Reliability Design and Simulation of a Surface-Mount Temperature Compensation Crystal Oscillator

SUI Jianping, CUI Wei, HASI Tuya, ZHENG Wenqiang, DUAN Youfeng Beijing Institute of Radio Metrology and Measurement, Beijing 100039

Abstract: This paper introduces the structure and principle of a surface-mount high-precision temperature compensation crystal oscillator, and specifically describes the reliability design of a quartz-crystal resonator used inside the surface-mount temperature compensation crystal oscillator, the reliability design of surface-mount ceramic base and the reliability design of hybrid micro-assembly. Finally, the thermal and mechanical simulation results and the experimental verification of this surface-mount temperature compensation crystal oscillator are provided, which prove that this crystal oscillator has high reliability.

Key words: temperature compensation crystal oscillator (TCXO), high precision, reliability

1 INTRODUCTION

With the broad application of aerospace devices medium and high-precision temperature compensation crystal oscillator(TCXO), it has raised a higher requirement for the TCXO in terms of size, temperature and frequency stability over temperature. Although the common constant temperature crystal oscillator has high frequency stability, the product is of large size,and it also has other shortages such as complicated structure,high cost and high power consumption, which cannot satisfy the requirement of compactness and high reliability. Therefore,compactness, high precision and high reliability of the crystal oscillator are important considerations that need to be considered during the research and development of a surface-mount high-precision TCXO. From its structure and principle, this paper introduces the reliability design and development method for a small SMD5032 high-precision TCXO, and also provides related mechanical and thermal simulation results.

2 STRUCTURE AND PRINCIPLE OF SURFACE-MOUNT HIGH-PRECISION TCXO

The surface-mount high-precision TCXO adopts the hybrid integrated package technique to integrate all circuits and quartz resonator in the SMD5032 base. The real product picture and overall structure are as shown in Figure 1. The filter circuit,principal oscillation circuit, temperature sensor and compensation network are all integrated in the temperature compensation oscillator chip, which is combined with the resonator and packed in the SMD5032 surface-mount ceramic base to form a complete crystal oscillator.

Figure 1 The real product picture (left) and overall structure (right)

The surface-mount high-precision TCXO mainly consists of a quartz resonator, oscillating circuit and compensating circuit,and its operating principle is as shown in Figure 2. The quartz resonator is a key component of entire crystal oscillator. By utilizing piezoelectric effect of the quartz crystal, it can generate a stable frequency signal by cooperating with the oscillating circuit,and the local frequency will be transformed to the required frequency for output.

The frequency of the crystal oscillator will change with the change of environmental temperature, which is mainly caused by the frequency change of resonator due to the temperature.In order to reduce the change of frequency caused by temperature, we need to adopt a frequency compensation network for frequency adjustment. The compensation network perceives the environment temperature through the temperature sensor and adjusts frequency according to the environment temperature, so that high frequency stability can be achieved within the entire temperature scope.

3 RELIABILITY DESIGN OF SURFACE-MOUNT HIGH-PRECISION TCXO

3.1 Reliability Design of Quartz Crystal Resonator

The shape and size of SMD5032 determine that its internal quartz wafer must have a proper size. On the one hand, the reliability design of compact quartz crystal resonator can guarantee its frequency temperature characteristic. On the other hand, it can improve the anti-vibration performance of crystal oscillator.

As the quartz resonator becomes more compact, it also has different frequency temperature characteristics from a large quartz crystal resonator. By changing the length and widthto-thickness ratio w/t of quartz wafer, we inhibit the coupled oscillation in its oscillation mode. During the design qualification stage, multiple tests were conducted by changing the shape and size of quartz wafer, and we compared the jump-point indices of quartz wafer and anti-vibration performance with difference sizes. Finally, we determine the optimal shape and size of wafer.Through the optimization of the design, we can control the curve jump-points of resonator to be within ±0.1×10-6, and the acceleration sensitivity less than 1×10-8/g, which can ensure the necessary compensation precision and reliability.

3.2 Reliability Design of Surface-Mount Ceramic Base

Figure 2 The TCXO operating principle

Reliability of surface-mount ceramic base defines the reliability of the crystal oscillator, so the reliability design of surface-mount ceramic base is the basis and key for the product to realize high reliability. According to its operating principle and characteristics, it is difficult to integrate the quartz resonator, so it must adopt the micro surface-mount ceramic base. In addition,due to smaller base size, the difficulty with the lead-out terminal, wiring and electromagnetic compatibility has been significantly increased. The TCXO has many external ports, which not only include the user ports such as output, power supply and ground connection, but also include other ports used for debugging, such as the SPI serial port and voltage control port. Among its internal ports connected to the chip,the setting of internal ports should be considered on various aspects of the chip, such as power, ground connection, control,debugging, input and output. During wiring to connect its internal ports and external ports, consideration should also be given on the aspects of EMC and board level. Finally, we should also consider other factors, such as the structural strength of the entire base, thermal design, wafer placement location and cover package.

