Surface mount crystal oscillator and method of manufacturing same

Abstract

A surface mount crystal oscillator comprises: a crystal unit having a crystal blank hermetically sealed in a package, and first external terminals formed on an outer bottom surface of the package; and a mounting substrate for containing an IC chip which has an oscillation circuit integrated therein, the oscillation circuit using the crystal blank. The mounting substrate includes second external terminals corresponding to the first external terminals on one main surface, and mounting terminals on the other main surface. The mounting substrate comprises a printed wiring board made up of a lower layer, an intermediate frame layer having an opening, and an upper layer laminated one on another. The IC chip is placed in a hollow space defined by the opening. The first external terminals are bonded to the second external terminals to integrate the crystal unit with the mounting substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view illustrating the exemplary configuration of a conventional surface mount crystal oscillator;

FIG. 1B is a plan view illustrating a mounting substrate used in the crystal oscillator illustrated in FIG. 1A;

FIG. 2 is a plan view of a crystal blank;

FIG. 3 is a cross-sectional view illustrating another exemplary configuration of a conventional surface mount crystal oscillator;

FIG. 4 is a cross-sectional view illustrating the configuration of a surface mount crystal oscillator according to one embodiment of the present invention;

FIG. 5A is a plan view illustrating a mounting substrate sheet; and

FIG. 5B is a partial cross-sectional view of the mounting substrate sheet when crystal units are mounted thereon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 4 which illustrates a surface mount crystal oscillator according to one embodiment of the present invention, the same components as those in FIGS. 1A and 1B are designated the same reference numerals, and repeated descriptions will be omitted.

The surface mount crystal oscillator illustrated in FIG. 4 comprises quartz crystal unit 3 and mounting substrate 2 which contains IC chip 1, and mounting substrate 2 is bonded to the bottom surface of crystal unit 3, as described above. Crystal unit 3 used herein is the same as the one described in connection with FIGS. 1A and 1B. Likewise, IC chip 1 used herein is similar to the one described above.

Mounting substrate 2 comprises a printed wiring board which has a substantially rectangular outer planar shape and uses a glass epoxy material as a base material. Specifically, mounting substrate 3 is made up of three sheets made of glass epoxy material in a trilaminar structure in which intermediate frame layer 14c is sandwiched between upper layer 14a and lower layer 14b. Each of upper layer 14a and lower layer 14b is formed flat, while intermediate frame layer 14a is formed with an opening which is completely sandwiched by upper layer 14a and lower layer 14b. The opening is therefore isolated from the outside to define hollow space 15. As will be later described, IC chip 1 is placed within hollow space 15.

Wiring patterns are formed on the surfaces of upper layer 14a, lower layer 14b, and intermediate frame layer 14c. The wiring patterns are formed by disposing copper foil over the entire surfaces of corresponding layers 14a to 14c, and patterning the copper foil into desired shapes by etching.

At four corners on one main surface of mounting substrate 2, i.e., the outer surface of upper layer 14a, external terminals 5 are formed for use in electrically and mechanically connecting with crystal unit 3. External terminals 5 are formed on the outer surface of substantially rectangular upper layer 14a to extend from the respective four corners to positions located above hollow space 15. In other words, external terminals 5 are formed to jut over hollow space 15 across upper layer 14a. At four corners on the other main surface of mounting substrate 2, i.e., the outer surface of lower layer 14b, mounting terminals 4 are disposed for use in surface-mounting the crystal oscillator onto a wiring board of a device in which the crystal oscillator is to be mounted. Like external terminals 5, mounting terminals 4 are also formed to extend to positions below hollow space 15 across lower layer 14b.

A plurality of circuit terminals are disposed in correspondence to the IC terminals of IC chip 1 at sites on the surface of lower layer 14b which face hollow space 15. The circuit terminals are electrically connected to mounting terminals 4 and external terminals 5 through the aforementioned circuit patterns, and via holes extended through respective layers 14a to 14c. Then, IC chip 1 is secured to the surface of lower layer 14b within hollow space 15 by electrically and mechanically bonding the IC terminals to the circuit terminals through ultrasonic thermo-compression bonding which involves bumps 6. Also, in this bonding process, electronic circuits within IC chip are electrically connected to mounting terminals 4 and external terminals 5.

