Printed wiring board and mounting structure

Abstract

This printed wiring board comprises an insulating layer, a first insulating film, and a second insulating film. The insulating layer has a first surface and a wiring pattern positioned on the first surface. The first insulating film is spaced apart from the wiring pattern and covers the first surface. The second insulating film covers the first surface from above the wiring pattern and the first insulating film. The first insulating film and the second insulating film may be made of the same material. The first insulating film may have a thickness that is equal to or greater than half the height of the wiring pattern and equal to or less than the height of the wiring pattern in a direction perpendicular to the first surface.

Description

Printed wiring board and mounting structure

[0001] The present disclosure relates to printed wiring boards and mounting structures.

[0002] In a printed wiring board, a plurality of wires electrically connecting pads extend on the wiring surface of each layer separated by an interlayer insulating layer. As the density and definition of the wiring on the printed wiring board increase, the spacing between the wires becomes narrower. JP 2014-179659 A discloses a technique for appropriately covering the unexposed wires and the spaces between the unexposed wires and the exposed wires with an insulating film to prevent short circuits from occurring in the unexposed wires, particularly the unexposed wires that are not connected to external terminals, etc.

[0003] One aspect of a printed wiring board according to the present disclosure comprises: (1) an insulating layer having a first surface and a wiring pattern located on the first surface; a first insulating film covering the first surface and spaced apart from the wiring pattern; and a second insulating film covering the first surface from above the wiring pattern and the first insulating film.

[0004] (2) In the printed wiring board described in (1) above, the first insulating film and the second insulating film are made of the same material. (3) In the printed wiring board described in (1) or (2) above, the first insulating film has a thickness that is at least half the height of the wiring pattern in a direction perpendicular to the first surface but is equal to or less than the height of the wiring pattern. (4) In the printed wiring board described in any of (1) to (3) above, the thickness of the second insulating film on the first insulating film in a direction perpendicular to the first surface is greater than the thickness of the second insulating film on the wiring pattern. (5) In the printed wiring board described in any of (1) to (4) above, the distance by which the first insulating film is separated from the wiring pattern is longer than the height of the wiring pattern. (6) In the printed wiring board described in any of (1) to (5) above, the second insulating film has a recess on the surface opposite the first surface at a portion where the first insulating film is separated from the wiring pattern. (7) In the printed wiring board according to any one of (1) to (6) above, the second insulating film has an opening that exposes a portion of the wiring pattern. (8) A mounting structure according to the present disclosure includes the printed wiring board according to (7) above and an electronic component mounted on the printed wiring board via the portion of the wiring pattern that is exposed in the opening.

[0005] FIG. 1 is a cross-sectional view illustrating a printed wiring board. FIG. 2 is a cross-sectional view illustrating a printed wiring board. FIG. 3 is a diagram illustrating the influence of two insulating layers. FIG. 4 is a diagram illustrating a method for manufacturing a printed wiring board. FIG. 5 is a diagram illustrating a method for manufacturing a printed wiring board. FIG. 6 is a diagram illustrating a method for manufacturing a printed wiring board. FIG. 7 is a diagram illustrating a method for manufacturing a printed wiring board. FIG. 8 is a diagram illustrating a method for manufacturing a printed wiring board.

[0006] 1 is a cross-sectional view illustrating a printed wiring board 100 according to the present embodiment. The cross-section includes two adjacent wirings on the printed wiring board 100.

[0007] As shown in FIG. 1A , printed wiring board 100 has insulating layer 2, first insulating film 4, and second insulating film 5. Insulating layer 2 may be an interlayer insulating layer, and another wiring layer and interlayer insulating layer may be located below the interlayer insulating layer. That is, printed wiring board 100 may have a laminated structure. In this case, via conductors connecting adjacent wiring layers may be located in the interlayer insulating layer. Insulating layer 2 may be an organic resin such as an epoxy resin, a polyimide resin, or a polyphenylene ether (PPE) resin.

