JPH04245460A - Package for electronic parts and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a package for housing electronic parts in which electronic parts can be easily mounted and which is less in the number of used parts and can be easily assembled. CONSTITUTION:A package main body 100 made of a ceramic material and provided with side walls 12a and 12b standing in the left and right middle sections on the upper surface of a bottom plate 10 for mounting electronic parts 40 is formed. At the same time, a cap 200 which covers the top of the bottom plate 10 between the side walls 12a and 12b is formed by bending a band plate to a U-shape. Since the front and rear sides of the bottom plate 10 are opened, the electronic parts 40 can be easily mounted on the plate 10. Since the cap 200 also acts as part of the side wall of the package main body 100, the number of parts used for the package can be reduced. When the side edges of the cap 200 are locked to the end faces, etc., of the side walls 12a and 12b, the cap 200 can be positioned to and supported by the package main body 100.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic component package for housing electronic components such as semiconductor chips, and a method for manufacturing the same.

0002

2. Description of the Related Art Conventionally, there are the following three types of packages as the above-mentioned packages. The first type of package is
It consists of a rectangular frame-shaped side wall made of a metal member, a bottom plate made of a metal member that covers the bottom surface of a cavity for housing electronic components inside the side wall, and a cap that covers the top surface of the cavity inside the side wall. It has a structure in which a ceramic terminal with a connection line is sealed in the notch. 2
This type of package has a structure in which the side wall and bottom plate of the above-mentioned type 1 package are integrally formed by cutting out a metal member, and ceramic terminals are sealed in the notches provided in the side wall. are doing. The package of type 3 has a structure in which the side wall and bottom plate are integrally molded using ceramic in the package of type 1 above, and a connection line is provided on the top surface of the bottom plate passing directly under the side wall.

0003

However, in the package of type 1 above, the side walls and the bottom plate are separate and independent, and the number of parts is large, requiring a great deal of effort and time to assemble. Further, in the above-mentioned type 2 package, it took a great deal of effort and time to integrally form the side wall and the bottom plate made of the metal member by cutting out the same. Furthermore, in the package of type 3 above, the process of integrally molding the side walls and bottom plate made of ceramic is complicated, and the molding process requires a great deal of effort and time.

The present invention has been made to solve these problems, and provides a versatile and inexpensive package for electronic components (hereinafter referred to as a package) that has a small number of parts, is easy to assemble, and is easy to assemble. , a manufacturing method for an electronic component package for manufacturing it (hereinafter referred to as the manufacturing method)
is intended to provide.

[0005]

[Means for Solving the Problems] In order to achieve the above object, a first package of the present invention includes side walls made of ceramic in the middle portions of the upper surface of the bottom plate made of ceramic on which electronic components are mounted. a package body, the front and rear end surfaces of which are positioned on the same plane as the front and rear end surfaces of the bottom plate, each standing upright, and a connection line passing through directly under at least a portion of the side walls of the bottom plate and provided with a connection line on the top surface of the bottom plate; The cap is formed by bending a band plate into a substantially U-shape, and the left and right side edges bent into a substantially U-shape and the front and rear edges thereof are the upper end surface of the side wall, the front and rear end surfaces thereof,
It is also characterized by comprising caps that are sealed to the front and rear end surfaces of the bottom plate and cover the top of the bottom plate inside the side wall.

[0006] The manufacturing method of the present invention involves standing up and stacking ceramic green sheets in the form of long thin strips in the front and back on the left and right midpoints of the upper surface of the ceramic green sheets in the form of long thick strips in the front and back, and stacking them. a package body forming member comprising a metallized line for firing a connection line is provided on the upper surface of the thick band-shaped ceramic green sheet passing directly under at least a portion of the thin band-shaped ceramic green sheet; The method is characterized in that the package body forming member is traversed to a predetermined width to form a package body forming body for firing the package body.

[0007] The second package of the present invention has side walls made of ceramic at the left and right middle parts of the upper surface of the bottom plate made of ceramic on which electronic components are mounted, and at the middle part of the rear part thereof, and the front end surfaces of the side walls at the left and right middle parts of the top surface of the ceramic bottom plate on which electronic components are mounted. A package main body, which is located on the same plane as the front end surface of the bottom plate, each of which stands upright continuously, and has a connection line on the top surface of the bottom plate passing directly under at least a part of the side wall thereof; and a strip plate. The cap is formed by bending the left and right edges into a substantially L-shape, and the left and right side edges bent into the substantially L-shape and the front and rear edges thereof are the upper end surface of the side wall, the front end surface thereof, and the front end surface of the bottom plate. and a cap that is sealed to the bottom plate and covers the top of the bottom plate inside the side wall.

