Electronic device and soldering method

Abstract

The present invention pertains to an electronic device formed by mounting, using soldering, a terminal of an electronic component on a printed wiring board having a plurality of through holes. The terminal has a flat plate-like flat plate part, and a plurality of projection parts branching from the flat plate part toward the plurality of through holes. The plurality of projection parts are joined to the plurality of through holes by soldering. At least one of the terminal and the printed wiring board is provided with a thermal resistance difference reduction part for reducing the difference in thermal resistance between each of the plurality of projection parts and a body part of the electronic component.

Description

Electronic device and soldering method

[0001] The present invention relates to an electronic device in which electronic components are mounted on a printed wiring board by solder bonding, and to a solder bonding method used in manufacturing the electronic device.

[0002] In order to manufacture electronic devices that meet desired electrical characteristics, various electronic components such as semiconductor elements and capacitors are commonly mounted on printed wiring boards by soldering. In such cases, if the electronic components have heat resistance limitations, methods of soldering each component individually (such as soldering iron methods) may be used to minimize the range of temperature effects during soldering.

[0003] When a printed wiring board is used in, for example, a power conversion device equipped with an inverter for an automobile, high reliability is required for the joints between the electronic components and the printed wiring board. To reduce the possibility of an unintentional open state of the joints after mounting, it is effective to form multiple joints for each electronic component. Furthermore, to improve productivity when soldering such electronic components, it is effective to simultaneously solder multiple joints with a single soldering iron.

[0004] A known technique for simultaneously soldering multiple joints is disclosed, for example, in Patent Document 1. Patent Document 1 discloses a mounting component having one or more pins, each of which is connected and fixed to a printed circuit board, thereby being mounted on the printed circuit board. At least one of the pins in the mounting component, a branch pin, is branched and extends into multiple parts, thereby forming multiple branch extensions. When the branch pin is connected and fixed to the printed circuit board, each of the branch extensions is connected and fixed to the printed circuit board. This makes it possible to form multiple joints.

[0005] Another known technique for simultaneously soldering multiple joints is, for example, Patent Document 2. Patent Document 2 discloses a soldering device that uses a cylindrical nozzle as a soldering iron, melts solder inside the nozzle, and solders a first conductor and a second conductor with the molten solder. This soldering device includes a nozzle with multiple parallel, linear solder piece supply passages, and a heating means that heats and melts the solder pieces supplied into each solder piece supply passage of the nozzle. This allows solder pieces to be supplied to multiple joints simultaneously, making it possible to simultaneously solder multiple joints.

[0006] Japanese Patent Publication No. 2011-181735 Japanese Patent Publication No. 2019-155381

[0007] The methods of Patent Documents 1 and 2 may cause temperature variations among multiple joints during soldering. Temperature variations during joining may lead to variations in the shape of the solder fillets formed at each joint. In order to improve the reliability and quality of electronic devices, it is necessary to minimize the variations in the shape of the solder fillets among joints, but Patent Documents 1 and 2 do not mention any measures to achieve this.

[0008] The present invention has been made in view of the above-mentioned problems, and a main object of the present invention is to realize an electronic device and a soldering method thereof that can achieve both high productivity and high reliability.

[0009] An electronic device according to a first aspect of the present invention comprises a printed wiring board having a plurality of through holes and a terminal of an electronic component mounted by solder bonding, the terminal having a flat plate portion and a plurality of protrusions branching from the flat plate portion toward each of the plurality of through holes, the plurality of protrusions being solder bonded to the plurality of through holes, and at least one of the terminal and the printed wiring board being provided with a thermal resistance difference reduction portion that reduces the difference in thermal resistance between each of the plurality of protrusions and the main body portion of the electronic component. An electronic device according to a second aspect of the present invention is formed by mounting a terminal of an electronic component by soldering on a printed wiring board having a plurality of through holes, wherein the terminal has a flat plate portion and a plurality of protrusions branching from the flat portion toward each of the plurality of through holes, and the plurality of protrusions are respectively joined to the plurality of through holes by soldering, and at least one of the terminal and the printed wiring board is provided with a thermal resistance increasing portion that increases the thermal resistance between a first protrusion of the plurality of protrusions that is closest to the main body portion of the electronic component along the extension direction of the terminal and the main body portion. A soldering method according to a third aspect of the present invention is a method for soldering a terminal of an electronic component to a printed wiring board having a plurality of through holes, wherein the terminal has a flat plate portion and a plurality of protrusions branching from the flat portion toward each of the plurality of through holes, and at least one of the terminal and the printed wiring board is provided with a thermal resistance difference reducing portion that reduces the difference in thermal resistance between each of the plurality of protrusions and a main body portion of the electronic component, and with the plurality of protrusions inserted into the plurality of through holes, solder pieces are supplied around the plurality of through holes, and the solder pieces are heated and melted in each of the plurality of through holes, thereby soldering the printed wiring board and the terminal.A soldering method according to a fourth aspect of the present invention is a method for soldering a terminal of an electronic component to a printed wiring board having a plurality of through holes, wherein the terminal has a flat plate portion and a plurality of protrusions branching from the flat portion toward each of the plurality of through holes, and at least one of the terminal and the printed wiring board is provided with a thermal resistance increasing portion that increases the thermal resistance between a first protrusion of the plurality of protrusions that is closest to a main body portion of the electronic component along the extension direction of the terminal and the main body portion, and with the plurality of protrusions inserted into the plurality of through holes respectively, solder pieces are supplied around the plurality of through holes respectively, and the solder pieces are heated and melted in each of the plurality of through holes to solder the printed wiring board and the terminal.

