Multilayer printed board

Abstract

To solve a problem in which, in a printed circuit board with multiple layers, if the GND layer in the middle layer is connected to a thermal via, heat may escape through the GND layer in the middle layer, making it difficult to remove the components.SOLUTION: The present invention is a multilayer printed board having three or more layers, including a surface layer to which components can be connected via thermal pads, a back layer opposite the surface layer and forming a ground layer, an intermediate layer arranged between the surface layer and the back layer and including one layer having a ground layer, and thermal vias that penetrate the surface layer, back layer, and intermediate layer, and the thermal vias connect the thermal pads on the surface layer to the ground layer on the back layer, but are not connected to the ground layer on the intermediate layer.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a multilayer printed circuit board. [Background technology]

[0002] The number of countries and regions around the world announcing carbon neutrality (CN) targets is rapidly increasing, accounting for 90% of the world's total GDP. Excessive reliance on increasingly unstable fossil fuels poses direct national risks in both security and economic terms, further accelerating the investment race toward achieving green transformation (GX). GX refers to a major shift in industrial and energy policy, shifting from a fossil fuel-centered industrial structure and socio-economy to one centered on clean energy sources with less impact on the natural environment, such as solar and hydrogen. In Japan, industry-specific GX initiatives are underway to achieve both CO2 emissions reduction and economic growth. In order to comply with European and American regulations, Japan aims to reduce CO2 emissions by 90% compared to 2000 levels by 2050. Therefore, there are high hopes for the simultaneous realization of decarbonization and economic growth, and the achievement of CN beyond that.

[0003] Looking ahead, circuit board design will be required to be even more energy-efficient and to build sustainable systems than ever before. For example, by making the components mounted on the board easily removable, it will be possible to prepare a system that allows reuse and recycling, and a structure can be created in which components can be recycled and used.

[0004] Surface mount components that generate heat, such as ICs and transistors, need to dissipate heat. For this reason, some surface mount components have pads with heat dissipation functions on the backside of their packages, and the heat is dissipated from the pads to the printed circuit board. This is a thermal pad, which is connected to the GND of the printed circuit board. Figures 1(a) and 1(b) show the conventional configuration of a surface mount component 63 with a thermal pad. The lead connection land 61 is a land portion for connecting lead wiring. The thermal pad 62 is a member for dissipating heat generated by the surface mount component. The printed circuit board 64 is a board to which various elements can be connected. In this way, the thermal pad 62 is soldered to the GND pattern on the component side, which is the surface layer of the printed circuit board 64, and dissipates heat.

[0005] To further improve the heat dissipation function of surface-mounted components with thermal pads, thermal vias are sometimes provided in the GND pattern. Figures 1(c) and 1(d) show a configuration in which a thermal via 66 is provided in a thermal pad 62. As shown in Figure 1(d), the thermal via 66 penetrates the inner layer of the printed circuit board 64 and is connected to the GND layer of the inner layer and the GND pattern on the solder side, which is the back side of the printed circuit board. This increases the area of ​​the GND pattern, thereby improving heat dissipation performance.

[0006] Patent Document 1 proposes that even if there is no through-hole on the GND wiring, the GND wiring can be connected to each other on the surface layer to serve as a substitute for the through-hole. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-066099 Summary of the Invention [Problem to be solved by the invention]

[0008] In multilayer printed circuit boards, heat dissipation performance can be improved by connecting not only the top and bottom layers but also the middle layers with thermal vias. This is because the area of ​​the GND pattern connected through thermal vias increases, making it easier for the heat applied to the solder to escape when removing the component. Also, since there are no lands outside the component outline, the thermal pad cannot be heated directly with a soldering iron, so it is necessary to heat the package of the surface-mounted component 63 or the bottom surface of the board.

[0009] In other words, it is difficult to heat with a soldering iron, and the thermal vias are connected to the GND patterns on multiple inner layers, which causes heat to escape. As a result, high temperatures are required to remove the components.

