Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-01-23
2003-04-01
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S340000, C310S348000
Reexamination Certificate
active
06541897
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a piezoelectric device, such as a piezoelectric resonator, piezoelectric oscillator and SAW device used for electronic appliances. In particular, the invention relates to a package structure for a piezoelectric device, wherein a lid made of a thin glass sheet is bonded to a base that mounts a resonator element made of piezoelectric material, such as quartz crystal, by using low-melting point glass, and the resonator element is then hermetically sealed.
2. Description of Related Art
In accordance with the related art, various types of information and communication equipment and electronic appliances, such as OA machines and household electronic appliances, extensively use piezoelectric devices as a clock source for electronic circuits, such as a piezoelectric resonator, an oscillator including a piezoelectric resonator element and an IC chip sealed in the same package, and a real time clock module. In particular, a piezoelectric device must be miniaturized, flattened and suitable for surface mounting on a circuit board in response to the downsizing of the overall device, such as compact information appliances, including for example mobile computers and IC cards, and communication appliances, including for example cellular phones.
In general, a surface-mounted piezoelectric device has a package structure in which a lid is bonded to a base that mounts a piezoelectric resonator element and, if necessary, an IC device to be sealed. In particular, a related art technique uses a transparent glass material for the lid so that laser beams can be irradiated onto the piezoelectric resonator element from the exterior of the device to adjust the frequency after the sealing. FIGS.
9
(A) and (B) show an example of a related art piezoelectric resonator having this package structure. The piezoelectric resonator has a package
3
including a base
1
made of insulating material and a lid
2
, in which a tuning fork type quartz crystal resonator element
4
is hermetically sealed. The base
1
is a rectangular box made of laminated thin sheets of insulating material, such as ceramics, on the bottom of which the quartz crystal resonator element
4
is fixed at its base portion
4
a
in a cantilever manner, with conductive adhesive. The rectangular thin sheet lid
2
is hermetically bonded at its lower surface to the top surface of the base
1
by using low-melting point glass
5
.
SUMMARY OF THE INVENTION
A package for a piezoelectric device is subject to, for instance, shock when dropped and external force when gripped by jigs during the mounting operation. Therefore, sealing strength is required for ensuring a sufficient hermetic state in the bonded and sealed part between the base and lid. However, the related art package structure described above simply bonds the top surface of a base to the lower surface of a lid using low-melting point glass. The horizontal joint surface receives the entire force, particularly, from the sides of the package. Therefore, to prevent the hermetic state at the bonded and sealed part from being impaired, which may result in performance loss or defects of the piezoelectric device, the lid is conventionally bonded to the top surface of the base with a predetermined sealing width (w1 in FIG.
9
(A)), which is then surrounded by low-melting point glass
5
having a sufficient width (w2 in FIG.
9
(A)), in order to enhance the strength of the device against lateral external force.
On the other hand, the base
1
is produced generally by laminating and bonding a predetermined number of large ceramic thin sheets
6
having a desired shape and size, and cutting it along the dotted lines
7
in
FIG. 10
into a number of bases having a predetermined length and width. Here, castrations (round through holes)
8
are made at the intersections of the horizontal and vertical dotted lines
7
to facilitate cutting. This leaves a quarter circle shaped cutout on the corners of the divided base
1
. When the rectangular lid is laid on it, as shown in FIG.
9
(C), the corner of the base
1
does not have enough margin to hold low-melting point glass
5
outside the lid
2
, or the corner of the lid
2
may even protrude from the outer limit of the base. In addition, if low-melting point glass
5
runs down the outer wall of the base
1
, as shown in FIG.
9
(C), a lateral external force may cause the low-melting point glass to crack, possibly impairing the hermetic state of the package. Conversely, if a margin is provided for the low-melting point glass to travel beyond the sealing width w1, sufficient bonding and sealing strength is ensured. However, this makes the package larger in length and width, limiting miniaturization of a piezoelectric device.
If less low-melting point glass is used to prevent the glass from running down the outer wall of the base
1
, the low-melting point glass may not reach the periphery of the lid
2
, and a space
10
may be defined between the lid
2
and the base
1
, as shown in FIG.
9
(D). If the package with the lid
2
and base
1
bonded in this way is subject to an external force from above, the space
10
may allow the initiation of cracking or breaking of the lid
2
that is made of glass material, and the hermetic state of the package may also be impaired.
The present invention addresses the problems in the related art described above, and an object of the invention is to provide a package structure for a piezoelectric device, that includes a package having a base on which a piezoelectric resonator element is mounted, and a lid that is made of glass material and bonded to the top surface of the base using low-melting point glass, the package structure ensuring sufficient bonding and sealing strength and enabling miniaturization and flattening of the device.
To attain the above object, the present invention provides a package for a piezoelectric device, including a base having a rectangular box shape, and a lid that is made of thin rectangular glass sheet and bonded onto the top surface of the base using low-melting point glass. A piezoelectric resonator element is hermetically sealed. The corners of the rectangular lid are cut off so that a margin with a predetermined width is defined entirely between the outer periphery of the base and the periphery of the lid.
Bonding the lid that is formed in this way to the base ensures a sufficient sealing width on the top surface of the base around the lid and a sufficient margin width for low-melting point glass to travel around. This provides sufficient bonding and sealing strength against a lateral external force applied to the package.
The corners of the lid can be formed differently than described above. The corners can be diagonally cut off in one embodiment, and rounded in another embodiment, for example. The lid can easily be processed into a desired shape using machining, chemical processes, such as etching, or monolithic molding, such as embossing.
In an embodiment, the cut off corners of the rectangular lid are tapered to the upper surfaces. This provides each corner of the package, which is highly possibly affected by a direct external force in the course of handling the piezoelectric device, with a sufficient sealing width and a margin at the corners of the lid described above. In addition, this tapered surface reduces horizontal force on each joint surface between the base top surface and the lid at each corner because it transforms the lateral force on the package into horizontal and vertical elements. In this way, a minor additional process to the lid ensures a practically sufficient bonding and sealing strength. In another embodiment, the lid is tapered to the upper surface along the periphery, providing enhanced bonding and sealing strength to the package.
In another embodiment, the lid is stepped on the upper surface thereof around the periphery. This reduces the possibility for the jigs to be trapped while handling the piezoelectric device for assembly, preferably reducing the risk of undesired ex
Addison Karen B
Dougherty Thomas M.
Oliff & Berridg,e PLC
Seiko Epson Corporation
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