Such mounting substrate 2 is formed by a known multi-layer wiring board formation technology. Upper layer 14a and intermediate frame layer 14c, as well as lower layer 14b and intermediate frame layer 14c are adhered to each other with the same resin 16 as that used in the glass epoxy material, i.e., prepreg on their respective laminated surfaces. Then, this resin 16 is also filled in hollow space 15 in which IC chip 1 is placed, such that IC chip 1 is adhered to lower layer 14 with a higher strength by the action of resin.

Mounting substrate 2 thus configured is electrically and mechanically bonded to crystal unit 3 by soldering external terminals 5 of mounting substrate 2 to external terminals 10 of crystal unit 3. This soldering causes the previously coated creamed solder to melt. Here, the creamed solder is coated on external terminals 5 formed on one main surface of mounting substrate 5, i.e., on the surface of upper layer 14a, by printing, and subsequently, crystal unit 3 is placed on mounting substrate 2. The resulting assembly is carried into a high-temperature pass-through type furnace for reflow soldering.

By bonding mounting substrate 2 to the bottom surface of crystal unit 3, crystal blank 8 within crystal unit 3 is electrically connected to the oscillation circuit within IC chip 1 to complete a crystal oscillator, as is the case with the aforementioned conventional crystal oscillator.

As described above, in the surface mount crystal oscillator of this embodiment, mounting substrate 2 is made of a glass epoxy material in a trilaminar structure, which is apt to cutting operations, hollow space 15 is defined in intermediate frame layer 14c sandwiched between upper layer 14a and lower layer 14b, and IC chip 1 is contained within this hollow space 15. In this event, hollow space 15 is isolated from external air by upper layer 14a and lower layer 14b. This crystal oscillator lends itself to improving the productivity because it can be manufactured by a manufacturing method which includes bonding crystal units onto a plurality of mounting substrates 2 defined on a single substrate sheet, and dividing the sheet into individual crystal oscillators. Also, since both main surfaces of mounting substrate 2 are flat, external terminals 5 of mounting substrate 2 can be readily bonded to external terminals 10 of crystal unit 3 using creamed solder which is coated by a printing method.

In this crystal oscillator, since external terminals 5 and mounting terminals 4 on mounting substrate 2 are all formed to extend from the four corners on mounting substrate 2 to positions over hollow space 15, external terminals 5 and mounting terminals 4 can be formed to have larger areas than those in the conventional crystal oscillator illustrated in FIGS. 1A and 1B. Accordingly, even if the crystal oscillator is reduced in size, a high bonding strength can be maintained between mounting substrate 2 and crystal unit 3, and a high bonding strength can also be maintained between mounting substrate 2 and wiring board. Since the external terminals and mounting terminals are not disposed at four corners on the surface of intermediate frame layer 14c, the opening, i.e., hollow space 15 in intermediate frame layer 14c can be increased in area, allowing mounting substrate 2 to receive relatively large IC chip 1, which may contain a temperature compensation mechanism. Therefore, according to this embodiment, the surface mount crystal oscillator lends itself to facilitating a reduction in size.

Next, a description will be given of a method of manufacturing the crystal oscillator. Generally, this method employs a mounting substrate sheet corresponding to a plurality of crystal oscillators, and includes collectively bonding a plurality of crystal units to the mounting substrate sheet, and subsequently cutting or dividing the mounting substrate sheet into individual mounting substrates with respective crystal oscillators, thereby producing a plurality of crystal oscillators at a time.

