Printed wiring board and mounting structure

Abstract

This printed wiring board comprises an interlayer insulating layer, a wiring pattern, a first solder resist, and a second solder resist. The wiring pattern is positioned on the interlayer insulating layer. The first solder resist has a first surface in contact with the interlayer insulating layer, and is positioned across a gap from the wiring pattern. The second solder resist covers a second surface opposite to the first surface of the first solder resist. The first solder resist has a third surface connecting the first surface and the second surface. The second surface has a curved concave portion, and the third surface has a curved convex portion.

Description

Printed wiring boards and mounting structures

[0001] This disclosure relates to printed circuit boards and mounting structures.

[0002] The outermost layer of a printed circuit board is a solder resist layer to prevent solder from adhering to the wiring during assembly. Normally, the solder resist layer is a single layer, but it may have a two-layer structure if thickness is required. Japanese Patent Publication No. 10-112580 discloses a thick solder resist layer with a two-layer structure as a measure against solder wetting and spreading.

[0003] One aspect of the present disclosure is as follows: [1] A printed circuit board comprising: an interlayer insulating layer; a wiring pattern located on the interlayer insulating layer; a first solder resist having a first surface in contact with the interlayer insulating layer and positioned with a gap between it and the wiring pattern; and a second solder resist covering a second surface of the first solder resist that is opposite to the first surface, wherein the first solder resist has a third surface connecting the first surface and the second surface, the second surface having a curved concave portion, and the third surface having a curved convex portion. [2] The printed circuit board of [1], wherein the third surface of the first solder resist has a constriction between the top of the convex portion and the first surface. [3] The printed circuit board according to [1] or [2], wherein the second solder resist has a fourth surface facing the first solder resist, a fifth surface opposite to the fourth surface, a sixth surface connecting the fourth surface and the fifth surface, and a projection along the sixth surface where the fourth surface is separated from the second surface, the fourth surface of the projection extending inward from the tip toward the interlayer insulating layer. [4] The printed circuit board according to any one of [1] to [3], wherein the second solder resist has a fourth surface facing the first solder resist and a fifth surface opposite to the fourth surface, the fifth surface having a curved convex portion. [5] A mounting structure comprising the printed circuit board according to any one of [1] to [4], and an electronic component mounted on the printed circuit board via a part of the wiring pattern.

[0004] It is a cross-sectional view of a printed wiring board. It is a cross-sectional view of a mounted structure. It is an enlarged cross-sectional view showing a solder resist and its periphery. It is a diagram showing a manufacturing procedure of a printed wiring board. It is a diagram showing a manufacturing procedure of a printed wiring board. It is a diagram showing a manufacturing procedure of a printed wiring board. It is a diagram showing a manufacturing procedure of a printed wiring board. It is a diagram showing a manufacturing procedure of a printed wiring board. It is a diagram showing a manufacturing procedure of a printed wiring board. It is a diagram showing a manufacturing procedure of a printed wiring board. It is a diagram showing a manufacturing procedure of a printed wiring board.

[0005] Hereinafter, embodiments will be described based on the drawings. FIG. 1A is a cross-sectional view of a printed wiring board 100. FIG. 1B is a cross-sectional view of a mounting structure S. As shown in FIG. 1A, the printed wiring board 100 includes an insulating layer 2 that is an interlayer insulating layer, and a wiring 3 and a solder resist 4 located on one surface of the insulating layer 2, here the upper surface 2a that is the +z side surface.

[0006] The insulating layer 2 is an interlayer insulating layer located between layers of a plurality of stacked wiring layers. The insulating layer 2 may be, for example, any one of an epoxy resin, a polyimide resin, and a polyphenylene ether (PPE) resin. The upper surface 2a of the insulating layer 2 is a plane along the xy plane perpendicular to the z direction.

