JP5520848B2 - Method for manufacturing flexible circuit board - Google Patents

Description

  The present invention relates to a method for manufacturing a flexible circuit board including an insulating layer and a wiring layer made of a patterned conductor.

  In recent years, flexible circuit boards have been widely used in electronic devices having movable parts such as mobile phones, hard disk devices, and printers. This flexible circuit board includes a flexible insulating layer and a wiring layer made of a patterned conductor and disposed on one surface of the insulating layer.

  The flexible circuit board is manufactured by patterning a conductive layer in a laminate including the insulating layer and a conductive layer disposed on one surface of the insulating layer by etching. Copper foil is often used for the conductor layer. The above laminate using a copper foil as the conductor layer is called a copper clad laminate. Hereinafter, the example in case the said laminated body is a copper clad laminated board is demonstrated.

  In recent years, there has been a strong demand for thinner and lighter electronic devices. Therefore, flexible circuit boards are required to be thin and have high flexibility and high bending resistance. In recent years, the amount of information handled in electronic devices has increased, and therefore, flexible circuit boards that transmit information are required to have finer and higher density wiring layers, and copper-clad laminates have finer conductor layers. It is demanded that patterning can be performed. In order to meet these requirements, the design and development of copper-clad laminates are progressing in the direction of making both insulating layers and conductor layers thinner and more flexible.

  For example, as described in Patent Document 1, it is known that in a copper-clad laminate, the bending resistance of the copper foil can be improved by annealing (heat treatment) the copper foil that is the conductor layer. . When the copper foil is annealed, the flexibility is improved as well as the bending resistance.

  Patent Document 2 describes a technique for annealing a thin film by applying pulsed electric energy to both ends of a thin film circuit formed on the substrate, although it is not related to a flexible circuit board. Note that the purpose of annealing in the technique described in Patent Document 2 is to relieve stress and recover defects in the thin film, not to improve the bending resistance and flexibility of the thin film.

JP 2007-59892 A JP-A-6-37096

  In the copper-clad laminate, if both the insulating layer and the conductor layer are made thin and the flexibility is increased, it becomes possible to meet the above-mentioned demand for the flexible circuit board. However, in this case, since the rigidity of the copper-clad laminate is lowered, the handling property of the copper-clad laminate is lowered. The decrease in handling properties of the copper clad laminate leads to a decrease in yield in the production of the copper clad laminate and the flexible circuit board.

  Further, even in a flexible circuit board, if flexibility is high, there arises a problem that handling property is lowered. The reduction in handling properties of the flexible circuit board leads to a reduction in the efficiency of the manufacturing work of electronic equipment using the flexible circuit board.

  As described above, in the conventional copper-clad laminate and flexible circuit board, it has been difficult to achieve both improvement in flexibility and bending resistance and improvement in handling properties.

  The present invention has been made in view of such problems, and a first object thereof is a laminate used for manufacturing a flexible circuit board while enhancing flexibility and bending resistance in a portion to be bent in the flexible circuit board. An object of the present invention is to provide a method of manufacturing a flexible circuit board that can improve the handling of the body.

  In addition, the second object of the present invention is to improve the handling properties of the laminate and the flexible circuit board used in the manufacture of the flexible circuit board, while increasing the flexibility and the bending resistance in the bent portion of the flexible circuit board. Another object of the present invention is to provide a method of manufacturing a flexible circuit board that can be made to operate.

  A flexible circuit board manufactured by the manufacturing method of the present invention includes an insulating layer having first and second surfaces facing away from each other and a patterned metal and is disposed on the first surface of the insulating layer. Wiring layer.

The method for producing a flexible circuit board of the present invention includes:
A step of producing a laminate including an insulating layer and a conductor layer made of a metal polycrystal constituting the wiring layer and disposed on the first surface of the insulating layer;
Forming a spare wiring layer by patterning the conductor layer;
In order that the average value of the maximum length in the thickness direction of the spare wiring layer of the metal crystal grains in at least a part of the spare wiring layer is larger than that before heating, the spare wiring layer becomes the wiring layer. And a step of heating at least a part.

  In the method for manufacturing a flexible circuit board of the present invention, the average value of the maximum length of the metal crystal grains in at least a part of the spare wiring layer in the thickness direction of the spare wiring layer is 1.1 times before heating and before heating. The thickness may be equal to or less than the thickness of the spare wiring layer.

  In the method for manufacturing a flexible circuit board according to the present invention, the longitudinal elastic modulus in at least a part of the spare wiring layer may be smaller than that before the heating by heating at least a part of the spare wiring layer. In this case, the longitudinal elastic modulus in at least a part of the spare wiring layer may be within a range of 0.5 to 0.95 times before heating after heating.

