JP2004335602A - Printed wiring board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a printed wiring board which is capable of restraining an adhesive agent that protrudes from a joint between a flexible board and a rigid board from increasing in amount, is capable of preventing voids from occurring in the above joint, is excellent in adhesion, hardly delaminates, and is very reliable. <P>SOLUTION: The flexible board 2 is sandwiched between rigid boards 3 and 4 through the intermediary of an adhesive sheet 5 for the formation of the printed wiring board 1. The adhesive sheet 5 is a three-layered laminated structure composed of a first adhesive layer 5a bonded to the flexible board 2, a second adhesive layer 5b formed of adhesive agent different from that of the first adhesive layer 5a and bonded to the rigid boards 3 and 4, and a third adhesive layer 5c which is lower in fluidity than the adhesive layers 5a and 5b and provided between the adhesive layers 5a and 5b. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a printed wiring board formed by joining a flexible substrate and a rigid substrate with an adhesive sheet. More specifically, the present invention relates to a bonding technique in which an adhesive sheet has a laminated structure including at least two or more adhesive layers, and prevents the adhesive from protruding and generating voids.
[0002]
[Prior art]
2. Description of the Related Art In recent years, various types of electric devices including mobile phones and digital cameras have been required to be further reduced in size and thickness. Correspondingly, printed circuit boards constituting electric circuits mounted in the housings of these electric devices have also been reduced in size. For example, there has been proposed a printed wiring board having a so-called laminated structure in which a flexible board having excellent bending properties and a rigid board having no bending properties are joined and integrated with an adhesive (for example, see Patent Document 1). ).
[0003]
FIG. 7 is an example of a printed wiring board in which a flexible board and a rigid board are stacked. This printed wiring board 101 has a laminated structure in which a flexible board 102 is sandwiched between two rigid boards 103 and 104 via an adhesive layer 105 from both sides thereof.
[0004]
The rigid substrates 103 and 104 have a wiring pattern 107 for forming an electric circuit on the front and back surfaces and an intermediate portion of a base 106 made of an insulating resin having a two-layer structure. In addition, metal projections for electrically connecting the wiring patterns 107 formed on these surfaces are formed on part of the wiring patterns 107 formed on the front and back surfaces and intermediate portions of the rigid substrates 103 and 104. 108 are formed.
[0005]
On the other hand, on the printed wiring board 102, a wiring pattern (not shown) and a land 109 connected to a part of the wiring pattern are formed. The land 109 is electrically connected to a metal protrusion 108 formed on the joint surface side of the rigid substrates 103 and 104.
[0006]
When manufacturing a printed wiring board having such a laminated structure, the adhesive layer 105 is required to have an appropriate resin flow property, a prevention property of voids, a high adhesive strength, a heat resistance to moisture absorption, and the like. .
[0007]
[Patent Document 1]
JP 2001-251053A (Pages 3 and 4, FIGS. 1 and 2)
[0008]
[Problems to be solved by the invention]
However, it is difficult to use the adhesive layer 105 satisfying these requirements. For example, assuming that the adhesive has a high resin flow property, the base portion (the portion surrounded by a broken line A in FIG. 7) of the flexible substrate 102 that protrudes outward from the end of the stacked printed wiring boards 101, The amount of protrusion of the adhesive increases. If a large amount of the adhesive protrudes from the base of the flexible substrate 102, it becomes difficult to bend the flexible substrate 102 with high flexibility.
[0009]
On the other hand, when an adhesive having a low resin flow property is used, the amount of the adhesive protruding at the base portion of the flexible substrate 102 is reduced, but voids (bubbles) are generated in the laminated internal adhesive layer 105, and Insufficient resin occurs in the vicinity of the wiring pattern 107 formed on the rigid boards 103 and 104 and the land 109 formed on the flexible board 102. Such voids may cause peeling in the substrate (peeling off from the bonded portion) at the time of reflow when mounting an electronic component or the like after moisture absorption.
[0010]
Further, since the flexible substrate 102 and the rigid substrates 103 and 104 have different base materials (different base material properties), it is difficult to use a resin (adhesive) having adhesive strength for both. It is. For this reason, it is conceivable to roughen the bonding surface of the flexible substrate 102 in order to increase the adhesive strength, but this leads to an increase in cost and lead time.