[0007] The wiring 3 is a conductor that transmits electrical signals and supplies power. The wiring 3 forms a wiring pattern of an appropriate shape according to the application of the printed wiring board 100 and the like. Each wiring pattern may be connected to a wiring pattern of another layer by a via conductor or the like penetrating the insulating layer 2 in the z direction. Here, a portion where a plurality of wiring patterns are arranged in parallel in the printed wiring board 100 is shown. The cross section perpendicular to the wiring direction of the wiring 3 may be substantially trapezoidal, and the two non-parallel sides of the trapezoid may have an inwardly concave shape.

[0008] The solder resist 4 is located on the insulating layer 2 with a gap between it and the wiring 3. The solder resist 4 can be any photosensitive solder resist. The solder resist 4 includes a first solder resist 41 and a second solder resist 42. The first solder resist 41 and the second solder resist 42 may be made of the same material. The first solder resist 41 and the second solder resist 42 are not mixed and an interface exists between them. The interface can be confirmed with an optical microscope. Since the first solder resist 41 and the second solder resist 42 are made of the same material and have a similar degree of polymerization, their deformation characteristics, such as expansion due to temperature changes during heating, may be similar. In the solder resist 4, the first solder resist 41 and the second solder resist 42 overlap in the z direction. The first surface 41a of the first solder resist 41 is the -z side surface located in contact with the upper surface 2a of the insulating layer 2. The second surface 41b of the first solder resist 41 is the surface located on the +z side opposite to the first surface 41a. The third surface 41c of the first solder resist 41 is the side surface connecting the first surface 41a and the second surface 41b.

[0009] The second solder resist 42 covers the second surface 41b of the first solder resist 41. The fourth surface 42a of the second solder resist 42 faces the second surface 41b. The fifth surface 42b of the second solder resist 42 is the +z side surface located opposite the fourth surface 42a. The sixth surface 42c of the second solder resist 42 is the side surface connecting the fourth surface 42a and the fifth surface 42b.

[0010] The thickness of the first solder resist 41 in the z direction may be greater than the thickness of the second solder resist 42 in the z direction. The thickness of the first solder resist 41 may be the maximum distance between identical positions in a plan view on the first surface 41a and the second surface 41b. The thickness of the second solder resist 42 may be the maximum distance between identical positions in a plan view on the fourth surface 42a and the fifth surface 42b. The upper end of the solder resist 4, i.e., the position on the fifth surface 42b that is furthest to the +z direction, may be located on the +z side of the upper surface position of the wiring 3.

[0011] The shape of the solder resist 4 in the y-direction may extend in a planar line according to the shape of the wiring 3, or it may be circular, elliptical, or oblong in plan view.

[0012] As shown in Figure 1B, in the mounting structure S, the upper surface 2a, which includes the wiring 3 and solder resist 4 located below the electronic component E, is covered with an underfill 5, which is a sealing resin. The underfill 5 covers the upper surface 2a of the insulating layer 2, the wiring 3, and the solder resist 4, except for the connection pads in the wiring 3 and other parts that are connected to external electronic components. The wiring 3 and the electronic component E are connected by a conductive bonding member C. The conductive bonding member C may be a brazing material such as solder.

[0013] Figure 2 is a cross-sectional view showing an enlarged view of the solder resist 4 and its surroundings. The second surface 41b of the first solder resist 41 has a curved concave portion, and accordingly, the fourth surface 42a of the second solder resist 42 has a curved convex portion. The irregularities referred to here are significantly larger than minute irregularities that occur during manufacturing, and may include partially flat portions. The concave portion of the second surface 41b and the convex portion of the fourth surface 42a may constitute the main parts of the second surface 41b and the fourth surface 42a, respectively, for example, at least half of them. The end of the fourth surface 42a along the sixth surface 42c may be separated from the second surface 41b. This separated portion extends inward from the periphery of the fourth surface 42a in the +z direction, that is, in the direction away from the insulating layer 2. As a result, the second solder resist 42 has a protruding portion 421 at the corner between the end of the fourth surface 42a and the lower end portion of the sixth surface 42c.