[0008]

[Operation] In the first package configured as described above, the left and right side edges of the cap bent into a substantially U-shape and their front and rear edges contact the upper end surface of the side wall, its front and rear end surfaces, and the front and rear end surfaces of the bottom plate, respectively. The caps, which are locked and sealed to the end faces thereof, can be positioned and supported above the bottom plate housing the electronic components inside the side wall so as not to move so as to cover the top of the bottom plate.

In the above manufacturing method, a large and small package body for baking the package body is provided with a large number or a small number of metallized lines for baking the connecting line across the package body forming member in various sizes and widths. Forming bodies can be formed freely.

[0010] In the second package having the above configuration,
A cap in which the left and right side edges bent into an approximately L-shape and the front side edges of the cap are brought into contact with and locked to the upper end surface of the side wall, the front end surface thereof, and the front end surface of the bottom plate, respectively, and are sealed to those end surfaces. can be positioned and supported so as not to move above the bottom plate that accommodates the electronic components inside the side wall so as to cover the top of the bottom plate.

[0011] Furthermore, in the first and second packages configured as described above, there is no side wall in the front and rear portions of the bottom plate or in the front portion of the bottom plate, and these portions are wide open.
Electronic components such as semiconductor chips can be easily mounted on the top surface of the bottom plate without being obstructed by the side walls.

[0012] In addition, the side wall and the bottom plate are integrally molded, and a part of the cap also serves as the side wall.
It has a small number of parts and can be easily assembled.

Furthermore, the cap that covers the upper part of the package is connected to the metallized layer provided on the back surface of the bottom plate through the cap sealing metallized layer provided on the end surface of the bottom plate and the end surface of the side wall that seals the side edges of the cap, and is grounded. However, almost the same electrical effect as that of a metal package in which the side walls and the bottom plate are formed by cutting out a metal member and the upper surface thereof is covered with a cap made of a metal member can be achieved.

[0014]

Embodiments Next, embodiments of the present invention will be described with reference to the drawings. 1 and 2 show a preferred embodiment of the first package of the present invention, with FIG. 1 showing an exploded perspective view thereof, and FIG. 2 showing a perspective view thereof. The embodiment shown in this figure will be described below.

In the figure, reference numeral 10 denotes a bottom plate made of ceramic such as alumina ceramic and having a band shape on which electronic components such as semiconductor chips are mounted.

On the upper surface of the left and right midway portions of the bottom plate 10, there are side walls 12a and 12b made of ceramic in the shape of narrow strips.
They are provided so as to cross the top surface of the bottom plate 10 and stand up integrally on the top surface of the bottom plate 10, respectively. And those side walls 12a,
The front and rear end surfaces of the side walls 12b are located on the same plane as the front and rear end surfaces of the bottom plate, and the front and rear end surfaces of the side walls 12a and 12b are connected to the front and rear end surfaces of the bottom plate 10 in a planar manner, respectively.

On the left and right upper surfaces of the bottom plate 10, connection lines 20 made of strip-shaped tungsten metallization are passed through directly under the side walls 12a, 12b, and connected to the side walls 12a, 12.
There are multiple pieces lined up inside and outside b.

The connection line 20 is connected to the side walls 12a and 12 according to the number of electrodes of electronic components mounted on the top surface of the bottom plate 10.
b, and may be provided side by side only on either one of the left and right upper surfaces of the bottom plate 10.

A metallized layer 30 made of tungsten or the like for cap sealing, which will be described later, is continuously provided on the upper end surfaces of the side walls 12a and 12b, their front and rear end surfaces, and the front and rear end surfaces of the bottom plate 10, respectively.

The bottom surface of the bottom plate 10 is widely provided with a metallized layer 32 made of tungsten or the like that is connected to the metallized layer 30 on the front and rear end surfaces of the bottom plate 10.