  In the method for manufacturing a flexible circuit board of the present invention, the step of heating at least a part of the spare wiring layer may heat only a part of the spare wiring layer. In this case, the wiring layer includes the first portion that has not been heated and the second portion that has been heated, and the average value of the maximum length of the metal crystal grains in the second portion in the thickness direction of the wiring layer. May be larger than the average value of the maximum lengths of the metal crystal grains in the first portion in the thickness direction of the wiring layer. The average value of the maximum length of the metal crystal grains in the second portion in the thickness direction of the wiring layer is 1 of the average value of the maximum length of the metal crystal grains in the first portion in the thickness direction of the wiring layer. It may be not less than 1 time and not more than the thickness of the wiring layer. Further, the longitudinal elastic modulus of the second portion may be smaller than the longitudinal elastic modulus of the first portion. In this case, the longitudinal elastic modulus of the second part may be in the range of 0.5 to 0.95 times the longitudinal elastic modulus of the first part.

  In the method for manufacturing a flexible circuit board of the present invention, the step of heating at least a part of the spare wiring layer may energize at least a part of the spare wiring layer.

  In the method for manufacturing a flexible circuit board of the present invention, the metal constituting the wiring layer may be copper. In this case, the conductor layer may be made of a rolled copper foil. The insulating layer may be made of a polyimide resin or a liquid crystal polymer.

  In the method for manufacturing a flexible circuit board of the present invention, after forming the preliminary wiring layer by patterning the conductor layer, the wiring layer is formed by performing a process of heating at least a part of the preliminary wiring layer. According to this manufacturing method, the softness | flexibility of the laminated body containing an insulating layer and a conductor layer can be made low. Thereby, the handleability of a laminated body can be improved. Further, according to the present invention, the average value of the maximum length in the thickness direction of the spare wiring layer of the metal crystal grains in at least a part of the spare wiring layer is heated by the step of heating at least a part of the spare wiring layer. It will be bigger than before. Thereby, in the flexible circuit board, it is possible to increase the flexibility and the bending resistance of the portion corresponding to at least a part of the wiring layer. This part can be applied to a bent part in the flexible circuit board. From the above, according to the method for manufacturing a flexible circuit board of the present invention, the handling of the laminate used for the manufacture of the flexible circuit board while enhancing the flexibility and the bending resistance in the bent portion of the flexible circuit board. It is possible to improve the performance.

  In the present invention, the step of heating at least a part of the spare wiring layer heats only a part of the spare wiring layer, and the wiring layer includes a first part that is not heated and a second part that is heated. And may be included. In this case, in the flexible circuit board, the flexibility of the portion corresponding to the first portion of the wiring layer can be reduced. Thereby, the handleability of the flexible circuit board can be improved as compared with the case where the longitudinal elastic modulus of the entire wiring layer is reduced to increase the flexibility and the bending resistance of the entire flexible circuit board. As a result, it is possible to improve the handling properties of the laminate and the flexible circuit board used in the manufacture of the flexible circuit board while increasing the flexibility and the bending resistance characteristics in the bent portion of the flexible circuit board. Play.

1 is a perspective view showing a schematic configuration of a flexible circuit board according to an embodiment of the present invention. It is a perspective view which shows a part of flexible circuit board shown in FIG. It is a perspective view which shows the 1st state at the time of use of the flexible circuit board shown in FIG. It is a perspective view which shows the 2nd state at the time of use of the flexible circuit board shown in FIG. It is a perspective view which shows the 3rd state at the time of use of the flexible circuit board shown in FIG. It is a perspective view which shows one process in the manufacturing method of the flexible circuit board which concerns on one embodiment of this invention. FIG. 7 is a perspective view showing a step that follows the step shown in FIG. 6. FIG. 8 is a perspective view showing a step that follows the step shown in FIG. 7. It is a top view which shows the sample used in the experiment for confirming the effect by this invention. It is sectional drawing which shows the cross section of the wiring layer in the sample 1 used by experiment. It is sectional drawing which shows the cross section of the wiring layer in the sample 2 used by experiment.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. First, the configuration of a flexible circuit board according to an embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a perspective view showing a schematic configuration of a flexible circuit board according to the present embodiment. FIG. 2 is a perspective view showing a part of the flexible circuit board shown in FIG. In FIG. 2, the hatched surface represents a cross section.