[0010] According to the present invention, an electronic device and a soldering method thereof that can achieve both high productivity and high reliability can be realized.

[0011] FIG. 1 is an external view of an electronic device according to a first embodiment of the present invention. FIG. 2 is an explanatory diagram of a soldering process for the electronic device according to the first embodiment of the present invention. FIG. 3 is an explanatory diagram of variations in fillet shapes of joints. FIG. 4 is a diagram showing the flow of heat in terminals during the soldering process for the electronic device according to the first embodiment of the present invention. FIG. 5 is a diagram showing modified examples of a thermal resistance difference reducing section and a thermal resistance increasing section in the first embodiment of the present invention. FIG. 6 is an external view of an electronic device according to a second embodiment of the present invention. FIG. 7 is an explanatory diagram of a cross-sectional structure of a printed wiring board according to the second embodiment of the present invention. FIG. 8 is an external view of a capacitor, which is an example of an electronic component used in a comparative example. FIG. 9 is an external view of an electronic device of a comparative example in which a capacitor is mounted on a printed wiring board.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The following description and drawings are examples for explaining the present invention, and some omissions and simplifications have been made as appropriate for clarity of explanation. The present invention can be implemented in various other forms. Unless otherwise specified, each component may be singular or plural.

[0013] In order to facilitate understanding of the invention, the position, size, shape, range, etc. of each component shown in the drawings may not represent the actual position, size, shape, range, etc. Therefore, the present invention is not necessarily limited to the position, size, shape, range, etc. disclosed in the drawings.

[0014] In the comparative examples and embodiments of the present invention described below, a capacitor will be used as an example of an electronic component that is soldered to a printed wiring board and mounted in an electronic device, but the present invention is not limited to capacitors and can also be applied to other electronic components.

[0015] The present invention aims to improve the reliability of an electronic device in which terminals of electronic components are simultaneously soldered to a printed wiring board at multiple joints by reducing temperature variations between the joints that occur during soldering, thereby suppressing variations in the shape of solder fillets formed at each joint. Here, the reason for temperature variations between multiple joints will be explained using the following comparative example with reference to Figures 8 and 9.

[0016] [Comparative Example] Fig. 8 is an external view of a capacitor 100C, which is an example of an electronic component used in a comparative example. In Fig. 8, (A) shows a front view of the capacitor 100C, and (B) shows a side view of the capacitor 100C. In Fig. 8, the x, y, and z axes of the capacitor 100C are defined as coordinate axis 1.

[0017] Capacitor 100C is configured by sealing a dielectric film of an internal electrode (not shown) and two terminals 110 joined to positive and negative external electrodes (metallicon) with resin 121, and covering with case 122. Hereinafter, the portion of capacitor 100C excluding terminals 110 will be referred to as body portion 120.

[0018] Each terminal 110 has a flat plate portion 111 and three protrusions 112 branching off from the flat plate portion 111. The flat plate portion 111 is bent approximately 90° along the way, thereby dividing into a flat plate portion 111a extending in the x-axis direction outside the main body portion 120 and a flat plate portion 111b extending in the y-axis direction and partially embedded in the main body portion 120. Each protrusion 112 is branched off from the flat plate portion 111a, and an end portion 113 is provided at the end of the portion of the flat plate portion 111b embedded in the main body portion 120.

[0019] Each terminal 110 is attached by soldering or welding to an external electrode of the main body 120 at an end portion 113 of the flat portion 111b. Generally, to increase productivity, the flat portion 111 and the protruding portion 112 are formed by pressing a flat plate, and then bending or the like is performed as necessary to separate the flat portion 111 into the flat portion 111a and the flat portion 111b, thereby manufacturing the terminals 110.

[0020] 9 is an external view of an electronic device 10C of a comparative example in which a capacitor 100C is mounted on a printed wiring board 200C. In FIG. 9, (A) shows a top view of the region of the printed wiring board 200C to be joined to the capacitor 100C, and (B) is a front view showing the relative positioning of the capacitor 100C and the printed wiring board 200C in the electronic device 10C after soldering. In FIG. 9, as in FIG. 8, the x, y, and z axes of the electronic device 10C are defined as coordinate axes 1.

[0021] The printed wiring board 200C is provided with a plurality of through holes 210 for inserting the plurality of protrusions 112 provided on each terminal 110 of the capacitor 100C, and lands 211 for soldering, and the other surfaces, namely the front and back surfaces, are protected by resist 212. After inserting the protrusions 112 into the through holes 210 of the printed wiring board 200C, the joints 300 between the protrusions 112 and the through holes 210 are soldered, thereby forming the electronic device 10C.

[0022] 9, of the two terminals 110, the terminal 110 located on the left side of the figure has three protrusions 112 provided on this terminal 110, which are indicated as protrusions 112a, 112b, and 112c in order of proximity to the flat plate portion 111b. Furthermore, the joints 300 between these protrusions 112a to 112c and the through-hole 210 are indicated as joints 300a, 300b, and 300c, respectively. Although these reference numerals are not shown for the terminal 110 located on the right side of the figure, the protrusions 112 and joints 300 of this terminal 110 also have similar structures.