The metallized layer 32 on the lower surface of the bottom plate 10 and the metallized layer 30 on the upper end surfaces of the side walls 12a, 12b pass through directly under the side walls 12a, 12b and connect the connection line 20 provided on the top surface of the bottom plate 10 for transmitting high-frequency signals, etc. It is formed into a railway line or microstrip line.

A metallized layer 34 made of tungsten or the like for bonding electronic components such as semiconductor chips is provided at the center of the upper surface of the bottom plate 10 inside the side walls 12a and 12b.

Metallized layers 3 are provided on the upper end surfaces of the side walls 12a and 12b, the front and rear end surfaces thereof, the front and rear end surfaces of the bottom plate 10, the lower surface thereof, and the center of the top surface of the bottom plate 10, respectively.
0, 32, 34 and the connection line 20 are each plated (not shown) such as nickel plating, which easily wets the brazing material.

[0024] These bottom plate 10, side wall 12a,
12b, connection line 20, metallized layers 30, 32, 34
etc. form the package body 100.

[0025] Reference numeral 200 is a cap formed by bending a thin metal band plate into a substantially U-shape. The cap 200 has its left and right side edges bent into a substantially U-shape and its front and rear edges flat against the top and front and rear end surfaces of the left and right side walls 12a and 12b on the upper surface of the bottom plate 10, and the front and rear end surfaces of the side walls 12a and 12b. Abuts on the front and rear end surfaces of the bottom plate 10 which are connected in a shape, respectively,
Lock it and position it above the bottom plate 10 so that it does not move,
The bottom plate 1 inside the left and right side walls 12a, 12b with the cap 200
Its shape and size are determined so that it can cover the area above 0 without any gaps. And 200 caps
The left and right side edges bent into a substantially U-shape and the front and rear edges thereof are connected to the upper end surfaces of the left and right side walls 12a, 12b on the upper surface of the bottom plate 10, the front and rear end surfaces thereof, and the front and rear end surfaces of the bottom plate 10, and are continuous to those end surfaces. Metalized layer 30 with pre-plated plating
The caps 200 are sealed by brazing, respectively, so that the cap 200 can cover the upper part of the bottom plate 10 inside the left and right side walls 12a and 12b without any gap.

The first package shown in FIGS. 1 and 2 is constructed as described above.

Next, an example of how this first package is used will be explained. As shown in FIG. 1, an electronic component 40 such as a semiconductor chip is die-bonded to the metallized layer 34 at the center of the top surface of the bottom plate 10 of the package body 100, and the electrodes of the electronic component 40 are connected to the side wall using a wire 50 or the like. 12a,1
It is connected to the connection line 20 on the upper surface of the bottom plate 10 inside 2b by wire bonding or the like. Next, the left and right side edges of the cap 200 bent into a substantially U-shape and the front and rear edges thereof are brought into contact with the upper end surfaces and front and rear end surfaces of the left and right side walls 12a and 12b of the package body 100, and the front and rear end surfaces of the bottom plate 10, respectively. The cap 200 is positioned and supported above the bottom plate 10 by contacting and locking, and the cap 200 is attached to the left and right side walls 12a, 1.
The upper part of the bottom plate 10 inside 2b is covered without any gap. As shown in FIG. 2, the left and right side edges of the cap 200 and the front and rear side edges thereof are fixed to the left and right side walls 12.
The upper end surfaces of a and 12b, their front and rear end surfaces, and the bottom plate 1
The front and rear end surfaces of 0 are each brazed using a brazing material such as silver solder or solder via the metallized layer 30 provided continuously on those end surfaces. Then, the electronic component 40 such as a semiconductor chip can be hermetically housed in the space inside the package surrounded by the bottom plate 10, the left and right side walls 12a, 12b, and the cap 200.

[0028] In the above-mentioned first package,
When using the cap 200, the side walls 12a, 12
The upper end surface of b and its front and rear end surfaces, as well as the front and rear end surfaces of the bottom plate 10, may be sealed using an adhesive or the like, or the electronic component 40 may be bonded to the center of the top surface of the bottom plate 10 using an adhesive or the like. In such a case, it is no longer necessary to provide the metallized layers 30 and 34 at the center of the end surfaces and the top surface, respectively.

FIG. 3 shows a preferred embodiment of the method for manufacturing the first package according to the present invention, and specifically shows an explanatory diagram of the manufacturing process. The embodiment shown in this figure will be described below.

