GB2174537A - Crystal oscillator - Google Patents

Abstract

A quartz crystal controlled oscillator contained in a hermetically sealed dual-in-line can (1,2) has its quartz resonator element (28) directly mounted upon a piece of flexible printed circuit board using electrically conductive resin (32,33,34). <IMAGE>

Description

SPECIFICATION Crystal oscillator This invention relates to the mounting of quartz crystal controlled oscillators in hermetically sealed enclosures such as metal cans.

In some designs of quartz crystal controlled oscillators the electronic oscillator circuitry is provided externally to the hermetically sealed enclosure which contains the quartz crystal resonator element, but in other designs the resonator element is encapsulated in a hermetically sealed can together with its associated electronic circuitry. The present invention is concerned with the latter design approach. Typically, though not essentially, the enclosure containing the resonator element and associated electronic oscillator circuitry takes the form of a DIL can. Within the can the oscillator circuitry takes the form of a hybrid circuit with thick film resistors and discrete components supported on a ceramic substrate. The quartz resonator element is spaced from the ceramic substrate, being supported between posts attached to the substrate.These posts need to be compliant so that they do not create temperature and time variables stresses which would degrade the frequency stability of the crystal resonator element from vibrating freely in shear mode.

The present invention is concerned with a design which avoids the requirement to use a ceramic substrate and reduces the number of discrete piece-parts.

According to the present invention there is provided a quartz crystal controlled oscillator in which the quartz crystal resonator element is directly mounted upon a piece of flexible printed circuit board (pcb) which also supports the components of the associated electronic oscillator circuitry, and wherein the piece of flexible pcb is contained within a hermetically sealed enclosure.

It is found that flexible pcb is adequately compliant to permit the direct mounting of the resonator element upon the pcb. A preferred form of enclosure is the dual-in-line DIL can, though other designs may alternatively be employed, including for instance a leadless chip carrier design of enclosure.

There follows a description of a quartz crystal oscillator embodying the invention in a preferred form. The description refers to the accompanying drawings in which Figure 1 is a schematic representation of the flexible printed circuit board of the oscillator, and Figure 2 is a schematic cross-section of the oscillator on the line A-A' of Figure 1.

The can of the crystal oscillator of the drawings is formed in two parts comprising a rectangular DIL can base 1 and a lid 2 hermetically sealed to the base by resistance welding. The base is penetrated near each corner by a terminal pin secured in a glass-to-metal seal (not shown). Housed inside the can is a piece of flexible printed circuit board (pcb) 4 constituted by a polyimide sheet 5 approximately 50 microns thick carrying a copper electrode pattern 6 approximately 25 microns thick.

The four terminal pins penetrate holes 7 in the corners of the polyimide sheet and are secured in position by solder or conductive resin fillets 8 which effect electrical connection between the pins 3 and four terminal pads 9, 10, 11 and 12, formed in the electrode pattern 6 of the pcb. Terminal pad 9 is connected to the positive rail, terminal pad 10 is connected to the output, terminal pad 11 is connected to the earth, and terminal pad 12 is a dummy terminal to which no connection is made.

The pcb carries an integrated circuit chip 13 which is conveniently mounted as a flip chip. This chip has six terminal connections 14 to 19. Terminal connection 14 is to the positive rail pad 9 by way of two intermediate pads 20 and 21. Terminal connection 15 is to the output terminal pad 10.

Terminal connection 16 is to the earth terminal pad 11 by way of a further intermediate pad 22. Terminal connection 17 is to a link pad 23, and terminal connections 18 and 19 are to terminal pads 24 and 25 respectively by which electrical connection is made with the electrodes 26 and 27 provided on the upper and lower faces of a quartz resonator disc element 28 designed to operate in shear mode, typically with a resonant frequency in the range 4 to 20MHz. For resonant frequencies above about 10MHz the resonator element will normally have flat upper and lower faces, whereas for lower frequencies a biconvex shape is preferred. Additionally the printed circuit includes a further pad 29 that is electrically connected by track 30 with the earth terminal connection 11.Pads 21 and 29 cooperate to provide connections for a decoupling capacitor having the form of a multilayer ceramic chip capacitor 31.

