A surface-mount crystal oscillator includes a quartz crystal unit, a mounting substrate on which an IC (Integrated Circuit) chip is mounted and which is bonded to the rear surface of the crystal unit, and an electronic components which is mounted on one end of the surface of the mounting substrate on which the crystal unit is bonded. The crystal unit includes a planar substrate, a quartz crystal blank held on one principal surface of the planar substrate, a metal film formed along the outer periphery of the principal surface, and a concave metal cover having an open-ended face. In the crystal unit, the open-ended face of the metal cover is brazed to the metal film, whereby the crystal blank is hermetically sealed between the metal cover and the planar substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a quartz crystal oscillator for supplying an oscillating frequency signal and which includes a quartz crystal unit and an IC (Integrated Circuit) chip that together with the crystal unit forms an oscillation circuit; and more particularly to a surface-mount crystal oscillator which is suitable for miniaturization wherein a mounting substrate, on which an IC chip is provided, is bonded to the rear surface of a crystal unit.

2. Description of the Related Art

Surface-mount crystal oscillators, and in particular, surface-mount temperature-compensated crystal oscillators (TCXOs) feature light weight, compact size, and an oscillation frequency having superior stability, and these devices are therefore widely used in communication equipment such as portable telephones which are used in a mobile environment. As one type of surface-mount crystal oscillator, there is a bonded-type surface-mount crystal oscillator in which a mounting substrate, on which an IC chip constituting an oscillation circuit has been mounted, is installed on the rear surface of a crystal unit.

FIGS. 1A and 1Bshow an example of the construction of this type of the conventional surface-mount crystal oscillator, andFIGS. 2A and 2Bshow a crystal unit that is used in this surface-mount crystal oscillator. The surface-mount crystal oscillator is provided with crystal unit1and mounting substrate2. Crystal unit1is constructed by accommodating quartz crystal blank3within casing4and covering the opening of casing4by metal cover5.

Casing4is shaped with a depression or recess formed in one of its principle surfaces, and is constructed from laminated ceramic in which bottom wall4aand frame walls4bare stacked together. Bottom wall4ais a substantially rectangular planar shape, and frame walls4bare formed as a substantially rectangular frame shape, whereby a substantially rectangular depression is formed in casing4. As shown inFIG. 2A, a pair of terminal electrodes6is formed on the bottom surface of the depression of casing4, i.e., the exposed surface of bottom wall4a.

Crystal blank3is, for example, a substantially rectangular AT-cut quartz crystal blank, and although not shown here, excitation electrodes are formed opposite each other on the two principal surfaces of the crystal blank. In addition, extension electrodes are formed on both sides of one end of crystal blank3to extend from the excitation electrodes. Both sides of one end of crystal blank3from which the extension electrodes extend are secured to terminal electrodes6on the bottom surface of the depression of casing4by means of conductive adhesive12. Crystal blank3is thus held horizontally and connected electrically and mechanically to casing4.

A pair of connection terminals7aare provided at diagonally opposite corners on the outer side of the bottom surface of casing4. These connection terminals7aare connected to respective terminal electrodes6by way of conductive paths formed in casing4. In addition, ground terminals7bare provided at the other diagonally opposite corners on the outer side of the bottom surface of casing4. Ground terminals7bare electrically connected to metal cover5. Typically, metal cover5is bonded to casing4by providing a metal ring on the upper surface of frame walls4band then seam welding metal cover5to this metal ring.

As with casing4, mounting substrate2is composed of laminated ceramic and has a depression or recess formed in one of its principal surfaces. The planar outer dimensions of mounting substrate2are greater than the planar outer dimensions of crystal unit1, and in particular, are set to allow a space at one longitudinal end of mounting substrate2that is not covered by crystal unit1when crystal unit1is bonded to mounting substrate2. In addition, connection terminals8awhich correspond to connection terminals7aof crystal unit1and ground terminals8bwhich correspond to ground terminals7bof crystal unit1are provided on the surface of mounting substrate2which is bonded to crystal unit1, i.e., on the surface in which the depression is not formed. Then, for example, two chip capacitors9are mounted in the space at the longitudinal end of mounting substrate2.

Mounting terminals10including terminals such as a power supply terminal, a ground terminal, an output terminal, and an AFC (Auto Frequency Control) terminal are provided on the upper surface of frame part2awhich surrounds the depression of mounting substrate2. IC chip11is secured inside the depression, by the way of, for example, ultrasonic thermocompression using bumps. An oscillation circuit which connects to crystal unit1and a temperature compensation mechanism for effecting temperature compensation of the oscillator frequency of the oscillation circuit are integrated on IC chip11. Mounting terminals10are electrically connected to the respective terminals of IC chip11. IC chip11is also electrically connected to connection terminals8aand ground terminals8b.

Mounting substrate2is installed on the rear surface of crystal unit1by bonding connection terminals7aand ground terminals7bof crystal unit1to connection terminals8aand ground terminals8bof mounting substrate2by means of, for example, solder.