[0030] In the manufacturing method shown in the figure, a ceramic green sheet 10 for firing the bottom plate, which is in the shape of a thick band long in the front and back, is used.
00 and the ceramic green sheet 1000
Ceramic green sheets 1200a, 1200 for firing side walls are provided in the middle of the left and right sides of the upper surface in the form of long thin strips in the front and back.
b are stacked in an upright manner.

At the same time, a metallized line 2000 for connecting line firing, which is made by applying metallizing paste in a narrow strip shape and drying it, on the left and right upper surfaces of the ceramic green sheet 1000 for firing the bottom plate, is attached to the ceramic green sheet 1000 for firing the side wall. A plurality of ceramic green sheets 1200a, 1200b are arranged at a predetermined pitch inside and outside the ceramic green sheets 1200a, 1200b for side wall firing, passing directly under the green sheets 1200a, 1200b.

Note that the metallized line 2000 is made of ceramic green sheets 1200a, 1200 for sidewall firing.
They may pass directly under either one of b and be arranged side by side on only one of the left and right upper surfaces of the ceramic green sheet 1000 for firing the bottom plate.

Next, a package body forming member 3000 consisting of the ceramic green sheet 1000 for firing the bottom plate, the ceramic green sheets 1200a and 1200b for firing the side walls, and the metallized line 2000 for firing the connection line is placed at one point in FIG. As shown by chain lines A-A and B-B, a part of the package body forming member 3000 is cut out across a predetermined width, and a package body forming body 40 is cut out.
00 is formed.

Thereafter, the cut out package body forming body 4000 is integrally fired to form the package body 100 for the first package described above, following a conventional general ceramic package manufacturing method.

According to this manufacturing method, the package body forming body 4 is cut out from the package body forming member 3000.
The width of the package body 100 to be formed by integrally firing the cut out package body forming body 4000 is adjusted to the size of the electronic component to be mounted on the top surface of the bottom plate 10 of the package body. The size can be adjusted freely. At the same time, the cutting width of the package body forming body 4000 cut out from the package body forming member 3000 is adjusted to be large or small, and the connecting line for firing is provided on the top surface of the ceramic green sheet 1000 for firing the bottom plate of the cut out package body forming body 4000. The number of connection lines 20 provided on the top surface of the bottom plate 10 of the package body 100 formed by adjusting the size of the metallized lines 2000 and integrally firing the package body forming body 4000 is determined by adjusting the number of connection lines 20 provided on the top surface of the bottom plate 10. You can freely adjust the size according to the number.

4 to 6 show other preferred embodiments of the first package of the present invention, FIG. 4 is an exploded perspective view thereof, FIG. 5 is a perspective view thereof, and FIG. 6 is a part of the package body. A partially cutaway front view is shown. The embodiment shown in this figure will be described below.

In the package shown in the figure, as shown in FIG. 6, a plurality of via holes 70 filled with conductors 60 such as metallization, which vertically penetrate the bottom plate 10, are provided in the front and rear parts of the bottom plate 10 on which the electronic components 40 are mounted. They are arranged side by side. Pads 62 and 64 made of metallization or the like are provided on the upper surface and the lower surface of the bottom plate 10, respectively, and are connected to the conductor 60 filled in the via hole 70. Bottom plate 1
A linear lead 80 is connected to the pad 64 on the bottom surface of the bottom plate 1.
0 They are erected downward and connected by brazing or the like. The via holes 70 filled with these leads 80 and conductors 60 form a connection line 90 through which power supply current flows and low frequency signals are transmitted.

A metallized layer 32 made of tungsten or the like is widely provided on the lower surface of the bottom plate 10, except for the periphery of the bottom plate 10 provided with the pad 64 to which the lead 80 is connected.

The rest of the structure is the same as that of the first package described above, and the example of its use is that of the electrodes of the electronic component 40 die-bonded to the top surface of the bottom plate 10, for power supply, low frequency signal, etc. The package is the same as the first package described above except that the electrodes are connected to the pads 62 provided on the top surface of the bottom plate 10 using wires 50 or the like, and the same members are given the same reference numerals and their explanations will be given below. omitted.

7 and 8 show a preferred embodiment of the second package of the present invention, with FIG. 7 showing an exploded perspective view thereof and FIG. 8 showing a perspective view thereof. The embodiment shown in this figure will be described below.

