JP3945417B2 - Package structure of piezoelectric device and method for manufacturing piezoelectric device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface-mount type piezoelectric device such as a piezoelectric vibrator and a piezoelectric oscillator used in various electronic devices, and more particularly to a package structure for mounting a piezoelectric vibrating piece in an airtight manner, and its The present invention relates to a method for manufacturing such a piezoelectric device.
[0002]
[Prior art]
Piezoelectric devices, such as piezoelectric vibrators, piezoelectric oscillators and real-time clock modules, which have been widely used in various electronic devices, for example, as clock sources for electronic circuits, are becoming more and more important as electronic devices become smaller and thinner. In addition to demands for miniaturization and thinning, surface-mounting types suitable for mounting on circuit boards and the like are frequently used. In general, a surface-mount type piezoelectric device has a piezoelectric vibrating piece mounted on a box-type base having a structure in which sheet materials of insulating materials are laminated, as described in the following Patent Documents 1 and 2, etc. This is provided with a package which is hermetically sealed by bonding a lid thereto.
[0003]
FIG. 7 shows a typical example of a conventional surface-mount type piezoelectric vibrator in which an AT-cut quartz crystal vibrating piece 2 is sealed inside a package 1. The package 1 includes a box-type base 3 having a two-layer structure in which a rectangular plate-shaped sheet material 3a and a rectangular frame-shaped sheet material 3b made of a ceramic material are laminated, and a seal ring 4 is attached to the upper end of the box-type base 3 by, for example, seam welding. It is comprised from the metal lid | cover 5 joined. The quartz crystal vibrating piece 2 has excitation electrodes 6 formed on both surfaces of a rectangular quartz crystal thin plate, and the lead electrodes from the excitation electrodes 6 are aligned with the connection electrodes 7 provided on the surface of the base 3 at the base end portions thereof to conduct electricity. It is mounted in a cantilever manner with the adhesive 8. The connection electrode 7 is electrically connected to the external electrode 10 formed on the back surface of the base 3 through a lead 9 wired on the surface of the ceramic sheet 3a.
[0004]
Further, as described in Patent Documents 3 and 4 below, as a surface-mount type piezoelectric device, a structure in which a piezoelectric vibrating piece is mounted on a package formed of a conductive material such as a metal plate and hermetically sealed is used. Are known. The surface mount type piezoelectric vibrator described in Patent Document 3 is provided with a through hole in the bottom surface of a base formed from a metal plate in a dish shape, and a conductive material is filled by fitting a hollow insulating material therein. A piezoelectric piece is fixedly held to the exposed end of the base plate with a conductive adhesive, an electrode is led out to the outer plate surface of the base, and the base opening is hermetically sealed with a metal lid. Further, this document describes a configuration in which an insulating film is applied to a predetermined position on the outer surface and the inner side of the base, a conductive film is formed at the predetermined position, and the inner and outer conductive films are electrically connected. . Patent Document 4 describes a crystal resonator in which a crystal resonator element is hermetically sealed with a base formed from a conductive material and a lid, as in Patent Document 3, and the base is inserted into the crystal resonator. A hole is provided, and a terminal is inserted into the insertion hole by being electrically insulated from the base by an insulating portion, and a crystal vibrating piece is connected to the terminal and supported in a cantilever manner.
[0005]
[Patent Document 1]
JP 2002-9576 A [Patent Document 2]
JP 2002-135074 A [Patent Document 3]
JP-A-3-113907 [Patent Document 4]
JP-A-2002-280868 gazette
[Problems to be solved by the invention]
In the conventional piezoelectric device package described above with reference to FIG. 7, each ceramic sheet material constituting the base is usually formed by firing a green sheet. However, the green sheet has a property of contracting in the vertical and horizontal directions when fired, and the contraction rate is not necessarily constant, so that the dimensional accuracy of the base on which the sheet material is finally laminated may be lowered. Furthermore, as the piezoelectric device becomes smaller and thinner, it is more difficult to ensure the dimensional accuracy of the base, that is, the package and further improve it.