[0011]
When the rigid substrates 103 and 104 are sandwiched and laminated from both sides of the flexible substrate 102, the locations where the metal projections 108 formed on the joint surfaces of the rigid substrates 103 and 104 face each other can be collectively laminated. In a place where the portion 108 is only on one of the rigid substrates 103 and 104 (a portion surrounded by a broken line B in FIG. 7), the flexible substrate 102 is bent, so that it is conceivable that interlayer short-circuit or interlayer insulating property is reduced.
[0012]
Further, since the adhesive layer 105 is made of only a simple adhesive, the laminated substrate may be warped.
[0013]
Therefore, the present invention has been made in view of such a problem, suppresses the amount of adhesive protruding and suppresses the occurrence of voids generated at the joint portion between the flexible substrate and the rigid substrate, and has a high adhesion force. An object of the present invention is to provide a highly reliable printed wiring board that does not peel off.
[0014]
[Means for Solving the Problems]
In the printed wiring board of the present invention, the adhesive sheet for joining the flexible board and the rigid board has a laminated structure including at least two or more adhesive layers. That is, the adhesive sheet is formed from the first adhesive layer on the side to be bonded to the flexible substrate and the second adhesive layer on the side to be bonded to the rigid substrate and made of an adhesive different from the first adhesive layer. And a laminated structure.
[0015]
According to the present invention, the adhesive sheet has a laminated structure of at least two layers, and is divided into a first adhesive layer on the side bonded to the flexible substrate and a second adhesive layer on the side bonded to the rigid substrate. Therefore, the thickness of each adhesive layer to be bonded to the flexible substrate and the rigid substrate can be made to be a thickness enough to adhere to each substrate with sufficient bonding strength, so that the amount of the adhesive protruding can be reduced. Can be minimized, and the occurrence of voids at each joint between the flexible substrate and the rigid substrate can be suppressed.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings.
[0017]
"Configuration of Printed Wiring Board"
As shown in FIG. 1, a printed wiring board 1 according to the present embodiment includes a flexible board 2, two rigid boards 3 and 4 sandwiching the flexible board 2 from both sides, and the flexible board 2 and the rigid boards 3 and 4. And a bonding structure in which the flexible board 2 is sandwiched between the rigid boards 3 and 4 from both sides via the bonding sheet 5 and joined.
[0018]
As shown in FIG. 1, the flexible substrate 2 has a structure in which insulating layers 7, 7 made of a flame-retardant material such as polyimide are formed on both surfaces of a wiring pattern 6 forming a predetermined electric circuit. The flexible substrate 2 is extremely excellent in bendability, and is disposed, for example, on a movable portion (hinge portion) of a foldable mobile phone. The flexible substrate 2 has lands 8 at several locations for electrical connection (conduction) with metal projections 15 on wiring patterns formed on rigid substrates 3 and 4 described later. The lands 8 are formed by, for example, plating an opening that exposes a part of the wiring pattern 6 outward when patterning the insulating layers 7 and 7 into a predetermined shape.
[0019]
As shown in FIG. 1, the rigid substrates 3 and 4 include bases 9 and 9 made of an insulating resin having a two-layer structure, wiring patterns 10, 11 and 12 formed on the bases 9 and 9, and It is composed of metal projections 13 and 14 for electrically connecting the wiring patterns 10, 11 and 12, and metal projections 15 for electrically connecting the lands 8 formed on the flexible substrate 2 and the wiring pattern 12. ing.
[0020]
Wiring patterns 10, 11, and 12 for forming a predetermined electric circuit are formed on the front and back surfaces of the base 9 having the two-layer structure and on the intermediate portion between the bases 9, 9, respectively. The wiring patterns 10, 11, and 12 are electrically connected to each other by contact with metal protrusions 13 and 14 formed through the bases 9 and 9, respectively. .
[0021]
The metal projection 15 formed on the side to be bonded to the flexible substrate 2 is formed at a predetermined position of the wiring pattern 12 formed on the side facing the flexible substrate 2, and the distal end of the metal projection 15 is formed on the flexible substrate 2. The second land 8 is electrically connected. As a result, the wiring patterns 10, 11, 12 of the rigid boards 3, 4 and the wiring pattern 6 of the flexible board 2 are electrically connected.
