TECHNICAL INFORMATION

Published in "AEI October 2005"
Technologies Reduce Size of SMD Quartz Crystal Resonators

 As mobile communication devices acquire more functionality and features and electronic equipment, such as digital tuners and digital cameras, evolve with more advanced technologies, higher-density packaging becomes a must for electronic devices. At the same time, the reduction in sizes of electronic parts and components continues. For quartz crystal resonators, often used as reference-signal sources in circuit applications, further downsizing in required.
 Because of the direct proportional relationship between the oscillation frequency and the current consumption of most circuits including oscillator circuits, lower frequencies mean lower power consumption than in higher frequencies. However, as MHz-band operating AT-cut quartz crystal resonators' sizes are scaled down, their quartz crystal blanks also become smaller. This makes it difficult to attain lower oscillation frequencies. Thus, crystal designers are faced with the challenge of developing small, yet low frequency, low-impedance quartz crystal resonators.

Technologies for Small, Low Frequency Resonators

 The sizes of SMD quartz crystal resonators manufactured by Daishinku Corp. are related to frequency segments (Fig.1). A comparison between the 8045- and 2520-sized quartz crystal resonators reveals that the larger 8045-sized resonators have superior resonant frequency reduction and impedance characteristics than the 2520-sized resonators. The latter incorporate a smaller quartz crystal blank with reduced impedance characteristics. Moreover, there is a limit to how closely quartz crystal blanks can be precision-machined. To resolve this problem, design engineers need to develop ways of ensuring the biggest possible space within the limited dimensions of the outer casing that can accommodate the largest possible quartz crystal blank.
 In this connection, a method for hermetically sealing the package becomes extremely important.
 Sealing methods for SMD crystal resonator packages include seam-weld sealing, low-melting-point glass sealing, and Au-Sn sealing. Although seam-weld sealing is the most commonly used for 3225-sized and smaller quartz resonator packages, Daishinku engineers, after conducting various studies, have decided to adopt the low-melting-point glass sealing method to reduce the sizes of resonators and lower their resonant frequency.
 This method offers several size-reduction advantages in terms of internal space, productivity, mechanical strength and environmental protection.

Model names and frequency ranges of Daishinku Corp.’s SMD quartz crystal resonators
Fig.1: Model names and frequency ranges of
Daishinku Corp.'s SMD quartz crystal resonators

Inner Cavity

 Previously, and in most cases, low-melting-point glass formed concave sealing caps. However, a sealing joint formed by this method retained sufficient strength even when the side walls of the base are reduced in thickness. Since it has become possible to reduce the thickness of the base's side walls, engineers have adopted a sealing method employing a flat-shaped cap and low-melting-point glass, which helped ensure spacious inner cavities. This has enabled Daishinku to place the industry's largest-sized quartz crystal blanks into those cavities.

Productivity

 Unlike seam weld sealing, which must be performed piece by piece, low-melting-point glass sealing can be done in a batch-processing fashion. Moreover, while the mechanism of seam weld sealing is extremely complex and expensive, the facility with which to do glass sealing is a simple continuous furnace, ensuring excellence in both price and throughput. This means that glass sealing offers an excellent means of sealing, and offers a huge cost advantage.

Mechanical Strength

 Conventional low-melting-point glass sealing used to be inferior to seam weld sealing in terms of on-board bending strength. This is because the on-board bending strength declines compared with seam weld sealing if the package size and the spaces between adjacent underside pads are large, on account of the brittleness of joints that are formed with glass. Daishinku has done away with the conventional concave cap shape in favor of a flat cap shape in order to increase the distance between the board and the joint.
 In addition, the company has increased bonding strength by forming inner meniscuses of sealing glass using a proprietary technology (Fig.2). As a result, the company achieved on-board bending strength and impact resistance equal to or better than those of seam weld sealing, and which do not become an issue in practical use.

