JP4668232B2 - Flexible printed circuit board - Google Patents

Description

  The present invention relates to a flexible printed circuit board used for hard disk wiring and the like, and more particularly to a flexible printed circuit board with improved flex resistance.

  Conventionally, a flexible printed circuit board on which a flexible printed circuit is formed has been used. The flexible printed circuit board is a printed circuit board that can be bent softly and bent, twisted, wound and overlapped. Specifically, it is used for wiring of multimedia products such as computer-related products, electronic communication devices and audio / visual products, cameras and automobiles.

  Recently, flexible printed circuit boards have been used particularly in fields that require high flexibility. For example, it is used as a wiring between a magnetic head inside a hard disk device and a main body circuit. In such an application, it is necessary to ensure electrical continuity between devices or elements while being repeatedly bent many times.

FIG. 4 is a schematic view showing a conventional method for producing a flexible printed board. Conventionally, when a flexible printed circuit board is manufactured, first, a copper foil is pasted on an adhesive base film 21 made of polyimide resin or polyethylene terephthalate (PET) resin using an adhesive 22. The base film 21 is also called CCL ( Copper Composite Laminate).

  Thereafter, an electronic circuit 23 corresponding to the purpose is formed on the copper foil by a lithography technique or the like. Next, a protective sheet in which an adhesive 24 is applied in advance to a flexible base film 25 made of polyimide resin or PET resin is placed on the base film 21 so that the adhesive 24 is on the lower side. Then, the adhesive 24 is cured while sandwiching the base films 21 and 25 at an appropriate pressure at an appropriate temperature. The base film 25 is also called CL (Cover Layer).

  The conventional flexible printed circuit board manufactured in this way has a structure in which an electronic circuit 23 is sandwiched between adhesives 22 and 24 and base films 21 and 25 as shown in FIG. The overall thickness is often 0.1 mm or less and has flexibility and reliability.

Japanese Patent Laid-Open No. 3-199353 JP 58-108785 A JP-A-7-188969 JP-A-8-277485

  However, if repeated bending is applied to a conventional flexible printed circuit board with a small bending radius, the conductor circuit (electronic circuit) deteriorates due to fatigue, and if the conditions are severe, a crack may occur in the conductor circuit, resulting in disconnection. There is a problem that there is also.

  In general, copper foil is manufactured by rolling to improve fatigue characteristics, but once a crack occurs, it often penetrates to the back of the copper foil, and the reliability when used under severe bending conditions is sufficient Not something.

  This invention is made | formed in view of this problem, Comprising: It aims at providing the flexible printed circuit board which can suppress the progress of a crack and can obtain high bending resistance.

A flexible printed circuit board according to the present invention is a flexible printed circuit board used for bending applications, and has a base film and a conductor circuit composed of a rolled copper alloy foil provided on the base film. The copper alloy foil has an average crystal grain size in the thickness direction of 2.1 μm or more and 10 μm or less, and when the crystal grain boundary is traced from the front surface to the back surface in the cross section, it takes 4 or more branch points. is intended to through, three or more crystal grains, wherein a plurality of crystal grain boundaries are formed by contacting each other is introduced region overlapping the T-shaped form rather than straight.

  In the present invention, even if a crack occurs in the conductor circuit from the front surface side and proceeds to the back surface side of the conductor circuit along the grain boundary, there are four or more branch points until the back surface reaches the back surface. Progress is hindered. For this reason, the progress of the crack to the back surface is remarkably reduced, and the bending resistance is improved.

The average crystal grain size is preferably 8.9 μm or less. In the present invention, the copper alloy foil may contain 0.05% by mass or more of at least one element selected from the group consisting of Sn, Zn, Be, Cd, Ag, and Nb .

As described above in detail, according to the present invention, even if an attempt is progress in the lower layer or the back surface side of the conductor circuit occurs from the surface layer or the surface side of the crack conductor circuit due to fatigue, region grain boundaries multiple crosses The progress can be suppressed. For this reason, by significantly reducing the progress of cracks to the lowermost layer or the back surface, the fatigue life can be significantly increased and the flex resistance can be improved.

  In order to solve the above problems, the inventors of the present application conducted a fatigue test of the flexible printed circuit board and observed the progress of cracks. As a result, it was conceived that cracks were generated on the surface of the copper foil where the strain due to bending was large and progressed from the inside to the back surface along the crystal grain boundaries in the copper foil.