The integrated circuit chip 13 is an oscillator that incorporates the resonator element 28 in its feedback path to control the frequency of its electrical output. The design of this chip is such that if terminal connection 17 is left floating, then the output frequency is equal to the resonant frequency, fO, of the resonator element 28.If however, terminal connection 17 is held at earth potential by means of a strap (not shown) between pads 22 and 23, then the output frequency of the oscillator is half fO. Alternatively, if terminal connection 17 is held at the potential of the positive rail by means of a strap (not shown) between pads 20 and 23, then the output frequency of the oscillator is quarter fO. One reason for providing this facility for producing an output signal that is a submultiple of the resonator resonant frequency f" is that it allows lower electrical signal frequencies to be generated without having to change from a flat disc-shaped type resonator to the more expensive bi-convex type resonator. Another reason is that it allows the generation of electrical frequencies down to 1Mhz with crystals down to 4MHz.

The resonator 28 is secured in position by means layers 32 and 33 of silver-loaded electrically conductive epoxy-based resin applied to the two resonator terminal pads 24 and 25. Resin layer 33 also serves to provide electrical connection between pad 25 and the electrode 27 on the lower face of the resonator element 28. Similarly resin layer 32 co-operates with a further silver-loaded electrically conductive epoxy-based resin layer 34 to provide electrical connection between pad 24 and the electrode 26 on the upper face of the resonator. If the thickness of these resin layers is not adequate to provide a safe spacing between the central region of the resonator element and the pcb, then additional clearance may be provided by forming the pcb with an aperture 35.The presence of such an aperture may also be beneficial in reducing the amount of mechanical damping provided by the pcb between the two electrodes of the resonator element. It may also be desirable to provide additional slots for the purpose of stress relief for some specialist precision parts.

In a preferred method of manufacture of the oscillator the flip chip integrated circuit 13 and the chip capacitor 31 are soldered to the flexible pcb 4, and then the pcb is mounted on the DIL can base 1. Uncured silver-loaded epoxy-based resin in the form of paste is next applied to provide the material of the fillets 8 and the layers 32 and 33 before the resonator element 28 is located in position on the pcb. Further paste is then applied to form the material of layer 34. At this step the assembly is ready for placing in an oven to cure the resin, after which the frequency of oscillation is adjusted by conventional means involving the deposition of evoprated silver upon the resonator to provide the requisite amount of additional loading. Finally the cap 2 is secured in position by resistance welding.

Claims (8)

1. A quartz crystal controlled oscillator in which the quartz crystal resonator element is directly mounted upon a piece of flexible printed circuit board (pcb) which also supports the components of the associated electronic oscillator circuitry, and wherein the piece of flexible pcb is contained within a hermetically sealed enclosure.

2. A quartz oscillator as claimed in claim 1, wherein the enclosure is a dual-in-line can.

3. A quartz oscillator as claimed in claim 1, wherein the enclosure is a leadless chip carrier construction of enclosure.

4. A quartz oscillator as claimed in claim 1, 2 or 3, wherein the piece of flexible pcb is provided with an aperture registering with the central region of the quartz resonator element.

5. A quartz oscillator as claimed in any preced ing claim, wherein the flexible pcb is made of po lyimide.

6. A quartz oscillator as claimed in any preced ing claim, wherein said components of the associ ated electronic circuitry consist of a integrated circuit chip and a decoupling multilayer ceramic capacitor chip.

7. A quartz oscillator as claimed in claim 6, wherein the integrated circuit chip incorporates one or more frequency dividers to provide an electrical oscillation frequency that is a submultiple of the resonant frequency of the quartz resonator ele ment.

8. A quartz crystal controlled oscillator substantially as hereinbefore described with reference to the accompanying drawings.