Patent Application
20180370463
The present disclosure relates to a circuit assembly and an electrical junction box.
Conventionally, an electrical junction box (which is also called a power distributor) for distributing power from a power source (battery) to loads such as headlights and windshield wipers is mounted in an automobile. The electrical junction box includes a busbar that is connected to the power source and is part of a power circuit, and a circuit board including a control circuit for controlling the flow of electric current in the power circuit. The control circuit includes a circuit pattern formed on the circuit board, and electronic components such as switching elements (e.g., relays and FETs (field effect transistors)) and control elements (e.g., microcomputers and control ICs (integrated circuits).
In recent years, in order to reduce the size of the electrical junction box, circuit assemblies in which a circuit board is arranged integrally with the top of a busbar have been developed. JP 2005-117719A discloses a circuit assembly manufactured by using an adhesive sheet to bond the busbar and the circuit board together.
A circuit assembly of the present disclosure is a circuit assembly in which a circuit board including a control circuit for controlling a flow of electric current in a power circuit is arranged integrally with a top of a plate-shaped busbar that is part of the power circuit, the circuit assembly including: a circuit board in which both sides are provided with circuit patterns and that is provided with a via hole for electrically connecting the circuit patterns to each other; a adhesive sheet that is interposed between the busbar and the circuit board and fixes the circuit board to the top of the busbar; a hole filling resin with which the via hole is filled; and a resist layer that is formed on at least a side of the circuit board that faces the busbar, covering the via hole filled with the hole filling resin, wherein the adhesive sheet includes a substrate made of an insulating material, and adhesive layers that are formed on both sides of the substrate and are sticky at room temperature.
An electrical junction box of the present disclosure includes: the above-mentioned circuit assembly of the present disclosure; a heat sink attached to the busbar; and a case accommodating the circuit assembly and the heat sink.
A typical example of the adhesive sheet in a conventional circuit assembly is an adhesive sheet in which a thermosetting epoxy-based adhesive is applied to both sides of a substrate made of a polyimide film. In the conventional circuit assembly, the busbar and the circuit board are bonded together by stacking the busbar and the circuit board with the adhesive sheet being sandwiched therebetween, and performing thermocompression bonding using a hot pressing apparatus.
The conventional circuit assembly is problematic in that manufacturing time is long due to thermocompression bonding and manufacturing cost increases because an apparatus such as a hot press apparatus is required, for example. In addition, residual stress may be generated in the circuit board and the solder for installing the electronic components due to repeated heating and cooling during thermocompression bonding, and cause the deformation of the circuit board and cracks in the solder. Therefore, there is a concern about an adverse influence on reliability. Furthermore, the epoxy-based adhesive is easy to deteriorate and needs to be stored at a low temperature, for example, and is thus difficult to store and handle.
Accordingly, the development of a circuit assembly that does not require thermocompression bonding and is superior in terms of productivity is in demand.
Therefore, an object of the present disclosure is to provide a circuit assembly that is superior in terms of productivity. In addition, another object of the present disclosure is to provide an electrical junction box including this circuit assembly.
The circuit assembly and the electrical junction box of the present disclosure are superior in terms of productivity.
The inventors of the present disclosure propose that in order to eliminate the need for thermocompression bonding, a gluing agent that is sticky at room temperature is used as an alternative material to a thermosetting adhesive (e.g., epoxy-based adhesive). First, embodiments of the present disclosure will be listed and described.
(1) A circuit assembly according to an aspect of the present disclosure is a circuit assembly in which a circuit board including a control circuit for controlling a flow of electric current in a power circuit is arranged integrally with a top of a plate-shaped busbar that is part of the power circuit, the circuit assembly including: a circuit board in which both sides are provided with circuit patterns and that is provided with a via hole for electrically connecting the circuit patterns to each other; a adhesive sheet that is interposed between the busbar and the circuit board and fixes the circuit board to the top of the busbar; a hole filling resin with which the via hole is filled; and a resist layer that is formed on at least a side of the circuit board that faces the busbar, covering the via hole filled with the hole filling resin, wherein the adhesive sheet includes a substrate made of an insulating material, and adhesive layers that are formed on both sides of the substrate and are sticky at room temperature.
