Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-11-15
2004-01-20
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S312000, C310S344000, C310S346000, C029S025350
Reexamination Certificate
active
06680560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quartz-crystal unit, and more particularly, to a crystal unit which is suitable for applications that require heat resistance.
2. Description of the Related Art
It is widely known that a quartz-crystal oscillator using a quartz-crystal unit provides a stable oscillation frequency. The influence exerted by a change in temperature is the largest factor which contributes to fluctuations in the oscillation frequency of the crystal oscillator. For example, a crystal oscillator using an AT cut crystal unit, which is most often employed, presents a change in frequency in a range of several tens to several hundreds of ppm to a change in temperature from −30° C. to +80° C. For this reason, a crystal oscillator for use in applications which require a stable frequency source such as measuring instruments, base stations of a mobile communication system, and the like is an oven-controlled type one which accommodates a crystal unit in a thermostatic oven. The oven-controlled type crystal oscillator prevents a change in oscillation frequency due to a change in temperature by accommodating the crystal unit in the thermostatic oven which is heated at a constant temperature of approximately +80° C. In this way, the crystal unit for use in the oven-controlled type crystal oscillator is used in a high temperature environment at approximately +80° C.
On the other hand, a crystal unit used at temperatures near a room temperature typically has excitation electrodes formed on both surfaces of a quartz crystal blank and extended to edges of their respective surfaces.
Then, the crystal blank is held at extreme ends of the extended excitation electrodes by holding members, each of which comprise a wire, a thin metal plate, or the like formed with a clip at a leading end thereof. A conductive adhesive is further applied on the sites at which the crystal blank is held to ensure secure fixation of the crystal blank and to make electric conduction between the holding members and excitation electrodes. The conductive adhesive for use in this case may be, for example, a mixture of an adhesive based on epoxy resin and thin pieces, grains, or the like of silver.
However, when a crystal unit using a conductive adhesive is placed in a high temperature environment, a gas component generated from organic components of the adhesive, in particular, sticks on the excitation electrodes, causing a change over time in the resonant frequency of the crystal unit, i.e., the oscillation frequency of a crystal oscillator over time. Therefore, if a crystal unit for use in room temperatures as described above is used in a oven-controlled type crystal oscillator, the crystal unit of which is exposed to high temperatures of approximately +80° C., the oscillation frequency will change over time.
To solve this problem, a structure as illustrated in
FIG. 1
is contemplated for a crystal unit for use in a thermostatic oven. The crystal unit illustrated in
FIG. 1
has two terminals
22
extending through base
21
, and holding member
23
attached at the leading end of each terminal
22
.
On the other hand, both side ends of disk-shaped crystal blank
24
are applied with a fillet comprised of a mixture of a low-melting glass and a silver filler, which is heated to a temperature exceeding 400° C., at which the fillet is molten, to form connections
26
. Since the low-melting glass is chemically similar to quartz in components, it can firmly secure and form connections
26
at predetermined positions of crystal blank
24
. Then, excitation electrodes
25
are formed by vapor deposition at the centers of top and bottom faces of crystal blank
24
, opposite to each other. Excitation electrodes
25
are extended in directions opposite to each other to positions spaced by a predetermined distance from connections
26
at the ends of the surfaces.
Then, connections
26
of crystal blank
24
are held by holding members
23
, and a brazing material composed of gold and germanium (Au—Ge), used to form a eutectic alloy, is heated to approximately 350° C. to bond holding members
23
to connections
26
.
When bonding by the gold-germanium brazing material is performed under the condition that excitation electrodes
25
directly contact with holding members
23
, excitation electrodes
25
are eroded by electrolytic etching, disadvantageously causing degeneration of excitation electrodes
25
and resulting gradual change in the resonant frequency. In extreme cases, crystal blank
24
could come off holding members
23
. However, when connections
26
mainly composed of silver are formed such that spacings are defined between connections
26
and excitation electrodes
25
, connections
26
can be securely bonded to holding members
23
using the gold-germanium brazing material without adversely affecting excitation electrodes
25
.
Then, metal thin films
27
are vapor deposited on the top and bottom faces of crystal blank
24
to cover the extended ends of excitation electrodes
25
and connections
26
, thereby providing electric conduction between excitation electrodes
25
and connections
26
.
Subsequently, a trace of metal film is additionally vapor deposited on excitation electrodes
25
to finely adjust the resonant frequency of crystal blank
24
to a target frequency by its mass addition effect.
Then, cover
28
having an open lower end is fitted over base
21
, and the opening end is bonded to a flange along the peripheral edge of base
21
by soldering, cold pressure welding, or the like, with the internal space of cover
28
placed in an inert gas or vacuum atmosphere, to hermetically encapsulate crystal blank
24
.
The conventional heat resistant crystal unit is assembled in the foregoing manner. In the conventional crystal unit, crystal blank
24
is held in a direction perpendicular to base
21
.
In recent years, however, surface mount devices tend to be more often used in a variety of electric devices for purposes of automated assembling processes, reduction in size, and the like. The surface mount type is also required for the crystal unit. A surface mount crystal unit employs, for example, a container which has a base made of ceramic. The base has outer shape in a rectangular parallelepiped and is formed with a recess on the top face. With this container, after a crystal blank is accommodated in the recess, a metal-made lid is seam welded along the opening of the recess to encapsulate the crystal blank. Therefore, in such a surface mount crystal unit, the crystal blank is accommodated in the recess of the base in parallel with the bottom face thereof. Thus, if connections of the crystal blank are secured to holding members formed on the bottom face of the recess with a gold-germanium brazing material for forming a heat resistant crystal unit of the structure described above, excitation electrodes below the crystal blank cannot be applied with vapor deposition and the like. This results in a problem that the underlying excitation electrode cannot be electrically connected to the holding member.
Also, some crystal units of a general type having lead lines, not for surface mounting, can hold a crystal blank horizontally on a holding member disposed on a base. Such a crystal unit is similar in that a lower surface of the crystal blank cannot be applied with vapor deposition and the like when the crystal blank is held by the holding member. In this structure, therefore, underlying excitation electrodes cannot either be electrically connected to the holding member.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a crystal unit which has good heat resistance, wherein a crystal blank is held horizontally to a base and securely fixed to holding members with a gold-germanium brazing material, and excitation electrodes on the top and bottom faces of the crystal blank are connected to the holding members by vapor deposition.
The object of the present invention is achieved by a crystal unit whic
Koki Genwa
Koyama Mituaki
Obara Shigeru
Saito Mikio
Dougherty Thomas M.
Knobbe Martens Olson & Bear LLP
Nihon Dempa Kogyo Co. Ltd.
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