FIG. 5A is a bottom view of a mounting substrate sheet used herein. Mounting substrate sheet 17 is a printed wiring board which comprises a plurality of the aforementioned mounting substrates arranged in the horizontal and vertical directions, and contains IC chip 1 in each of areas which are to be assembled into crystal oscillators. Then, individual crystal oscillators are completed through a first to a fourth step described below.

First, mounting substrate sheet 17 will be described. Like the one illustrated in FIG. 4, mounting substrate sheet 17 is made of a glass epoxy material and has a trilaminar structure made up of upper layer 14a, intermediate frame layer 14c, and lower layer 14b which are laminated one on another. An area corresponding to the mounting substrate for each crystal oscillator cut from mounting substrate sheet 17 is called “mounting substrate portion” 2A. Accordingly, mounting substrate sheet 17 has mounting substrate portions 2A arranged in the vertical and horizontal directions. Then, division grooves 18 are drawn on mounting substrate sheet 17 along four sides of the periphery of each mounting substrate portion 2A for facilitating the division of mounting substrate sheet 17. Division grooves 18 may extend through both main surfaces of mounting substrate sheet 17, or formed in the shape of linear recess on the surface of mounting substrate sheet 17.

For each mounting substrate portion 2A, hollow space 15 is formed through intermediate frame layer 14c, and IC chip 1 has been previously embedded within hollow space 15 in a manner similar to the foregoing. For each mounting substrate portion 2A, upper layer 14a and lower layer 14b are formed flat. IC chip 1 comprises IC terminals, similar to those described above, which include a power supply terminal, a ground terminal, an oscillation output terminal, a pair of connection terminals for connection to a crystal unit, and the like. Also, in each mounting substrate portion 2A, external terminals 5 are formed at four corners on the surface of upper layer 14a, and mounting terminals 4 are formed at four corners on the surface of lower layer 14b. Among the IC terminals, the power supply terminal, ground terminal, oscillation output terminals and the like are electrically connected to mounting terminals 4. In each mounting substrate portion 2A, external terminals 5 located at both ends of one diagonal are electrically connected to the connection terminals within the IC terminals of IC chip 1, while the remaining two of external terminals 5 are connected to the ground terminal within mounting terminals 4.

A plurality of measurement terminals 19 are arranged in a row along one edge of mounting substrate sheet 17. These measurement terminals 19 are electrically connected to mounting terminals 4 of each mounting substrate portion 2A through wiring paths, i.e., a wiring pattern formed on mounting substrate sheet 17.

Next, a description will be given of a process of assembling surface mount crystal oscillators using the foregoing mounting substrata sheet 17. It is assumed that a plurality of crystal units which have the same structure as described above are previously prepared.

First, in a first step, a mask is applied to the surface of upper layer 14a in mounting substrate sheet 17, such that external terminals 5 alone expose on each mounting substrate portion 2A, and creamed solder is collectively coat on external terminals 5 by printing.

Next, in a second step, respective crystal units 3 are mounted on respective mounting substrate portions 2A such that external terminals 10 of crystal units 3 are aligned to external terminals 5 of mounting substrate portions 2A. Mounting substrate sheet 17 mounted with the crystal units are carried into a high-temperature pass-through type furnace to melt the creamed solder, thereby electrically and mechanically bonding external terminals 5 of mounting substrate 17 to external terminals 10 of crystal units 3. In this way, a plurality of crystal units 3 are bonded to mounting substrate sheet 17, as illustrated in a cross-sectional view of FIG. 5B, to form a plurality of crystal oscillators on mounting substrate sheet 17.

Next, each of the crystal oscillators formed on mounting substrate sheet 17 in the second step is tested for oscillation characteristics in a third step. The test for oscillation characteristics is conducted by connecting mounting substrate sheet 17 to a measuring instrument, not shown, through measuring terminals 19 which are formed to serve as connectors in a row along one edge of mounting substrate sheet 17, and measuring electric characteristics such as oscillation frequency for each crystal oscillator. Subsequently, in a fourth step, mounting substrate sheet 17 is cut along division grooves 18, for example, by a dicing saw for separation of respective mounting substrate portions 2A from one another, thereby dividing mounting substrate sheet 17 into individual mounting substrates 2. Line A-A in FIG. 5B indicates an exemplary cutting position.