  In many conventionally used rolled foils, the crystal grain size is not intentionally controlled, and a structure in which crystal grains are thinly stretched in the rolling direction is observed immediately after rolling. However, the heat treatment after the base film (CL) is attached is performed under press pressure under conditions of a temperature of 100 to 180 ° C. and a time of several tens of minutes to several hours. In order to further cure the adhesive, a further heat treatment may be performed under the conditions of a temperature of 100 to 180 ° C. and a time of several tens of minutes to several hours. Recrystallization occurs by these heat treatments, and the average crystal grain size in the thickness direction of the copper foil often exceeds 10 μm. Therefore, in the copper foil having a thickness of about 5 to 35 μm depending on the application, there are only a few crystal grains in the thickness direction. For this reason, it is possible for cracks to propagate along the crystal grain boundaries very easily from the front surface to the back surface.

  In addition to the rolling method, there is an electrolysis method as a method for producing the copper foil used in the conductor circuit, but the metal foil produced by the electrolysis method grew in a dendrite shape in the thickness direction during electrodeposition. Since crystals are present, the crystal grain boundaries are connected by a simple path in the thickness direction, and when recrystallization proceeds by the above-described heat treatment, cracks progress more easily than in the rolled foil.

Therefore, the inventors of the present application have further conducted extensive experimental research, and as a result, the conductor layer constituting the circuit is a laminate of three or more layers, or the average grain size in the thickness direction is 2.1 μm or more and 10 μm or less. with the four or more the number of grain boundaries of the branch point, and Heading that can improve the flexibility of the flexible printed circuit board.

  Hereinafter, a flexible printed circuit board according to an embodiment of the present invention will be specifically described with reference to the accompanying drawings. FIG. 1 is a schematic view showing a flexible printed circuit board according to a reference example of the present invention.

  In the reference example, an adhesive 2 is applied on a base film 1 made of polyimide resin or PET resin, and a multilayer conductor circuit 3 is formed thereon. The multilayer conductor circuit 3 is composed of a laminate of three or more layers of metal foil, and has, for example, a five-layer structure. Each metal foil 3a thru | or 3e is formed from copper foil or silver foil, for example. These lamination methods are not particularly limited and may be all copper foils, or copper foils and silver foils may be alternately laminated.

  Further, a base film 5 made of polyimide resin or PET resin is attached on the base film 1 with an adhesive 4 that covers the multilayer conductor circuit 3.

In the reference example configured as described above, even if a crack occurs in the metal foil 3e located in the outermost layer and penetrates the metal foil 3e completely along the crystal grain boundary, it is located in the second layer. Since the probability that the crystal grain boundary of the metal foil 3d to be present is at the crack penetration position of the metal foil 3e is low, the progress of the crack stops at the interface between the metal foil 3e and the metal foil 3d in many cases. Further, even if the crack penetrates the metal foil 3d, in many cases, the progress stops at the interface with the metal foil 3c . Therefore, the probability that a crack generated in the metal foil 3e located on the outermost layer penetrates all the metal foils is extremely low. As a result, the bending resistance of the multilayer conductor circuit 3 and the bending resistance of the flexible printed circuit board are significantly improved.

  However, if the laminate of metal foils has a two-layer structure, if the crack can overcome one interface, it will penetrate the whole easily, so the laminate requires three or more layers of metal foil. is there.

  In addition, the laminated body of metal foil can be manufactured with the following method. The first method is a method in which foils having a predetermined number of layers of three or more layers are previously stacked and rolled to a desired thickness. The second method is a method of manufacturing a laminate with the same material by performing electroplating while repeating interruption and restart. The third method is a method of manufacturing a laminate with a plurality of types of materials by performing electroplating while changing the electrolyte solution. Moreover, it is also possible to manufacture a laminated body similarly to the said 2nd or 3rd method by vacuum processes, such as a vacuum evaporation method or sputtering method.

  When manufacturing a flexible printed circuit board, first, a metal foil manufactured as described above using an adhesive 2 on a flexible base film 1 made of polyimide resin or polyethylene terephthalate (PET) resin or the like. The laminated body of 3a thru | or 3e is affixed.

