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Among the Worldfs Smallest Quartz Crystal Units

Small Quartz Crystal Units FCX-07

Introduction

The dominant trends in todayfs electronics are toward smaller, slimmer, and lighter products, primarily in mobile devices such as smartphones. Other key trends include expanding functions and higher performance, as technologies cross-pollinate and find innovative new applications involving various media in various devices. Of the electronic components used to build these devices, quartz crystal units play key roles in regulating device frequencies and providing clock signals.
Because these units rely on the mechanical oscillation of a quartz wafer or blank, they must be produced in hollow, sealed packages. These requirements have delayed miniaturization compared to other components such as capacitors or resistors. In responding to trends in electronics, manufactures have energetically sought to identify or create smaller, thinner components, driving advances in smaller quartz crystal units.
In turn, we developed the ultra compact FCX-07, which we introduce here. This AT-cut, MHz range surface-mount crystal unit is among the worldfs smallest.

Product Overview

Photo 1 illustrates the FCX-07. Figure 1 provides its dimensions.

FCX-07
  • Photo 1: FCX-07 appearance

Shaped for easy surface mounting, the unit measures 1.6~1.2~0.4 mm. Compared to conventional AT-cut MHz range quartz crystal units at 2.0~1.6~0.5 mm, the product occupies 60% of the projected area and accounts for 48% of the volume of typical units.

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  • Figure 1: FCX-07 dimensions

Table 1 lists FCX-07 electrical characteristics.
With a frequency range of 24-80 MHz, the unit has a frequency deviation (f/F) of }10 ppm, an indicator of frequency precision. In the future, we hope to achieve even greater precision (for example, on the level of }5 ppm), lower ESR values, and wider supported frequency range.

  • Table 1: FCX-07 electrical characteristics

Product Highlights

Structurally, the FCX-07 consists of a rectangular blank with a surface-mounted drive electrode, connected to a ceramic package that incorporates an interconnect and mounting terminals. The blank is sealed inside the ceramic package by soldering a metal lid to a metalized layer.
An electron beam is used to seal the package (see Figure 2). The workpiece is rapidly scanned in a vacuum by an electron beam, controlled by magnetic deflection, to weld the metal portions and seal the lid shut.

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  • Figure 2: E-beam welding to seal the package

Quartz crystal units produced by e-beam sealing offer several performance advantages, highlighted as follows:

    1. Smaller components
  • The fine-focused electron beam emitted enables precise control over soldering width. In quartz crystal units, this enables a narrower sealing width than conventional seam welding, a significant factor for smaller components.
  • 2. Lower ESR values
  • Smaller packages also require smaller blanks. However, for blanks that oscillate in thickness-shear mode, such as AT-cut blanks, smaller blanks have higher ESR values, reducing the oscillation margin relative to negative resistance in the circuit. With e-beam sealing, packages are, in principle, sealed under a vacuum, so that they maintain a high-vacuum state after sealing. This eliminates gas that would otherwise interfere with blank oscillation and in turn keeps the ESR value low.
  • 3. Easier to support low frequencies
  • With AT-cut quartz crystal units, the lower the frequency (that is, the thicker the blank), the higher the ESR value. Another advantage of the lower ESR value from vacuum sealing mentioned in (2) is that it is easier for these units to support low frequencies.
    To cite an example in short-range wireless systems - where manufacturers are especially eager to find smaller components - the smallest package in a 24 MHz quartz crystal unit for a Bluetooth module was formerly 2.0~1.6 mm. Now, the FCX-07 has the smallest package supporting this frequency.
  • 4. Higher frequency precision
  • With e-beam sealing, each package is sealed quickly in 10 milliseconds. Thanks to localized heating with the fine e-beam, any thermal stress to which blanks are subject is rendered negligible, minimizing any change in frequency to the quartz crystal units during the sealing process due to thermal stress. The resulting components support specifications at higher frequency precision, with frequency deviations of }10 ppm.
  • 5. Aging performance
  • Frequencies of quartz crystal units tend to change over time, due to the gas derived from moisture or oxygen, whether present initially or released over time. However, e-beam sealing reduces this residual or released gas to trace amounts for better aging performance and high-precision, highly reliable quartz crystal units offering the higher frequency precision described in (4).
  • 6. High frequency support
  • To support higher frequencies in AT-cut quartz crystal units, blanks must be cut thinner: for example, 21 m for 80 MHz. From the standpoint of convenient fabrication and mechanical strength, the FCX-07, with a small blank, surpasses conventional components.
  • 7. Enables high productivity and lower costs
  • Because e-beam sealing is much faster than conventional seam welding (as described in [4]), a single e-beam sealing station can be highly productive. The process eliminates the need for a seam ring (as with seam-welded packages) and costly materials (as with packages soldered with gold and tin alloys), reducing material costs.
    RIVER ELETEC first used e-beam sealing for the 3.2~2.5 mm FCX-04. While the technique was ideal for miniaturization, we have variously refined it over time to enable even smaller products. The e-beam spot diameter has been reduced, beam power optimized, and the deflection coil and control system improved, enhancing the precision of beam deflection. Together, these advances make it possible to reduce the area of the FCX-07 to nearly 1/4 the area of the FCX-04.

Besides e-beam sealing, the FCX-07 incorporates a range of new elemental technologies and innovative techniques that further efforts to achieve miniaturization.
The performance of quartz crystal units depends to a significant extent on the performance of the blanks themselves. Smaller quartz crystal units call for smaller blanks, but since blank performance (ESR values in particular) is linked to blank oscillation area, reducing this area generally increases ESR values. While miniaturization was a goal for the FCX-07, we succeeded in reducing blank size significantly while suppressing a rise in ESR value to maintain performance. One way to lower ESR values in smaller blanks is through beveling. This is a type of barrel polishing, but because the shape afterward greatly affects the performance of quartz crystal units (in terms of ESR values, temperature characteristics, and other factors), the machined shape must be controlled with great precision. In consideration of the smaller blank in the FCX-07, an optimal shape was determined, and new fabrication conditions and equipment were introduced to achieve this. Additionally, the process for efficient fabrication of small blanks from raw synthetic quartz was refined, and the raw material is now used more efficiently. Moreover, enhanced mechanical precision of quartz crystal unit assembly and precision of the material itself has enabled fabrication at finer scales. These advances also contribute to significantly smaller product dimensions.

Conclusion: Implementation Examples, Future Prospects

Examples of FCX-07 applications include many mobile devices for which small components are critical. FCX-07 applications include digital tuners and wireless modules such as WLAN modules. In the circuitry of mobile phones, these wireless functions are generally implemented by mounting modular components on the main board, making smaller, thinner modules essential. Recently, some tiny modules measuring less than 10~10 mm have been used. The need for even smaller quartz crystal units in these modules is more pressing than in other applications, making the FCX-07 a highly sought-after component. Looking ahead, we anticipate use in electronic devices that need to be smaller and more advanced, including compact medical devices. As part of efforts to meet a wide range of market needs, our targets include a broader supported frequency range, higher frequency precision (for communication devices and other applications), greater reliability (for in-vehicle systems), and other advances.


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