JP2023124189A - Print circuit board - Google Patents

Abstract

To provide a print circuit board capable of suppressing solder from flowing from a surface having a thermal pad to the opposite surface through thermal vias and protruding from the opposite surface by soldering components in a reflow process.SOLUTION: A print circuit board has a thermal via in a hole that opens at a mounting position where a component with a heat dissipation pad is mounted on one main surface and passes through between one main surface and the other main surface, and the hole includes a first hole portion extending from one main surface to the other main surface and terminating within the print circuit board, and a second hole portion that is formed continuously with the first hole portion and extends to the other main surface and has a larger hole cross section than the first hole portion in a direction perpendicular to the hole penetration direction.SELECTED DRAWING: Figure 1

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printed wiring board for mounting components with thermal pads.

2. Description of the Related Art In recent years, printed wiring boards are often mounted with components having heat-dissipating pads. Components with thermal pad include, for example, QFN (Quad Flat Non-leaded Package), DFN (Dual Flatpack No-leaded), LGA (Land Grid Array), QFP (Quad Flat Package) and the like. Mounting of these components with thermal pads to printed wiring boards is generally performed by soldering and fixing the thermal pads of the components to thermal pads containing thermal vias of the printed wiring board.

One main surface of the printed wiring board is the solder surface, and the other main surface is the component surface. A thermal pad having a size corresponding to the shape of is formed. Further, the printed wiring board is formed with thermal vias penetrating from the thermal pad to the surface of the component, and the heat dissipation effect is enhanced by dissipating heat through the thermal vias.

A reflow process is used to mount a component with a heat dissipation pad on the soldering surface of a printed wiring board. In the reflow process when the component with heat dissipation pads is, for example, a QFP, solder printing is first performed on the solder surface of the printed wiring board using a metal mask (also referred to as a "stencil"). After solder printing, components are mounted, and during mounting, alignment is performed so that the leads (or terminals) of the QFP and heat dissipation pads are aligned with the positions of the respective pads on the printed wiring board. After the component is mounted, the solder is heated to melt the solder, and the QFP lead is joined to the lead connection pad on the solder surface, and the QFP heat dissipation pad is joined to the thermal pad on the solder surface. This fixes the QFP to the solder surface of the printed wiring board.

JP 2012-227349 A

By the way, as described above, thermal vias penetrating between the solder surface and the component surface are formed in the thermal pad of the printed wiring board. In the reflow process, solder melted from the soldering surface may flow out through the thermal vias to the opposite side of the component, and the solder may protrude in a hemispherical shape from the component surface. If such a phenomenon occurs, when mounting other components on the component surface of the printed wiring board, the solder protrudes in a hemispherical shape, and the adhesion of the metal mask used for solder printing deteriorates, resulting in poor adhesion of the printed solder. becomes unstable, and the required amount of solder cannot be printed. As a result, there is a problem that the mounting of the component on the component surface is defective.

Patent Document 1 discloses a printed wiring board structure intended to suppress protrusion of solder onto one main surface. In the printed wiring board structure of Cited Document 1, a thermal pad is provided on the component mounting surface, and a through hole is formed through the component mounting surface and the surface opposite to the mounting surface having a solder resist, The through-holes are provided within the range to which the thermal pads of the printed wiring board are soldered, and on the opposite side, a solder-resist is provided in an area other than the surrounding areas including the through-hole holes, and the solder-resist openings inside the solder-resist are provided. An area surrounding the through-hole is provided with a copper solid pattern that absorbs solder. In a printed wiring board with such a structure, part of the surplus solder that has spread on the thermal pad of the printed wiring board reaches the opposite side through the through-holes during soldering in the reflow process. Wet and spread on the copper solid pattern without doing.

However, in the printed wiring board structure of the cited document 1, while the solder volume is ensured at the portion where the solder spreads on the opposite surface, the direction of the solder flowing out changes by 90 degrees at the edge of the opening on the opposite surface, so the solder concentrates and stays. , localized solder can protrude on the opposite side. In this case, as described above, the adhesion of the metal mask for solder printing is deteriorated on the opposite surface.

Therefore, an object of the present invention is to focus on such problems and to suppress solder from flowing from a surface having a thermal pad to the opposite surface through thermal vias and protruding from the opposite surface during soldering of components in a reflow process. It is to provide a printed wiring board capable of

The printed wiring board of the present invention has a thermal via in a hole that opens at a mounting position where a component with a heat dissipation pad is mounted on one main surface and penetrates between the one main surface and the other main surface. A printed wiring board, wherein the hole includes a first hole portion extending from the one main surface toward the other main surface and terminating within the printed wiring board; and the first hole portion. a second hole portion formed continuously to reach the other main surface and have a hole cross section larger than that of the first hole portion in a direction perpendicular to the through-hole direction of the hole. and

According to the printed wiring board of the present invention, since the hole cross section of the second hole portion which is continuous with the first hole portion of the thermal via is larger than the cross section of the first hole portion, the soldering of the components in the reflow process becomes difficult. Excess solder melted at the time flows into the first hole portion of the thermal via, and from the first hole portion to the second hole portion (hole enlarged portion), forming a hemispherical shape in the second hole portion. It is possible to prevent the solder from flowing out to the opposite surface, thereby suppressing the protrusion from the opposite surface.