[0007]
In addition, the conventional metal packages described in Patent Documents 3 and 4 are electrically insulated and hermetically sealed with an insulating member such as glass between a hole provided in the base and a terminal passed through the hole. To do. For this reason, the airtightness between the base and the insulating member or between the insulating member and the terminal is impaired particularly by long-term use, and the reliability and life of the piezoelectric device may be reduced, and the performance may be degraded or completely impaired. Further, as the piezoelectric device becomes smaller and thinner, it becomes more difficult to ensure sufficient airtightness between the base and the insulating member and between the insulating member and the terminal at the time of manufacture.
[0008]
Accordingly, an object of the present invention is to further improve the dimensional accuracy of a surface-mount type piezoelectric device in which a piezoelectric vibrating piece is mounted in a package, while meeting the demand for downsizing and thinning. Another object of the present invention is to provide a package structure capable of ensuring and maintaining higher dimensional accuracy.
Another object of the present invention is to provide a method of manufacturing a piezoelectric device having a package structure capable of ensuring high dimensional accuracy and reliability while achieving a reduction in size and thickness.
[0009]
[Means for Solving the Problems]
According to the present invention, in order to achieve the above object, the base includes a base for mounting the piezoelectric vibrating piece, and a lid joined to the upper portion of the base to hermetically seal the piezoelectric vibrating piece inside. Is formed of a metal material and has an insulating layer whose surface is insulated. A connection electrode for connecting the piezoelectric vibrating piece on the insulating layer is provided on the back surface of the piezoelectric vibrating piece mounting surface of the base. External electrodes are formed on the insulating layer, the connection electrodes and the corresponding external electrodes are electrically connected via via holes formed in the base, and the via holes are hermetically sealed with a filling material. A package structure for a closed piezoelectric device is provided.
[0010]
Thus, even if the base made of a metal material is reduced in size and thickness, it can be processed into a desired outer shape with high dimensional accuracy, so that the dimensional accuracy of the package can be increased. In addition, since the insulating layer formed by the surface insulating treatment is integrated with the base material of the base, it is difficult to peel off, which is convenient. Further, since the via hole can be disposed at any appropriate position on the base surface, the degree of freedom in designing the connection electrode, the external electrode, and the wiring pattern connected to them is improved. Further, since the via hole can be used to seal the inside of the package after the lid is joined, it can be applied to both the thickness-shear mode piezoelectric vibrating piece and tuning fork type vibrating piece.
[0011]
The metal material forming the base is preferably aluminum from the viewpoint of processability, cost, etc. In that case, the insulating layer is formed of an anodized film formed by, for example, anodizing treatment, and its corrosion resistance and surface hardness. From the viewpoint of adhesion to the base material, etc. Further, when the metal material is aluminum, the insulating layer has a further enhanced corrosion resistance than the alumite because the tafram film formed by the Tafram (registered trademark) treatment impregnated with Teflon (registered trademark) after the anodization is performed. preferable. Further, when the insulating layer has a thickness of at least about 5 μm, sufficient insulation can be ensured.
[0012]
In one embodiment, the base is formed of a flat plate and the lid is formed in a box shape, and by joining them, a space for mounting the piezoelectric vibrating piece is defined therein. A flat base can be more easily manufactured.
[0013]
In another embodiment, the base has a box shape and the lid is formed of a flat plate, and a space for mounting the piezoelectric vibrating piece is defined inside the base. The flat base is easier to manufacture than the box-type lid, can reduce costs, and can reduce the welding margin for joining to the base, thereby further reducing the size of the package. The box shape of the base can be constituted by a structure in which a plurality of metal plates are laminated in the same manner as the base made of the conventional ceramic material described above with reference to FIG. Moreover, since the box-shaped shape of the base has good workability of the metal plate, it is possible to use an integral structure in which a recess is processed on one surface thereof.
[0014]
Further, according to the present invention, the metal plate is processed into the outer shape of the base of the piezoelectric device, two through holes are formed in the metal plate, the surface of the metal plate including the through holes is insulated, and the through hole is formed. A conductive material is formed on the inner surface of the metal plate to form a via hole, a connection electrode for connecting the piezoelectric vibrating piece to the piezoelectric vibrating piece mounting surface of the metal plate, and an external electrode on the back surface of the metal plate in a predetermined pattern, respectively. And forming the base by forming the corresponding connection electrode and the external electrode to be electrically connected via the via hole, and mounting the piezoelectric vibrating piece on the base, and covering the lid There is provided a manufacturing method including a process of joining and hermetically sealing, and a process of hermetically closing a via hole.
