Piezoelectric devices, such as piezoelectric quartz filters, piezoelectric quartz resonators and the like; typically include a piece of piezoelectric material mounted to a substrate. In quartz devices, the quartz element of necessity has thin metallic electrodes attached to it through which electrical signals are coupled into and out of the piezoelectric quartz material. Common problems with piezoelectric devices are adequately isolating the piezoelectric device from mechanical shock and dealing with thermal expansion coefficient mismatches between the piezoelectric material and the substrate material.
Quite often, the piezoelectric devices, such as a piezoelectric quartz material and the substrate have mismatched thermal expansion coefficients. This mismatch can cause mechanical stresses to be induced in the quartz as time goes by during the life of such devices, as the quartz and substrate expand and contract over temperature variations. Further, mechanical shock transferred to the quartz through its mounting structure can increase mechanical stresses that in addition to the thermal stress, adversely affect the output frequency and accuracy of these devices.
Various attempts over the years have been developed in an attempt to compliantly mount piezoelectric quartz devices to a substrate. For purposes of this application, a compliant mount for a piezoelectric device, is a mounting device, apparatus or other mounting means that attempts to reduce or minimize mechanical stresses on the piezoelectric quartz element. Some prior art compliant mounting devices have used thin foil tabs that act as spring-type mounting structures that attempt to isolate the quartz element from its substrate. Other types of compliant mounting structures have attempted to use substrate materials having thermal expansion coefficients which more closely match the thermal expansion coefficient of the quartz material itself.
Most, if not all of the prior art compliant mounting schemes are difficult to use because of the small physical dimensions that modern piezoelectric quartz elements have. Using bent foil tabs alone, for example, to compliantly mount a small silver of quartz onto a substrate is not a structure that lends itself to economic mass production of quartz crystal devices.
A prior art overtone crystal mount structure is shown in FIG. 1. The crystal mount structure 10 includes a substrate 12, conductive pads 14 attached to an external portion of the substrate for electrical connection to the structure 10, a pair of C-shaped mounts 16, a piezoelectric element 18 including a left wraparound electrode 20 and a right wraparound electrode 22 coupled to the C-shaped mounts by an upper adhesive 24. A lower adhesive 26 securely couples the C-shaped mounts 16 to the conductive pads 14. The adhesives 24 and 26 are epoxy.
When an AC voltage is applied across the pads 14, the piezoelectric element 18 vibrates acoustically at a certain displacement. The resonant frequency of the piezoelectric element 18 drifts about a nominal frequency, in part due to changes of temperature, stresses, exposure to mechanical shock, microcracks in the quartz and the like.
There is a need for an improved method of mounting a piezoelectric device with a substrate, to: (i) minimize the mechanical stresses induced due to the thermal expansion mismatches between the two; (ii) provide a mechanically sufficient coupling such that the device can withstand mechanical shocks; and (iii) provide a method of crystal attachment which is adaptable to mass production.
Accordingly, a low cost, readily-manufacturable, compliant mount for a piezoelectric device would be an improvement over the prior art. A method by which quartz devices can be easily and readily attached to a substrate and which isolates the quartz element from mechanical stresses would be an improvement over the art.