[0023] As is clear from FIG. 9B , in the comparative example, the distances from the joints 300a-300c between the protrusions 112a-112c and the printed wiring board 200C to the end portion 113 joined to the main body 120 of the capacitor 100C are different for each of the protrusions 112a-112c. Therefore, when the joints 300a-300c are soldered, the thermal resistance along the path of heat flow through the terminal 110 to the main body 120 also differs among the protrusions 112a-112c. As a result, even if the solder heating time is the same, the soldering temperatures of the joints 300a-300c differ. Thus, in the comparative example, when the protrusions 112a-112c are soldered to the printed wiring board 200C, temperature variations occur among the joints 300a-300c.

[0024] The temperature variations during soldering as described above may lead to variations in the shape of the solder fillets formed at the joints 300a to 300c. In order to further improve the reliability and quality of the electronic device 10C, it is necessary to minimize the variations in the shape of the solder fillets.

[0025] [First Embodiment] Fig. 1 is an external view of an electronic device 10 according to a first embodiment of the present invention. In Fig. 1, (A) shows a front view, and (B) shows a side view. Like the electronic device 10C of the comparative example, the electronic device 10 of this embodiment is configured by mounting a capacitor 100 on a printed wiring board 200. Note that in Fig. 1, the definitions of the axes of the electronic device 10 are as shown in coordinate axis 1.

[0026] The printed wiring board 200 has a structure similar to that of the printed wiring board 200C described in the comparative example. That is, the printed wiring board 200 is provided with a plurality of through holes 210 for inserting the plurality of protrusions 112 provided on each terminal 110 of the capacitor 100, and lands 211 for joining solder, as in Fig. 9A, and the other front and back surfaces are protected by resist 212.

[0027] Like the capacitor 100C described in the comparative example, the capacitor 100 is configured by sealing a dielectric film of an internal electrode (not shown) and two terminals 110 joined to positive and negative external electrodes (metallicon) with resin 121 and covering the case 122. The resin 121 is, for example, epoxy resin. The main body 120 is configured by the portion of the capacitor 100 excluding the terminals 110.

[0028] Each terminal 110 has a flat plate portion 111 and three protrusions 112 branching off from the flat plate portion 111. These structures are similar to those of the comparative example. That is, the flat plate portion 111 is divided into a flat plate portion 111a extending in the x-axis direction outside the main body portion 120 and a flat plate portion 111b extending in the y-axis direction and partially embedded in the main body portion 120. Furthermore, each protrusion 112 is branched off from the flat plate portion 111a, and an end portion 113 is provided at the end of the portion of the flat plate portion 111b embedded in the main body portion 120.

[0029] Each terminal 110 is attached to an external electrode of the main body 120 at an end portion 113 of the flat portion 111b by soldering or welding. The terminal 110 is made of, for example, Cu or a Cu alloy, and its surface may be plated to improve solderability. While the tip of each protrusion 112 is angular in FIG. 1 , the tip of each protrusion 112 may be acutely angled to improve insertion into a through-hole 210 of the printed wiring board 200 during soldering. Although the flat portion 111a and the flat portion 111b of the terminal 110 have the same width in the z direction in FIG. 1 , they may be different widths.

[0030] The protrusions 112 of each terminal 110 and the through-holes 210 of the printed wiring board 200 are joined to each other via solder at the inner surfaces of the through-holes 210 and at lands 211 provided on the front and back surfaces of the printed wiring board 200. This forms solder joints 300 between the protrusions 112 and the through-holes 210. For example, Sn-Ag-Cu based lead-free solder is used for the joints 300.

[0031] In the electronic device 10 of this embodiment, the terminal 110 of the capacitor 100 has slits 114 formed in the flat plate portion 111a corresponding to the three protrusions 112 (protrusions 112a to 112c). The slits 114 are formed by cutting out a portion of the flat plate portion 111a along the extension direction (z-axis direction) of the protrusions 112, thereby partially removing the flat plate portion 111a between the protrusions 112 and the main body portion 120. In FIG. 1, the slits 114 formed corresponding to the protrusions 112a to 112c are shown as slits 114a to 114c, respectively.

[0032] In the electronic device 10 of this embodiment, the slits 114a-114c are provided in the flat plate portion 111a of the terminal 110, and the slits 114a-114c can function as thermal resistance difference reducing portions that reduce the difference in thermal resistance between the protrusions 112a-112c and the main body portion 120 of the capacitor 100. The slit 114a can also function as a thermal resistance increasing portion that increases the thermal resistance between the protrusion 112a and the main body portion 120. This makes it possible to reduce the temperature variation during soldering between the joints 300a-300c, as described in the comparative example. This point will be described in detail later.

[0033] Here, it is not necessary to provide slits 114 for all protrusions 112 in flat plate portion 111a. For example, in FIG. 1, of slits 114a to 114c, slit 114c has substantially no difference in shape from the original flat plate portion 111a shown in the comparative example. Therefore, it can be considered that, of protrusions 112a to 112c, slit 114 is not provided for protrusion 112c that is located furthest from main body 120 of capacitor 100 along the extension direction of terminal 110. Even in this case, the effect of reducing temperature variation during soldering between joints 300a to 300c can be obtained as described above.