[0010] As mentioned above, in a printed circuit board with multiple layers, if the GND layer of the middle layer is connected to a thermal via, heat may escape through the GND layer of the middle layer, making it difficult to remove the component. In order to remove the component, it must be heated to a higher temperature so that it can be removed even if heat escapes from the GND layer of the middle layer, which may lead to thermal destruction of the component. Furthermore, the same can be said for peripheral components when components are mounted nearby, such as on a printed circuit board with high density mounting.

[0011] The present invention has been made in view of the above, and has an object to improve the ease of removal of components having thermal pads while suppressing thermal damage in the components. [Means for solving the problem]

[0012] The present invention is a multilayer printed circuit board having three or more layers, comprising a surface layer to which components can be connected via thermal pads, a back layer opposite the surface layer and forming a ground layer, an intermediate layer arranged between the surface layer and the back layer and including one layer having a ground layer, and thermal vias penetrating the surface layer, the back layer, and the intermediate layer, wherein the thermal vias connect the thermal pads on the surface layer and the ground layer on the back layer, but are not connected to the ground layer on the intermediate layer. [Effects of the Invention]

[0013] According to the present invention, it is possible to improve the ease of removal of a component having a thermal pad while suppressing thermal damage. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing an example of a conventional configuration of a component having a thermal pad and a thermal via. [Figure 2] 1A and 1B are diagrams illustrating an example of a component configuration of a printed circuit board on which components having thermal pads are mounted. [Figure 3] FIG. 1 is a diagram illustrating an example of a printed circuit board structure. [Figure 4] FIG. 2 is a diagram showing an example of a wiring pattern on a first layer of a printed circuit board. [Figure 5] FIG. 2 is a diagram showing an example of a wiring pattern on a second layer of a printed circuit board. DETAILED DESCRIPTION OF THE INVENTION

[0015] Each embodiment of the present invention will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all of the combinations of features described in each embodiment are necessarily essential to the solution of the present invention. In this embodiment, an image forming apparatus will be used as an example of an information processing apparatus, but the present invention is not limited to this.

[0016] Example 1 FIG. 2 shows a top view of components with thermal pads mounted on a multilayer printed circuit board 1 according to this embodiment. A multilayer printed circuit board is a printed circuit board with three or more layers. Although not shown, for example, a six-layer printed circuit board has a signal layer on the first layer, a GND layer on the second layer, a power supply layer on the third layer, a signal layer on the fourth layer, a GND layer on the fifth layer, and a signal layer on the sixth layer. Although not shown, for example, a four-layer printed circuit board has a signal layer on the first layer, a GND layer on the second layer, a power supply layer on the third layer, and a signal layer on the fourth layer. A DC-DC converter 72, a motor driver 73, and a CPU 74 are mounted on the multilayer printed circuit board 1 as components with thermal pads.

[0017] The DC-DC converter 72 converts DC (direct current) to DC (direct current) and converts voltages from DC to DC. For example, it converts DC 24V to DC 5V or DC 3.3V to DC 1.8V. The motor driver 73 drives and controls the motor. The CPU 74 is a control unit that controls the device to which the printed circuit board 1 is attached. For example, in a multifunction peripheral, programs such as the operating system (OS) and application software are stored on a storage device such as a hard disk drive (HDD). The CPU 74 loads the programs stored on the storage device into RAM and executes them. The CPU 74 controls a scanner unit (not shown) to scan an original document and generate image data, and then controls a printer unit based on the generated image data to print an image on paper. The current consumption and temperature characteristics of these components with thermal pads and the temperature rise characteristics of the printed circuit board 1 vary depending on the configuration. For example, the CPU 74 consumes a high amount of current compared to other components in the printed circuit board 1. Therefore, it is desirable to provide a thermal via in the thermal pad and connect it to the GND layer inside the printed circuit board 1.

