우주용 전장품의 솔더접합부 구조 신뢰성 평가를 위한 수명예측 기법에 관한 연구

Abstract

The Spaceborne electronic equipment controls its intended functions for the spacecraft during the mission lifetime. During the launch, the on-board electronic equipment in the spacecraft experiences various mechanical loads, such as the steady-state static load due to the engine propulsion, sinusoidal and random vibration occurred by engine cut-off, noise of exhaust, and turbulent flows along the launcher. For assuring a successful mission of the satellite, the on-board electronic equipment shall be designed to survive under severe launch vibration environment. For this, a highly reliable life prediction is essential to prevent fatigue failure of electronic equipment induced by an accumulated damage on the solder joint of lead wires of component during the launch. In addition, various types of high-density packages have recently been applied to the spaceborne electronic equipment to improve the functional performance as well as its compactness. Therefore, a highly reliable design procedure is necessary through an early diagnosis of potential risks prior to the manufacturing of qualification model. In the previous study, a Steinberg’s fatigue failure theory has been widely applied to assess the vibration reliability of spaceborne electronic equipment. Numerous vibration tests and finite element model based studies have indicated that the fatigue life of electronic component is mostly related to the dynamic displacements of PCB. Therefore, this theory defines when the maximum displacement of PCB is designed to be less than allowable value, the component can endure 10 and 20 million cycles for sinusoidal and random vibration, respectively. For the structural design of electronic equipment based on the Steinberg’s theory, a structural safety has been evaluated through the margin of safety estimation with respect to the maximum displacement of PCB calculated from the finite element analysis and the allowable displacement calculated from the empirical formula proposed by Steinberg. In addition, a simplified finite elements consisted of rigid element and concentrated mass has been used for modeling the electronic component. However, a higher stiffness value than the actual condition can be applied due to the rigid element as the component size increases. And the accumulated stiffness as the number of mounted components increases is the main factor that causes an error in the vibration response of PCB and the MoS estimation results. In addition, the assessment technique based on the structural safety estimation has some limitation to predict the reliability of electronic equipment because accumulated fatigue damage factor under vibration is not considered. Therefore, this approach is not sufficient for further reliability and life prediction of electronic equipment. To predict the launch vibration fatigue life of electronic equipment, several life prediction techniques based on the theoretical analysis and finite element method were proposed and validated through the actual life tests. However, most of these approaches were validated only for the particular component package type. This means that the reliability of life prediction techniques proposed in those previous studies cannot be assured if the packaging type changes. Therefore, a highly reliable design technique for the life prediction of spaceborne electronic equipment is essential that can reduce the development cost and time. This study investigates the reliable life prediction technique to estimate the structural reliability of spaceborne electronic equipment, which can overcome the technical limitations of the conventional design approaches. For this, the highly integrated spaceborne electronic PCB was selected as an example to demonstrate various fatigue life prediction techniques performed by previous research. As a part of the life prediction, a sherlock, which commercial analysis tool for an integrative life prediction of electrical equipment, was used for the life prediction of the PCB Specimen. Highly reliable life prediction of each failure mechanism resulting from the packaging structure of actual on-board electronic parts is possible by using the sherlock based on the PoF (Physics of Failure). And this has been widely used in diverse areas such as automotive, national defence, medical as well as aerospace. However, the application has not been made in domestic space research region of South Korea. In this study, the reliability estimation results of spaceborne electronic PCB using sherlock was compared with those of conventional life prediction technique. To validate the effectiveness of the conventional life prediction approaches, launch vibration life test of the manufactured PCB Specimen was performed and the fatigue life of solder joint was evaluated through the daisy-chain resistance measurement. The most effective fatigue life prediction technique was derived through the comparison between the prediction and test results of solder joint fatigue life.