Introduction
Crystal units are essential electronic components used in a wide range of applications, including automobiles, smartphones, personal computers, industrial equipment, and communication systems.
As a key frequency reference device, a crystal unit provides the precise timing and frequency control required for modern electronic equipment. To ensure high frequency stability, accuracy, and reliability, crystal units must be manufactured through highly controlled and precise production processes.
Despite their widespread use, few people have the opportunity to learn how crystal units are actually made.
In this article, we take a closer look at the crystal unit manufacturing process, explaining the 10 key manufacturing stages that transform raw synthetic quartz into high-performance crystal devices.
1. Crystal Growth (Growing Synthetic Quartz)
The synthetic quartz used as the raw material for crystal units is grown in a special high-temperature, high-pressure vessel called an autoclave.
Natural quartz chips known as lascas are dissolved under high temperature and pressure and then recrystallized onto seed crystals to produce large, high-purity synthetic quartz crystals.
This process is one of the most critical steps because it has a significant impact on the performance of the finished crystal unit.
2. Cutting and Angle Measurement
The grown synthetic quartz is cut to the specified angle and thickness according to product requirements.
The cutting angle has a direct impact on crystal unit performance, including temperature stability and frequency characteristics, making highly precise machining essential.
After cutting, the angle is measured and the crystal blanks are sorted according to specifications. For example, the widely used AT-cut is produced by cutting the crystal parallel to a plane inclined approximately 35°15′ from the Z-axis.
3. Polishing
The operating frequency of a crystal unit is determined by the thickness of the quartz blank.
In general, thinner quartz blanks produce higher oscillation frequencies.
To achieve precise frequency control and optimal crystal unit performance, the quartz blank is polished with micron-level accuracy using carefully selected abrasive materials and grain sizes.
4. Shaping
The polished quartz blank is machined into the required shape and dimensions according to its intended application.
Circular and rectangular quartz blanks are commonly used in crystal units.
In some cases, bevel machining (edge chamfering) is also applied to improve vibration characteristics and suppress unwanted vibration modes.
5. Etching and Cleaning
The cutting and polishing processes introduce microscopic machining stresses into the quartz surface.
Because these stresses can adversely affect vibration characteristics, they are removed through chemical etching1.
The quartz is then thoroughly cleaned to remove any residual chemicals and contaminants, ensuring a high-quality surface.
1 Chemical etching is a process that removes unwanted material by dissolving it with chemical solutions such as acids or alkalis.
6. Electrode Formation
A crystal unit requires electrodes in order to vibrate in response to electrical signals.
The electrodes are formed using either a sputtering system2 or a vacuum deposition system3. These processes deposit metal electrodes onto the quartz blank, allowing voltage to be applied and enabling stable oscillation through the piezoelectric effect.
2 Sputtering: A thin-film deposition process in which ions strike a metal target in a vacuum, ejecting metal atoms that are deposited onto the quartz surface.
3 Vacuum deposition: A thin-film deposition process in which metal is heated and evaporated in a vacuum, allowing the vaporized metal to condense on the quartz surface.
7. Assembly
The quartz blank with formed electrodes is bonded to the electrode terminals on the package base during the crystal unit assembly process.
Conductive adhesive is commonly used to securely mount the quartz blank while providing a reliable electrical connection between the crystal electrodes and the package terminals.
This assembly step requires extremely high precision to maintain optimal vibration characteristics, frequency stability, and overall crystal unit performance.
8. Frequency Adjustment
Frequency adjustment is performed to compensate for slight variations that occur during manufacturing.
Small differences in quartz blank thickness and electrode deposition can cause frequency deviations, making precise frequency trimming essential for achieving the required performance and frequency stability.
Typically, the electrode thickness is finely adjusted to bring the oscillation frequency within the specified frequency tolerance defined by the product specifications.
9. Sealing
To protect the assembled crystal unit from external environmental factors such as moisture, contamination, and oxidation, the package is hermetically sealed.
Common crystal unit sealing methods include seam welding, glass sealing, and gold-tin (AuSn) sealing. The most suitable sealing technology is selected based on the required hermeticity, long-term reliability, and application requirements.
During the crystal unit packaging process, the package cavity is either evacuated or filled with an inert gas atmosphere⁴. This helps prevent electrode oxidation, moisture-related degradation, and contamination, ensuring stable frequency performance and long-term crystal unit reliability.
4 An inert gas atmosphere is an environment in which oxygen and moisture are removed and replaced with inert gases such as nitrogen, argon, or helium.
10. Inspection
The final step in the crystal unit manufacturing process is inspection, which includes comprehensive electrical performance testing and quality control procedures.
Only crystal units that meet all quality and reliability standards are approved for shipment. They are then used in a wide range of electronic devices, including automobiles, smartphones, personal computers, industrial equipment, and communication systems.
Key Factors in Crystal Unit Manufacturing
The following factors are particularly important in manufacturing high-quality crystal units:
- High-purity synthetic quartz growth
- Precise crystal cutting angle control
- Micron-level polishing technology
- Highly accurate frequency adjustment
- Rigorous quality inspection
Through these manufacturing processes, crystal units achieve outstanding frequency stability and reliability.
Conclusion: High-Performance Crystal Units Start with Precision Manufacturing
Crystal unit manufacturing involves a series of highly precise processes, from synthetic quartz growth and crystal cutting to polishing, electrode formation, assembly, hermetic sealing, and final inspection.
Each stage of the manufacturing process plays a critical role in achieving the frequency stability, reliability, and performance required for today's electronic devices. Advanced manufacturing technologies and rigorous quality control are essential to ensure consistent product quality.
At NDK, we operate a fully integrated manufacturing system that covers everything from synthetic quartz growth to the production of finished crystal devices. This one-stop manufacturing approach enables us to deliver high-quality, high-precision crystal units and crystal devices for a wide range of applications, including automotive, industrial, communication, and consumer electronics.