[0034] However, it is desirable to provide slit 114a at least in protrusion 112a, of protrusions 112a to 112c, that is closest to main body 120 of capacitor 100 along the extension direction of terminal 110. In this way, the thermal resistance of joint 300a between protrusion 112a and through hole 210 can be increased compared to the other joints 300b and 300c, thereby reducing temperature variations during soldering among joints 300a to 300c.

[0035] Furthermore, in terminal 110, slit 114 can be formed simultaneously with flat plate portion 111 and protrusion portion 112 by press working. Therefore, it is possible to manufacture capacitor 100 having terminal 110 that can be used in electronic device 10 of this embodiment without increasing costs.

[0036] 2 is an explanatory diagram of the soldering process of the electronic device 10 according to the first embodiment of the present invention. In FIG. 2, (A) shows the solder piece supplying process in the soldering process, (B) shows the solder piece melting process in the soldering process, and (C) shows the state of the joint 300 after the soldering process. These figures each show the state of the X-X' cross section in FIG.

[0037] The printed wiring board 200 includes a plurality of wiring layers. In this embodiment, the printed wiring board 200 includes four wiring layers 221 to 224 as shown in FIG. 2A, but the number of wiring layers is not limited to this. In the printed wiring board 200, through holes 210 are formed so as to penetrate the wiring layers 221 to 224. The wiring layers 221 to 224 are insulated from each other by insulating layers 225, and the wiring layers 221 and 224 arranged on the front and back surfaces of the printed wiring board 200 are covered with resist 212.

[0038] 2A, in the solder piece supply process, nozzle 400 heated by a heater is placed in contact with wiring layer 221 (resist 212) arranged on the surface of printed wiring board 200. Nozzle 400 has three solder supply holes 410a-410c at positions corresponding to protrusions 112a-112c, respectively. The inner diameters of solder supply holes 410a-410c are set smaller than lands 211 arranged around through-hole 210, and preferably are approximately the same as the inner diameter of through-hole 210.

[0039] 2A, nozzle 400 is pressed against wiring layer 221 for a certain period of time. Then, with protrusions 112a-112c inserted into through-holes 210, solder pieces 420 are simultaneously inserted into solder supply holes 410a-410c. As a result, solder pieces 420 are supplied around each of through-holes 210 corresponding to solder supply holes 410a-410c.

[0040] Next, as shown in Fig. 2(B), in the solder piece melting process, the solder piece 420 is melted by the heat of the nozzle 400 and penetrates into the inside of the through-hole 210. Through this process, as shown in Fig. 2(C), the protrusions 112a-112c are completely solder-joined to the wiring layers 221-224 and the land 211 around the through-hole 210, and joints 300a-300c having solder fillets are formed. Note that the joints 300a-300c are made of the same material as the solder piece 420, and the finished state of the solder can be evaluated based on the shape of the solder fillet, etc.

[0041] 3 is an explanatory diagram of the variation in the fillet shape of the joint 300. The variation in the fillet shape of the joints 300a to 300c will be described below with reference to FIG. 3. Note that FIG. 3 is the same as the cross-sectional view of the joint 300 after the soldering process shown in FIG. 2(C).

[0042] 3, line YY' in the drawing indicates the surface of wiring layer 224 disposed on the back side of printed wiring board 200. Furthermore, the intersections of line YY' and the center lines of protrusions 112a to 112c are indicated by reference numerals 115a to 115c, respectively.

[0043] During soldering, the areas near the wiring layer 224 and the intersections 115a-115c of the protrusions 112a-112c are away from the contact surface of the printed wiring board 200 with the nozzle 400, and therefore have a lower temperature than the wiring layer 221 arranged on the front side of the printed wiring board 200. In electronic devices 10 used in in-vehicle inverters, etc., the wiring layers 221-224 are formed relatively thick (e.g., 0.1 mm or more) on the printed wiring board 200 to accommodate large currents. Therefore, compared to the wiring layer 221, the temperature drop at the wiring layer 224 and the intersections 115a-115c arranged on the back side of the printed wiring board 200 is more significant. In particular, because the nozzle 400 does not directly contact the protrusions 112a-112c, the temperature at the intersections 115a-115c is less likely to rise than at the wiring layer 224. On the other hand, in order for the solder that has penetrated into the inside of through-hole 210 at joints 300a to 300c to wet and spread around protrusions 112a to 112c on the back surface of printed wiring board 200, the temperature of protrusions 112a to 112c must be at or above the melting point of the solder up to the vicinity of intersections 115a to 115c. Therefore, variations in the temperature of intersections 115a to 115c will lead to variations in the shape of the solder fillets on the back surface of printed wiring board 200 at joints 300a to 300c.

[0044] Here, the thermal resistance of the joints 300a to 300c is defined as the temperature difference between the terminal ends 113 and the intersections 115a to 115c of the protrusions 112a to 112c of the terminals 110 during soldering, divided by the amount of heat input from the nozzle 400 to the electronic device 10. If it is difficult to experimentally measure the temperatures of the intersections 115a to 115c, the thermal resistance of the joints 300a to 300c can also be evaluated by computer simulation.

