Patent Application
20060272850
The present invention relates to an interlayer connector of a multilayer flexible printed circuit board (hereafter, referred to as FPC) on which a variety of surface-mounting type electric components are mounted, particularly an interlayer connection conductor and manufacturing method of the same, which is used to connect wiring layers of a multilayer FPC requiring high connection reliability.
Recently, as electronic devices have decreased in size and weight and become sophisticated, the wiring concentration of the FPC involved is likely to increase. Miniaturization of a wiring layer is not enough to increase the wiring concentration of an FPC. Attention is given to a multilayer FPC having increased wiring concentration that is formed by a process of layering wiring layers, disposing interlayer connectors on an insulating layer between the wiring layers, and connecting the wiring layers in three dimensions.
In the related art, a multilayer FPC is formed by a process of forming through holes on adhesive layers formed of a polyimide film, applying copper plate on the through hole walls, and connecting the wiring layers on both side of the adhesive layers in three dimensions (Patent Document 1). Plating-through hole method is the most common method of interlayer connection.
Manufacturing method of the plating-through hole includes two main processes of applying conductivity to an insulating through hole by nonelectrolytic plating and copper thickening plating by electrolytic plating. The above-mentioned method has a feature that it has improved connection reliability against heat because of the same thermal expansive rate of the copper plating film on the inside of the through hole and the insulating layer having the through hole.
However, during the copper thickening plating, the copper plating film on the inside of the through hole increases in thickness and the thickness of a copper film that is a raw material of the wiring layer also increases. Therefore, it is difficult to miniaturize the wiring with a subsequent etching. Further, the process involved is complicated, and thus problems still remain in terms of productivity.
Method of applying melt-solidification by printing solder paste inside a through hole to overcome the above problems has been proposed (for example, Patent Document 2). The method has a feature that the productivity is improved because it requires a simple process compared with the above plating-through hole method, and the thickness of a copper in is not affected by anything in during process and miniaturization of the wiring layer is not prevented due to interlayer connection after forming the wiring layer.
However, since the thermal expansive rate of a solder is larger than that of an insulating layer, the solder inside a through hole expands more than the wiring layer. Accordingly, the joining interface between the wiring layer on the insulating layer and the solder may be separated. Therefore, as for the method using a solder, reliability in not satisfied due to heat.
JP-A-5-175636
JP-A-7-176847
Method of connecting layers by forcing substantially spherical conductor into through holes to overcome the problem that the connection reliability is not secured due to heat has been proposed. The substantially spherical conductor is formed of a core of metal member covered with a solder metal with uniform volume.
Thickening solder plating by barrel solder plating is generally known method to apply a solder metal to the surface of a metal member but it requires long time to plate to form a solder plating film with uniform thickness on the surface of the metal member. Plating solution needs to be controlled for plating film thickness control and quality control. However it is complicated and causes problems in terms of productivity.
In order to achieve a spherical shape after solder plating, the shape of a metal member of core portion needs to be spherical. However, in general method of manufacturing a metal spherical body with uniform volume to prevent non-uniform particle diameter is not suitable to mass production.
In order to achieve a spherical shape after solder plating, the shape of a metal member of core portion needs to be spherical. However, in general, method of manufacturing a metal spherical body with uniform volume to prevent non-uniform particle diameter is not suitable to mass production.
An object of the present invention is to provide an interlayer connection conductor having high connection reliability, optimal miniaturization of a wiring layer, and improved productivity, and manufacturing the interlayer connection conductor.
According to an aspect of the invention, there is provided an interlayer connection conductor, which is formed in a substantially spherical shape and is press-fitted into a through hole formed in a flexible printed circuit board in a thickness direction so as to serve an interlayer connector, the flexible printed circuit board having a wiring layer that is formed on at least one surface of an insulating layer. The interlayer connection conductor includes a metal core that has a cylindrical small piece formed by cutting a metal fine wire, and a solder metal that covers the surface of the metal core.
According to another aspect of the invention, a method of manufacturing an interlayer connection conductor includes forming a metal core by cutting a metal fine wire in a predetermined length, disposing the cut metal core on a thermal resistant substrate having a recess, coating a solder metal to cover the metal core disposed in the recess, and covering the metal core with the solder metal in a substantially spherical shape by heating and melting the solder metal.