  Thereafter, the multilayer conductor circuit 3 corresponding to the purpose is formed on the laminated body by a lithography technique or the like. Next, a protective sheet in which the adhesive 4 is previously applied to the flexible base film 5 made of polyimide resin or PET resin is placed on the base film 1 so that the adhesive 4 is on the lower side. Then, the flexible printed circuit board according to the first embodiment can be manufactured by curing the adhesive 4 while sandwiching the base films 1 and 5 at an appropriate pressure at an appropriate temperature.

  Next, examples of the present invention will be described. In the embodiment of the present invention, a fine grain conductor circuit 6 having fine crystal grains is provided instead of the multilayer conductor circuit 3. FIG. 2 is a schematic view showing a flexible printed circuit board according to an embodiment of the present invention. In the embodiment shown in FIG. 2, the same components as those in the reference example shown in FIG.

In the embodiment of the present invention, an adhesive 2 is applied on a base film 1 made of polyimide resin or PET resin, and a fine crystal conductor circuit 6 is formed thereon. The fine crystal grain conductor circuit 6 is formed of a copper alloy foil having an average grain size in the thickness direction of crystal grains of 2.1 μm or more and 10 μm or less, for example, 2.1 to 3 μm. Moreover, in any cross section, there are four or more branch points in any path when the crystal grain boundary is traced from the front surface to the back surface of the copper alloy foil. In the copper alloy foil, for example, 0.05% by mass or more of Sn, Zn, Be, Cd, Ag, or Nb is added to Cu alone or in combination.

In the embodiment of the present invention thus configured, the number of crystal grains per unit volume is larger than that of a conventional conductor circuit, and a plurality of crystal grains are formed by contacting three or more crystal grains with each other. An infinite number of regions where the boundaries overlap in a T shape rather than a straight line are introduced. In such an overlapping region, for example, a crack that has propagated along one crystal grain boundary extending in the thickness direction has to be changed in a vertical direction, for example. Accordingly, cracks are difficult to transfer to intersecting crystal grain boundaries, and it is difficult to further progress in the thickness direction. As described above, the progress of the crack in the thickness direction is obstructed, and as a result, the bending resistance of the fine grain conductor circuit 6 and the bending resistance of the flexible printed circuit board are remarkably improved.

  In addition, copper alloy foil can be manufactured with the following method. The first method is to roll a copper alloy material to which an element such as Sn, Zn, Be, Cd, Ag, or Nb, which raises the recrystallization temperature of Cu, is added alone or in combination. It is a method. The kind and amount of the additive element at this time are set so that the recrystallization temperature of the copper alloy foil is higher than the temperature in the heat treatment after the flexible printed circuit board is manufactured. Since the grain size in the thickness direction of the foil becomes fine by rolling, if the recrystallization after rolling is prevented, the average grain size in the rolling direction is 10 μm or less even after the flexible printed circuit board is manufactured. It has become. In addition, the kind of additive element is not limited to the above. The second method is a method of performing electrodeposition using a plating bath in which elements such as Sn, Zn, Be, Cd, Ag, or Nb are added alone or in combination. Copper alloy foils produced by this method are already commercially available. In addition, since the copper alloy foil obtained by electrodeposition is harder to recrystallize than the thing by a 1st method, said element does not need to be added. However, it is desirable to be added to stabilize the quality. Moreover, the kind of additive element is not limited to the above.

  As described above, the additive element is added to prevent recrystallization in the heat treatment after manufacturing the flexible printed circuit board. In other words, the heat treatment for curing the adhesive at the time of manufacturing the flexible printed board is performed at a temperature lower than the recrystallization temperature of the copper alloy foil.

Note that, as in the embodiment of the present invention, a conductive circuit may be formed by laminating three or more metal foils having an average crystal grain size in the thickness direction of 2.1 μm or more and 10 μm or less .

  Hereinafter, the effects of the embodiment of the present invention will be specifically described in comparison with a comparative example that departs from the scope of the claims of the present invention. FIG. 3 is a schematic view showing a fatigue test method.

  First, the flexible printed circuit board 11 to be tested was attached to the lower jig 12 and the upper jig 13 arranged in parallel with each other so that the bending radius thereof was 2 mm. The thickness of the base film made of two polyimide resins on the upper and lower sides of the flexible printed board 11 is 25 μm, the thickness of the conductor circuit is 35 μm, and the thickness of the adhesive between the conductor circuit and the two base films. All were 15 μm.