[0045] In this embodiment, the aforementioned slits 114a-114c are provided in the terminal 110 corresponding to the protrusions 112a-112c, respectively. If the lengths from the tips of the protrusions 112a-112c to the bottoms of the slits 114a-114c (slit depths) are Ia, Ib, and Ic, respectively, the shapes and dimensions of the slits 114a-114c are determined so that the relationship Ia > Ib > Ic is satisfied. This reduces the thermal resistance difference between the joints 300a-300c, suppresses temperature variations at the intersections 115a-115c, and ultimately reduces variations in the solder fillet shapes at the joints 300a-300c. This makes it possible to manufacture a highly reliable electronic device 10.

[0046] The reduction of the difference in thermal resistance between the joints 300a to 300c by the slits 114a to 114c will be described below with reference to FIG.

[0047] FIG. 4 is a diagram illustrating the flow of heat in terminal 110 during the soldering process of electronic device 10 according to the first embodiment of the present invention. In the soldering process described with reference to FIG. 2 , heat from nozzle 400 is transferred directly from nozzle 400 or from nozzle 400 through printed wiring board 200 to liquid solder 421 generated by melting solder piece 420 in FIG. 2 , and then transferred via liquid solder 421 to protrusions 112a-112c. This heat is transferred from protrusions 112a-112c to terminal 110 through flat portions 111a and 111b to end portion 113, and then flows from end portion 113 to main body 120 of capacitor 100. Note that a portion of the heat transferred from nozzle 400 to printed wiring board 200 diffuses without being transferred to protrusions 112a-112c, as indicated by arrows 520.

[0048] The inventors of the present invention conducted a computer simulation to analyze the heat flow from protrusions 112a to 112c to end portion 113 in terminal 110. As a result, they found that when slits 114a to 114c are not formed in terminal 110 and terminal 110 has a structure as described in the comparative example, the heat flux flowing in from protrusions 112a to 112c changes direction suddenly at the connection with flat plate portion 111a, as shown by arrows 500a to 500c in Fig. 4, respectively, and then passes through flat plate portions 111a and 111b to continue to end portion 113.

[0049] In contrast, when slits 114a-114c are provided in terminal 110 as in this embodiment, these slits divert the heat flow from protrusions 112a and 112b. As a result, the heat flow from protrusions 112a and 112b through flat plate portion 111a toward flat plate portion 111b and end portion 113 changes from arrows 500a and 500b to arrows 510a and 510b, respectively. On the other hand, as described above, since slit 114c is substantially the same as in the comparative example, the heat flow from protrusion 112c through flat plate portion 111a toward flat plate portion 111b and end portion 113 remains unchanged from arrow 500c. Due to these changes in heat flux, the length of the thermal path as seen from junctions 300a and 300b corresponding to protrusions 112a and 112b increases compared to the comparative example, increasing the thermal resistance of junctions 300a and 300b. Although the thermal resistance of the joint 300c is also slightly affected by the slits 114a and 114b, the heat flow flowing in from the protrusion 112c spreads toward the protrusion 112b, so the degree of influence of these slits is smaller than that of the joints 300a and 300b.

[0050] Here, the slit depths la to lc of the slits 114a to 114c are set so that the relationship la > lb > lc is satisfied, as described above. Because the greater the slit depth, the greater the thermal resistance compared to a state without slits, the rate of increase in thermal resistance of the junctions 300a to 300c due to slits 114a to 114c is in the order 300a > 300b > 300c. On the other hand, when there are no slits, as in the comparative example, the magnitude of the thermal resistance of the junctions 300a to 300c is determined by the path length of heat transmitted from the protrusions 112a to 112c through the flat portions 111a and 111b to the end portion 113, so the order is 300a < 300b < 300c. From these relationships, it can be seen that providing slits 114a to 114c in the terminal 110 can reduce the difference in thermal resistance of the junctions 300a to 300c. That is, the slits 114 a to 114 c function as thermal resistance difference reducing portions that reduce the difference in thermal resistance between the protrusions 112 a to 112 c and the main body 120 of the capacitor 100 .

[0051] To reduce the thermal resistance difference between the junctions 300a-300c using the above-described mechanism, it is desirable to provide the slits 114a-114c corresponding to the protrusions 112a-112c, respectively, closer to the main body 120 of the capacitor 100 along the extension direction of the terminal 110 than the connection between the protrusions 112a-112c and the flat portion 111a, i.e., closer to the flat portion 111b and the end portion 113. In this way, the flow of heat flowing in from the protrusions 112a-112c is diverted by the slits 114a-114c, respectively, thereby increasing the thermal resistance of the junctions 300a-300c at an increasing rate corresponding to the depth of the slits 114a-114c. Therefore, by setting the relationship between the slit depths of the slits 114a-114c to be the opposite of that when there are no slits, it is possible to reduce the thermal resistance difference between the junctions 300a-300c as described above.

[0052] Furthermore, of the protrusions 112a-112c, the protrusion 112a is closest to the main body 120 of the capacitor 100 along the extension direction of the terminal 110. That is, of the thermal resistances of the joints 300a-300c corresponding to the protrusions 112a-112c, the thermal resistance of the joint 300a is the smallest. Therefore, by providing the slit 114a for at least the protrusion 112a among the slits 114a-114c, it is possible to reduce the difference in thermal resistance between the joints 300a-300c. In this case, the slit 114a functions as a thermal resistance increasing portion that increases the thermal resistance between the protrusion 112a and the main body 120 of the capacitor 100. In order to further reduce the difference in thermal resistance between the joints 300a to 300c, it is desirable to provide slits 114b and 114c in the protrusions 112b and 112c, respectively, and gradually shorten the lengths of these slits from the protrusion 112a side toward the protrusion 112c side, so that the relationship la>lb>lc holds.