According to the aspects of the invention, the interlayer connection conductor for electric connection between wiring layers of a multilayer FPC includes the metal core that has a cylindrical small piece formed by cutting the metal fine wire, and the solder metal that covers the surface of the metal core. Accordingly, the interlayer connection conductor having a substantially spherical conductor with uniform volume can be obtained. Therefore, with deformation by pressure, the spherical conductor can be filled into the hole with no gap. As a result, the solder metal can be securely adhered to the wiring layer. Further, high connection reliability can be obtained because the metal core is formed of a cylindrical small piece with uniform volume.
Since the metal core has a cylindrical small piece formed by cutting the metal fine wire, the conductor can have the optimum size to the volume of the through hole. Further, since the cut metal fine wire can also be used, the productivity can be improved compared with a case where a metal ball is used as the metal core.
An interlayer connection conductor according to an embodiment of the invention will be described below with reference to
As shown in
The condition in use of an interlayer connection conductor according to an embodiment of the invention will be described below with reference to
Manufacturing method of a multilayer FPC using an interlayer connection conductor 1 according to an embodiment of the invention will be described below with reference to
As shown in
As shown in
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A basic material of a metal fine wire for the metal core 2 may be a soft metal, such as gold, silver, copper or copper alloy, and tin or tin alloy. As for an interlayer connection conductor 1 having soft metal core 2, the conductor may be easily forced in the hole and deform under low pressure, and thus filling the inside of the hole 8 with no gap. Therefore, the metal post-shaped interlayer conductor 20 securely jointed to the wiring layer 6 and having high conductivity is achieved. As a result, it secures joining. In particular, when a copper-based metal is used for the metal fine wire, the reliability with respect to the thermal cycle deformation becomes high and it is suitable for a product that requires high reliability because the thermal expensive rate of the insulating layer is equal to the conductor.
Surface treatment, such as gold plating, nickel plating, or solder treatment, is applied to the metal fine wire. Improved wettability is obtained in coating of the solder metal 3 on the metal core 2 of a small cylindrical piece formed by cutting the metal fine wire due to the surface treatment for the metal fine wire. As a result, the metal core 2 and the solder metal 3 may be securely integrally formed.
However, even though the solder metal 3 is not coated on the metal core 2, the interlayer connection conductor is available unless it affects the connection between the wiring layers 6.
In the present embodiment, the metal core 2 is formed of a small piece of cylindrical body formed by cutting a metal fine wire so that the metal core 2 having optimum volume may be uniformly formed with ease in the interlayer connection conductor 1 by making the metal core 2 of a small cylindrical body big or long depending on the thickness of the FPC of the diameter of the through hole 8 of the multilayer FPC 10.
Accordingly, the interlayer connection conductor 1 having uniform volume can be manufactured.
Because the metal core is formed by only cutting a metal fine wire, the metal core 2 having improved productivity at low cost is achieved comparing with conventional manufacturing method using a metal ball.
In the interlayer connection conductor 1 according to the present embodiment, uniform volume is required for the forcing and deforming. However, the interlayer connection conductor 1 may not be a complete sphere when the interlayer connection conductor 1 can be handled and positioned, and elliptical sphere is also available.
The metal core 2 is formed by cutting a small cylindrical metal fine wire with a diameter the same as or more than height. Therefore, it is possible to maintain the handling of the interlayer connection conductor 1 and the position of the small cylindrical body in positioning. Accordingly, the metal core 2 of a small piece of cylindrical body is stably disposed in the through hole 8. Because the interlayer connection conductor 1 becomes a metal post-shaped internal conductor 20 having high conductivity that fills the through hole 8 with no gap, the interlayer connection conductor 1 and the wiring layer 6 are stably joined and high connection reliability is obtained. In particular, when a copper-based metal is used for the metal fine wire, the reliability with respect to the thermal cycle deformation becomes high because the thermal expensive rate of the insulating layer 11 is equal to the conductor, and it is suitable for the interlayer connection conductor 1 of a product that requires high reliability.
The interlayer connection conductor 1 smoothly deforms with ease, because it is composed of a metal core 2 formed of a soft metal and a solder metal 3 surrounding the metal core. Therefore, the wiring layer 6 is not affected during following processes and the deformed solder metal is securely joined to the wiring layer 6. As a result high reliability in the wiring layer 6 is obtained.