With the above-mentioned circuit assembly, the busbar and the circuit board can be bonded together at room temperature using a adhesive sheet including adhesive layers that are sticky at room temperature without thermocompression bonding, thus making it easy to fix the circuit board to the top of the busbar. Accordingly, the thermocompression bonding can be omitted, and thus the manufacturing time can be reduced. In addition, an apparatus such as a hot pressing apparatus is also not required, thus making it possible to reduce production cost. Therefore, the above-mentioned circuit assembly is superior in terms of productivity. Furthermore, since the thermocompression bonding is not performed, the deformation of the circuit board and cracks in the solder caused by repeated heating and cooling can be prevented.
With the above-mentioned circuit assembly, the via hole is filled with the hole filling resin, thus making it possible to improve the reliability of insulation between the busbar and the circuit board. It is necessary to ensure electric insulation between the busbar and the circuit board in the circuit assembly. When a resist layer is formed on a side of the circuit board provided with the via hole, the side facing the busbar, the resist layer is not formed on the portion in which the via hole is provided, and the dielectric breakdown voltage between the circuit board and the busbar thus decreases. Therefore, sufficient electric insulation cannot be ensured in some cases. In the case of the circuit board in which the via hole is filled with the hole filling resin, the resist layer can be formed to cover the via hole due to the hole filling resin. Accordingly, with the above-mentioned circuit assembly, the resist layer is formed to cover the via hole, thus making it possible to suppress the decrease in the dielectric breakdown voltage due to the via hole and ensure the electric insulation between the busbar and the circuit board.
(2) In an embodiment of the above-mentioned circuit assembly, the adhesive layers are made of an acrylic gluing agent.
It is sufficient if a gluing agent that has electric insulating properties and is sticky at room temperature is used as the gluing agent for the adhesive sheet, and examples thereof include an acrylic gluing agent, a silicone-based gluing agent, and a urethane-based gluing agent. The adhesive layers are required to have a heat resistance against a solder reflow temperature at which the electronic components are installed. Furthermore, it is desired that the gluing agent is less likely to deteriorate at room temperature, has excellent shelf life, and is inexpensive. The acrylic gluing agent is favorable because it meets these required characteristics and is very sticky.
(3) In an embodiment of the above-mentioned circuit assembly, the substrate is a nonwoven fabric made of cellulose.
It is sufficient if a substrate that has electric insulating properties and a heat resistance against a solder reflow temperature is used as the substrate for the adhesive sheet, and examples thereof include nonwoven fabrics and resin films. Examples of the nonwoven fabrics include nonwoven fabrics containing cellulose fibers, nonwoven fabrics containing resin fibers, and nonwoven fabrics containing glass fibers, and examples of the resin fibers include polyimide fibers and polyamideimide fibers. Examples of the resin films include a polyimide film and a polyamideimide film. A nonwoven fabric made of cellulose, which is a sheet made of cellulose fibers, is favorable because it has a heat resistance against a solder reflow temperature and is relatively inexpensive.
(4) An electrical junction box according to an aspect of the present disclosure includes: the circuit assembly according to any one of (1) to (3) above; a heat sink attached to the busbar; and a case accommodating the circuit assembly and the heat sink.
The above-mentioned electrical junction box includes the above-mentioned circuit assembly according to an aspect of the present disclosure and is thus superior in terms of productivity. Moreover, in the above-mentioned electrical junction box, the heat sink is attached to the busbar of the circuit assembly, and therefore, heat generated in the circuit assembly can be dissipated to the heat sink, and the high reliability is achieved.
Hereinafter, specific examples of the circuit assembly and the electrical junction box according to embodiments of the present disclosure will be described with reference to the drawings. In the figures, components having the same name are denoted by the same reference numeral. It should be noted that the present disclosure is not limited to these embodiments and is defined by the scope of the appended claims, and all changes that fall within the same essential spirit as the scope of the claims are intended to be included therein.
Circuit Assembly
A circuit assembly of Embodiment 1 will be described with reference to
Busbar
The busbar 10 is a plate-shaped component that is part of a power circuit. In this embodiment, as shown in
Circuit Board
As shown in
As shown in
In this embodiment, another part of the terminals of FETs 31 is directly joined to the top of the busbar 10 through soldering. Therefore, as shown in
Hole Filling Resin
As shown in
Resist Layer
As shown in
The thickness of the resist layer 26 is preferably 5 μm or more and more preferably 25 μm or more from the viewpoint of ensuring the electric insulation between the busbar 10 and the circuit board 20 (circuit pattern 22). The upper limit of the thickness of the resist layer 26 is 65 μm, for example, from the viewpoint of the adhesion with the circuit board 20 and the workability. The insulation resistance of the resist layer 26 is 500 MΩ or more, for example.