  As shown in FIG. 2, the flexible circuit board 1 according to the present embodiment includes an insulating layer 2 made of an insulating material and having flexibility, and a wiring layer 3. The insulating layer 2 has a first surface 2a and a second surface 2b that face away from each other. The wiring layer 3 is made of a patterned conductor and is disposed on the first surface 2 a of the insulating layer 2. Although not shown, the flexible circuit board 1 may further include a protective layer made of an insulating material and having flexibility. The protective layer is disposed so as to cover the first surface 2 a and the wiring layer 3. The flexible circuit board 1 may further include a second wiring layer made of a patterned conductor and disposed on the second surface 2b of the insulating layer 2. In this case, the flexible circuit board 1 may further include a second protective layer that covers the second surface 2b and the second wiring layer.

  FIG. 1 shows an example of the overall shape of the flexible circuit board 1. In this example, the flexible circuit board 1 has a first end 1A and a second end 1B. The second end 1B is wider than the first end 1A. FIG. 1 shows a simplified pattern of the wiring layer 3. The flexible circuit board 1 has flexibility as a whole, but particularly has a portion (hereinafter, referred to as a bent portion) 1C that is bent during use.

  The insulating layer 2 is made of an insulating resin, and is preferably made of a polyimide resin or a liquid crystal polymer having good flexibility and heat resistance. The insulating layer 2 may be a single layer or a plurality of layers. The insulating layer 2 may be formed, for example, by applying a polyimide resin or a liquid crystal polymer resin solution onto a metal foil such as a copper foil that will be patterned later to form the wiring layer 3 and then drying it. Alternatively, the insulating layer 2 may be formed by applying a polyimide resin precursor solution on the metal foil and drying and imidizing the precursor solution by heat treatment. Alternatively, the insulating layer 2 may be formed by bonding a commercially available insulating film made of polyimide resin or liquid crystal polymer to the metal foil. In this case, the insulating film may be bonded to the metal foil using an adhesive made of an epoxy resin, a thermoplastic polyimide resin, or the like as necessary.

  Although the thickness of the insulating layer 2 is not specifically limited, It is preferable to exist in the range of 5-100 micrometers. It is preferable that the thickness of the insulating layer 2 in the portion of the flexible circuit board 1 where particularly high flexibility (flexibility) is required is in the range of 5 to 30 μm. If the thickness of the insulating layer 2 is less than 5 μm, the insulating property of the insulating layer 2 may not be sufficiently obtained. On the other hand, since the flexibility decreases when the thickness of the insulating layer 2 increases, the upper limit of the thickness of the insulating layer 2 is appropriately determined according to the degree to which the flexible circuit board 1 is bent. As a material for the protective layer, a polyimide resin with an adhesive is generally used, and the thickness of the protective layer is appropriately determined as in the case of the insulating layer 2.

  The flexible circuit board 1 is manufactured by patterning a conductive layer in a laminated body including an insulating layer 2 and a conductive layer disposed on the first surface 2a of the insulating layer 2 by etching. The conductor layer is made of a metal constituting the wiring layer 3. For the conductor layer, for example, a metal foil, particularly a copper foil is used. A laminate using copper foil as the conductor layer is called a copper clad laminate. Copper foil used for the conductor layer includes rolled copper foil and electrolytic copper foil. In general, rolled copper foil has higher bending resistance than electrolytic copper foil. Therefore, it is preferable to use a rolled copper foil as the copper foil used for the conductor layer. The thickness of the copper foil used for the conductor layer is preferably in the range of 5 to 30 μm, and more preferably in the range of 9 to 15 μm. When the thickness of the copper foil is less than 5 μm, the rigidity of the copper-clad laminate becomes too small, and the handling properties of the copper-clad laminate at the time of manufacturing the flexible circuit board 1 are lowered. On the other hand, when the thickness of the copper foil exceeds 30 μm, the flexibility and bending resistance of the flexible circuit board 1 are lowered and the cost is increased.

  Next, a usage example of the flexible circuit board 1 according to the present embodiment will be described with reference to FIGS. 3 to 5 are perspective views showing first to third states when the flexible circuit board 1 is used. In this example, a connector 6 is attached to the first end 1A of the flexible circuit board 1, and a rigid printed board 7 is attached to the second end 1B. In use, the flexible circuit board 1 is bent at an arbitrary angle in the bent portion 1C shown in FIG.

