Ultra-compact wearable devices used close to the body, such as hearing aids, smart rings, and blood glucose sensors, need to accommodate numerous electronic components within extremely limited space. This has created demand for further miniaturization and lower profiles in 32.768 kHz crystal resonators used as reference clocks.

However, miniaturizing a crystal resonator is not simply a matter of reducing its external dimensions. In the ultra-compact range of 1210 size and smaller, the space inside the package becomes increasingly limited, making it difficult to ignore the effects of even trace amounts of outgassing and other factors that are less likely to pose problems in larger packages.

River Eletec addresses these challenges with MDS (solid-phase diffusion bonding) technology, which eliminates the need for conductive adhesives. This article explains how MDS works, its proven track record in mass production with the TFX-05X, and its application to AT-cut crystal resonators.

​​​ Miniaturization of Crystal Resonators for IoT and Wearable Devices

Ultra-compact wearable devices such as hearing aids, smart rings, and blood glucose sensors need to accommodate numerous components, including communication modules, sensors, power supply components, and batteries, within a limited PCB area. As devices become smaller and thinner, individual electronic components are also required to take up less space and have lower profiles.

The 32.768 kHz tuning-fork crystal resonator is another essential component, serving as a reference clock for RTCs and low-power operation. As applications expand from conventional watches and wristband-type devices to devices designed to be worn in closer integration with the body, crystal resonators must be designed not only for smaller size, but also with lower profiles and reliability in mind.

​​​Growing Demand for Smaller Electronic Components in High-Density Mounting Applications

Ultra-compact wearable devices such as hearing aids, smart rings, and blood glucose sensors need to incorporate numerous functions within limited PCB space. As communication modules, sensors, power supply components, and other components are mounted at increasingly higher densities, crystal resonators must offer both a smaller footprint and a lower profile.

In fact, demand for crystal resonators has expanded from the conventional 1610 size to 1210 and smaller sizes. As surrounding components such as capacitors shrink to 0603 and even 0402 sizes, the height of the crystal resonator can become a determining factor in the overall thickness of the PCB assembly.

Further reductions in device size and thickness therefore require crystal resonators with a smaller footprint and lower profile.

​​​Package Structures Need to Be Reconsidered for 1210 and Smaller Sizes

Miniaturizing crystal resonators to 1210 and smaller sizes is difficult to achieve simply by scaling down conventional structures. As external dimensions decrease, there is less internal space and less flexibility in component placement, making it increasingly difficult to ignore the effects of trace amounts of gases and other factors that pose little concern in larger packages.

For ultra-miniaturization, simply reducing component dimensions is not enough. The entire package must be designed with careful consideration of factors including the internal materials and bonding methods.

In the 1210-and-smaller range, it is important to shift the focus from “how far can conventional structures be scaled down?” to “how can we create a structure optimized for ultra-miniaturization?”

​​​The Impact of Trace Outgassing in Ultra-Miniaturized Crystal Resonators

Conventional-size crystal resonators have a certain amount of space inside the package, allowing the required performance and reliability to be maintained even in structures that use conductive adhesives. However, as crystal resonators are miniaturized to 1210 and smaller sizes, the internal volume decreases, making it increasingly difficult to ignore the effects of trace outgassing that are less likely to pose problems in larger packages.

With ultra-miniaturization, it is important to consider not only the external dimensions, but also the internal materials and overall package structure as part of the design process.

​​​The Effects of Internal Gases Become More Pronounced with Ultra-Miniaturization

As the internal volume of a package decreases, the relative impact of the same amount of outgassing on the internal environment becomes greater. Outgassing refers to trace amounts of gases released from materials such as adhesives.

Even amounts that are unlikely to cause problems in conventional-size packages can alter the internal environment of ultra-compact packages, potentially affecting frequency characteristics due to factors such as trace substances adhering to the crystal surface. For 1210 and smaller sizes, designs must minimize the use of materials that can generate outgassing and maintain a stable internal environment.