Adhesive Sheet
As shown in
Substrate
The substrate 41 of the adhesive sheet 40 is made of a material that has a heat resistance against a solder reflow temperature (e.g., 260° C.) and electric insulating properties, and examples thereof include nonwoven fabrics containing cellulose fibers, nonwoven fabrics containing resin fibers, nonwoven fabrics containing glass fibers, a resin film made of polyimide, and a resin film made of polyamideimide. Examples of the resin fibers include polyimide fibers and polyamideimide fibers. Of these, a nonwoven fabric made of cellulose, which is a sheet made of cellulose fibers, is practical because it has a heat resistance against a solder reflow temperature and is relatively inexpensive. In this embodiment, a nonwoven fabric made of cellulose is used as the substrate 41. It is sufficient if the thickness of the substrate 41 is selected as appropriate such that the electric insulation between the busbar 10 and the circuit board 20 can be ensured, and the thickness of the adhesive sheet 40 including the adhesive layers 42 is set to 50 μm or more, for example.
The adhesive layers 42 of the adhesive sheet 40 are made of a gluing agent that has a heat resistance against a solder reflow temperature and electric insulating properties, and that is sticky at room temperature. Examples of the gluing agent include an acrylic gluing agent, a silicone-based gluing agent, and a urethane-based gluing agent. Of these, the acrylic gluing agent containing an acrylic polymer is practical because it is very sticky, has excellent shelf life due to the capability of being stored at room temperature, and is inexpensive. In this embodiment, the adhesive layers 42 are made of an acrylic gluing agent. The adhesive layers 42 are formed by applying a gluing agent to both sides of the substrate 41.
In this embodiment, as shown in
Method for Manufacturing Circuit Assembly
An example of a procedure for manufacturing the circuit assembly 1 of Embodiment 1 shown in
(1) The busbar 10, the circuit board 20, and the adhesive sheet 40 are prepared (see
The circuit board 20 is produced as follows. A board material is prepared that has been processed into a predetermined shape by forming the component openings 29 (see
Next, as shown in
The adhesive sheet 40 is produced by cutting, into a predetermined shape as shown in
(2) The busbar 10 and the circuit board 20 are bonded together using the adhesive sheet 40, and the circuit board 20 is thus fixed to the top of the busbar 10 (see
(3) After the busbar 10 and the circuit board 20 are integrated, the electronic components are installed on the circuit board 20 (see
Functions and Effects of Circuit Assembly
The circuit assembly 1 of Embodiment 1 exhibits the following effects.
(1) With the circuit assembly 1, as shown in
(2) With the circuit assembly 1, as shown in
(3) The adhesive layers 42 of the adhesive sheet 40 are made of an acrylic gluing agent, thus making it possible to firmly fix the circuit board 20 to the top of the busbar 10. The acrylic gluing agent is very sticky, has excellent shelf life due to the capability of being stored at room temperature, and is inexpensive, thus making it possible to improve the productivity and reduce the manufacturing cost.
(4) A nonwoven fabric made of cellulose is used as the substrate 41 of the adhesive sheet 40, thus making it possible to reduce the cost of the adhesive sheet 40 and reduce the manufacturing cost. For example, a polyimide film used as the substrate of the adhesive sheet in a conventional circuit assembly is expensive, whereas a nonwoven fabric made of cellulose is inexpensive. Therefore, use of the adhesive sheet in which the substrate is constituted by the nonwoven fabric made of cellulose makes it possible to reduce the cost of materials compared with the case where a conventional adhesive sheet is used.
Electrical Junction Box
Next, an electrical junction box 100 of Embodiment 1 will be described with reference to
The heat sink 60 is attached to the busbar 10 of the circuit assembly 1. The heat sink 60 is made of a highly heat-conductive metal material such as aluminum or copper. In this embodiment, the heat sink 60 is an aluminum plate. There is no particular limitation on the shape of the heat sink 60, and the heat sink 60 may have a plate shape or a block shape. The heat sink 60 mainly serves to prevent the temperatures of the electronic components (e.g., FETs 31) installed in the circuit assembly 1 and the temperature of the solder for installing the electronic components from exceeding the acceptable temperatures. The size of the heat sink 60 is set to be suitable for heat dissipation, for example.