  FIG. 3 shows a state (first state) in which the flexible circuit board 1 is not bent at the bent portion 1C. 4 and 5 respectively show a state (second and third states) where the flexible circuit board 1 is bent at the bent portion 1C. The second state shown in FIG. 4 is a state in which a portion of the flexible circuit board 1 closer to the connector 6 (first end 1A) than the bent portion 1C is rotated 90 degrees from the state shown in FIG. It is. The third state shown in FIG. 5 is a state in which the portion of the flexible circuit board 1 closer to the connector 6 (first end 1A) than the bent portion 1C is rotated 180 degrees from the state shown in FIG. It is.

  As shown in FIG. 2, the wiring layer 3 in the present embodiment includes a first portion 3A and a second portion 3B that exist at different positions on the first surface 2a of the insulating layer 2. May be. The longitudinal elastic modulus of the second portion 3B is smaller than the longitudinal elastic modulus of the first portion 3A. The second portion 3B includes at least a portion of the wiring layer 3 that exists in the bent portion 1C shown in FIG. The longitudinal elastic modulus of the second portion 3B is, for example, in the range of 0.5 to 0.95 times the longitudinal elastic modulus of the first portion 3A.

  In the present embodiment, in particular, the wiring layer 3 is made of a polycrystal of a metal such as copper. The average value of the maximum length of the metal crystal grains in the second portion 3B in the thickness direction of the wiring layer 3 is the maximum length of the metal crystal grains in the first portion 3A in the thickness direction of the wiring layer 3. Greater than average value. The thickness direction of the wiring layer 3 is the direction in which the insulating layer 2 and the wiring layer 3 are laminated. The average value of the maximum length of the metal crystal grains in the second portion 3B in the thickness direction of the wiring layer 3 is, for example, the maximum value of the metal crystal grains in the first portion 3A in the thickness direction of the wiring layer 3. It is 1.1 times or more of the average value of the length and less than the thickness of the wiring layer 3. Hereinafter, the average value of the maximum length of the metal crystal grains in the wiring layer 3 (first portion 3A, second portion 3B) in the thickness direction of the wiring layer 3 is referred to as an average crystal grain size. For the average crystal grain size, the maximum length in the thickness direction of the wiring layer 3 is measured for each of a plurality of metal crystal grains in one or more, preferably a plurality of cross-sections of the wiring layer 3, and an average value thereof is obtained. Calculated.

  Note that the wiring layer 3 in the present embodiment does not include the first portion 3A and the second portion 3B, and the whole may have the same properties as the second portion 3B.

  Next, with reference to FIG. 6 thru | or FIG. 8, the manufacturing method of the flexible circuit board 1 which concerns on this Embodiment is demonstrated. In this method of manufacturing the flexible circuit board 1, first, as shown in FIG. 6, the insulating layer 2 and the polycrystal of the metal constituting the wiring layer 3 are formed on the first surface 2 a of the insulating layer 2. The laminate 10 including the conductor layer 30 arranged is produced. The method for forming the insulating layer 2 and the conductor layer 30 is as described above. The laminate 10 using copper foil for the conductor layer 30 is a copper-clad laminate. Next, as shown in FIG. 7, the conductor layer 30 is patterned by etching to form a spare wiring layer 31.

  Next, as shown in FIG. 8, the average value of the maximum length of the metal crystal grains in at least a part of the spare wiring layer 31 in the thickness direction of the spare wiring layer 31 becomes larger than that before heating. At least a part of the preliminary wiring layer 31 is heated so that the layer 31 becomes the wiring layer 3. Hereinafter, this process is referred to as a heat treatment process. Similarly to the case of the wiring layer 3, the average value of the maximum length of the metal crystal grains in the spare wiring layer 31 in the thickness direction of the spare wiring layer 31 is also called the average crystal grain size.

  When forming the wiring layer 3 including the first portion 3A and the second portion 3B, only a part of the preliminary wiring layer 31 is heated in the heat treatment step. When the wiring layer 3 having the same properties as the second portion 3B is formed as a whole, the entire preliminary wiring layer 31 is heated in the heat treatment step.

  As a method of heating at least a part of the spare wiring layer 31, for example, there is a method of energizing at least a part of the spare wiring layer 31. Hereinafter, this method will be described with reference to FIG. Here, a case where only a part of the spare wiring layer 31 is heated will be described. FIG. 8 shows a plurality of lines 31a constituting the spare wiring layer 31 and including a portion to be heated. In this method, the current-carrying electrodes 41 and 42 are brought into contact with both ends of the heated portion of each line 31a. The electrodes 41 and 42 have, for example, a shape that is long in one direction. Thereby, the electrodes 41 and 42 can be simultaneously brought into contact with the plurality of lines 31a, respectively. Next, a predetermined voltage is applied between the electrodes 41 and 42 to energize the portion between the electrodes 41 and 42 of each line 31a. By using the electrodes 41 and 42 having a long shape in one direction, it is possible to energize a part of each of the plurality of lines 31a. The energized portion of each line 31a generates Joule heat and is heated by the Joule heat. Since portions other than between the electrodes 41 and 42 in each line 31a are not energized, no Joule heat is generated. In this way, only a part of the spare wiring layer 31 is heated.