[0014] As a result, even if a force acts in the +z direction on the underfill 5 that fills the gap between the solder resist 4 and the wiring 3, the force is applied in a direction that causes the underfill 5 to catch on the protrusion 421. Therefore, due to the presence of the protrusion 421, the underfill 5 is less likely to peel off from the insulating layer 2 than if the protrusion 421 were absent.

[0015] The third surface 41c of the first solder resist 41 has a convex portion that extends outward in cross-sectional view near the center in the z direction. The outermost apex 411 of the convex portion extends circumferentially along the side surface of the first solder resist 41. The position of the apex 411 does not have to be constant in the z direction along the circumferential direction. The first solder resist 41 is a continuous surface that is curved in the ±z direction across the apex 411. The first solder resist 41 may have a constricted portion 412 between the boundary with the first surface 41a and the apex 411. That is, at the constricted portion 412, the third surface 41c of the first solder resist 41 is narrowed in an annular and locally concave shape. The third surface 41c may be discontinuously bent across the constricted portion 412, or it may be a continuous curved surface. The convex portion and constricted portion 412 of the third surface 41c are clearly larger than any minute irregularities that may occur during manufacturing. The apex 411 may be located outside the first surface 41a in a plan view. The constricted portion 412 may be closer to the boundary with the first surface 41a than the apex 411.

[0016] Thus, by having a protrusion on the side surface, the underfill 5 becomes less likely to separate from the -z side of the protrusion, i.e., from the insulating layer 2 side. Therefore, the robustness of the underfill 5 in this printed circuit board 100 is increased. In addition, because the third surface 41c has a constricted shape, the reduction in the bonding area between the first solder resist 41 and the insulating layer 2 is reduced. Therefore, the decrease in the bonding strength of the solder resist 4 with the insulating layer 2 is reduced, and the possibility of the solder resist 4 peeling off from the insulating layer 2 can be reduced.

[0017] The fifth surface 42b of the second solder resist 42 may have a convex portion that is curved in the +z direction. The protrusion width of this convex portion is clearly larger than minute irregularities in manufacturing, but may be smaller than the protrusion width of the convex portion of the fourth surface 42a. The fifth surface 42b does not have to be curved overall. For example, in a plan view, part of the fifth surface 42b near the center may be substantially flat, and the portion along the periphery other than the aforementioned part may have a curved shape.

[0018] The rounded top surface of the solder resist 4 reduces the possibility of the upper corner of the solder resist 4 catching on something and peeling it away from the insulating layer 2 during component mounting or transportation. Even if solder adheres to the fifth surface 42b during electronic component mounting, the curved convex portion causes the solder to flow off, reducing solder adhesion and thus reducing solder bridging.

[0019] Figures 3A to 3D and 4A to 4D show the manufacturing procedure for the printed circuit board 100. Each figure is a cross-sectional view identical to that of Figures 1A and 2 above.

[0020] An insulating substrate, which will serve as the insulating layer 2, is prepared as shown in Figure 3A. Conductors 300 for the wiring pattern may be pre-positioned on the upper surface 2a of the insulating layer 2. If conductors 300 are not positioned on the upper surface of the insulating layer 2, a thin film layer of conductors 300 may be formed on the upper surface 2a of the insulating substrate.

[0021] As shown in Figure 3B, the conductor 300 is patterned to obtain wiring 3. As shown in Figure 3C, a solder resist layer 410 is formed to cover the upper surface 2a and wiring 3. The formed solder resist layer 410 is heated and semi-cured. Here, semi-curing is sufficient to prevent the outflow of the liquid solder resist layer 410, and does not completely harden the solder resist layer 410. As shown in Figure 3D, the solder resist layer 410 is exposed to light using a mask M corresponding to the position of the solder resist 4. After that, the area outside the photosensitive range is removed by developing with a developer, and as shown in Figure 4A, a solder resist layer 41p corresponding to the first solder resist 41 is obtained. Due to the difference in the flow state of the developer, the solder resist layer 41p remains in a wider area near the joint surface with the upper surface 2a than in the area away from the joint surface. In this state, the solder resist layer 41p is not heat-cured.