[0018] On the other hand, there are cases where the DC-DC converter 72 requires large power consumption, and cases where it only requires small power consumption. In this embodiment, a configuration that makes it easy to remove parts will be described using a DC-DC converter with low current consumption as an example.

[0019] 3 is a diagram showing the structure (cross section) of a multilayer printed circuit board 1 according to this embodiment, with the first layer 23 (surface layer including a signal layer) on which an IC2 having a thermal pad 12 is mounted facing up. In FIG. 3, the multilayer printed circuit board 1 is the printed circuit board of this embodiment. The IC2 is an IC package having a thermal pad and a heat sink 21.

[0020] IC2 is an IC that requires a package with high heat dissipation, such as an operational amplifier, DC-DC converter, motor driver, or buffer. The package can be a TSSOP-14 package, SP-8 package, or other package with a thermal pad. As mentioned above, this example describes a DC-DC converter with low current consumption.

[0021] The printed circuit board 1 is shown as an example of a typical four-layer structure of 1.6 mm thick FR material, but it can also be a multi-layer structure with various numbers of layers, such as two or six. Solder resist 10 is an ink that is applied to the surface of the printed circuit board and forms an insulating film. Lead connection lands 11 are copper foil areas used to solder terminals of surface-mounted components, etc., to the printed circuit board to connect lead wiring. Thermal pads 12 for ICs are placed on the first layer of the printed circuit board 1.

[0022] Soldering 14 shows that cream solder melts during reflow mounting and solders the lead connection land 11 to the lead electrode 22 of IC 2. Cream solder is a paste-like solder that is a mixture of powdered metal and flux, and is also called solder paste.

[0023] The member 15 shows that the cream solder melts during reflow mounting and flows into the thermal vias 16, filling the thermal vias 16 with solder.

[0024] Reflow mounting is a component mounting method in which a component is placed on unmelted solder and heated to melt it. The gap between the IC 2 and the thermal pad 12 is determined by the package standard. The thermal via 16 is, for example, a through via with a diameter of 0.3 mm, e.g., a through via with a diameter of 0.25 mm, and is plated on the inside with a thickness of 15 μm. The copper foil 18 is copper foil arranged on the second layer (ground layer (GND layer)) of the printed circuit board 1. The pattern 19 is the third layer (power wiring layer). The third layer is not connected to the thermal via 16 of the IC 2. In this embodiment, the first intermediate layer is referred to as the second layer, and the second intermediate layer is referred to as the third layer. Here, the number of intermediate layers is two, but this is not limited to two. Furthermore, a configuration in which no thermal vias are connected to any of the intermediate layers, or at least one layer, is not connected, may be adopted.

[0025] The GND layer 18 is not connected to the thermal via 16 of IC2. By not connecting the thermal via 16 to the GND layer 18, it is possible to reduce the heat applied to the thermal pad 12 when removing IC2. The thermal via 16 penetrates to the fourth layer (back surface layer) and is connected to the GND layer 20 of the fourth layer. The GND layer (ground pattern) of the fourth layer is not connected to the GND of the inner layer via a via, but only the GND of the fourth layer. This makes it easier to melt the solder of the thermal pad 12 of IC2 by performing a heating process that applies heat to the GND pattern of the fourth layer using, for example, a soldering iron. The back surface layer is the layer on the opposite side of the surface layer with respect to the board.

[0026] FIG. 4(a) is a diagram (perspective view from the first layer) showing the wiring pattern of the first layer on which an IC with a thermal pad is mounted on a printed circuit board. The IC component outline 71 is the outline of IC2 with a thermal pad. The thermal pad 12 is located inside the outline of IC2. Therefore, when removing the mounted IC, the thermal pad 12 cannot be heated directly.

[0027] In contrast, FIG. 4(b) is a diagram (perspective view seen from the first layer) showing the wiring pattern of the first layer on which IC2 of the printed circuit board in this embodiment is mounted. The heating land 51 is a heating land connected to the thermal pad 12. The heating land is provided to heat the thermal pad 12, and supplying solder to the heating land 51 transfers heat to the thermal pad 12, making it easier to melt the solder on the thermal pad 12. This makes it easier to heat the thermal pad portion of surface-mounted components that have thermal pads, preventing thermal damage when the surface-mounted components are removed. The heating land is located away from the location where the elements are located and is connected to the ground layer on the back surface layer.