[0022]
As shown in the enlarged view of FIG. 2, the adhesive sheet 5 includes a first adhesive layer 5a on the side bonded to the flexible substrate 2 and a second adhesive layer 5b on the side bonded to the rigid substrates 3 and 4. , And a third adhesive layer 5c provided therebetween. The first adhesive layer 5a is adhered to the flexible substrate 2, and the second adhesive layer 5b is made of an adhesive different from the first adhesive layer 5a, and adheres to the rigid substrates 3 and 4.
[0023]
For the first adhesive layer 5a, an adhesive having a high adhesive strength to the flexible substrate 2 is used in order to increase the bonding strength with the flexible substrate 2. On the other hand, an adhesive having a high adhesive strength to the rigid substrates 3 and 4 is used for the second adhesive layer 5b in order to increase the bonding strength with the rigid substrates 3 and 4. That is, since the flexible substrate 2 and the rigid substrates 3 and 4 have different base materials (physical properties), an adhesive having good adhesion to the respective base materials is used.
[0024]
The first adhesive layer 5a and the second adhesive layer 5b are applied to the flexible substrate 2 and the rigid substrates 3 and 4 by vacuum hot pressing. In order to firmly bond the substrate 2 and the rigid substrates 3 and 4, an adhesive having a relatively high flow property (easiness of resin flow) is used.
[0025]
For the first adhesive layer 5a on the side to be bonded to the flexible substrate 2, for example, an imide-based adhesive is used. In addition, an adhesive such as an epoxy resin can be used for the first adhesive layer 5a. Further, the thickness of the first adhesive layer 5a can ensure a sufficient adhesive strength with the flexible substrate 2, and when the flexible substrate 2 and the rigid substrates 3 and 4 are subjected to vacuum hot pressing, the adhesive is It is preferable that the thickness be such that it does not protrude from the end of the printed wiring board 1. For example, the thickness of the first adhesive layer 5a is preferably set to 10 to 30 μm.
[0026]
For the second adhesive layer 5b on the side to be bonded to the rigid substrates 3 and 4, an adhesive such as an epoxy resin-based prepreg (a suitable amount of resin added to glass fiber) is used. In addition, an adhesive such as a phenolic resin (when the substrate is a phenolic resin) can be used for the second adhesive layer 5b. The thickness of the second adhesive layer 5b is preferably substantially equal to or greater than the thickness of the wiring pattern 12 formed on the surface to be bonded to the flexible substrate 2. . Of course, the thickness of the second adhesive layer 5b is of course such that the adhesive does not protrude from the end of the printed wiring board 1 by vacuum hot pressing. For example, the thickness of the second adhesive layer 5b is preferably set to 10 to 30 μm.
[0027]
For the third adhesive layer 5c, it is preferable to use an adhesive having less flowability than the first adhesive layer 5a and the second adhesive layer 5b. As the adhesive used for the third adhesive layer 5c, for example, an epoxy resin is used. If an adhesive having a low flow property is used for the third adhesive layer 5c, the resin flow during vacuum hot pressing is small, so that the adhesive can be prevented from protruding, and the third adhesive layer 5c can be used as a core ( It functions as a core) and can suppress the warpage generated in the printed wiring board 1 due to the pressure during the vacuum heat press.
[0028]
As described above, the first adhesive layer 5a on the side to be bonded to the flexible substrate 2 and the second adhesive layer 5b on the side to be bonded to the rigid substrates 3 and 4 are bonded to the first adhesive layer. A three-layer structure of a third adhesive layer 5c provided between the layer 5a and the second adhesive layer 5b, and the first adhesive layer 5a and the second adhesive layer 5b have an adhesive having a relatively high flow property. And the third adhesive layer 5c is made of an adhesive having a lower flow property than those of the first adhesive layer 5c, so that each of the first adhesive layers 5a to be adhered to the flexible substrate 2 and the rigid substrates 3 and 4 The thickness of the second adhesive layer 5b can be made smaller than that of a conventional printed wiring board having a single-layered adhesive layer. It is possible to suppress the protrusion from the end.
[0029]
In addition, since the first adhesive layer 5a and the second adhesive layer 5b use an adhesive having a high flow property, voids are hardly generated in the adhesive during vacuum hot pressing, so that the wiring pattern 12 Insufficient resin in the vicinity and the vicinity of the land 8 can be avoided. For this reason, voids are not generated in the adhesive sheet 5 or even if they are generated, the flexible substrate 2 and the rigid substrates 3 and 4 are separated by reflow when mounting the electronic component after absorbing moisture. Can be prevented. In addition, no void is formed in the vicinity of the wiring pattern 12 and the vicinity of the land 8, so that an interlayer short-circuit and a decrease in interlayer insulation can be avoided.