[0053] It should be noted that the thermal resistance difference reducing portion and the thermal resistance increasing portion may be provided in the terminal 110 by a method other than forming the slits 114a to 114c. Figure 5 is a diagram showing modified examples of the thermal resistance difference reducing portion and the thermal resistance increasing portion in the first embodiment of the present invention.

[0054] 4, it can be seen that in order to increase the thermal resistance by lengthening the heat dissipation path of junction 300, which has a small thermal resistance, it is effective to provide slit 114 on the capacitor 100 side along the extension direction of terminal 110 at the connection between protrusion 112 corresponding to junction 300 and flat plate portion 111a. Therefore, for example, as shown in FIG. 5A, slits 114a1 and 114b1 may be provided in protrusions 112a and 112b, and slit 114b1 may be formed only in the vicinity of protrusion 112b, not extending to the connection with protrusion 112a.

[0055] 5(B), for example, in order to suppress the flow of heat from the protrusions 112a and 112b indicated by arrows 500a and 500b in FIG. 4, holes 114a2 and 114b2 may be provided through the flat plate portion 111a instead of the slits. In this case, the diameters of the holes 114a2 and 114b2 are set to 114a2 > 114b2 so that the diameters become smaller the further away from the main body portion 120 in the extension direction of the terminal 110. Even in this case, the same effect as when the slits 114a and 114b are provided can be obtained.

[0056] Alternatively, the difference in thermal resistance between the joints 300a to 300c may be reduced by reducing the thermal resistance between the joints 300b and 300c rather than increasing the thermal resistance between the joints 300a and 300b. For example, the protrusions 112b and 112c may be provided with padding 114b3 and 114c3, respectively, so that the heat flow from the protrusions 112b and 112c passes through the padding 114b3 and 114c3, thereby shortening the heat dissipation path. In other words, by providing the padding 114b3 and 114c3 on the flat plate portion 111a, the rate of reduction in thermal resistance between the joints 300a to 300c may be in the order of 300a < 300b < 300c, thereby reducing the difference in thermal resistance between the joints 300a to 300c.

[0057] 4 and 5 to be selected as the thermal resistance difference reducing portion or the thermal resistance increasing portion can be flexibly determined depending on the desired functions and performance (e.g., inductance) of the electronic device 10.

[0058] Second Embodiment Fig. 6 is an external view of an electronic device 10A according to a second embodiment of the present invention. Fig. 6 shows the external appearance of a capacitor 100 and a printed wiring board 200 included in the electronic device 10A of this embodiment. In Fig. 6, (A) shows a top view of the region of the printed wiring board 200 to be bonded to the capacitor 100, and (B) is a front view showing the positional relationship between the capacitor 100 and the printed wiring board 200 in the electronic device 10A after soldering. Note that in the following description, portions that overlap with the first embodiment will be omitted.

[0059] The second embodiment of the present invention is characterized in that the thermal resistance difference reducing portion and the thermal resistance increasing portion described in the first embodiment are provided on the printed wiring board 200 side, rather than on the terminal 110 of the capacitor 100. This makes it possible to obtain the same effect as in the first embodiment, even without providing the slit 114 in the terminal 110.

[0060] Specifically, in this embodiment, as shown in FIG. 6A, thermal cut-off portions 230a and 230b that suppress heat conduction are arranged around through-hole 210 in printed wiring board 200. Thermal cut-off portions 230a and 230b are provided in each of wiring layers 221 to 224 to which through-hole 210 is connected in printed wiring board 200. This allows thermal cut-off portions 230a and 230b to function as thermal resistance difference reducing portions and thermal resistance increasing portions, thereby reducing the thermal resistance difference between bonding portions 300a to 300c. This point will be described below.

[0061] As described in the first embodiment, a portion of the heat transferred from the nozzle 400 to the printed wiring board 200 diffuses without being transferred to the protrusions 112a-112c, as indicated by arrows 520 in FIG. 4 . This heat diffusion occurs in each of the wiring layers 221-224 of the printed wiring board 200 not only in the x direction shown in FIG. 4 but also in the y direction. Therefore, in this embodiment, thermal cutouts 230a and 230b are provided around the through holes 210 into which the protrusions 112a and 112b are inserted, respectively, in the printed wiring board 200. This suppresses heat diffusion around these through holes 210 and enables heat to be efficiently concentrated at the joints 300a and 300b. As a result, the temperatures of the intersections 115a and 115b in FIG. 3 described in the first embodiment also rise in the protrusions 112a and 112b. 6A, by dividing the thermal cutting portions 230a and 230b into multiple portions and making the spacing da of the thermal cutting portion 230a narrower than the spacing db of the thermal cutting portion 230b, it is possible to make the temperature rise at the intersection 115a greater than that at the intersection 115b. As a result, the thermal cutting portions 230a and 230b function as thermal resistance difference reducing portions and thermal resistance increasing portions, thereby reducing the thermal resistance difference between the junctions 300a to 300c.