​​​Ultra-Miniaturization Creates New Design Challenges

There is nothing inherently problematic about conventional structures that use conductive adhesives. In products measuring 1610 or larger, where there is sufficient internal space, conventional structures can provide the required performance and reliability.

These challenges become more apparent when moving into the ultra-compact range of 1210 and smaller sizes. As the available internal space becomes extremely limited, greater consideration must be given to factors such as outgassing and material placement that are less likely to pose significant issues in conventional-size products.

MDS is not a technology designed to compensate for shortcomings in conventional products. Rather, it is a structure developed to meet the new challenges associated with ultra-miniaturization

​​​MDS (Solid-Phase Diffusion Bonding): Breaking Through the Barriers to Ultra-Miniaturization

Ultra-compact crystal resonators require a fundamental reconsideration of not only their external dimensions but also their internal structure. River Eletec’s MDS (solid-phase diffusion bonding) technology integrates three crystal wafers—the cap, resonator, and base—into a single unit, using the crystal itself as the package.

Because MDS does not use conductive adhesives, it eliminates a source of adhesive-derived outgassing, which can become an issue with ultra-miniaturization, while enabling a structure optimized for the limited internal space available.

​​​A Three-Layer “Crystal × Crystal × Crystal” Case

One of the key features of MDS is its “crystal case,” in which crystal is used for all three layers: the cap, resonator, and base. In conventional structures, the case and crystal element are separate components, whereas MDS uses the crystal itself as part of the package.

The required structures are formed on each of the three crystal wafers, which are then stacked with the resonator sandwiched between the cap and base. This integrates the space needed to protect the resonator with the package itself. In essence, the case is constructed entirely from “crystal × crystal × crystal.”

Even in ultra-compact applications where the number of components and available internal space are severely constrained, this approach facilitates miniaturization through the package structure itself.

​​​How Solid-Phase Diffusion Bonding Integrates the Three Layers

Solid-phase diffusion bonding, the core technology behind MDS, is used to integrate the three crystal wafers into a single unit. A metal layer is formed on the bonding surface of each wafer. The polished surfaces are then brought together and bonded by applying heat and pressure in a vacuum.

Solid-phase diffusion bonding is a process in which metals are joined without melting, with heat causing atoms to diffuse across the bonding interface and form an integrated structure. MDS requires no applied voltage during bonding and does not use conductive adhesives.

By firmly bonding the cap, resonator, and base into a single unit, MDS makes it easier to maintain a simple structure even in ultra-compact packages.

​​​Adhesive-Free Structure Supports Ultra-Miniaturization and Stable Performance

Because MDS does not use conductive adhesives, it eliminates the source of adhesive-derived outgassing altogether. In ultra-compact packages with limited internal volume, even trace amounts of gas can potentially affect performance, making the adhesive-free structure a significant advantage in achieving further miniaturization.

Another key feature of solid-phase diffusion bonding is its ability to minimize stress at the bonding interface. By reducing unwanted stress on the crystal, this approach helps maintain excellent characteristics, including at high frequencies, as well as long-term stability.

Rather than simply scaling down a conventional structure, MDS is a technology that fundamentally redesigns the package structure for ultra-miniaturization.

​​​TFX-05X: Demonstrating MDS as a Mass-Production Technology

MDS is not a technology limited to the research or prototyping stage. River Eletec mass-produces the TFX-05X, a 32.768 kHz tuning-fork crystal resonator featuring a crystal case based on MDS technology.

With maximum dimensions of just 1.2 × 1.0 × 0.35 mm, the TFX-05X is used in ultra-compact wearable devices with stringent mounting-space and height constraints, including hearing aids, smart rings, and blood glucose sensors. In addition to its compact size and low profile, the TFX-05X demonstrates the practical viability of MDS through its mounting strength, long-term stability, and proven track record in the market.