In the package shown in the figure, side walls 12c, 12d, and 12e made of ceramic in the form of narrow strips are continuously erected in a U-shape at the left and right midpoints of the upper surface of the bottom plate 10a and at the midway back thereof. It is equipped with and,
The front end surfaces of the side walls 12c and 12e at the left and right middle portions are the bottom plate 10a.
The front end surfaces of the side walls 12c and 12e at the left and right middle portions are connected to the front end surface of the bottom plate 10a in a planar manner.

Tungsten or the like is plated (not shown) on the upper end surfaces of the side walls 12c, 12d, 12e, the front end surfaces of the side walls 12c, 12e, and the front end surface of the bottom plate 10a that extends in a planar manner to these front end surfaces. Each of the metallized layers 30 is continuously provided.

[0043] On the left and right sides of the bottom plate 10a and the rear upper surface thereof, there is a thin band-shaped tungsten metallized plate that passes directly under the side walls 12c, 12d, 12e and is plated (not shown) on the inside and outside of the side walls 12c, 12d, 12e. A plurality of connection lines 20 such as the like are arranged side by side.

The connection line 20 is connected to the side walls 20c, 2 according to the number of electrodes of the electronic components mounted on the top surface of the bottom plate 10a.
0d, 20e, and may be provided side by side on only one or two of the left and right sides of the bottom plate 10a and the rear upper surface thereof.

[0045] These bottom plate 10a and side wall 12c
, 12d, 12e, the metallized layer 30, the connection line 20, etc., form the package body 100a.

The cap 200a is formed by bending a thin metal strip into a substantially L-shape. cap 200
a has its left and right side edges bent into an approximately L-shape, and its front and rear edges thereof, abutting and engaging the upper end surfaces of the side walls 12c, 12d, and 12e, their front end surfaces, and the front end surface of the bottom plate 10a, respectively. position above the bottom plate 10a, and use the cap 200a to cover the bottom plate 1 inside the side walls 12c, 12d, 12e.
Its shape and size are determined so that it can cover the area above 0a without any gaps.

The rest of the structure is the same as that of the first package shown in FIGS. 1 and 2 described above, and the same members are given the same reference numerals and their explanations will be omitted.

Next, an example of how this second package is used will be explained. In the same manner as in the first package described above, as shown in FIG.
Mount on the top surface of 0a. Next, approximately L of the cap 200a
The left and right side edges bent in a letter shape and the front and rear side edges are
2c, 12d, 12e upper end surfaces and side walls 12c, 12
e and the front end surface of the bottom plate 10a to position and support the cap 200a above the bottom plate 10a.
, 12e so as to cover the upper part of the inner bottom plate 10a without any gap. As shown in FIG.
A brazing material such as silver solder or solder is applied to the upper end surface of the upper end surface of the side wall 12e, the front end surface of the side walls 12c and 12e, and the front end surface of the bottom plate 10a through a metallized layer 30 provided continuously on these end surfaces. braze. Then, the electronic component 40 such as a semiconductor chip is attached to the bottom plate 10a, the side wall 12c,
It can be airtightly housed in the space inside the package surrounded by the cap 200a and the cap 200a.

[0049]

As explained above, according to the first and second packages of the present invention, the bottom plate and the side wall are integrated, and a part of the cap is also used as the side wall.
It is possible to provide a versatile and inexpensive package with a small number of parts, which is easy to assemble.

[0050] Furthermore, without providing side walls on the front and rear parts of the top surface of the bottom plate on which electronic components such as semiconductor chips are mounted, or on the front part thereof, these parts are left wide open so that the top surface of the bottom plate can be covered without being obstructed by the side walls. By die-bonding electronic components, etc., or connecting the electrodes of the electronic components to the connection line on the top of the bottom plate,
It can be connected via wire etc.

At the same time, it is no longer necessary to provide side walls on the front and rear portions of the top surface of the bottom plate, or on the front portion thereof, making it easier and faster to form the package body made of ceramic.

Furthermore, the cap covering the upper part of the package is connected to the metallized layer provided on the back surface of the bottom plate, etc., through the cap sealing metallized layer provided on the end surface of the bottom plate and the end surface of the side wall, which seals the side edges of the cap. However, it is possible to provide almost the same electrical effect as a metal package in which the bottom plate and side walls are formed of metal members and the top surface thereof is covered with a cap made of metal members.