As for the solder metal 3, any one of a eutectic solder, a high-temperature solder, or lead-free solder may be used under circumstances. Accordingly, when the interlayer connection conductor 1 according to the present embodiment is used, high reliability in joining is obtained with miniaturized joining of a wiring layer.
An example when the above-mentioned multilayer FPC is further layered will be described below with reference to
As shown in
The multilayer FPC 21 formed as described above has high connection reliability because the solder metals 3 around the surface of the interlayer connection bump 17 come in contact with each of the wiring layers 6 in layering and securely electrically come in contact with the surface of the wiring layer 6 through the pressed-deformation. Because the contacting portion is the solder metal 3, when it undergoes heating/cooling in layered and contact conditions, the interlayer bumps 17 and the wiring layers are simply joined with each other through melt-solidification of the solder metal 3. Therefore, the connection reliability is further improved. The above multilayer FPC 21 has high connection reliability and may be further decreased in size because the components involved are reliable in joining and miniaturized wiring layers are provided.
Manufacturing method of an interlayer connection conductor according to an embodiment of the invention will be described hereafter, with reference to
Manufacturing method of an interlayer connection conductor according to an embodiment of the invention using a solder paste as a solder metal will be described below with reference to
As shown in
As shown in
The thermal resistant substrate 24 may be a metal plate such as a stainless plate. In the present embodiment, a solder is not attached when melted and SUS304 is used considering thermal resistance, chemical resistance, and machinability, but not limited.
As shown in
As shown in
As described above, an interlayer connection conductor is manufactured through a simple process including forming the metal cores 2 by cutting a metal fine wire, disposing the metal cores 2 at predetermined positions on the thermal resistant substrate 24, applying a predetermined amount of solder paste on the metal cores 2, and integrally forming the solder paste with the metal core 2 in a spherical shape by heating and melting the solder paste. Therefore, an interlayer connection conductors having uniform volume can be manufactured with improved productivity.
Manufacturing method of an interlayer connection conductor according to an embodiment of the invention using solder balls as a solder metal will be described below with reference to
As shown in
As shown in
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Manufacturing method of an interlayer connection conductor according to en embodiment of the invention using a metal fine wire coated with a solder metal will be describe below with reference to
As shown in
In the solder-coated metal core 30, because the diameter of the metal fine wire 22 composing the solder-coated metal fine wire 28 and the thickness of the solder metal layer 27 coating the surface of the metal fine wire 22 are constant and the solder-coated metal fine wire 28 is cut into a predetermined length by adjusting the amount of its movement, uniform volume of the metal core 2 and the solder metal layer 29 is simultaneously obtained. Further, the solder-coated metal core 30 can be manufactured through a simple process of the moving of the solder-coated metal fine wire 28 and cutting of the cut mold 23, and the terms of regular amount and independence in the metal core 2 and the solder metal are simultaneously obtained. Accordingly, it improves the productivity.
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In
In the manufacturing method of the interlayer connection conductor 1 according to embodiments of the invention through the processes as described above, the interlayer connection conductor 1 having improved productivity can be manufactured through simple processes of cutting of a metal fine wire and heating and melting of a solder metal because it has a substantially spherical shape having uniform volume. Therefore, the interlayer connection conductor 1 can be used as an interlayer connection conductor of multilayer FPC having small wiring layers with high connection reliability.
Hereinafter, a method of manufacturing a multilayer flexible printed circuit board according to another embodiment of the present invention will be described. First, the multilayer flexible printed circuit board according to the embodiment of the present invention will be described with reference to
In the multilayer EPC 100 shown in
Next, the method of manufacturing the multilayer FPC 100 according to the present embodiment will be described in detail with reference to FIGS. 8 to 12. According to the embodiment, the electrical connection between the upper wiring layer 103 and the lower wiring layer 104 can be realized with high reliability.
FIGS. 8 to 12 are views illustrating the manufacturing process of the multilayer FPC 100 according to the present embodiment.