[0015]
The metal plate can be processed by, for example, etching or machining, and the base can obtain high dimensional accuracy. Therefore, the piezoelectric device can meet high demands for downsizing and thinning, and has high dimensional accuracy and low cost. Can be manufactured. Further, the arrangement of via holes is relatively free, and correspondingly, connection electrodes, external electrodes, and wiring patterns can be designed and formed more freely. Further, since the via hole is closed after the lid is joined to the base, the inside of the package can be hermetically sealed with an appropriate gas or vacuum atmosphere, and the piezoelectric device has a thickness-shear mode piezoelectric vibrating piece or tuning fork type. Any of the vibrating pieces can be mounted.
[0016]
In one embodiment, the metal plate is made of aluminum, and can be easily insulated by subjecting it to an anodizing process, and has a base excellent in corrosion resistance, surface hardness, adhesion to a base material, and the like. Can be formed. In another embodiment, similarly, the metal plate is made of aluminum, which can be easily insulated by a taffron treatment impregnated with Teflon after anodizing to form a base with higher corrosion resistance. Can do.
[0017]
In the manufacturing method of the present invention, the package of the piezoelectric device can be configured such that the base is flat and the lid is box-shaped, or the base is box-shaped and the lid is flat. In the case where the base is formed into a box shape, in some embodiments, the box shape can be formed by laminating a frame-shaped metal plate on the piezoelectric vibrating piece mounting surface of the metal plate. In another embodiment, in order to form the box shape, a recess is formed in the surface of the metal plate on the side where the connection electrode is to be formed, and then the metal plate is insulated.
[0018]
Further, in one embodiment, a plurality of bases can be formed on the metal plate at the same time, thereby enabling a reduction in size and thickness while ensuring high dimensional accuracy, and stabilizing a high-quality piezoelectric device. Can be produced in large quantities.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1A and 1B schematically show a preferred embodiment of a crystal resonator to which the present invention is applied. An AT-cut crystal resonator element 12 is hermetically sealed in a package 11. The package 11 includes a substantially rectangular flat base 13 formed of aluminum and a lid 14 bonded to the upper surface thereof. The lid 14 is formed in a box shape in which a space for accommodating the crystal vibrating piece 12 is defined, for example, by drawing aluminum.
[0020]
The base 13 is provided with two via holes 15a and 15b having a conductive film formed on the inner surface in the vicinity of one longitudinal end thereof. The surface of the base 13 is insulated in advance including the via hole portion. In this embodiment, after anodizing by anodizing treatment, a Tafram (registered trademark) treatment for impregnating Teflon (registered trademark) is performed, whereby an insulating layer 16 made of a tafram film is formed to a predetermined thickness. . This thickness is at least about 5 μm, preferably about 15 μm, to achieve the desired electrical insulation effect. Further, the base 13 can be subjected to a desired insulation process only by anodizing.
[0021]
A pair of connection electrodes 17a and 17b are formed on the quartz resonator-piece mounting surface of the base 13 in a pattern that is electrically connected to the corresponding via hole on the one longitudinal end portion side. As shown in FIG. 2 (A), a pair of external electrodes 18a and 18b are formed on the back surface of the base 13 in the vicinity of both ends in the longitudinal direction. The pair of external electrodes 18a and 18b are electrically connected by the corresponding via holes and leads 19a and 19b, respectively. It is connected to the. Thereby, the corresponding connection electrodes 17a and 17b and the external electrodes 18a and 18b are electrically connected to each other. These electrodes and leads are generally formed by patterning a metal wiring material such as W or Mo on the surface of the base 13 by screen printing and plating Ni or Au thereon.
[0022]
FIG. 2B shows a pattern of external electrodes 18a and 18b different from FIG. 2A. Both external electrodes are extended near the longitudinal end of the base 13 near the via holes 15a and 15b. Is provided. Thus, the external electrodes 18a and 18b can be designed and wired in various patterns on the back surface of the base 13 as required. Also, the via holes 15a and 15b can be arranged at positions different from those of the above-described embodiment corresponding to the external electrodes 18a and 18b and the wiring pattern, and correspondingly on the crystal resonator element mounting surface, the corresponding connection is made. Wiring can be performed so as to be connected to the electrodes 17a and 17b.