[0053] Also, the left and right side edges of the cap and the front and rear edges thereof, or the front side edges thereof, are brought into contact and locked with the upper end surface of the side wall and the front and rear end surfaces thereof, or the front end surface thereof, and the front and rear end surfaces of the bottom plate, or the front end surface thereof, respectively. Thus, the cap can be easily positioned and supported above the bottom plate without using a positioning jig or the like. Then, the peripheral side edges of the cap are sealed to the side wall and the peripheral end surface of the bottom plate by brazing or the like, so that the cap can accurately cover the upper part of the bottom plate on which electronic components are mounted without any gaps.

Further, according to the manufacturing method of the present invention, the width of the package body formed body cut out from the package body forming member is adjusted to be large or small, and the size of the package body formed by integrally firing the package body formed body is adjusted. The number of connection lines provided on the top surface of the bottom plate of the package body of the sheath can be freely adjusted in size according to the size of electronic components such as semiconductor chips accommodated in the package and the number of electrodes thereof.

[Brief explanation of the drawing]

FIG. 1 is an exploded perspective view of a first electronic component package of the present invention.

FIG. 2 is a perspective view of the first electronic component package of the present invention.

FIG. 3 is an explanatory diagram showing the first method of manufacturing an electronic component package of the present invention.

FIG. 4 is an exploded perspective view of the first electronic component package of the present invention.

FIG. 5 is a perspective view of the first electronic component package of the present invention.

FIG. 6 is a partially cutaway front view of the package body of the first electronic component package of the present invention.

FIG. 7 is an exploded perspective view of the second electronic component package of the present invention.

FIG. 8 is a perspective view of a second electronic component package of the present invention.

[Explanation of symbols]

10 Bottom plate 10a Bottom plate 12a Side wall 12b Side wall 12c Side wall 12d Side wall 12e Side wall 20 Connection line 30 Metallized layer 32 Metallized layer 34 Metallized layer 40 Electronic component 60 Conductor 70 Via hole 80 Lead 90 Connection line 100 Package body 100a Package body 200 Cap 200a Cap 1000 Ceramic green sheet 1200a shaped like a thick strip Ceramic green sheet 1200b shaped like a thin strip Ceramic green sheet 2000 shaped like a thin strip Metallized line 3000 Package body forming member 4000 Package body forming body

Claims (3)

[Claims] [Claim 1] A side wall made of ceramic is provided halfway between the left and right of the top surface of the bottom plate made of ceramic on which electronic components are mounted.
The front and rear end surfaces of those side walls are positioned on the same plane as the front and rear end surfaces of the bottom plate, and are respectively erected, and
The package body includes a connecting line passing directly under at least a portion of the side wall of the bottom plate and a connecting line on the upper surface of the bottom plate, and a cap formed by bending a band plate into a substantially U-shape. An electronic component package comprising a cap that covers the upper part of the bottom plate inside the side wall by sealing the curved left and right side edges and the front and rear edges thereof to the upper end surface of the side wall, the front and rear end surfaces thereof, and the front and rear end surfaces of the bottom plate. [Claim 2] Ceramic green sheets in the form of long thin strips are stood up and stacked on the left and right midpoints of the upper surface of the ceramic green sheets in the form of long thick strips in the front and back, and at least some of them are stacked. A package body forming member having a metallized line for firing a connecting line is provided on the upper surface of the thick band plate-shaped ceramic green sheet passing directly under the thin band plate-shaped ceramic green sheet, and forming the package body. 1. A method of manufacturing a package for electronic components, comprising: crossing a member to a predetermined width to form a package body forming body for firing the package body. 3. Ceramic side walls are provided at the left and right middle parts of the upper surface of the ceramic bottom plate on which electronic components are mounted, and at the rear part thereof, and the front end surfaces of the side walls at the left and right middle parts are flush with the front end surface of the bottom plate. A package main body, which is located on top of the main body and has a connecting line on the upper surface of the bottom plate passing through directly under at least a part of the side wall thereof, and a strip plate folded into a substantially L-shape. The cap is formed by bending the left and right side edges bent into an approximately L shape, and the front and rear edges thereof to be sealed to the upper end surface of the side wall, the front end surface thereof, and the front end surface of the bottom plate, respectively, to form a side wall. and a cap that covers above the bottom plate on the inside.