우주용 전장품의 솔더 접합부 피로수명 보장을 위한 고댐핑 적층형 전자기판에 관한 연구

Abstract

A wedge lock is used to mechanically fasten a printed circuit board (PCB) in an electronics. The two wedge locks are mounted at the edge of the PCB using screws or rivets. Then, the PCB is inserted into the chassis of an electronics housing structure. Finally, the PCB is fastened by applying the tightening torque to the wedge and making it in close contact with the chassis. This fastening principle has advantages from the point of view of vibration damping induced by friction between the wedge and the chassis, and easier assembly process. In addition, the wedge lock is able to act as conductive thermal path of the electronics. Due to these effectiveness of the wedge lock, it has been widely used in aeronautical, automotive, defense, as well as space industries. Spaceborne electronics is subjected to various extreme vibrations like sine vibration, random vibration, pyro-shock during flight after launch. These vibrations make the PCB in the electronics bent repetitively, which could lead to fatigue fracture of a solder joint connecting an electronic component and the PCB. Thus, high reliable mechanical design of the electronics is necessary for space mission success. Generally, additional mechanical fixations at various locations on the PCB or additional stiffeners are applied in order to secure fatigue life of the solder joint by reducing a dynamic displacement of the PCB under the vibration by increasing a stiffness of the PCB. However, it is difficult to ensure the fatigue life on the solder joint on the PCB applying the wedge lock, although the wedge lock helps to dissipate the vibration energy in some extent. This is because only the edge of the PCB where the wedge locks are located is constrained due to the assembly method of the PCB to the housing. To this day, additional stiffeners which have higher stiffness compared to the PCB like an aluminum, have been mounted on the PCB to assure the fatigue life of solder joint by decreasing the displacement of the PCB under the vibration. However, this design approach induces increase in volume and weight of the PCB assembly. Furthermore, in case of the electronics that accommodates a number of the PCBs, the housing structure would become more bulky. A recent trend of satellite development in space industry has been changed from large satellites of high cost to small satellites relatively economic. According to the change, a demand for small and lightweight electronics has also increased. However, the conventional design approach of PCB has a limitation in miniaturizing the electronics. Therefore, new mechanical design approach for the electronics which could assure fatigue life of solder joint under the vibration and minimize the size of electronics compared to the conventional mechanical design approach is necessary. In this study, a high damping multi-layered PCB using a viscoelastic tape was proposed for the fatigue life of solder joint on the PCB with wedge lock under the vibration, as well as lightweight electronics design. In the proposed PCB, multiple constrained layers are laminated on the PCB employing the tape to realize the high damping capability. This multi-layered structure is effective to attenuate the dynamic displacement of the PCB, because the friction occurred between the constrained layer and the tape under the vibration dissipates the vibration energy. Therefore, the proposed PCB is effective in ensuring the fatigue life of solder joint under the vibration due to high damping capability. Furthermore, it is much smaller and lighter than the conventional PCB assembly. In order to validate the effectiveness of the design, the PCB specimens having different number of the constrained layers were manufactured. A basic characteristic test at different temperature was performed to compare the 1st eigenfrequency and damping ratio. Also, through the random vibration test using the specimens without the electronic components, the damping capability was experimentally validated. Thorough the launch vibration fatigue test on the specimens assembling QFP208 and PBGA388 components, the fatigue life extension effect of electronic components induced by the high damping characteristics was validated, and the structural safety of the PCB related to tape delamination under continuous vibration was confirmed, as well. Finally, the capability of the proposed PCB in enhancing the fatigue life was also validated by predicting the fatigue life based on Critical Strain Theory.