[0023]
FIG. 2 (C) shows a modification of FIG. 2 (A). The substantially entire back surface of the base 13 except for the vicinity of the end portions in the longitudinal direction where the external electrodes 18a and 18b are arranged is an appropriate insulating material. The resin is coated with the insulating film 20 by spray coating. As a result, most of the via holes 15a and 15b and the leads 19a and 19b are insulated, so that current leakage and short circuit can be prevented, and more stable operation of the crystal unit can be ensured.
[0024]
The quartz crystal vibrating piece 12 is formed of a rectangular quartz thin plate, and excitation electrodes 21 are formed on both sides thereof, and extraction electrodes 22 and 22 from the excitation electrodes are formed on the left and right sides at one end portion, that is, the base end portion. Has been. The quartz crystal vibrating piece 12 is fixed in a cantilever manner and electrically connected with a conductive adhesive 23 by aligning each extraction electrode 22 with the corresponding connection electrode 17a, 17b of the base 13 at the base end.
[0025]
The crystal vibrating piece 12 mounted on the base 13 in this manner is frequency-adjusted and then sealed by bonding the lid 14 to the base 13 in a nitrogen or vacuum atmosphere. Next, each via hole 15a, 15b is airtightly closed with a suitable filler 24. As a result, the inside of the package 11 is hermetically sealed in a nitrogen or vacuum atmosphere. As the filler 24, for example, a low melting point metal such as Au · Sn or Au · Ge is suitable. A ball of such a metal is arranged in the opening of the via holes 15a and 15b from the outside of the package, and this is used as a laser. The via holes 15a and 15b can be filled by melting with a beam or the like.
[0026]
The lid 14 formed of aluminum can be joined to the base 13 by welding using, for example, an electron beam. If the lid 14 is formed of the same metal as the base 13 in this way, the coefficient of thermal expansion is the same, which is advantageous when welding them. Moreover, the lid | cover 14 can also be joined by another well-known means using metal brazing materials, such as solder, and low melting glass. Furthermore, if the surface of the lid 14 is insulated in the same manner as the base 13, the possibility of an electrical short circuit that increases as the package 11 is reduced can be suppressed.
[0027]
3A to 3C show modified examples of the via hole and the electrode or wiring connected to the via hole. In the via hole 15a (15b) in FIG. 3A, the opening portions 25 on the quartz resonator element mounting surface side and the back surface side of the base 13 are chamfered. The conductive film 26 of the via hole 15a (15b) is usually formed by plating a conductive material on the inner surface of the through-hole formed in the base 13, but it is made better by enlarging the opening by chamfering. A film can be formed, and the connection electrode 17a (17b) and the lead 19a (19b) can be well and reliably connected in the vicinity of the opening. Furthermore, the filler 24 can be filled more easily and more reliably in an airtight manner.
[0028]
In FIG. 3B, the conductive film 26 is formed by sputtering of a conductive material. The connection electrode 17a (17b) and the lead 19a (19b) are formed by the above-described method so as to overlap the conductive film 26 formed in the vicinity of the opening of the via hole 15a (15b). Thus, by forming each electrode and wiring pattern separately by a method different from the conductive film 26 of the via hole, the degree of freedom of the pattern and design is improved.
[0029]
3C, the conductive film 26 in the via hole 15a (15b), the connection electrode 17a (17b) and the lead 19a (19b) connected to the conductive layer 26 are formed on the base layer 27a by sputtering of a conductive material. And a plating layer 27b is laminated thereon. Thus, by increasing the film thickness of each electrode, lead, and conductive film, the conductivity is improved, and a more reliable operation of the crystal resonator can be ensured.
[0030]
According to the present invention, a large number of crystal resonators of FIG. 1 can be manufactured simultaneously according to the following process. First, as shown in FIG. 4, a large number of small aluminum plate pieces 29 serving as a base material for an aluminum plate 28 having a predetermined size are respectively connected to the frame portion of the aluminum plate 28 by connecting portions 28a at the center of both sides in the longitudinal direction. The outer shape of the base 13 is processed in a state of being coupled to 28b, and through holes 30 for forming the via holes are formed in each. The outer shape of the base 13 and the through hole 30 can be processed by wet etching or machining the aluminum plate 28. As described above, the aluminum plate is easy to process and inexpensive, and the base 13 having high dimensional accuracy can be obtained as compared with the package made of the conventional ceramic material described above with reference to FIG. The manufacturing cost can be reduced.
