| Hybrid integrated circuit(HIC) has been widely used in the fields of aerospace and aeronautical engineerings, military equipment, automotive electronics and household electrical appliances, due to characteristics of HIC, such as small volume, high packaging density, high power and high reliability; in particular the hermetical metalpackaging(or sealing)structure can effectively insulate the HIC without the contact to oxygen and moisture in air and corrosive medium. However, during the service, the HIC hermetical metal sealing structure may fail frequently in the form of cracking or separation between the cover plate and base structure due to the vibration load, which may consequently lead to the failure of HIC.Thus far, very limited work on thefailure issue of HIC hermetical metal sealing structure under random vibration load can be seen in China. In this thesis study, both finite element simulation and experimental techniques are employed to develop an assessment methodology to predict the fatigue life of HIC hermetical sealing structure under random vibration loading, and reliability optimization design is carried out to improve fatigue performance of the structure.Thedevelopment of fatigue life prediction method consists of following steps. The first was to estabilish athree-dimensional(3D) finite-element model of the sealing structure based on an extensive literature survey and measurement of the sample's dimensions. Then,the numerical simulation method was used to perform modal analysis and random vibration analysis. Afterward,the obtained modal parameters(data) and random vibration response data were verified by modal test and random vibration experiment. Furthermore, after having a nicecoincidencebetweennumerical simulation andexperiment results, the stress response power spectral density(PSD) at the critical position of the sealing structure was obtained and then transformed to load-time data in time domain through the inverse Fourier transform. Finally,the fatigue life of the sealing structure was predicted by using rainflow-counting algorithm, thefatigue S-N curve of Kovar alloy(i.e., the material used in the cover plate), Miner's damage accumulation rule,which is determined to be 496 hours under the given vibration load in this study. In addition, the factors influencing the fatigue failure of the sealing structure was analyzed, which include stress concentration, surface condition of the structure, geometry and dimension of the structure, measurement condition and service environment; and the cracking mechanism in the sealing structure under vibration load was also discussed.The optimization designfor the sealing structure has beencarried out in two ways so as to improve its fatigue performance. At first, based on finite element analysis results,the effect of the width of weld beam on the first-order modal natural frequency of the sealing structurecan be deterimined, and the results show that the use of weld beam width over a critical value can prevent the sealing structure fromthe mechanical resonancein the frequency range of 20~2000Hz. Then, the optimized weld beam width can be obtained by adjusting the weldingprocess parameters, and consequently the alleviation of structural resonance can be achieved. The second way to optimize the sealing structure is to minimize the stress response under vibration condition, and geometry optimizationhas been performed on the cover plate to reduce the maximum stress response. The simulation results show that the sealing structure with small stress response can be obtained by decreasing the thickness of the lower part or increasing the thickness of the edge partof the cover plate.Thus, the stress response on the critical part of the cover plate can be reduced effectively, which may bring about an alleviation of damage due to vibration stress and the increase of the reliability of the sealing structure under vibration load. |
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