  Of the preliminary wiring layer 31, a portion that is not heated in the heat treatment step is referred to as a first portion 31A, and a portion that is heated in the heat treatment step is referred to as a second portion 31B. Prior to the heat treatment step, the average crystal grain sizes of the metal in the first portion 31A and the second portion 31B are substantially equal. In the heat treatment step, only the second portion 31B of the preliminary wiring layer 31 is heated, and the average crystal grain size of the second portion 31B after heating becomes larger than that before heating. As a result, the average crystal grain size of the metal in the second portion 31B becomes larger than the average crystal grain size of the metal in the first portion 31A by the heat treatment step. After the heat treatment step, the first portion 31A of the auxiliary wiring layer 31 becomes the first portion 3A of the wiring layer 3, and the second portion 31B of the auxiliary wiring layer 31 becomes the second portion 3B of the wiring layer 3. Therefore, in the wiring layer 3, the average crystal grain size of the metal in the second portion 3B is larger than the average crystal grain size of the metal in the first portion 3A.

  In addition, before the heat treatment step, the longitudinal elastic modulus, flexibility, and bending resistance of the first portion 31A and the second portion 31B are substantially equal. By the heat treatment step, the average crystal grain size of the second portion 31B after heating becomes larger than that before heating, so that the longitudinal elastic modulus of the second portion 31B after heating becomes smaller than that before heating, and after the heating, The flexibility and bending resistance of the second portion 31B are higher than before heating. As a result, the longitudinal elastic modulus of the second portion 31B is smaller than that of the first portion 31A, and the flexibility and bending resistance of the second portion 31B are higher than those of the first portion 31A due to the heat treatment process. Therefore, in the wiring layer 3, the longitudinal elastic modulus of the second portion 3B is smaller than that of the first portion 3A, and the flexibility and bending resistance of the second portion 3B are higher than those of the first portion 3A.

  In addition, in the polycrystalline body of metal such as copper used for the wiring layer 3, the heating increases the crystal grain size, decreases the longitudinal elastic modulus, and increases the flexibility and bending resistance. This is common technical knowledge in the technical field related to the manufacture of circuit boards.

  When the entire spare wiring layer 31 is heated by the method shown in FIG. 8, the entire spare wiring layer 31 may be energized. In this case, the entire wiring layer 3 formed by heating the preliminary wiring layer 31 has the same properties as the second portion 3B.

  As described above, in the method of manufacturing the flexible circuit board 1 according to the present embodiment, the step of heating at least a part of the preliminary wiring layer 31 after patterning the conductor layer 30 to form the preliminary wiring layer 31. Then, the wiring layer 3 is formed. According to this manufacturing method, at the stage of the laminate 10 including the insulating layer 2 and the conductor layer 30, the average crystal grain size of the metal in the conductor layer 30 can be reduced, and the flexibility of the laminate 10 can be reduced. . Thereby, the handleability of the laminated body 10 can be improved.

  Further, according to the present embodiment, the step of heating at least a part of the spare wiring layer 31 increases the average crystal grain size of the metal in at least a part of the spare wiring layer 31 than before heating. Thereby, in the flexible circuit board 1, it is possible to increase the flexibility and the bending resistance of the portion corresponding to at least a part of the wiring layer 3. This portion can be a bent portion 1 </ b> C in the flexible circuit board 1.

  From the above, according to the method for manufacturing the flexible circuit board 1 according to the present embodiment, the flexible circuit board 1 is used for manufacturing the flexible circuit board 1 while enhancing the flexibility and the bending resistance in the bent portion 1C. It becomes possible to improve the handling property of the laminated body 10 to be obtained.