In summary, we optimized the lead-out terminal design,wiring design and electromagnetic design of base, and designed a micro surface-mount ceramic base which can satisfy the product reliability performance, so that the performance of TCXO can be ensured. The base model is as shown in Figure 3.

Figure 3 The TCXO base model

3.3 Reliability Design of Hybrid Microassembly

In order to realize reliability design required by the product,an integrated chip and compact resonator are adopted, which raises a higher requirement for the assembly technology of hybrid integrated circuit. Hybrid microassembly technology is the key to realize product manufacturing, which is also one of the critical production processes. The hybrid microassembly technology for TCXO consists of oscillator manufacturing and crystal oscillator assembly. The oscillator manufacturing process involves cleaning and removing various contaminants from the wafer surface； then, vacuum coating, formation of electrode membrane on the wafer surface, and create quartz oscillator with resonance function. During the crystal oscillator assembly,the quartz oscillator and differential dual-output chip are integrated and assembled in the based through various processes such as chip mounting, chip bonding, adhesive and dispensing,frequency fine tuning and packaging, and a parallel welded package is provided to encapsulate the physical crystal oscillator product.

4 THERMAL AND MECHANICAL RELIABILITY SUMULATION OF SURFACE-MOUNT TCXO

4.1 Thermal Reliability Simulation of Surface-Mount TCXO

In order to ensure that the surface-mount TCXO could maintain high frequency stability over temperature within a wide temperature scope, we use the Ansys software to conduct finite-element analysis of the surface-mount TCXO model and obtain the internal thermal distribution of surface-mount TCXO.By setting the heat source and environment temperature of thermal model, we can obtain the temperature change of various components under different environmental temperatures.Based on that, we can determine the influence of the thermal environment on the frequency stability over temperature of the oscillator, and modify the design of compensating circuit and wafer according to the simulation results. Figures 4 - 6 show the temperature distribution of various components when the oscillator works at normal temperature.

According to the above simulation analysis, we can see that the temperature compensation chip produces the highest heat,which affects the thermal field distribution of quartz wafer. In accordance with the final temperature distribution cloud chart of surface-mount TCXO under room-temperature operating state, the temperature increase has limited influence on the quartz wafer under room temperature, which can satisfy the product requirement for frequency stability over temperature. It verifies the reasonability of the thermal design of surface-mount high-precision TCXO. The experiment result shows that, this TCXO can work between -55℃ to +125℃. The high environmental temperature adaptability of the product is realized.

4.2 Mechanical Reliability Simulation of Surface-Mount TCXO

We input the complete surface-mount TCXO model into the finite-element simulation analysis software, set the mechanical parameters of assembled surface-mount TCXO model, and conducted modal analysis of crystal oscillator. According to the modal simulation results, it can be seen that the lowest-order frequency of crystal oscillator is 1380 Hz, and the second-order resonant frequency is 1416.2 Hz, which satisfies most systems'requirement for the resonant frequency of crystal oscillator components and the reliability requirement. The mode shapes of various orders of resonant frequency of surface-mount TCXO structure are as shown in Figure 7.

Figure 4 The temperature distribution of base

Figure 5 The temperature distribution of crystal

Figure 6 The temperature distribution of TCXO

Figure 7 The mode shapes of various orders of resonant frequency of TCXO

Figure 7 The mode shapes of various orders of resonant frequency of TCXO

Figure 8 The displacement along the three axes of TCXO

After grasping the related information such as the inherent frequency, we can conduct random vibration simulation. According to the qualification test scale required under the national military standard, we conducted the simulation of displacement along the vibration direction to observe the displacement of various parts of crystal oscillator. Figure 8 shows the displacement along the three axes of x, y and z when the vibration direction is perpendicular to the wafer of crystal oscillator (x-axis), and the unit is m in this chart.

According to the simulation results as shown in Figure 9, it can be seen that the displacement along y-axis under random vibration is bigger than that along the other two directions. In the structure, the biggest deformation value is only 0.017 μm with very small displacement, which will not affect the product reliability. When the vibration direction is parallel to the wafer of crystal oscillator (y-axis, z-axis), none of the biggest displacements along various axes exceeds 0.01 μm. The simulation results prove that the surface-mount TCXO has small displacement under the vibration operating state, which satisfies the product requirement for vibration index, and it verifies the reasonability of the mechanical design of surface-mount TCXO.The experiment result shows that, this TCXO can withstand shocks in 10000 g, and the acceleration sensitivity also less than 1×10-8/g. The high mechanical environmental adaptability of the product is realized.

5 CONCLUSION

The surface-mount high-precision TCXO has adopted the hybrid microassembly technology to integrate the temperature compensation oscillator chip with compact quartz resonator and pack them in the SMD5032 surface-mount ceramic base to form a complete compact high-reliability TCXO. Through the thermal and mechanical simulation, we prove that this surface-mount high-precision TCXO has high reliability. The experimental result shows that, this TCXO can withstand shocks in 10000 g, and the acceleration sensitivity also less than 1×10-8/g.The high mechanical environmental adaptability of the product has been realized. which can satisfy the requirements for compactness and high reliability.