The heat sink 60 is attached to the circuit assembly 1 (busbar 10) through bonding using a adhesive sheet having a configuration similar to that of the adhesive sheet 40 (see
The case 80 accommodates the circuit assembly 1 and the heat sink 60. In this embodiment, as shown in
Method for Manufacturing Electrical Junction Box
An example of a procedure for manufacturing the electrical junction box 100 of Embodiment 1 shown in
After the heat sink 60 is bonded to the lower side of the busbar 10 of the circuit assembly 1, the circuit assembly 1 is attached and fixed to the inside of the lower case 82 using screws. Then, the case 80 is assembled by fitting the upper case 81 to the lower case 82. The electrical junction box 100 shown in
Circuit assemblies of samples 1 to 3 as described below were produced and evaluated.
Sample 1 was the above-described circuit assembly of Embodiment 1. In the circuit assembly of sample 1, a adhesive sheet in which an acrylic gluing agent was applied to both sides of a substrate constituted by a nonwoven fabric made of cellulose was used, and the busbar and the circuit board were bonded together using the adhesive sheet. The via hole of the circuit board was filled with the hole filling resin made of an epoxy resin, and a resist layer made of an epoxy resin was formed on the lower side of the circuit board to cover the via hole. The adhesive sheet had a thickness of 50 μm, and the resist layer had a thickness of 25 μm.
Sample 2 was produced in the same manner as in sample 1, except that the via hole of the circuit board was not filled with the hole filling resin made of an epoxy resin.
In sample 3, an adhesive sheet in which an epoxy-based adhesive was applied to both sides of a substrate made of a polyimide film was used instead of the adhesive sheet of sample 1. The adhesive sheet was sandwiched between the busbar and the circuit board, and then the busbar and the circuit board were bonded together through thermocompression bonding. In the circuit assembly of sample 3, similarly to sample 2, the via hole of the circuit board was not filled with the hole filling resin made of an epoxy resin. The substrate of the adhesive sheet had a thickness of 25 μm.
The circuit assemblies of samples 1 to 3 were subjected to a withstand voltage test in which a D.C. voltage of 0.8 to 2.0 kV is applied between the circuit board and the busbar, and the electric insulation was evaluated. When dielectric breakdown occurred, the electric insulation was evaluated as “A”, and when dielectric breakdown did not occur, the electric insulation was evaluated as “B”. Table 1 shows the results.
It is found from the results shown in Table 1 that dielectric breakdown did not occur at a voltage of up to 2.0 kV in the circuit assembly of sample 1, and high electric insulation for DC 2 kV or more was achieved. In contrast, dielectric breakdown occurred at a voltage of 1.8 kV in the circuit assembly of sample 2. The dielectric breakdown voltage of this circuit assembly was lower than that of the circuit assembly of sample 1, and the electric insulation decreased. It is thought that sufficient electric insulation was ensured due to the resist layer and the adhesive sheet in sample 1 because the via hole was filled with the hole filling resin and the resist layer was formed to cover the via hole filled with the hole filling resin.
Dielectric breakdown did not occur at a voltage of up to 2.0 kV in the circuit assembly of sample 3, and similarly to sample 1, high electric insulation was achieved. However, in sample 3, it was necessary to perform thermocompression bonding. Therefore, compared with sample 1, operations were more complicated, and the productivity was poorer. In addition, the polyimide film used as the substrate of the adhesive sheet in sample 3 was expensive, and the adhesive sheet was thus expensive. In contrast, in sample 1, a nonwoven fabric made of cellulose was used as the substrate of the adhesive sheet, and therefore, the adhesive sheet was more inexpensive than the adhesive sheet, thus making it possible to reduce the cost of materials.
Applications of Circuit Assembly and Electrical Junction Box
The circuit assembly and the electrical junction box according to the embodiment of the present disclosure can be favorably used in an electrical junction box for an automobile.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-244888 | Dec 2015 | JP | national |
This application is the U.S. national stage of PCT/JP2016/086460 filed Dec. 7, 2016, which claims priority of Japanese Patent Application No. 2015-244888 filed on Dec. 16, 2015, which is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/086460 | 12/7/2016 | WO | 00 |