  In the present embodiment, the step of heating at least a part of the spare wiring layer 31 may heat only a part of the spare wiring layer 31. In this case, the wiring layer 3 includes a first portion 3A that has not been heated and a second portion 3B that has been heated. The average crystal grain size of the second portion 3B is larger than the average crystal grain size of the first portion 3A, and the longitudinal elastic modulus of the second portion 3B is smaller than the longitudinal elastic modulus of the first portion 3A. In this case, in the flexible circuit board 1, the flexibility of the portion corresponding to the first portion 3A of the wiring layer 3 can be reduced. Thereby, the handling property of the flexible circuit board 1 can be improved as compared with the case where the longitudinal elasticity modulus of the entire wiring layer 3 is reduced and the flexibility and bending resistance of the entire flexible circuit board 1 are increased. As a result, it is possible to improve the handleability of the laminate 10 and the flexible circuit board 1 used for manufacturing the flexible circuit board 1 while increasing the flexibility and bending resistance of the bent portion 1C in the flexible circuit board 1. Become.

  Next, an experiment conducted for confirming the effect of the present invention will be described. FIG. 9 is a plan view showing the appearance of a plurality of samples prepared in the experiment. The sample corresponds to the flexible circuit board 1. The sample includes an insulating layer 2 made of polyimide resin and a wiring layer 13 made of a patterned conductor and disposed on the first surface 2a of the insulating layer 2. The shape of the 1st surface 2a of the insulating layer 2 is a rectangle long in one direction (left-right direction in FIG. 9). The wiring layer 13 has a meander shape. More specifically, the wiring layer 13 has a plurality of linear portions 13a extending in the longitudinal direction (left and right direction in FIG. 9) of the first surface 2a of the insulating layer 2 and the entire wiring layer 13 has a meander shape. And a connecting portion 13b for connecting the ends of two adjacent linear portions 13a. Here, the width of the linear portion 13a (the vertical dimension in FIG. 9) is defined as a line width LW, and the interval between two adjacent linear portions 13a is defined as a line width SW. In the sample, the line width LW and the line width SW are both 75 μm. The insulating layer 2 has a thickness of 16 μm, and the wiring layer 13 has a thickness of 12 μm.

  In the experiment, four samples, namely Sample 1, Sample 2, Sample 3, and Sample 4, were prepared under different conditions. Specifically, Samples 1 to 4 were produced as follows. First, the laminated body which consists of the insulating layer 2 and the conductor layer which consists of a rolled copper foil arrange | positioned on the 1st surface 2a of this insulating layer 2 was produced. In addition, such a laminate is also sold as Espanex (registered trademark) M series MB12-16-12PRQ (product name) manufactured by Nippon Steel Chemical Co., Ltd. The foil was removed by etching. Next, the conductor layer was patterned by etching to form a preliminary wiring layer having the same pattern as the wiring layer 13 shown in FIG. For sample 1, sample 1 was completed using the spare wiring layer as it was as wiring layer 13 without performing heat treatment for heating the spare wiring layer. When the samples 2 to 4 were produced, the wiring layer 13 was formed by performing a heat treatment for heating the preliminary wiring layer, and the samples 2 to 4 were completed. The heat treatment for the spare wiring layer was performed by bringing two energization electrodes into contact with both ends of the spare wiring layer, applying a predetermined voltage between the two electrodes, and energizing the spare wiring layer. Thus, the preliminary wiring layer was heated by Joule heat, and the preliminary wiring layer was used as the wiring layer 13.

  In the process of preparing Samples 2 to 4, the voltage applied between the two electrodes was increased from 0 V to 30 V over a predetermined time (hereinafter referred to as 30 V arrival time) and held at 30 V for 5 minutes. The 30 V arrival times when producing Sample 2, Sample 3, and Sample 4 were 86 seconds, 24 seconds, and 76 seconds, respectively. Moreover, the resistance values before heating of the preliminary wiring layers in Sample 2, Sample 3, and Sample 4 were 61.49Ω, 61.12Ω, and 62.18Ω, respectively. The temperature of the preliminary wiring layer in Sample 2, Sample 3, and Sample 4 when 30 V was applied between the two electrodes was all about 200 ° C.

  In the experiment, for samples 1 to 4, the longitudinal elastic modulus of the wiring layer 13 and the average crystal grain size of the metal in the wiring layer 13 were measured. The average crystal grain size was calculated as an average value obtained by measuring the maximum length in the thickness direction of the wiring layer 13 for each of the plurality of metal crystal grains in the four cross sections of the wiring layer 13.

In the experiment, the length of the interface of metal crystal grains per unit cross-sectional area (75 μm × 12 μm = 900 μm ² ) in the wiring layer 13 was measured for samples 1 to 4 (hereinafter referred to as interface length).