As shown in
In FIGS. 8 to 12, the double-sided flexible printed circuit board 101 includes an insulating layer 102 made of a polyimide film on whose both sides the upper wiring layer 103 and the lower wiring layer 104 are formed, and the through hole 106 is formed through the insulating layer 102, the upper wiring layer 103, and the lower wiring layer 104. The classification mask 105 has the classification mask opening 107 for classifying and positioning the substantially spherical conductor 108 with respect to the through hole 106 in the double-sided flexible printed circuit board 101. A hot plate 109 is a means for heating and pressing the substantially spherical conductor 108 that is inserted in the through hole 106.
To manufacture the multilayer FPC 100 according to the present embodiment, as shown in
Next, as shown in
Next, after the substantially spherical conductor 108 is classified to be accommodated in the classification mask opening 107 of the classification mask 105, as shown in
Next, as shown in
In the manufacturing method, an insulated plastic film with high flexibility is used as the insulating layer 102 that is used in the double-sided flexible printed circuit board 101 with wiring layers on both sides thereof The insulating plastic film includes polyimide film, polyethylene film, polyethylene terephthalate film, polyethylene naphthalate film, polyether nitryl film, etc.
The double-sided flexible printed circuit board 101 is not limited to having the insulating layer 102 with wiring layers on both sides thereof That is, the double-sided flexible printed circuit board 101 may use an insulating film with wiring layers on both sides thereof, or may use a pair of insulating films attached to each other, and each insulating film has a wiring layer on one side thereof.
The metal used in forming wiring layers may be various kinds of conductive metal such as copper, gold, and nickel. The substantially spherical conductor 108 may be a solder ball, a copper ball, a copper core solder ball, a copper core metal ball, a resin core solder ball, a resin core metal ball, etc.
In general, since the conductor for connecting layers of the multilayer FPC should be smoothly introduced in the through hole for interlayer connection, the conductor is preferably a spherical conductor having high sphericity and little variations in particle diameter. However, a method of manufacturing the spherical conductor satisfying the above-mentioned requirements is not suitable for mass production, which causes cost of the spherical conductor to rise.
However, according to the present invention, the conductor can be classified and arranged at the same time by the classification mask 105, which may reduce the requirements for the substantially spherical conductor 108 used as the conductor. Therefore, it is possible to use a substantially spherical conductor that is obtained by, for example, an atomizing method suitable for mass production, which may contribute to cost reduction.
As a method of forming a desirably sized through hole 106 in a predetermined position of the double-sided flexible printed circuit board 101, conventional methods such as drilling, punching, and laser processing can be used. The shape of the through hole 106 is not particularly limited, but the substantially spherical conductor 108 is preferably formed in a circular shape so as to be evenly press-fitted in the two wiring layers. The diameter of the through hole 106 may differ according to the thickness of the multilayer FPC, preferably, in the range of 100 μm to 500 μm.
The classification mask 105 may be formed as an opening with the size of 0.2 mm to 1.0 mm is formed at a metal thin plate with the size of 0.1 mm to 0.3 mm, such as 42 alloy, stainless, and coppers, by laser. In the present invention, the diameter of the classification mask opening 107 of the classification mask 105 is preferably larger than that of the though hole 106 of the double-sided flexible printed circuit board 101 Otherwise, when the diameter of the classification mask opening 107 is equal to or smaller than that of the through hole 106 of the double-sided flexible printed circuit board 101, the substantially spherical conductor 108 dropped through the classification mask opening 107 in the through hole 106 of the double-sided flexible printed circuit board 101 cannot thoroughly fill the through hole 106 of the double-sided flexible printed circuit board 101, which may deteriorate interlayer connection.
According to the present embodiment, the method of manufacturing the multilayer FPC 100 has characteristics as follows. As the substantially spherical conductor 108 is classified using the classification mask 105, interlayer connection inside the through hole 106 is optimized, thereby obtaining high reliability. Further, ultrasonic vibration is applied to have the substantially spherical conductor 108 accommodated in the classification mask opening 107 of the classification mask 105, which makes unnecessary maintenance such as a jig for accommodation. Therefore, the substantially spherical conductor 108 can be efficiently accommodated in the classification mask opening 107 of the classification mask 105, thereby achieving higher productivity. In addition, as the classification mask opening 107 of the classification mask 105 is aligned with the through hole 106 of the double-sided flexible printed circuit board 101 using the positioning pin 111, suction is performed by controlling vacuum pressure into the through hole 106, which eliminate the possibility that the substantially spherical conductor 108 is wrongly arranged in the through hole 106. Therefore, the substantially spherical conductor 108 can be easily and reliably arranged in the through hole 106, which leads to higher reliability and productivity.