[0008] A wiring pattern 21 is located on the first surface 2a of the insulating layer 2. The direction perpendicular to the first surface 2a is the z-direction, and the first surface 2a is located on the +z side of the insulating layer 2. The wiring pattern 21 includes wiring, a ground plane, connection pads, and the like in the wiring layer. This cross section includes two wires 211 and 212. The wiring pattern 21 may be made of copper, which is commonly used. The multiple wires 211 and 212 may be locally parallel. The specific shape of the wiring pattern is not particularly limited. Here, a portion where two wires 211 and 212 are arranged in parallel is illustrated as an example of a portion where the effects of the present disclosure are significant. The cross-sectional shape of each wire 211 and 212 may be approximately trapezoidal, and the side of the trapezoid may be recessed.

[0009] The first insulating film 4 is a solder resist and covers a part of the first surface 2a of the insulating layer 2 between the wiring patterns 21 that is not hidden by the wiring patterns 21. The second insulating film 5 is a solder resist and covers the first surface 2a of the insulating layer 2 from above the wiring patterns 21 and the first insulating film 4.

[0010] The first insulating film 4 and the second insulating film 5 may be photosensitive solder resist. The first insulating film 4 and the second insulating film 5 may be made of the same material. The first insulating film 4 and the second insulating film 5 are not mixed and have an interface. When the first insulating film 4 and the second insulating film 5 are made of the same material and have the same degree of polymerization, the characteristics related to deformation, such as expansion in response to temperature changes during heating, become similar. The interface can be confirmed with an optical microscope. The degree of polymerization can be confirmed using Fourier transform infrared spectroscopy (FTIR) or the like.

[0011] The first insulating film 4 is spaced apart from the wiring pattern 21 and covers a portion of the insulating layer 2. In other words, the first insulating film 4 is not in contact with the wiring pattern 21. The thickness of the first insulating film 4 in the z direction may be at least half the height, which is the width of the wiring pattern 21 in the z direction, and may be equal to or less than the height of the wiring pattern 21. For example, the thickness of the first insulating film 4 may be 60 to 70% of the height of the wiring pattern 21. The distance by which the first insulating film 4 is spaced apart from the wiring pattern 21 may be longer than the height of the wiring pattern 21. The upper surface of the first insulating film 4 opposite to the insulating layer 2 may be rough, i.e., may have irregularities.

[0012] The second insulating film 5 covers the +z side, which is the upper surface side of the insulating layer 2, including the first insulating film 4. Therefore, the second insulating film 5 has a portion covering the wiring pattern 21, a portion in direct contact with the insulating layer 2, and a portion covering the first insulating film 4. As shown in FIG. 1B , the second insulating film 5 may have an opening that exposes a portion of the wiring pattern 21. An external electronic component E or the like is mounted on the wiring pattern 21 in the opening using a bonding material C. The bonding material C may be a brazing material such as solder. In this way, a mounting structure S including the printed wiring board 100 and the electronic component E is obtained.

[0013] The second insulating film 5 also has unevenness on the upper surface opposite to the side facing the first surface 2a, depending on the height of the wiring pattern 21 and the thickness of the first insulating film 4. Specifically, the second insulating film 5 has a recess in a portion where the first insulating film 4 is separated from the wiring pattern 21, where the upper surface is partially lower in the z direction than the portion that overlaps with the first insulating film 4 or the wiring pattern 21 in a planar view. When the height of the wiring pattern 21 is greater than the thickness of the first insulating film 4, the upper surface of the second insulating film 5 on the wiring pattern 21 is located furthest to the +z side. In this case, the thickness of the second insulating film 5 on the first insulating film 4 may be greater than the thickness of the second insulating film 5 on the wiring pattern 21.

[0014] 2 is a diagram illustrating the influence of two insulating layers. Two wirings 211, 212 are insulated from each other by a first insulating layer 4 and a second insulating layer 5. However, when the distance between the two wirings 211, 212 becomes narrow, copper in the wiring pattern 21 can be ionized and move across the interface between the insulating layer 2 and the solder resist layer due to ion migration, depending on the voltage between the wirings 211, 212. When copper ions connect the two wirings 211, 212, insulation deteriorates, causing a short circuit and possibly generating an abnormal signal.