As chip capacitors9, a variety of large-capacitance capacitors which are difficult to incorporate inside IC chip11may be arranged, including bypass capacitors between power supply and ground, capacitors for coupling with a succeeding stage, or CR filter capacitors for suppressing noise which is produced by the temperature compensation mechanism.

In a surface-mount crystal oscillator of the above-described construction, however, there is the problem that, with the development of ever-smaller devices, the space for mounting chip capacitors9becomes difficult to maintain, such as in a case in which the planar outer dimensions of the oscillator are reduced to a size of 3.2 mm×2.5 mm.

This problem does not arise if crystal unit1becomes correspondingly smaller with the decrease in the planar outer dimensions. Decreasing the size of crystal unit1entails a choice between either correspondingly reducing the outer dimensions of crystal blank3or leaving the size of crystal blank3unchanged and increasing the ratio of the inside volume of casing4to the outer dimensions of the crystal oscillator. However, since the oscillation characteristics of a crystal blank improve with the increase in the outer dimensions of the crystal blank, decreasing the outer dimension of crystal blank3has an adverse effect on the oscillation characteristics of crystal blank and thus results in design problems. More specifically, decreasing the dimensions of crystal blank3brings about an increase in the crystal impedance (CI) of crystal blank3and the occurrence of spurious oscillation. On the other hand, increasing the ratio of the inside volume of casing4necessitates a corresponding decrease of the thickness of frame walls4b. Frame walls4bare composed of ceramic, and reducing the thickness not only reduces strength, but also raises problems in fabrication. Frame walls4bmust maintain a particular fixed thickness. Thus, in either case, reducing the size of crystal unit1is problematic, and a reduction of the outer dimensions of mounting substrate2thus prevents mounting of chip capacitors9.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compact surface-mount crystal oscillator in which the crystal unit is reduced in size to allow mounting of electronic components.

The object of the present invention is achieved by a crystal oscillator for surface mounting which includes: a crystal unit, a mounting substrate on which an IC chip is mounted and which is bonded to the rear surface of the crystal unit, and an electronic component which is mounted on one end of the surface of the mounting substrate on which the crystal unit is bonded; wherein the crystal unit includes: a planar substrate, a crystal blank secured to one principal surface of the planar substrate, a metal film formed along the outer periphery surrounding the principal surface, and a concave metal cover having an open-ended face; wherein the open-ended face is bonded to the metal film by brazing to hermetically seal the crystal blank between the metal cover and the planar substrate.

Because the thickness of the metal cover can be made smaller than the thickness of the frame walls of laminated ceramic, the size of the crystal unit can be reduced while maintaining the size of the crystal blank unchanged, whereby a compact surface-mount crystal oscillator can be constructed which maintains space for mounting electronic components on the mounting substrate.

DETAILED DESCRIPTION OF THE INVENTION

InFIGS. 3 and 4, which show a surface-mount crystal oscillator according to a preferable embodiment of the present invention, constituent elements which are identical to elements in the oscillator shown inFIGS. 1A,1B,2A, and2B are identified by the same reference numerals, and redundant detailed explanation regarding these elements is not repeated here.

As previously described, in the surface-mount crystal oscillator according to the present embodiment, mounting substrate2which incorporates IC (Integrated Circuit) chip11is bonded to the rear surface of quartz crystal unit1, and chip capacitors9are mounted on one end of mounting substrate2. Crystal unit1used in this construction is made up from planar substrate13, concave metal cover14which is provided to cover planar substrate13, and quartz crystal blank3which is sealed between planar substrate13and metal cover14. As in the previously described crystal oscillator of the prior art, a substantially rectangular AT-cut quartz crystal blank may be employed as crystal blank3.

Planar substrate13is formed from laminated ceramic composed of first layer13aand second layer13b. A pair of terminal electrodes6for connecting to both sides of the end of crystal blank3from which the extension electrodes extend are provided on one of the surfaces of planar substrate13, i.e., on the surface of first layer13a. In addition, metal film15surrounding the outer periphery of first layer13ais formed on the surface of first layer13a. The width W of metal film15is, for example, 0.15 mm. Terminal electrodes6and metal film15are simultaneously formed with planar substrate13when sintering planar substrate13, which is composed of laminated ceramic. Frame-shaped brazing filler metal16, which is, for example, a gold-tin (AuSn) eutectic alloy, is bonded to metal film15.

A pair of connection terminals7aare provided at both ends of a one diagonal of the outside bottom surface of second layer13bof planar substrate13. Ground terminals7bare provided at both ends of the other diagonal of the outside bottom surface of second layer13b. These connection terminals7aand ground terminals7bextend to the side surface of second layer13b. Connection terminals7aelectrically connect to terminal electrodes6by way of through-holes which are provided in first layer13aand second layer13b. By using conductive adhesive12to secure both sides of one end of crystal blank3, from which extension electrodes extend, to terminal electrodes6, crystal blank3is held horizontally and is electrically and mechanically connected to planar substrate13.