  In the experiment, samples 1 to 4 were subjected to a bending test by the MIT method, and each bending life was measured. The bending test by the MIT method is performed by fixing the first end portion in the longitudinal direction of the sample and reciprocatingly rotating a part of the sample including the second end portion opposite to the first end portion. . In the bending test, the wiring layer 13 is energized and the resistance value of the wiring layer 13 is detected. Then, when the resistance value of the wiring layer 13 becomes a predetermined value or more, it is determined that the wiring layer 13 is broken. In the bending test, the number of reciprocating rotations of a part of the sample from the start of the test until the wiring layer 13 breaks, that is, until the resistance value of the wiring layer 13 becomes a predetermined value or more is measured as the bending life. .

  The results of the experiment are shown in Table 1 below. In Table 1, “magnification” in the longitudinal elastic modulus, average crystal grain size, interface length, and bending life represents the ratio of those values in samples 2 to 4 to those values in sample 1.

  FIG. 10 shows a cross section of the wiring layer 13 in the sample 1. FIG. 11 shows a cross section of the wiring layer 13 in the sample 2. Lines other than the outer edges in FIGS. 10 and 11 indicate crystal grain boundaries. The cross section shown in FIG. 10 can be said to be an example of the cross section of the first portion 3A and the cross section of the second portion 3B before heating. Moreover, it can be said that the cross section shown in FIG. 11 is an example of the cross section of the 2nd part 3B after a heating.

  As shown in Table 1, the samples 2 to 4 in which the wiring layer 13 is formed by performing heat treatment on the spare wiring layer are longer than the samples 1 in which the wiring layer 13 is formed without performing heat treatment on the spare wiring layer. The elastic modulus and interface length are small, and the average crystal grain size and bending life are large. From the experimental results, it can be seen that as the average crystal grain size increases, that is, as the interface length decreases, the longitudinal elastic modulus decreases and the bending life increases.

  Also, from the experimental results, if the modulus of longitudinal elastic modulus is 0.95 or less, the magnification of average crystal grain size is 1.1 or more, and the magnification of interface length is 0.79 or less, the flex life is longer than that of sample 1. It can be seen that is significantly increased. Although there is a limit to reducing the longitudinal elastic modulus by increasing the average crystal grain size, from the experimental results, it is possible to reduce at least the magnification of the longitudinal elastic modulus to 0.5. It can be seen that the bending life can be increased as compared with Sample 1. From this, it can be said that the longitudinal elastic modulus of the second portion 3B is preferably in the range of 0.5 to 0.95 times the longitudinal elastic modulus of the first portion 3A. Similarly, it can be said that the longitudinal elastic modulus in the second portion 31B of the auxiliary wiring layer 31 is preferably in the range of 0.5 to 0.95 times before heating after heating by the heat treatment step.

  Further, the upper limit of the average crystal grain size is equal to the thickness of the wiring layer 3 (preliminary wiring layer 31). Therefore, from the experimental results, the average crystal grain size of the metal in the second portion 3B is 1.1 times or more the average crystal grain size of the metal in the first portion 3A and less than the thickness of the wiring layer 3. It can be said that it is preferable. Similarly, the average crystal grain size of the metal in the second portion 31B of the spare wiring layer 31 is 1.1 times or more of that before the heating and less than the thickness of the spare wiring layer 31 after the heating in the heat treatment step. Can be said to be preferable.

  From the experimental results, it can be said that the interface length in the second portion 3B is preferably in the range of 0.35 to 0.79 times the interface length in the first portion 3A. Similarly, it can be said that the interface length in the second portion 31B of the auxiliary wiring layer 31 is preferably within a range of 0.35 to 0.79 times before heating after heating by the heat treatment step.

  From the experimental results described above, by heating the preliminary wiring layer 31 to form the wiring layer 3, the average crystal grain size of the metal in the wiring layer 3 can be made larger than that of the preliminary wiring layer 31. It can be seen that the longitudinal elastic modulus of the layer 3 can be reduced, and the flexibility and bending resistance of the wiring layer 3 can be increased.

  In the present embodiment, as described above, only a part of the spare wiring layer 31 is heated, and the first part 3A including the unheated part of the spare wiring layer 31 and the spare wiring layer 31. The wiring layer 3 including the second portion 3B made of the heated portion may be formed. In this case, as described above, the flexibility and bending resistance of the portion corresponding to the second portion 3B of the wiring layer 3 are made higher than that of the portion corresponding to the first portion 3A of the wiring layer 3. This makes it possible to improve the handling properties of the flexible circuit board 1 while increasing the flexibility and bending resistance of the bent portion 1C.