[0022] As shown in Figure 4B, a solder resist layer 420 is applied to cover the top surface 2a, the wiring 3, and the solder resist layer 41p. As shown in Figure 4C, pressure is applied from above the solder resist layer 420 using a pressing member P to compress and deform the solder resist layer 420 and the solder resist layer 41p. By applying heat at the same time, the solder resist layer 420 is partially hardened, and the deformation can be made plastic deformation. Due to this compression, the solder resist layer 41p takes on a shape with irregularities that is almost the final first solder resist 41.

[0023] Using a photomask, the area of ​​the solder resist 4 is exposed to light, and the unexposed areas are removed with a developer. As a result, a solder resist 4 is obtained in which the first solder resist 41 and the second solder resist 42 are laminated, as shown in Figure 4D. This solder resist 4 is then heat-cured to form the printed circuit board 100.

[0024] As described above, the printed circuit board 100 of this embodiment comprises an insulating layer 2, wiring 3 forming a wiring pattern, a first solder resist 41, and a second solder resist 42. The wiring pattern is located on the insulating layer 2. The first solder resist 41 has a first surface 41a that is in contact with the insulating layer 2 and is located with a gap between it and the wiring pattern. The second solder resist 42 covers the second surface 41b of the first solder resist 41, which is opposite to the first surface 41a. The first solder resist 41 has a third surface 41c that connects the first surface 41a and the second surface 41b. The second surface 41b has a curved concave portion, and the third surface 41c has a curved convex portion. A locally thick solder resist layer is prone to stress concentration at its base, which can cause cracks or even collapse. This can lead to a deterioration in insulation reliability. In this type of printed circuit board 100, the solder resist 4 has a two-layer structure, and the lower layer, the first solder resist 41, is heat-cured twice. This reduces the possibility of insufficient curing inside the solder resist compared to when a single thick solder resist is formed. Therefore, this solder resist 4 is more firmly bonded to the insulating layer 2. In addition, since the second surface 41b of the first solder resist 41 and the fourth surface 42a of the second solder resist 42 are joined on a curved surface, the bonding strength is improved compared to bonding between flat surfaces. In particular, when stress is applied due to expansion and contraction during heating, stress is generated not only perpendicular to the second surface 41b and the fourth surface 42a, but also in a direction along the second surface 41b and the fourth surface 42a. Because the interface between the second surface 41b and the fourth surface 42a is curved, the strength against such shear stress is improved, and the unity of the first solder resist 41 and the second solder resist 42 is improved.

[0025] Furthermore, the third surface 41c of the first solder resist 41 may have a constricted portion 412 between the top of the convex portion 411 and the first surface 41a. When the printed circuit board 100 is covered with underfill 5, the underfill 5 hardened at the constricted portion 412 will catch on the top 411 and become difficult to remove, thereby improving the reliability of the connection between the printed circuit board 100 and the electronic component E.

[0026] Furthermore, the second solder resist 42 has a fourth surface 42a facing the first solder resist 41, a fifth surface 42b opposite to the fourth surface 42a, and a sixth surface 42c connecting the fourth surface 42a and the fifth surface 42b. The second solder resist 42 may have a protrusion 421 along the sixth surface 42c where the fourth surface 42a is separated from the second surface 41b. The portion of the protrusion 421 on the fourth surface 42a may extend inward from the tip, i.e., the peripheral portion, in a direction away from the insulating layer 2, i.e., towards the +z side. As a result, the protrusion 421 located on the periphery of the second solder resist 42 contacts the underfill 5 in such a way that it prevents the underfill 5 from peeling off from the insulating layer 2. Therefore, peeling of the underfill 5 is reduced, and the reliability of the connection between the printed circuit board 100 and the electronic component E is improved.

[0027] Furthermore, the fifth surface 42b may have a curved convex portion. Having the fifth surface 42b, which is the uppermost surface of the solder resist 4, convex reduces the possibility of the corners of the solder resist 4, which form irregularities on the insulating layer 2, getting caught when transporting the printed circuit board 100. This reduces damage to the solder resist 4 and makes it less likely for defective products to occur. Even if solder adheres to the fifth surface 42b during electronic component mounting, the curved convex portion causes the solder to flow off, reducing solder adhesion and thus reducing solder bridging.