Metal cover14is composed of, for example, kovar, nickel silver or the like and is formed in a concave form by a process such as deep drawing. More specifically, metal cover14has a substantially rectangular bottom wall and side walls which extend from and substantially perpendicular to the bottom wall, these side walls thus forming a shape having an open-ended face. The planar outer dimensions of metal cover14are smaller than the planar outer dimensions of planar substrate13. The inside dimensions of the open-ended face of metal cover14are substantially identical to the inside dimensions of metal film15. In addition, the thickness of metal cover14, and in particular, the thickness D of the side-wall portions is less than thickness W of metal film15. This metal cover14is electrically connected to ground terminals7b.

Mounting substrate2used in this embodiment is identical to the mounting substrate used in the surface-mount crystal oscillator shown inFIGS. 1A,1B,2A, and2B. When crystal unit1of this embodiment is bonded to mounting substrate2, an area which is not covered by crystal unit1, i.e., a space, is formed on one longitudinal end of mounting substrate2, and two chip capacitors9are mounted in this space in the similar manner as shown inFIG. 1B. As previously described, the bonding of crystal unit1and mounting substrate2is realized by using solder or the like to join connection terminals7aand ground terminals7bof crystal unit1to connection terminals8aand ground terminals8bof mounting substrate2.

When assembling this surface-mount crystal oscillator, crystal unit1is first formed and crystal unit1and mounting substrate2are then bonded together. When fabricating crystal unit1, both sides of one end of crystal blank3are secured to planar substrate13by conductive adhesive12, following which the open-ended face of metal cover14is positioned inside the outer edges of planar substrate13to contact brazing filler metal16, and then heated. The application of heat melts brazing filler metal16, forms a fillet on the outer peripheral surface, and bonds the open-ended face of metal cover14to planar substrate13to hermetically seal crystal blank3.

In this construction, planar substrate13and metal cover14constitute a container which accommodates crystal blank3in crystal unit1. The thickness of the frame walls in the casing which was composed of laminated ceramic in the prior art was, for example, 0.35 mm, but the use of metal cover14in the present embodiment enables a reduction in the thickness D of the surrounding walls compared to the example of the prior art. For example, the use of metal cover14enables reduction of thickness D of the surrounding walls to 0.08 mm. Thus, as a result of adopting the present embodiment, the inside volume of the container which accommodates crystal blank3can be maintained without alteration and crystal blank3of the same size as the prior art can be used despite reduction of the outer planar area of crystal unit1.

Thus, despite reduction of the size of mounting substrate2, the use of the present embodiment allows the size of crystal unit1to be reduced while suppressing increase in CI and the occurrence of spurious oscillation, and further, allows space for mounting chip capacitors9to be secured on one end of mounting substrate2. A surface-mount crystal oscillator can be obtained that allows mounting of chip capacitors9and that features both compact size and high added value.

Explanation next regards a surface-mount crystal oscillator according to another embodiment of the present invention. In the above-described embodiment, a case was described in which the thickness of metal cover14of crystal unit1was substantially the same throughout, but the thickness of metal cover14may also vary at different positions as shown inFIG. 5. In the crystal unit shown inFIG. 5, a process of crushing the outer edge of the open-ended face of metal cover14produces a flared shape in which the thickness increases toward the edge. In this case, the thickness of the portion of increased thickness is on the same order as thickness D of the metal cover in the above-described embodiment, and the thickness D1of the main body of metal cover14is smaller than thickness D of the metal cover in the above-described embodiment. By adopting this form, the length, i.e., a so-called seal path, of the bonding surface of the open-ended face of metal cover14and metal film15can be maintained unchanged from the above-described embodiment and a hermetic seal can be reliably achieved. In addition, setting the thickness of metal cover14to a thickness D1that is less than thickness D enables a surface-mount crystal oscillator to be obtained in which the height dimension is less than in the above-described embodiment, and miniaturization in the direction of height of the crystal oscillator can therefore be advanced.

In the above embodiments, an example was described in which two chip capacitors9were mounted in the space at one end of mounting substrate2, but the number of mounted chip capacitors is not limited to two. For example, just one capacitor may be mounted. Further, the present invention does not limit the components which can be mounted in this space to chip capacitors, and any chip-type electronic component such as inductors and thermistors can be mounted as necessary.

The use of laminated ceramic of two-layer structure as planar substrate13improves the degree of hermetic sealing of the casing of the crystal unit, but ceramic of single-layer structure may be employed, the terminal electrodes and connection electrodes may be electrically connected by way of, for example, via-holes, and a dielectric film may be printed over the via-holes to ensure an airtight seal in the via-hole portions. In this case, the height of the crystal oscillator may be further reduced.