  Moreover, according to the manufacturing method of the flexible circuit board 1 which concerns on this Embodiment, it becomes possible to make the softness | flexibility of the conductor layer 30 and the laminated body 10 (copper clad laminated board) containing this low. Thereby, it becomes possible to improve the handleability of the laminated body 10 (copper clad laminated board).

  By the way, if the laminated body 10 is manufactured by sufficiently performing the heat treatment on the conductive layer 30 so that the average crystal grain size of the metal in the conductive layer 30 is sufficiently large, the cost of the laminated body 10 is increased. On the other hand, in this Embodiment, in order to make the softness | flexibility of the laminated body 10 low, the one where the average crystal grain diameter of the metal in the conductor layer 30 is smaller is preferable. Therefore, according to the present embodiment, it is not necessary to sufficiently perform the heat treatment on the conductor layer 30, thereby reducing the cost of the multilayer body 10.

  In addition, this invention is not limited to the said embodiment, A various change is possible. For example, the method of heating at least a part of the spare wiring layer 31 is not limited to a method of energizing at least a part of the spare wiring layer 31, and high energy light such as a laser beam is applied to at least a part of the spare wiring layer 31. Other methods such as an irradiation method may be used. Moreover, when heating the whole preliminary wiring layer 31, you may heat using a heating furnace.

  DESCRIPTION OF SYMBOLS 1 ... Flexible circuit board, 1C ... Bending part, 2 ... Insulating layer, 3 ... Wiring layer, 3A ... 1st part, 3B ... 2nd part, 10 ... Laminated body, 30 ... Conductive layer, 31 ... Reserve wiring layer , 41, 42 ... electrodes.

Claims (12)

A method of manufacturing a flexible circuit board, comprising: an insulating layer having first and second surfaces facing away from each other; and a wiring layer made of a patterned metal and disposed on the first surface of the insulating layer Because
Producing a laminate including the insulating layer and a conductor layer made of a metal polycrystal constituting the wiring layer and disposed on the first surface of the insulating layer;
Patterning the conductor layer to form a preliminary wiring layer;
The average value of the maximum length in the thickness direction of the spare wiring layer of the metal crystal grains in at least a part of the spare wiring layer is larger than before heating, so that the spare wiring layer becomes the wiring layer, And a step of heating at least a part of the preliminary wiring layer.   The average value of the maximum length of the metal crystal grains in at least a part of the spare wiring layer in the thickness direction of the spare wiring layer is 1.1 times or more before heating after heating, and the spare wiring layer The method for manufacturing a flexible circuit board according to claim 1, wherein the thickness is equal to or less than the thickness of the flexible circuit board.   3. The flexible circuit board according to claim 1, wherein the step of heating at least a part of the spare wiring layer reduces a longitudinal elastic modulus in at least a part of the spare wiring layer before heating. 4. Production method.   4. The method of manufacturing a flexible circuit board according to claim 3, wherein the longitudinal elastic modulus in at least a part of the preliminary wiring layer is within a range of 0.5 to 0.95 times after heating.   The step of heating at least a part of the preliminary wiring layer heats only a part of the preliminary wiring layer, and the wiring layer includes a first part that is not heated and a second part that is heated. The average value of the maximum length of the metal crystal grains in the second portion in the thickness direction of the wiring layer is the maximum length of the metal crystal grains in the first portion in the thickness direction of the wiring layer. 5. The method for manufacturing a flexible circuit board according to claim 1, wherein the average value is greater than an average value of   The average value of the maximum length of the metal crystal grains in the second portion in the thickness direction of the wiring layer is the maximum length of the metal crystal grains in the first portion in the thickness direction of the wiring layer. 6. The method for manufacturing a flexible circuit board according to claim 5, wherein the average value is 1.1 times or more and not more than the thickness of the wiring layer.   The method for manufacturing a flexible circuit board according to claim 5, wherein a longitudinal elastic modulus of the second portion is smaller than a longitudinal elastic modulus of the first portion.   8. The method of manufacturing a flexible circuit board according to claim 7, wherein the longitudinal elastic modulus of the second portion is in a range of 0.5 to 0.95 times the longitudinal elastic modulus of the first portion. .   9. The method of manufacturing a flexible circuit board according to claim 1, wherein the step of heating at least a part of the spare wiring layer energizes at least a part of the spare wiring layer.   The method for manufacturing a flexible circuit board according to claim 1, wherein the metal constituting the wiring layer is copper.   The method for manufacturing a flexible circuit board according to claim 10, wherein the conductor layer is made of a rolled copper foil.   The method for manufacturing a flexible circuit board according to claim 1, wherein the insulating layer is made of a polyimide resin or a liquid crystal polymer.

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