According to the present embodiment, the flexible printed circuit board does not have a connection layer among the insulating layer 102, the upper wiring layer 103 and the lower wiring layer 104. However, the kind of flexible printed circuit board is not limited thereto in the present invention. For example, a flexible printed circuit board having a connection layer or a pair of flexible printed circuit boards attached to each other—each flexible printed circuit board has a wiring layer on one side thereof—can be used, otherwise, a desired type of flexible printed circuit board can be used.
According to further embodiment, the above-described multilayer FPC is further laminated.
In
In the multilayer FPC 300, component members composed of the multilayer FPC 100a, 100b, and 200a have high interlayer connection reliability and fine wiring layers. Also, component members composed of the multilayer FPC 200a and the one-sided printed circuit board 500a have high interlayer connection reliability. Therefore, even though the multilayer FPC 300 is formed of more wiring, layers, as compared to that of the first embodiment, the multilayer FPC is realized with high interlayer connection reliability and fine wiring layers.
To manufacture the multilayer FPC 300, as shown in
The interlayer connector 110 of
Next, as shown in
Next, the multilayer FPC 200a is laminated on the one-sided printed circuit board 500a for bonding through the connection layer 121. Accordingly, the multilayer FPC 200a can be electrically connected to the wiring layer 131 of the one-sided printed circuit board 500a by the interlayer connection bump 115, which makes easy to form a multilayer. The multilayer FPC 100 in which the multilayer FPC 200a is laminated on the one-sided printed circuit board 500a is laminated on the multilayer FPC 100b for bonding through the connection layer 122. Further, the multilayer FPC 100 in which the printed circuit board 500a, the multi layer FPC 200a, and the multilayer FPC 100b are laminated on each other is laminated on the multilayer FPC 100a for bonding through the connection layer 123. By this, the multilayer FPC 300 having more wiring layers can be easily manufactured. The order of laminations is not particularly limited, and can be modified.
In
In the multilayer FPC 400 component members composed of the multilayer FPC 100c, 100d, and 100e and the one-sided printed circuit board 500b have high interlayer connection reliability and fine wiring layers. Therefore, even though the multilayer FPC 400 is formed of more wiring layers, as compared to that of the first embodiment, the multilayer FPC is realized with high interlayer connection reliability and fine wiring layers.
To manufacture the multilayer FPC 400, as shown in
Next, the multilayer FPC 200d is laminated on the one-sided printed circuit board 500b for bonding through the connection layer 124, so that the wiring layer 131 of the one-sided printed circuit board 500b is in contact with the interlayer connector 110 of the multilayer FPC 200d. Accordingly the multilayer FPC 200d can be electrically connected to the wiring layer 131 of the one-sided printed circuit board 500b by the interlayer connector 110, which makes easy to form a multilayer. The multilayer FPC in which the multilayer FPC 200d is laminated on the one-sided printed circuit board 500b is laminated on the multilayer FPC 200c for bonding through the connection layer 125.
Further, the multilayer FPC in which the printed circuit board 500b, the multi layer FPC 200d, and the multilayer FPC 200c are laminated on each other is laminated on the multilayer FPC 200b for bonding through the connection layer 126. By this, as wiring layers of the component members are electrically connected to each other, the multilayer FPC 400 having more wiring layers can be easily manufactured. The order of laminations is not particularly limited, and can be modified.
An interlayer connection conductor according to the invention is preferable to connection between wiring layers of multilayer FPC on which a variety of surface-mounting type components are mounted because of high connection reliability, optimally miniaturized wiring layers, and improve productivity.
This application is based upon and claims the benefit of priorities of Japanese Patent Application No. 2005-165125 filed on Jun. 6, 2005 and No. 2005-277758 filed on Sep. 26, 2006, the contents of which are incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005/277758 | Sep 2005 | JP | national |
| 2005/165125 | Jun 2005 | JP | national |