[0031]
Next, the insulating layer 16 is formed on the surface of each of the small plate pieces 29 including the inner surface of the through hole 30 by a tuffing process. Further, a conductive film is formed on the inner surface of the through hole 30 to form the via hole, and the connection electrode, the external electrode, and the lead are patterned on both surfaces of the small plate piece 29, respectively, and then the connecting portion 28a is cut. Thus, the base 13 of FIG. 1 is obtained. After the crystal vibrating piece 12 is mounted on each base 13 and the frequency is adjusted, the lid 14 is joined by the method described above and sealed inside the package 11. Finally, the via holes 15a and 15b are hermetically closed with a filler such as the above-described low melting point metal in a nitrogen atmosphere, thereby completing the crystal resonator of FIG.
[0032]
In the present embodiment, both the through holes 30 are formed to have the same size, but in another embodiment, only one of the through holes 30 can have a small diameter. In this case, the smaller through-hole is hermetically closed by filling with a conductive material when the connection electrode or the external electrode is formed. As described above, the other through hole having the normal size is used to seal the inside of the package in an airtight manner by sealing with a filler after the lid is joined to the base.
[0033]
5A and 5B show another embodiment of the crystal resonator according to the present invention. In this embodiment, the package 31 includes a substantially rectangular box-shaped base 32 in which aluminum plates are stacked, and a metal lid 33 joined to the upper end of the base. The base 32 is formed by laminating a rectangular frame-shaped frame plate 32b, which is also formed of aluminum, on a bottom plate 32a made of an aluminum thin plate having the same configuration as the base 13 of FIG. It is formed in a box-type structure in which a space for accommodating the space is defined. This laminated structure can be easily manufactured by welding an aluminum plate with, for example, an electron beam. It is more convenient if the surface of the frame plate 32b is similarly insulated.
[0034]
The lid 33 is joined to the base 32 by welding using, for example, an electron beam. However, if the lid 33 is made of aluminum as in the embodiment of FIG. 1, the coefficient of thermal expansion is the same as that of the base 32, which is advantageous during welding. Further, the lid 33 can be joined by another known means using a metal brazing material such as solder or low melting point glass, and if the surface thereof is insulated in the same manner as the bottom plate 32a, an electrical short circuit can be achieved. This is advantageous because the possibility of
[0035]
In the present embodiment, by joining the flat lid 33 to the box-shaped base 32, the welding margin necessary for this can be made smaller than in the case of the embodiment of FIG. 1, and thus the size of the package 31 can be made smaller. Further, since the drawing process for forming the lid 14 into a box shape is not required in the embodiment of FIG. 1, the manufacturing cost can be reduced accordingly.
[0036]
FIG. 6 shows a modification of the crystal resonator of FIG. In this modification, the base 34 of the package 31 is formed into a box-shaped structure in which a space for accommodating the quartz crystal vibrating piece 12 is defined by forming an aluminum plate, as shown in FIG. Different from the example. Such a void is formed by half-etching or machining the surface of the aluminum plate on which the connection electrode is to be formed to a predetermined depth by wet etching or dry etching, for example. In particular, in the case of wet etching, when the outer shape of the base is processed on the aluminum plate by wet etching, it can be processed along the flow of the process, which is advantageous. The surface of the aluminum plate formed into a box shape is subjected to insulation treatment in the same manner as in the embodiment of FIG. The through holes for forming the via holes 15a and 15b may be formed either before or after the aluminum plate is formed into a box shape.
[0037]
The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments, and various modifications and changes are made to the above-described embodiments within the technical scope thereof. can do. For example, the present invention can be similarly applied to a vibrating piece made of various piezoelectric materials such as lithium tantalate and lithium niobate other than quartz, and a piezoelectric device having a tuning fork type vibrating piece. The base of the package can be formed of a suitable metal material such as magnesium in addition to aluminum.