1 is a cross-sectional view of a printed wiring board according to the present invention; FIG. 2 is a cross-sectional view showing a method of forming an enlarged portion of the printed wiring board of FIG. 1; FIG. FIG. 2 is a cross-sectional view showing a mounting method for mounting a component with a heat dissipation pad on the printed wiring board of FIG. 1; 2 is a cross-sectional view showing solder printing on the component surface of the printed wiring board of FIG. 1; FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a cross section of a printed wiring board 10 according to the invention. Such a printed wiring board 10 is composed of four layers and has three base materials 11 , 12 and 13 . The substrates 11, 12, and 13 are plate-like ones arranged parallel to each other. The substrates 11, 12, and 13 are made of, for example, a fire-resistant paper-based epoxy laminate or a natural glass-based epoxy resin laminate. One main surface of the substrate 11 is the solder surface 16 on which the first surface layer is formed, and the other main surface of the substrate 13 is the component surface 17 on which the fourth surface layer is formed. A copper foil 18 forming a second inner layer is formed between the base material 11 and the base material 12 . A copper foil 19 forming a third inner layer is formed between the base material 12 and the base material 13 .

A component pad 21 and a thermal pad 22 are formed as a first layer on the solder surface 16 . Each of component pads 21 and thermal pads 22 is copper foil. Moreover, the solder surface 16 is covered with a solder resist 23 so as to surround each area of the component pads 21 and the thermal pads 22 . Component pads 25 and conductor patterns 26 made of copper foil are formed as a fourth layer on the component surface 17 .

A plurality of thermal vias 28 are formed perpendicularly to the surface of each of the substrates 11 , 12 , 13 within the formation range of the thermal pad 22 . The thermal via 28 has a through hole penetrating between the solder surface 16 and the component surface 17 via the substrates 11, 12, 13 and the copper foils 18, 19. FIG. The cross-sectional shape of the through hole of the thermal via 28 in the direction perpendicular to the penetrating direction is circular in this embodiment. An inner wall surface 29 of the thermal via 28 is plated with copper. The copper-plated portion is the first hole portion that functions as a thermal via and reaches the base material 13 .

Each of the through-holes of the thermal via 28 forms a hole-enlarged portion 28a having an enlarged hole diameter (hole cross-section) at the end portion on the component surface 17 side, that is, a second hole portion. The enlarged hole portion 28a extends from the position of the base material 13 to the part surface 17, and the inner wall surface thereof is not plated. For example, the enlarged hole portion 28a has a diameter of 0.5 to 1.0 mm and a depth of 0.3 mm or more. The shortest distance between edges of adjacent enlarged hole portions 28a on the part surface 17 is 0.4 to 0.6 mm. The length of the enlarged hole portion 28a, which is the second hole portion, is shorter than the length from the component surface 17, which is the other main surface, to the inner layer of the copper foil connected to the thermal via 28 closest to the component surface 17. This is because the performance of the thermal via 28 is not degraded.

Next, a method for forming hole enlarged portion 28a of printed wiring board 10 of FIG. 1 will be described with reference to FIG. FIG. 2A shows printed wiring board 10 before formation of enlarged hole portion 28a. In the printed wiring board 10 of FIG. 2A, each thermal via 28 penetrates between the solder surface 16 and the component surface 17 with the same diameter. In order to additionally machine the printed wiring board 10 in this state to form the enlarged hole portion 28a, the printed wiring board 10 is set on a table (not shown) with the component surface 17 facing upward.

A drill 31 is prepared for additional machining. The diameter of the blade of the drill 31 is 0.5-1.0 mm. In the additional machining, the tip of the drill 31 is positioned at the opening of the thermal via 28 on the part surface 17, and the hole diameter of the thermal via 28 is enlarged by rotating the blade of the drill 31. During the enlargement, the plated inner wall surface 29 is scraped off by the blade of the drill 31 . Further, the enlargement is performed up to the position of the substrate 13, and the depth is, for example, 0.3 mm. As a result of this additional processing, an enlarged hole portion 28a is formed as shown in FIG. 2(B).

Next, a mounting method for mounting a component 40 with heat dissipation pads on the solder surface 16 of the printed wiring board 10 of FIG. 1 will be described with reference to FIG.

First, with the solder surface 16 of the printed wiring board 10 of FIG. As shown in FIG. 3A, the metal mask 41 has openings at positions corresponding to the parts of the component pads 21 and the thermal pads 22 to be soldered. Solder printing is performed on the part of the component pad 21 and the thermal pad 22 . In the solder printing, a squeegee 42 is used to apply cream solder 43 to the solder surface 16 covered with the metal mask 41 , thereby filling the openings of the metal mask 41 with the cream solder 43 . During solder printing, the cream solder 43 slightly enters into the thermal vias 28 . Then, the metal mask 41 is removed, leaving cream solder 43 on the component pads 21 and thermal pads 22 of the solder surface 16 .