[0062] 7 is an explanatory diagram of the cross-sectional structure of a printed wiring board 200 according to a second embodiment of the present invention. Fig. 7 shows the X-X' cross section of Fig. 6. Strictly speaking, in this cross section, the wiring layers 221 to 224 are not present at the positions of the thermal-cutting portions 230a and 230b. However, in order to visualize the positional relationship with the thermal-cutting portions 230a and 230b, Fig. 7 shows the positions of the thermal-cutting portions 230a and 230b, which are present in the y direction of the paper, projected onto the wiring layers 221 to 224.

[0063] In the example of Figure 7, thermal cut-off portions 230a and 230b are provided for all of the wiring layers 221 to 224. However, if any of the wiring layers 221 to 224 has a large area in the xy plane and therefore relatively small thermal diffusion, a thermal cut-off portion need not be provided for that wiring layer. Also, in Figure 7, the thermal cut-off portions 230a and 230b are provided at the same x-coordinate position for all of the wiring layers 221 to 224, but they do not have to be at the same position. Furthermore, in order to increase the temperature on the back side of the printed wiring board 200 and facilitate the formation of the joint 300, it is desirable to make the distance between the protrusion 112 and the thermal cut-off portion 230 (and thus the distances da and db in Figure 6) smaller for the wiring layer 224, which is located closest to the back side of the wiring layers 221 to 224, than for the other wiring layers.

[0064] In addition, in the electronic device 10A of this embodiment, the joint 300 can be formed by soldering each protrusion 112 to the through hole 210 using a soldering process similar to that described in Figure 2 in the first embodiment.

[0065] In the first and second embodiments of the present invention described above, examples have been described in which the slits 114 and the thermal cut-off portions 230 that function as a thermal resistance difference reducing portion and a thermal resistance increasing portion are separately provided in the terminals 110 and the printed wiring board 200, respectively, but these may also be provided together. That is, the terminals 110 having the slits 114 described in the first embodiment or the modifications described in Fig. 5 formed therein and the printed wiring board 200 having the thermal cut-off portion 230 described in the second embodiment formed therein may be used in combination in one electronic device.

[0066] According to the embodiment of the present invention described above, the following advantageous effects are achieved.

[0067] (1) Electronic device 10, 10A is configured by soldering terminal 110 of capacitor 100, which is an electronic component, to printed wiring board 200 having a plurality of through holes 210. Terminal 110 has flat plate portion 111 and a plurality of protrusions 112 branching from flat plate portion 111 toward each of the plurality of through holes 210. The plurality of protrusions 112 are respectively soldered to the plurality of through holes 210, and at least one of terminal 110 and printed wiring board 200 is provided with a thermal resistance difference reducing portion that reduces the difference in thermal resistance between each of the plurality of protrusions 112 and main body portion 120 of capacitor 100, or a thermal resistance increasing portion that increases the thermal resistance between a first protrusion (protrusion 112 a) of the plurality of protrusions 112 that is closest to main body portion 120 of capacitor 100 along the extension direction of terminal 110 and main body portion 120. This makes it possible to suppress variations in the shape of the solder fillet at the joint 300 between each protrusion 112 and through-hole 210, thereby realizing an electronic device that is both highly productive and highly reliable.

[0068] (2) The thermal resistance difference reducing portion or the thermal resistance increasing portion is formed by partially removing material from the flat plate portion 111a between the protrusion 112 and the main body portion 120. Specifically, the thermal resistance difference reducing portion or the thermal resistance increasing portion is a slit 114 formed by cutting out a portion of the flat plate portion 111a along the extension direction of the protrusion 112. Alternatively, the thermal resistance difference reducing portion or the thermal resistance increasing portion is a hole 114a2, 114b2 penetrating the flat plate portion 111a. Alternatively, the thermal resistance difference reducing portion or the thermal resistance increasing portion is formed by disposing a thermal cut portion 230 that suppresses heat conduction around the through hole 210. This configuration allows the thermal resistance difference reducing portion or the thermal resistance increasing portion to be provided in the terminal 110 or the printed wiring board 200 without significantly modifying their structures.

[0069] (3) In a method for soldering a terminal 110 of a capacitor 100, which is an electronic component, to a printed wiring board 200 having a plurality of through holes 210, at least one of the terminal 110 and the printed wiring board 200 is provided with a thermal resistance difference reducing portion that reduces the difference in thermal resistance between each of the plurality of protrusions 112 and the main body 120 of the capacitor 100, or a thermal resistance increasing portion that increases the thermal resistance between a first protrusion (protrusion 112a) of the plurality of protrusions 112 that is closest to the main body 120 of the capacitor 100 along the extension direction of the terminal 110 and the main body 120. Then, with the plurality of protrusions 112 inserted into the plurality of through holes 210, solder pieces 420 are supplied around the plurality of through holes 210 ( FIG. 2(A) ), and the solder pieces 420 are heated and melted in each of the plurality of through holes 210 ( FIG. 2(B) ), thereby soldering the printed wiring board 200 and the terminal 110 ( FIG. 2(C) ). This makes it possible to suppress variations in the solder fillet shape at the joint 300 between each protrusion 112 and through hole 210, thereby realizing a soldering method for electronic devices that can achieve both high productivity and high reliability.