[0028] 5(a) is a diagram (perspective view seen from the fourth surface) showing the wiring pattern on the fourth surface side of a conventional printed circuit board on which an IC with a thermal pad is mounted. GND pattern 53 is a GND pattern that includes thermal vias for the IC, and includes GND vias (ground vias) 54 that connect to an inner layer (e.g., ground 18 on the second layer). In this configuration, the thermal vias are connected to the inner layer, making it difficult to remove IC2.

[0029] In contrast, Figure 5(b) is a diagram (perspective view seen from the fourth surface) showing the wiring pattern on the fourth surface side where an IC with a thermal pad is mounted on a printed circuit board in this embodiment. The GND pattern 52 is a GND pattern including thermal vias 16. By not connecting to the GND vias on the fourth layer in this way, it is made difficult for the heat from the thermal pad 12 heated from the heating land 51 to escape. In addition, heating the GND pattern 52 transfers heat to the thermal pad 12, making it easier to melt the solder on the thermal pad 12.

[0030] According to the present invention, IC2 having a thermal pad is not connected to the inner GND layer 18, the GND pattern on the fourth layer is not connected to the inner GND pattern, and the first layer is provided with a heating land 51 connected to the thermal pad 12. In this configuration, by heating the heating land 51 on the first layer, it is possible to improve the ease of heating work and thermal efficiency, and it becomes possible to easily remove IC2.

[0031] (Other embodiments) While various examples and embodiments of the present invention have been shown and described, the spirit and scope of the present invention is not limited to the specific descriptions within this specification. [Explanation of symbols]

[0032] 1 Printed circuit board 12 Thermal Pads 16 Thermal vias 18 Second layer GND layer 19 Third layer power wiring layer 20 4th layer GND layer

Claims

1. In a multilayer printed circuit board having three or more layers, a surface layer to which components can be connected via thermal pads; a back surface layer opposite to the surface layer and forming a ground layer; an intermediate layer disposed between the front layer and the back layer, the intermediate layer including one layer having a ground layer; a thermal via penetrating the front surface layer, the back surface layer, and the intermediate layer; 10. A multilayer printed circuit board, comprising: a thermal via that connects the thermal pad on the surface layer to the ground layer on the back surface layer, and that does not connect to the ground layer on the intermediate layer.

2. The intermediate layer comprises at least two intermediate layers, 2. The multilayer printed circuit board according to claim 1, wherein, of the at least two intermediate layers, a first intermediate layer is a layer having the ground layer, and a second intermediate layer is a layer having power wiring.

3. 3. The multilayer printed circuit board according to claim 2, wherein the thermal vias do not connect any of the intermediate layers including the first intermediate layer and the second intermediate layer.

4. the surface layer has a heating land disposed at a position away from the component and capable of receiving a heat treatment; 2. The multilayer printed circuit board according to claim 1, wherein the thermal pad on the surface layer is connected to the heating land.

5. the back surface layer has a ground via connected to the ground layer of the intermediate layer; 2. The multilayer printed circuit board according to claim 1, wherein the thermal vias are not connected to the ground vias.

6. 2. The multilayer printed circuit board according to claim 1, wherein the component is a DC-DC converter.

7. 2. The multilayer printed circuit board according to claim 1, wherein the component is a motor driver.

8. 2. The multilayer printed circuit board according to claim 1, wherein the component is a CPU that controls a scanner that reads an original and generates image data.

9. 2. The multilayer printed circuit board according to claim 1, wherein the thermal vias are through vias whose insides are plated.

10. 2. The multilayer printed circuit board according to claim 1, wherein the thermal vias and the thermal pads have a heat dissipation function.

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