Next, as shown in FIG. 3B, the component 40 with thermal pads is mounted on the cream solder 43 of the solder surface 16. Then, as shown in FIG. In this embodiment, the component 40 with thermal pads is a QFN. Positional deviation is adjusted for the component 40 with the heat radiation pad, and the component 40 with the heat radiation pad is set at a predetermined position (position corresponding to the component pad 21 and the thermal pad 22) on the solder surface 16. FIG. As a result, the terminals 40 a and the heat dissipation pads 40 b of the component 40 with heat dissipation pads come into contact with the cream solder 43 .

After mounting the component 4 with heat radiation pads, as shown in FIG. 3C, solder is melted in a reflow furnace (not shown). That is, cream solder 43 is melted because it is heated in a reflow furnace. By melting the cream solder 43, the terminals 40a and the heat dissipation pads 40b of the component 40 with heat dissipation pads are soldered to the component pads 21 and the thermal pads 22 of the solder surface 16, respectively. As a result, the terminal 40a and the heat dissipation pad 40b are bonded to the component pad 21 and the thermal pad 22 of the solder surface 16, respectively.

In solder melting, excess melted solder 44 flows into the thermal via 28 without accumulating in the joint portion. The excess solder 44 escapes to the thermal via 28, protrudes from the thermal via 28 to the enlarged hole portion 28a, and bulges in a hemispherical shape within the enlarged hole portion 28a as indicated by reference numeral 45 in FIG. 3C. If the solder 44 that has flowed into the thermal via 28 is so large that it cannot maintain its semispherical shape, it will separate from the solder 44 in the thermal via 28 and fall down as a solder ball 46 . Therefore, it is possible to prevent the solder 44 that has flowed into the thermal via 28 from protruding from the component surface 17 or reaching above the component surface 17 .

After the component 40 with heat dissipation pads is fixed to the soldering surface 16 by soldering, as shown in FIG. After application, solder printing is performed. Since this solder printing is performed in a state where the solder 44 does not protrude onto the component surface 17, the adhesion of the metal mask 47 to the component surface 17 is not impaired. As described above, since the solder 44 remains in a hemispherical state within the enlarged hole portion 28a, printing solder on the component surface 17 can be stably performed, and the necessary amount of solder can be printed. As a result, it is possible to stably mount the components on the component surface 17 without causing any problems.

In the above-described embodiment, a four-layer multilayer printed wiring board is shown, but the number of layers of the printed wiring board of the present invention may be two or more. Further, in the embodiment, the hole cross section of the thermal via 28 is circular, but the shape of the hole cross section of the thermal via of the printed wiring board of the present invention is not limited to circular, and may be rectangular, hexagonal or the like. A rectangular shape is also acceptable.

In the above-described embodiment, the component 40 with heat dissipation pads is mounted on the soldering surface 16, but it may be mounted on the component surface 17, in which case the enlarged hole portion 28a is formed on the soldering surface 16. be.

10 Printed wiring boards 11, 12, 13 Base material 16 Solder surface 17 Component surface 21 Component pad 22 Thermal pad 23 Solder resist 25 Component pad 26 Conductor pattern 28 Thermal via 28a Expanded hole 31 Drill 40 Component with heat dissipation pad 40a Terminal 40b Heat dissipation Pads 41, 47 Metal mask 42 Squeegee 43 Cream solder 44 Solder 45 Semispherical solder 46 Solder ball

Claims (7)

A printed wiring board having a thermal via in a hole that opens at a mounting position where a component with a heat dissipation pad is mounted on one main surface and penetrates between the one main surface and the other main surface,
The hole has a first hole portion extending from the one main surface toward the other main surface and terminating within the printed wiring board, and a first hole portion formed continuously with the first hole portion. and a second hole portion that reaches the other main surface and has a hole cross section larger than that of the first hole portion in a direction perpendicular to the through-hole direction of the hole. 2. The printed wiring board according to claim 1, wherein the cross section of said hole is circular. 3. The printed wiring board according to claim 1, wherein the inner wall surface of said first hole portion is plated and the inner wall surface of said second hole portion is not plated. The diameter of the second hole portion is added by drilling from the other main surface side to the thermal via having the diameter of the first hole portion between the one main surface and the other main surface. 3. The printed wiring board according to claim 2, wherein the printed wiring board is enlarged by machining. 5. The printed wiring board according to claim 1, wherein a plurality of said thermal vias are provided. 6. The printed wiring board according to any one of claims 1 to 5, wherein the printed wiring board is a multilayer printed wiring board, and an end of the first hole portion reaches a base material having a surface forming the other main surface. 1. The printed wiring board according to 1. The length of the second hole portion is shorter than the length from the other main surface to the inner layer of the copper foil connected to the thermal via closest to the other main surface. 7. The printed wiring board according to any one of 6.

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