[0015] The printed wiring board 100 of the present disclosure has, between the wirings 211 and 212, not only an interface between the first insulating film 4 and the insulating layer 2, but also an interface between the first insulating film 4 and the second insulating film 5. In particular, the printed wiring board 100 has only an interface between the insulating layer 2 and the second insulating film 5 in the immediate vicinity of the wirings 211 and 212, and has two insulating layer interfaces in the intermediate portions of the wirings 211 and 212. This splits ion migration into two paths, r1 and r2. As a result, the movement of copper ions is delayed, and insulation degradation is reduced in the printed wiring board 100.

[0016] 3A to 3C and 4A to 4D are diagrams illustrating a method for manufacturing printed wiring board 100. As shown in Fig. 3A, a conductor layer 20 is formed on insulating layer 2. Alternatively, a substrate having conductor layer 20 on insulating layer 2 is prepared. This conductor layer 20 is patterned to obtain wiring pattern 21 as shown in Fig. 3B.

[0017] As shown in Figure 3C, a solder resist layer 40 is applied to the surface of the insulating layer 2 on the wiring pattern 21 side. At this time, the solder resist layer 40 is lightly heated and cured, i.e., semi-cured. "Lightly" here means just enough to prevent the liquid solder resist layer 40 from flowing out, but the solder resist layer 40 is not completely cured. As shown in Figure 4A, a photomask P is used to expose the area of ​​the first insulating film 4. As shown in Figure 4B, this solder resist layer 40 is developed and the area outside the exposed area is removed, leaving a solder resist layer 40a in the portion corresponding to the first insulating film 4.

[0018] As shown in Fig. 4C, a solder resist layer 50 is applied to the upper surface of the substrate 1. If necessary, the solder resist layer 50 is semi-cured. The area of ​​the solder resist layer 50 that corresponds to the second insulating film 5 is exposed to light and developed. Thereafter, as shown in Fig. 4D, the solder resist layer 40a and the solder resist layer 50 are heat-cured, i.e., cured, to obtain the first insulating film 4 and the second insulating film 5.

[0019] In this way, the printed wiring board 100 is formed by curing the solder resist layers 40 and 50 together while they remain separate, thereby obtaining the first insulating film 4 and the second insulating film 5. This allows each layer to have an independent two-layer structure, while the two layers tend to have uniform characteristics. Therefore, the first insulating film 4 and the second insulating film 5 are subject to approximately the same amount of deformation when heated, making the printed wiring board 100 less susceptible to defects such as cracking or peeling.

[0020] As described above, the printed wiring board 100 of this embodiment includes an insulating layer 2, a first insulating film 4, and a second insulating film 5. The insulating layer 2 has a first surface 2a, and the wiring pattern 21 is located on the first surface 2a. The first insulating film 4 covers the first surface 2a while being spaced apart from the wiring pattern 21. The second insulating film 5 covers the first surface 2a from above the wiring pattern 21 and the first insulating film 4. In this way, the solder resist layer has a two-layer structure between the wiring patterns 21, so that the printed wiring board 100 can reduce the movement of metal ions between wirings due to ion migration. Therefore, the printed wiring board 100 of the present disclosure can reduce deterioration of the insulation state.

[0021] Furthermore, the first insulating film 4 and the second insulating film 5 may be made of the same material. When the first insulating film 4 and the second insulating film 5 have similar characteristics, the occurrence of cracks due to differences in thermal expansion, etc. is reduced. Therefore, the printed wiring board 100 is less susceptible to deterioration and can be used stably for a longer period of time.

[0022] Furthermore, the first insulating film 4 may have a thickness that is at least half the height of the wiring pattern 21 in the z direction perpendicular to the first surface 2a, but not greater than the height of the wiring pattern 21. When the first insulating film 4 has an appropriate thickness, the printed wiring board 100 can adequately separate the ion migration paths without becoming thicker than necessary. Furthermore, since the second insulating film 5 has irregularities that correspond to the irregularities of the first insulating film 4, the bonding area is increased and the bonding strength is improved.

[0023] Furthermore, the thickness of the second insulating film 5 on the first insulating film 4 in the z direction perpendicular to the first surface 2 a may be greater than the thickness of the second insulating film 5 on the wiring pattern 21. If the second insulating film 5 on the wiring pattern 21 is not too thick for the electronic component E to be mounted on the wiring pattern 21, the electronic component E can be easily and stably mounted.