[0038]
As is apparent from the above description, according to the package structure of the piezoelectric device of the present invention, the base can be processed into a desired outer shape with high dimensional accuracy, so that the dimensional accuracy of the package can be improved regardless of its downsizing and thinning. In addition, the manufacturing cost can be greatly reduced, and the wiring structure using via holes improves the design flexibility of the electrodes and wiring patterns, and can be hermetically sealed. Any tuning-fork type resonator element can be mounted, and a low-cost piezoelectric device that can exhibit high reliability and stability can be obtained.
In addition, according to the method for manufacturing a piezoelectric device of the present invention, the base can be formed with high dimensional accuracy and at low cost by processing a metal plate, and with high dimensional accuracy while meeting the demand for downsizing and thinning. And it can be manufactured at low cost.
[Brief description of the drawings]
FIG. 1A is a plan view showing a preferred embodiment of a crystal resonator according to the present invention, with a package lid omitted, and FIG. 1B is a cross-sectional view taken along the line II.
FIGS. 2A, 2B, and 2C are bottom views showing different examples of external electrodes, respectively.
FIGS. 3A, 3B and 3C are partial enlarged sectional views showing different embodiments of via holes and electrodes or wirings connected to the via holes, respectively.
FIG. 4 is a plan view showing a part of an aluminum plate in which an outer shape of a base and a through hole are formed.
5A is a plan view showing another embodiment of the crystal resonator according to the present invention, with the package lid omitted, and FIG. 5B is a cross-sectional view taken along the line IV-IV.
6 is a cross-sectional view similar to FIG. 5B, showing a modification of the crystal resonator according to the present invention.
FIG. 7 is a cross-sectional view showing an example of a conventional crystal resonator.
[Explanation of symbols]
1, 11, 31 package; 2, 12, crystal vibrating piece; 3, 13, 32, 34 base; 5, 14, 33 lid; 7, 17a, 17b connection electrode; 8, 23 conductive adhesive; 9, 19a, 19b Lead; 10, 18a, 18b External electrode; 15a, 15b Via hole; 16 Insulating layer; 24 Filler; 32a Bottom plate; 32b Frame plate

Claims (8)

In order to mount the piezoelectric vibrating piece , a base having a box shape in which a recess is processed on the surface of a metal plate, and an insulating layer whose surface is insulated, and
A flat lid that is joined to the upper part of the base and hermetically seals the piezoelectric vibrating piece inside the recess ,
A connection electrode for connecting the piezoelectric vibrating piece on the insulating layer is formed on the surface where the recess of the base is processed, and an external electrode is formed on the insulating layer on the back surface, respectively.
The connection electrode and the corresponding external electrode are electrically connected via a via hole formed in the base, and
A package structure of a piezoelectric device, wherein the via hole is hermetically closed with a filling material.   The package structure for a piezoelectric device according to claim 1, wherein the metal material is aluminum, and the insulating layer is made of an alumite film. 2. The package structure of a piezoelectric device according to claim 1, wherein the metal material is aluminum, and the insulating layer is made of a tafram film in which an alumite film is impregnated with Teflon .   4. The piezoelectric device package structure according to claim 1, wherein the insulating layer has a thickness of at least about 5 [mu] m. A recess is formed on the surface of the metal plate and formed into a box shape, two through holes are formed in the metal plate, the surface of the metal plate including the through hole is insulated, and an inner surface of the through hole is formed. the conductive material is deposited to form the via hole, an external electrode on the rear surface of the connecting electrode Oyobi the metal plate for connecting the piezoelectric resonator element the said surface by processing recess of the metal plate, each of the predetermined Forming the base of the piezoelectric device by forming the connection electrode corresponding to the pattern and the external electrode to be electrically connected via the via hole; and
A process of mounting a piezoelectric vibrating piece in the recess of the base and sealing a hermetic seal by bonding a plate-like lid thereto.
And a method of manufacturing the piezoelectric device, wherein the via hole is hermetically closed. The method for manufacturing a piezoelectric device according to claim 5 , wherein the metal plate is made of aluminum, and the insulation treatment is an alumite treatment. 6. The method for manufacturing a piezoelectric device according to claim 5 , wherein the metal plate is made of aluminum, and the insulating treatment is a taffron treatment in which Teflon is impregnated after anodizing treatment. The method for manufacturing a piezoelectric device according to claim 4 , wherein a plurality of the bases are simultaneously formed on the metal plate.

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