Conceptual cross-sectional renderings showing inner meniscuses of sealing glass
 
Fig2: Conceptual cross-sectional renderings showing inner meniscuses of sealing glass

Environmental Protection Measures

 Lead contained in sealing glass material used in low-melting-point glass sealing, which is found in glass sealing material for electronic parts, is exempted from the RoHS Directive. However, because a number of set manufacturers want to purchase only lead-free products, Daishinku has developed lead-free glass sealing, which is being introduced into the company's volume production lines.

SMD Quartz Crystal Resonators With Lower Resonant Frequency

 The 3225-sized DSX321G is a product series developed using the previously discussed technology. Requests for a smaller product have led the company to develop the 2520-sized DSX221G as an extension of the design concept of the DSX321G. External dimension drawings are shown in Fig.3, while specifications are shown in Table 1.
 The two quartz crystal resonator product series have the following features:

Dimensions of DSX321G and DSX221G
 
Fig.3: Dimensions of DSX321G and DSX221G

 

Table 1: Standard specifications of DSX321G and DSX221G

Standard specifications of DSX321G and DSX221G

DSX321G

Range of Resonating Frequencies Available
 The 3.2*2.5*0.75mm DSX321G resonators are available at frequencies 9.8MHz and higher, which gives the product an industry-leading low-frequency resonation capability. A 9.8MHz quartz crystal resonator used in remote keyless entry (RKE) systems comes with temperature characteristic (Fig.4), and 60Ω and higher in impedance, which can be rated as an extremely excellent characteristic.

Temperature characteristics of 9.84375MHz DSX321G
Fig.4: Temperature characteristics of 9.84375MHz DSX321G

Frequency-Pulling Sensitivity Characteristic
 The ability to put a large quartz crystal blank inside a package offers advantages in terms of frequency-pulling sensitivity. For example, the frequency-pulling sensitivity characteristic of a DSX321G-series resonator at 26MHz (Fig.5) compares favorably with the 5032-sized DSX531S resonator of the seam welding sealed type in terms of the above-mentioned characteristic.

Frequency-pulling sensitivity of DSX321G and DSX531S at 26MHz
Fig5: Frequency-pulling sensitivity of DSX321G and DSX531S at 26MHz

DSX221G

Range of Resonating Frequencies Available
 DSX221G-series quartz crystal resonators, which measure 2.5*2.0*0.75mm, are available at specific frequencies from 16MHz and up even with 38 percent less volume than their DSX321G-series counterparts.
 These DSX221G-series resonators perform very well in the market for 2520-sized resonators in terms of the range of available resonant frequencies. At present, the company is delivering 14.31818MHz samples and is focusing research and development efforts to achieve lower frequency limits.

Small, Low-profile SMD Quartz Crystal Resonators
 To address customer requests for smaller SMD quartz crystal resonators, Daishinku has also products with low profiles. The company used vacuum seam weld sealing instead of glass sealing for these products. Typically measuring 0.5mm (maximum of 0.55mm) in height, these SMD quartz crystal resonators are available in two series: the DSX321SL (3.2*2.5*0.5mm) and the DSX221S (2.5*2.0*0.5mm).
 Despite their small sizes and low profiles, these resonators are being prepared by the company for introduction to market segments where low-impedance characteristics are at a premium.

Future Developments

 With each passing year, market demand is growing for electronic parts that are built smaller and at lower costs than their predecessors.
 The products introduced by Daishinku were developed using optimized versions of conventional construction methods. This enabled costs to be kept at a minimum, while electrical characteristics, high reliability and similar characteristics similar to those of larger products are retained.
 In order to support the development of applications, Daishinku is committed to broadening of its product lines that cater to the needs of the market with emphasis on the lower end of the radio frequency spectrum.
 The company welcomes feedback and requests from its customers and readers as it works on the development of small quartz crystal resonators.

About This Article
 The author of this article is Makoto Miyagawa. He works at Section1, Engineering Department of Daishinku Corp.