[0024] Furthermore, the distance by which the first insulating film 4 is separated from the wiring pattern 21 may be longer than the height of the wiring pattern 21. By having the first insulating film 4 have a certain width relative to the wiring pattern 21, the solder resist on the wiring pattern 21 can easily flow into this separated portion when the solder resist layer 50 is applied. This makes it easy to make the second insulating film 5 thinner on the wiring pattern 21 than on the first insulating film 4.

[0025] Furthermore, the second insulating film 5 may have a recess 5a on the surface opposite to the first surface 2a in a portion where the first insulating film 4 is separated from the wiring pattern 21. This makes it possible to improve the adhesive strength between the second insulating film 5 and an underfill resin that is injected between the electronic component E and the second insulating film 5 to improve mounting reliability after the electronic component E is mounted, for example.

[0026] The second insulating film 5 may have an opening that exposes a part of the wiring pattern 21. The electronic component E can be easily bonded to the printed wiring board 100 through this opening.

[0027] Furthermore, the mounting structure S of this embodiment includes the above-described printed wiring board 100 and an electronic component E mounted on the printed wiring board 100 via a portion of the wiring pattern 21 exposed in the opening. Such a mounting structure S reduces insulation deterioration of the printed wiring board 100, allowing the electronic component E to operate stably for a longer period of time.

[0028] The above embodiment is merely an example, and various modifications are possible. For example, the material of the solder resist forming the first insulating film 4 and the material of the solder resist forming the second insulating film 5 may be different from each other.

[0029] The thickness of the second insulating film 5 on the first insulating film 4 may be the same as that on the wiring pattern 21. Alternatively, the thickness of the second insulating film 5 on the wiring pattern 21 may be greater than that on the first insulating film 4. The thickness may also be non-uniform.

[0030] The distance between the first insulating film 4 and the wiring pattern 21 may be shorter than the height of the wiring pattern 21. This distance may vary depending on the shape of the wiring pattern 21, and may not be uniform.

[0031] The second insulating film 5 does not necessarily have to have the recess 5 a. The upper surface of the second insulating film 5 does not necessarily have to be located at a height corresponding to the height of the wiring pattern 21 and the thickness of the first insulating film 4.

[0032] The wiring pattern may be made of a metal other than copper in which the occurrence of ion migration cannot be ignored.

[0033] In addition, the specific details of the structure, configuration, material, size, etc. shown in the above embodiment can be appropriately changed without departing from the spirit of this disclosure. The scope of the present invention includes the scope of the invention described in the claims and its equivalents.

[0034] The present disclosure can be used for printed wiring boards and mounting structures.

[0035] REFERENCE SIGNS LIST 1 substrate 2 insulating layer 2a first surface 4 first insulating film 5 second insulating film 5a recess 20 conductor layer 21 wiring pattern 40, 40a, 50 solder resist layer 100 printed wiring board 211, 212 wiring C bonding member E electronic component P photomask S mounting structure

Claims

1. A printed wiring board comprising: an insulating layer having a first surface and a wiring pattern located on the first surface; a first insulating film covering the first surface and spaced apart from the wiring pattern; and a second insulating film covering the first surface from above the wiring pattern and the first insulating film.

2. The printed wiring board according to claim 1, wherein the first insulating film and the second insulating film are made of the same material.

3. A printed wiring board according to claim 1 or 2, wherein said first insulating film has a thickness in a direction perpendicular to said first surface that is equal to or greater than half the height of said wiring pattern and equal to or less than the height of said wiring pattern.

4. A printed wiring board according to any one of claims 1 to 3, wherein the thickness of the second insulating film on the first insulating film in a direction perpendicular to the first surface is greater than the thickness of the second insulating film on the wiring pattern.

5. The printed wiring board according to any one of claims 1 to 4, wherein the distance by which said first insulating film is separated from said wiring pattern is longer than the height of said wiring pattern.

6. A printed wiring board according to any one of claims 1 to 5, wherein the second insulating film has a recess on the surface opposite the first surface in a region where the first insulating film is separated from the wiring pattern.

7. The printed wiring board according to any one of claims 1 to 6, wherein the second insulating film has an opening that exposes a part of the wiring pattern.

8. A mounting structure comprising: the printed wiring board according to claim 7; and an electronic component mounted on said printed wiring board via a portion of said wiring pattern exposed in said opening.

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