[0030]
In addition, an adhesive having good adhesion to the flexible substrate 2 can be used, and an adhesive having good adhesion to the rigid substrates 3 and 4 can be used. 4 can be sufficiently secured. As described above, since the adhesive sheet 5 corresponding to different base materials of the flexible substrate 2 and the rigid substrates 3 and 4 can be formed by one sheet, cost increase and lead time can be greatly reduced.
[0031]
"Method of manufacturing printed wiring boards"
Next, a method of manufacturing the printed wiring board according to the present embodiment will be briefly described.
[0032]
First, as shown in FIG. 3 (a), patterning, exposure processing, development processing, etching processing, and the like are performed to form front and back surfaces of bases 9, 9 having a two-layer structure made of an insulating resin. , 9 are formed at respective intermediate portions, and metal protrusions 13, 14 for electrically connecting the wiring patterns 10, 11, 12 of each layer to each other are formed. Formed on each base 9. Then, as shown in FIG. 3B, a metal projection 15 for connecting the wiring pattern 12 on the side facing the flexible substrate 2 to the land 8 formed on the flexible substrate 2 is formed.
[0033]
Next, the adhesive sheet 5 having a three-layer structure shown in FIG. 2 is formed. That is, a first adhesive layer 5a made of an imide-based adhesive is formed on the side bonded to the flexible substrate 2, and an adhesive such as an epoxy resin prepreg is formed on the side bonded to the rigid substrates 3 and 4. Is formed, and a third adhesive layer 5c made of an adhesive such as an epoxy resin is formed between the first adhesive layer 5a and the second adhesive layer 5b.
[0034]
Subsequently, as shown in FIG. 3C, the adhesive sheet 5 is adhered to the rigid substrates 3 and 4 so that the second adhesive layer 5b faces the flexible substrate 2. When attaching the adhesive sheet 5 to the rigid substrates 3 and 4, the metal protrusion 15 is penetrated so that the tip end projects slightly from the adhesive sheet 5.
[0035]
Next, as shown in FIG. 4, a wiring pattern 6 forming a predetermined electric circuit by performing patterning, exposure processing, development processing, etching processing, and the like, and a wiring pattern 12 formed on the rigid substrates 3 and 4. A land 8 for electrical connection with the metal protrusion 15 is formed, and insulating layers 7 and 7 made of polyimide are formed on both surfaces of the wiring pattern 6 to form a flexible substrate 2 having high flexibility. .
[0036]
Next, as shown in FIG. 1, after the flexible substrate 2 is sandwiched between the two rigid substrates 3 and 4 from both sides thereof, the rigid substrates 3 and 4 and the flexible substrate 2 are bonded and fixed by a vacuum hot press. At this time, the adhesive sheet 5 includes a first adhesive layer 5a that is in close contact with the flexible substrate 2, a second adhesive layer 5b that is in close contact with the rigid substrates 3 and 4, and a second adhesive layer that is in contact with the first adhesive layer 5a. Since it has a three-layer structure with the third adhesive layer 5c provided between the layers 5b, the respective adhesives bonded to the flexible substrate 2 and the rigid substrates 3, 4 are necessary and sufficient for bonding. Therefore, the adhesive is prevented from being melted out by the vacuum hot press and protruding from the edge of the substrate.
[0037]
For the first adhesive layer 5a, an adhesive having high adhesion to the flexible substrate 2 is used, and for the second adhesive layer 5b, an adhesive having high adhesion to the rigid substrates 3, 4 is used. Therefore, sufficient adhesive strength can be secured to each of the flexible substrate 2 and the rigid substrates 3 and 4 having different base materials. Further, the first adhesive layer 5a and the second adhesive layer 5b may have a bonding amount sufficient to secure a sufficient bonding strength to the flexible substrate 2 and the rigid substrates 3 and 4, so that voids may be contained in the adhesive. Does not occur. For this reason, at the time of reflow in which electronic components and the like are mounted on the wiring patterns 10 of the rigid boards 3 and 4, swelling of the adhesive can be prevented, and peeling in the board can be prevented.