[0028] Furthermore, the mounting structure S of this embodiment may also include the printed circuit board 100 and electronic components E mounted on the printed circuit board 100 via a portion of the wiring pattern. This mounting structure S can improve connection reliability through the structure of the printed circuit board 100.

[0029] The above embodiment is illustrative and can be modified in various ways. For example, the fourth surface 42a of the second solder resist 42 does not have to be separated from the second surface 41b of the first solder resist 41 at its periphery. Consequently, the second solder resist 42 does not have to have a protrusion 421. Even with only the protruding shape of the third surface 41c of the first solder resist 41, the occurrence of peeling of the underfill 5 can be reduced.

[0030] Furthermore, the first solder resist 41 does not necessarily have to have a constricted portion 412. Even with this shape, the necessary bonding strength to the insulating layer 2 on the first surface 41a can be obtained. Also, as long as the third surface 41c has a convex portion, it is possible to reduce peeling of the underfill 5 from the insulating layer 2 even without the constricted portion 412.

[0031] Furthermore, the second solder resist 42 may have a flat or concave shape on its upper surface, the fifth surface 42b, or it may have an uneven shape with both convex and concave portions.

[0032] Furthermore, the degree of unevenness of the third surface 41c may be non-uniform depending on each cross-section of the first solder resist 41, including the z-axis direction. That is, the degree of unevenness may differ between the cross-section along the x-axis and the cross-section along the y-axis, and even within the cross-section along the x-axis, the degree may be non-uniform depending on the position along the y-axis direction.

[0033] Furthermore, the method for manufacturing the printed circuit board 100 is not limited to the above embodiments. Other methods may be used. The wiring 3 may be formed by a semi-additive method or the like, instead of a subtractive method. Any conventionally known or novel method may be used for printing the solder resist 4. In addition, specific details such as the structure, configuration, materials, and size shown in the above embodiments can be modified as appropriate without departing from the spirit of this disclosure. The scope of the present invention includes the scope of the invention as described in the claims and its equivalents.

[0034] This disclosure can be used in printed circuit boards and mounting structures.

[0035] 2 Insulating layer 2a Top surface 3 Wiring 4 Solder resist 5 Underfill 5a Through hole 41 First solder resist 41p Solder resist layer 42 Second solder resist 100 Printed circuit board 300 Conductor 410, 420 Solder resist layer 411 Top 412 Narrowed portion 421 Protruding portion C Conductive bonding member E Electronic component M Mask P Pressing member S Mounting structure

Claims

1. A printed circuit board comprising: an interlayer insulating layer; a wiring pattern located on the interlayer insulating layer; a first solder resist having a first surface in contact with the interlayer insulating layer and positioned with a gap between it and the wiring pattern; and a second solder resist covering a second surface of the first solder resist that is opposite to the first surface, wherein the first solder resist has a third surface connecting the first surface and the second surface, the second surface has a curved concave portion, and the third surface has a curved convex portion.

2. The printed circuit board according to claim 1, wherein the third surface of the first solder resist has a constricted portion between the top of the convex portion and the first surface.

3. The printed circuit board according to claim 1 or 2, wherein the second solder resist has a fourth surface facing the first solder resist, a fifth surface opposite to the fourth surface, a sixth surface connecting the fourth surface and the fifth surface, and a projection along the sixth surface where the fourth surface is separated from the second surface, and the fourth surface of the projection extends inward from its tip in a direction away from the interlayer insulating layer.

4. The printed circuit board according to any one of claims 1 to 3, wherein the second solder resist has a fourth surface facing the first solder resist and a fifth surface opposite to the fourth surface, and the fifth surface has a curved convex portion.

5. A mounting structure comprising: a printed circuit board according to any one of claims 1 to 4; and an electronic component mounted on the printed circuit board via a portion of the wiring pattern.

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