[0070] In the above embodiment, the flat portion 111 of the terminal 110 is bent at approximately 90° midway to separate it into the flat portion 111a and the flat portion 111b, i.e., the terminal 110 has an L-shape. However, the present invention is not limited to this. The present invention can be applied to terminals 110 of any shape as long as a plurality of joints 300 are formed in the terminal 110 by soldering and the thermal resistance of each joint 300 differs.

[0071] Furthermore, the present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

[0072] The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention.

[0073] 10, 10A, 10C: Electronic device 100, 100C: Electronic component (capacitor) 110: Terminal of electronic component 120: Main body of electronic component 121: Resin 122: Case 111, 111a, 111b: Flat plate portion 112: Protruding portion 113: End portion (thermal resistance definition point) 114: Thermal resistance difference reducing portion, thermal resistance increasing portion (slit, hole) 115: Intersection of the wiring layer on the back side of the printed wiring board and the center line of the protruding portion (thermal resistance definition point) 200, 200C: Printed wiring board 210: Through hole 211: Land 212: Resist 221: Wiring layer (surface of printed wiring board) 222: Wiring layer 223: Wiring layer 224: Wiring layer (back side of printed wiring board) 225: Insulating layer 230: Thermal resistance difference reducing portion, thermal resistance increasing portion (thermal cut portion) 300: Joint 400: Nozzle (soldering iron) 410: Solder supply hole of nozzle 420: Solder piece

Claims

1. An electronic device in which terminals of electronic components are mounted by soldering on a printed wiring board having a plurality of through holes, wherein the terminals have a flat plate portion and a plurality of protrusions branching from the flat portion toward each of the plurality of through holes, and the plurality of protrusions are respectively joined to the plurality of through holes by soldering, and at least one of the terminals and the printed wiring board is provided with a thermal resistance difference reducing portion that reduces the difference in thermal resistance between each of the plurality of protrusions and a main body portion of the electronic component.

2. An electronic device in which terminals of electronic components are mounted by soldering on a printed wiring board having a plurality of through holes, wherein the terminals have a flat plate portion and a plurality of protrusions branching from the flat portion toward each of the plurality of through holes, and the plurality of protrusions are respectively joined to the plurality of through holes by soldering, and at least one of the terminal and the printed wiring board is provided with a thermal resistance increasing portion that increases the thermal resistance between a first protrusion of the plurality of protrusions that is closest to the main body portion of the electronic component along the extension direction of the terminal and the main body portion.

3. An electronic device according to claim 1 or claim 2, wherein the thermal resistance difference reducing portion or the thermal resistance increasing portion is formed by partially removing material from the flat plate portion between the protrusion and the main body portion.

4. An electronic device according to claim 3, wherein the thermal resistance difference reducing portion or the thermal resistance increasing portion is a slit formed by cutting out a part of the flat plate portion along the extension direction of the protrusion portion.

5. An electronic device according to claim 4, wherein a plurality of slits are formed in the flat plate portion so that the length from the printed wiring board to the bottom of the slit becomes shorter the further away from the main body portion in the extension direction of the terminals.

6. An electronic device according to claim 3, wherein the thermal resistance difference reducing portion or the thermal resistance increasing portion is a hole penetrating the flat plate portion.

7. An electronic device according to claim 6, wherein a plurality of holes are formed in the flat plate portion so that the diameter of the holes decreases with increasing distance from the main body portion along the extension direction of the terminals.

8. An electronic device according to claim 1 or claim 2, wherein the thermal resistance difference reducing portion or the thermal resistance increasing portion is formed by arranging a thermal cut portion that suppresses heat conduction around the through hole.

9. An electronic device as claimed in claim 8, wherein a plurality of said thermal cut-off portions are respectively arranged around at least two of said plurality of through-holes so that the distance between said thermal cut-off portions increases as the distance between said thermal cut-off portions increases from the main body portion along the extension direction of said terminal to the protrusion portion of said through-hole.

10. A method for soldering a terminal of an electronic component to a printed wiring board having a plurality of through holes, wherein the terminal has a flat plate portion and a plurality of protrusions branching from the flat portion toward each of the plurality of through holes, and at least one of the terminal and the printed wiring board is provided with a thermal resistance difference reducing portion that reduces the difference in thermal resistance between each of the plurality of protrusions and a main body portion of the electronic component, and with the plurality of protrusions inserted into the plurality of through holes, solder pieces are supplied around the plurality of through holes, respectively, and the solder pieces are heated and melted in each of the plurality of through holes, thereby soldering the printed wiring board and the terminal.

11. A method for soldering a terminal of an electronic component to a printed wiring board having a plurality of through holes, wherein the terminal has a flat plate portion and a plurality of protrusions branching from the flat portion toward each of the plurality of through holes, and at least one of the terminal and the printed wiring board is provided with a thermal resistance increasing portion that increases the thermal resistance between a first protrusion of the plurality of protrusions that is closest to a main body portion of the electronic component along the extension direction of the terminal and the main body portion, and with the plurality of protrusions inserted into the plurality of through holes respectively, solder pieces are supplied around the plurality of through holes, and the solder pieces are heated and melted in each of the plurality of through holes, thereby soldering the printed wiring board and the terminal.

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