[0038]
Further, since the third adhesive layer 5c uses an adhesive having a relatively low flow property as compared with the first adhesive layer 5a and the second adhesive layer 5b, the flow of resin during vacuum hot pressing is small. Also, the strength is higher than the first adhesive layer 5a and the second adhesive layer 5b. Therefore, the printed wiring board 1 having the laminated structure is unlikely to be warped.
[0039]
[Other embodiments]
As described above, the specific embodiments to which the present invention is applied have been described. However, the present invention is not limited to the above-described embodiments, and can be variously modified.
[0040]
For example, as shown in FIG. 5, glass fibers 16 may be mixed into the third adhesive layer 5c constituting the adhesive sheet 5. When the glass fibers 16 are mixed into the third adhesive layer 5c, as shown in FIG. 6, the portions where the metal protrusions 15 formed on the rigid substrates 3 and 4 are not opposed to each other (indicated by a broken line C in FIG. 6). In the enclosed portion), no bending occurs in the flexible substrate 2. This is because the strength of the third adhesive layer 5c is increased by mixing the glass fibers 16 into the adhesive. Therefore, if this adhesive sheet 5 is used, it is possible to further prevent the interlayer short circuit and the interlayer insulating property from being lowered.
[0041]
Further, the manufacturing method of the above-described embodiment is an example, and does not limit the manufacturing method of the rigid substrates 3 and 4, the flexible substrate 2, and the adhesive sheet 5.
[0042]
【The invention's effect】
According to the printed wiring board of the present invention, the adhesive sheet has a laminated structure of at least two or more layers, and the first adhesive layer on the side bonded to the flexible substrate and the second adhesive layer on the side bonded to the rigid substrate Since the thickness of each of the adhesive layers to be bonded to the flexible substrate and the rigid substrate can be made thick enough to adhere to each substrate with sufficient bonding strength, the adhesive The amount of protrusion can be minimized. In addition, it is possible to suppress the generation of voids at each joint portion between the flexible substrate and the rigid substrate, prevent peeling in the substrate, and provide a printed wiring board having high bonding strength and excellent reliability.
[Brief description of the drawings]
FIG. 1 is an enlarged sectional view of a printed wiring board to which the present invention is applied.
FIG. 2 is an enlarged sectional view of a main part of an adhesive sheet constituting the printed wiring board of the present invention.
FIGS. 3A and 3B are cross-sectional views illustrating a manufacturing process of a rigid board constituting the printed wiring board of the present invention, wherein FIG. 3A illustrates a step of forming a wiring pattern on a base having a two-layer structure, and FIG. Shows a step of forming a metal projection on the wiring pattern on the side to be bonded to the flexible substrate, and FIG.
FIG. 4 is a cross-sectional view showing a step of manufacturing a flexible substrate.
FIG. 5 is an enlarged sectional view of a main part of an adhesive sheet in which glass fibers are mixed in a third adhesive layer.
FIG. 6 is an enlarged cross-sectional view of a printed wiring board in which a rigid board and a flexible board are bonded and fixed using an adhesive sheet in which glass fibers are mixed into a third adhesive layer.
FIG. 7 is an enlarged sectional view of a conventional printed wiring board having a laminated structure.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Printed wiring board 2 ... Flexible board 3, 4 ... Rigid board 5 ... Adhesive sheet 5a ... First adhesive layer 5b ... Second adhesive layer 5c ... Third adhesive layer 10, 11, 12 ... Wiring pattern 13, 14, 15: metal protrusion 16: glass fiber

Claims (4)

In a printed wiring board that is made by bonding a flexible board and a rigid board with an adhesive sheet,
A second adhesive layer made of an adhesive different from the first adhesive layer on the first adhesive layer at least on the side to be bonded to the flexible substrate and the first adhesive layer on the side to be bonded to the rigid substrate; A printed wiring board having a laminated structure in which are laminated. The printed wiring board according to claim 1,
A printed wiring board, wherein a third adhesive layer having a lower flow property than the adhesive layers is provided between the first adhesive layer and the second adhesive layer. The printed wiring board according to claim 1,
A printed wiring board, wherein glass fibers are mixed in the third adhesive layer. The printed wiring board according to claim 2,
A printed wiring board, wherein the flexible board is sandwiched and joined from both sides of the flexible board via the adhesive sheet.

Images