  Then, the lower jig 12 was fixed, and the upper jig 13 was reciprocated in the horizontal direction at a stroke of 20 mm and a speed of 1500 times / minute. During this time, the fluctuation of the resistance value in the conductor circuit was measured, and the number of reciprocations when it increased by 10% from the initial value was measured.

Reference Example In a reference example, a flexible printed circuit board to be tested was used which has a multilayer conductor circuit as in the reference example shown in FIG. The multilayer conductor circuit is produced by laminating a predetermined number of foils having a thickness of 0.5 mm and then repeating the rolling process until the total thickness becomes 35 μm as described above. The number of laminations and the lamination conditions at this time are shown in Table 1 below. Moreover, the above-mentioned number of measurements is shown in Table 2 below.

  As shown in Table 2 above, the life of the reference example was more than twice that of Comparative Example 11 which is a single layer. And as in Reference Examples 2 and 3, the lifetime dramatically increased as the number of layers increased. Further, as in Reference Examples 4 to 6, even when the copper foil and the silver foil were alternately laminated, the life was extended as the number of laminations increased.

  Furthermore, for each of the reference examples and comparative examples, when the cross section of the fatigue test that was stopped at 1,000,000, or 1,000 times was observed, the progress of cracks generated on the surface layer due to fatigue in any of the examples Was stopped at the interface with the lower layer.

Example In the example, a flexible printed circuit board to be tested was used which has a fine grain conductor circuit as in the example of FIG. The average crystal grain size in the thickness direction at this time, the minimum number of branch points of the crystal grain boundary from the front surface to the back surface, and their contents are shown in Table 3 below. Further, the above-mentioned measurement times are shown in Table 4 below.

  As shown in Table 4 above, in Examples 7 to 9, an element (Ag, Sn, or Nb) that increases the recrystallization temperature of 0.05 mass% or more is added, and the average crystal grain size in the thickness direction is 10 μm. Since the conductor circuit was formed using the following rolled foil, the bending life exceeded 10 million times. Since the crystal grains are refined by addition of Zn, Be and Cd, the same effect is considered to be obtained.

  On the other hand, in Comparative Example 12, the conductor circuit is formed using a commercial rolled foil having a Cu content of 99.8% by mass and an average crystal grain size in the thickness direction exceeding the upper limit of the range of the present invention. Since only one or two crystal grains existed in the thickness direction and the cracks progressed until they reached the back surface easily, the bending life was extremely short at 2.18 million times.

  In Comparative Example 13, since the conductor circuit is formed using a commercially available electrolytic foil having a dendritic structure with a coarse metal structure and an average crystal grain size in the thickness direction exceeding the upper limit of the range of the present invention, the flex life is Compared with Comparative Example 12, it was remarkably low.

  Furthermore, for each of the examples and comparative examples, when the cross section of the fatigue test that was stopped at 1,000,000, or 1,000,000 times was observed, cracks caused by fatigue had progressed along the grain boundaries, In any of the examples, the progress was stopped at the intersections of many grain boundaries existing in the metal structure.

It is a schematic diagram which shows the flexible printed circuit board which concerns on the reference example of this invention. It is a schematic diagram which shows the flexible printed circuit board based on the Example of this invention. It is a schematic diagram which shows the method of a fatigue test. It is a schematic diagram which shows the manufacturing method of the conventional flexible printed circuit board.

Explanation of symbols

1, 5, 21, 25; base film 2, 4, 22, 24; adhesive 3, 6, 23; circuit 3a, 3b, 3c, 3d, 3e; metal foil 11; flexible printed circuit board 12, 13;

Claims (3)

A flexible printed circuit board used for bending applications, comprising a base film and a conductor circuit composed of a rolled copper alloy foil provided on the base film , the copper alloy foil having a thickness is intended to through take four or more branch points any path when tracing the grain boundary from the surface to the back surface in the average crystal grain size is 10μm or less than 2.1μm cross-section in the direction, three A flexible printed circuit board, wherein a region where a plurality of crystal grain boundaries are overlapped in a letter shape instead of a straight line is formed by contacting the above crystal grains . The flexible printed circuit board according to claim 1, wherein the average crystal grain size is 8.9 μm or less. The copper alloy foil, Sn, Zn, Be, Cd , according to claim 1 or 2 at least one element selected from the group consisting of Ag and Nb, wherein the benzalkonium be contained more than 0.05 wt% A flexible printed circuit board according to 1.

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