With the manufacturing method as described above, both main surfaces of mounting substrate sheet 17 are flat except for the division grooves, so that the creamed solder can be coated on external terminals 5 formed on the surface of mounting substrate sheet 17 by a printing method. Therefore, the step for coating the creamed solder can be facilitated, as compared with a conventional coating step which involves coating creamed solder using a dispenser. Also, since this manufacturing method collectively bonds a plurality of crystal units 3 onto mounting substrate sheet 17 to create a plurality of crystal oscillators, integrally tests each oscillator for oscillation characteristics through measurement terminals 19, and eventually divides mounting substrate sheet 17 into individual crystal oscillators, the productivity can be improved in the manufacturing of the surface mount crystal oscillators.

In the respective embodiments described above, a glass epoxy material is used as a base material for making the printed wiring boards which constitute mounting substrate 2 and mounting substrate sheet 17, but the present invention is not limited to this particular material. Any arbitrary printed wiring board can be used for mounting substrate 2 and mounting substrate sheet 17 if it is formed with a wiring pattern on a laminate made up of an insulating material and metal foil adhered thereto.

Claims

1. A surface mount crystal oscillator comprising: crystal unit having a crystal blank hermetically sealed in a package, and first external terminals formed on an outer bottom surface of the package; andmounting substrate for containing an IC chip which has an oscillation circuit integrated therein, said oscillation circuit using said crystal unit, said mounting substrate including second external terminals corresponding to the first external terminals on one main surface, and mounting terminals on the other main surface,wherein said mounting substrate comprises a printed wiring board made up of a lower layer, an intermediate frame layer, and an upper layer laminated one on another, said intermediate frame layer having an opening, said opening defining a hollow space sandwiched by said lower layer and said upper layer, said IC chip being contained in said hollow space, andsaid first external terminals are bonded to said second external terminals to integrate said crystal unit with said mounting substrate.

2. The crystal oscillator according to claim 1, wherein said first external terminals and said second external terminals are bonded to each other by soldering using creamed solder.

3. The crystal oscillator according to claim 2, wherein said creamed solder is coated on said first external terminals and/or said second external terminals by a printing method.

4. The crystal oscillator according to claim 1, wherein both said crystal unit and said package body have a substantially rectangular planar geometry, said first external terminals are formed at four corners on an outer bottom surface of said package, said second external terminals are formed at four corners on the one main surface of said mounting substrate, and said mounting terminals are formed at four corners on the other main surface of said mounting substrate.

5. The crystal oscillator according to claim 5, wherein said first external terminals are formed to extend from the four corners on the one main surface to positions above said hollow space, and said mounting terminals are formed to extend from the four corners on the other main surface to positions below said hollow space.

6. The crystal oscillator according to claim 1, wherein said printed wiring board is a printed wiring board which uses a glass epoxy material as a base material.

7. A method of manufacturing the crystal oscillator according to claim 4, comprising: using a mounting substrate sheet having a plurality of hollow spaces corresponding to a plurality of said mounting substrates, said mounting substrate sheet being made up of the lower layer, intermediate frame layer, and upper layer which are commonly provided for said plurality of mounting substrates, said IC chip being contained in each of the hollow spaces, said mounting substrate sheet having measurement terminals electrically connected to the IC chips along at least one edge thereof;coating creamed solder by printing on the second external terminals formed on each area which is to serve as said each mounting substrate on said mounting substrate sheet;collectively mounting said crystal units such that the first external terminals are in alignment to the second external terminals in each said area, and bonding said crystal units to said mounting substrate sheet with the melted creamed solder to create a plurality of oscillators;testing each oscillator for oscillation characteristics through the measurement terminals; anddividing, after the test, said mounting substrate sheet into the respective areas so as to separate said mounting substrate sheet into individual mounting substrates.