyanate. As the blocking agent, one selected from alcohols, phenols, acid amides, oximes, and active methylenes can be used. Furthermore, a curing catalyst such as an organic tin compound can be used in combination.
In the present embodiment, a microcapsule type curing agent (trade name NOVACURE HX-3941 manufactured by Asahi Kasei E-Materials Co., Ltd.) in which an imidazole derivative is coated with a thermoplastic resin is used as a curing agent.

  Moreover, when using the microcapsule type hardening | curing agent (Asahi Kasei E Materials Co., Ltd. brand name Novacure HX-3941) which coat | covered the imidazole derivative with the thermoplastic resin as a hardening | curing agent, content with respect to paste hardened | cured material is 0.01- The content is preferably 1% by weight, and more preferably 0.02 to 0.1% by weight. By setting it as such a structure, favorable flexibility can be ensured, improving heat resistance and adhesiveness with respect to paste hardened | cured material.

After preparing a solution by dissolving the binder resin described above in a solvent, add conductive particles, uniformly mix using a rotary stirring defoamer, add a curing agent and further mix, and then add three solutions. A conductive paste is produced by passing through a roll.
The viscosity of the conductive paste is preferably 30 to 200 Pa · s, more preferably 50 to 100 Pa · s.
The viscosity can be measured using a TVE viscometer manufactured by Toki Sangyo Co., Ltd., with the cone used as cord 7, the number of rotations as 1 rpm, and the measurement temperature as 25 degrees.
By setting it as such a structure, it can be set as the electroconductive paste with favorable electroconductivity, fluidity | liquidity, sclerosis | hardenability, and adhesiveness. Therefore, it is possible to fill the hole L formed in the first cover lay 4a with high accuracy with a good fluidity only by applying the conductive paste once to the portion where the jumper wiring 5 is formed. At the same time, a circuit (mainly 0.6 mm or less) having a small circuit pitch M can be formed on the surface of the first cover lay 4a due to good adhesion and viscosity.

That is, conventionally, in a flexible printed wiring board provided with jumper wiring, when a conductive paste is pressed into a hole formed in an insulating layer in a coating process such as screen printing, the conductive paste is caught on the hole wall. In other words, there is a problem that a gap is generated between the conductive paste and the conductive paste in the hole serving as the stepped portion is insufficient, and the jumper wiring is cracked or scraped. Similar problems also occur when the aperture diameter of the hole is wide.
Insufficient filling of the conductive paste reduces connection reliability, and cracks and scraping cause disconnection of jumper wiring, resulting in deterioration of the quality of flexible printed wiring boards. In general, the application process of the adhesive paste is performed a plurality of times.

On the other hand, by adopting the configuration of the present invention, the jumper wiring 5 is formed which can cope with high-density conductor wiring in a single coating process and has a small circuit pitch M and can ensure high connection reliability. It can be set as the electrically conductive paste which can be used, the flexible printed wiring board 1 provided with the jumper wiring 5 formed with this conductive paste, and an electronic device provided with this flexible printed wiring board 1.
Therefore, the flexible printed wiring board 1 and an electronic device (not shown) can be made thin, highly functional, compact, lightweight, improved in production efficiency, and reduced in cost.

Next, with reference to FIG. 3 and FIG. 4, the manufacturing process of the flexible printed wiring board 1 which concerns on embodiment of this invention is demonstrated.
First, as shown in FIG. 3A, a substrate 2 is prepared which is a so-called single-sided copper-clad laminate in which a copper foil is laminated on a surface of a resin film with a heat-resistant adhesive resin (not shown).
Next, the laminated metal foil is etched by a conventional method to form conductor wirings 3a to 3f as shown in FIG.

Next, as shown in FIG. 3C, the first cover lay 4 a is laminated on the substrate 2. At this time, a hole L having the upper surface of the conductor wirings 3a and 3e as the bottom surface is formed in the upper surface portion of the conductor wirings 3a and 3e that are connected to the jumper wiring 5. At this time, although not shown, a hole L whose bottom surface is the top surface of the conductor wirings 3b and 3f is also formed in the top surface portion of the conductor wirings 3b and 3f that are connected to the jumper wiring 5.
As a method for forming the hole L, for example, shearing with a male / female mold, irradiation with a laser, or the like can be used.

Next, as shown in FIG. 4A, the hole L is filled with the conductive paste, and the coating process of applying the conductive paste to the surface of the first coverlay 4a by screen printing is performed once, and the heat treatment is performed. Then, the jumper wiring 5 having a predetermined circuit pitch M (mainly 0.6 mm or less) and thickness is formed by curing.
Next, as shown in FIG. 4B, the second cover lay 4b is bonded onto the first cover lay 4a so as to cover the jumper wiring 5.
As described above, the flexible printed wiring board 1 according to the embodiment of the present invention is formed.
Note that the method of applying the conductive paste is not limited to screen printing, and may be configured to use intaglio printing, flat printing, a dispenser, or the like. From the viewpoint of fineness, film thickness, and productivity of the jumper wiring 5 to be formed. Therefore, it is desirable to use screen printing.

  As shown in FIG. 5, the flexible printed wiring board 1 having such a configuration has terminals (not shown) of electronic components 6 connected to connecting portions (not shown) of the conductor wiring 3 so that the first cover layer 1 The electronic component 6 is mounted on the component mounting surface 4a and disposed inside an electronic device (not shown).

  According to the present invention, the circuit pitch that can be used for high-density conductor wiring is minute, high connection reliability can be secured, and both flexible printed wiring boards and electronic devices can be made thin and cost can be reduced. Jumper wiring can be formed by a single coating process, and has high industrial applicability in the field of flexible printed wiring boards having jumper wiring formed of conductive paste and electronic devices having the flexible printed wiring boards. .

DESCRIPTION OF SYMBOLS 1 Flexible printed wiring board 2 Board | substrate 3 Conductor wiring 3a Conductor wiring 3b Conductor wiring 3c Conductor wiring 3d Conductor wiring 3e Conductor wiring 3f Conductor wiring 4 Coverlay 4a 1st coverlay 4b 2nd coverlay 5 Jumper wiring 6 Electronic component L Hole M Circuit pitch

Claims (5)

  Conductivity comprising conductive particles, a binder resin for dispersing the conductive particles, a solvent for dissolving the binder resin, and a curing agent for curing the binder resin in which the conductive particles are dispersed. A conductive paste, wherein the conductive paste has a viscosity of 30 to 200 Pa · s.   2. The conductive paste according to claim 1, wherein the conductive particles are made of silver powder, have an average particle diameter of 1 to 10 μm, and a content of 91 to 94% by weight.   3. The conductive paste according to claim 1, wherein the binder resin is made of an epoxy resin, has a molecular weight of 10,000 to 100,000, and a content of 6.5 to 7.5 wt%.   The jumper wiring which bridge | crosslinks and electrically connects between specific conductor wiring among several conductor wiring is formed using the electrically conductive paste in any one of Claims 1-3, It is characterized by the above-mentioned. Flexible printed wiring board.   An electronic apparatus comprising the flexible printed wiring board according to claim 4.

Images