Uncertainty and Reliability Analysis of Chip Scale Package Subjected to Board-level Drop Test

• Main Authors: Masafumi Sano, 佐野雅文
• Other Authors: Kuo-Ning Chiang
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/45241684843926660184

碩士 === 國立清華大學 === 動力機械工程學系 === 97 === Trends in electronic equipment development tend to focus on decreasing size, weight, as well as multiple functionalities. As a result, electronic handheld and portable devices, which are more prone to be dropping during their useful service life, are widely carried out with more capabilities and functions. Therefore, study of the dynamic response of electronic equipment and packages in reliability analysis has become increasingly important. The Board-level drop test method was proposed by the Joint Electron Device Engineering Council (JEDEC) to assess the drop performance of surface-mounted IC packages. However, in actual experiment, the uncertainties of drop conditions such as the impact pulse and screw conditions, etc, may cause the difficulties of repeatability of experiment, which may affect the reliability of the resulting packages. Therefore, the modified input-G method, which considered the screw binding force, was proposed in this research to discuss the uncertainty of drop conditions. After validating the method through an experiment, dynamic simulation of WLCSP structure was conducted using various design parameters to improve the reliability of the package. The input-G method is widely used for conducting drop test dynamic analyses. Nonetheless, there are some limitations to the input-G method. The input-G method is unable to consider the uncertainty of the screw binding condition due to the simplicity of drop test equipment. Therefore, the free-drop model and the modified input-G method were proposed in this research. Moreover, experiment was also carried out to verify these methods. For the reliability analysis, this study focused on the mechanical behavior of the solder joints and investigated the effects of drop conditions and various design parameters, such as chip size, chip thickness, dielectric layer, and ball layout. The results indicate that the simulation from free-drop model and the modified input-G method were close to the experiment data. It can thus be proven that these methods are reliable. In uncertainty analysis, the uncertainty of screw conditions consequently influences the dynamic behavior of the printed circuit board (PCB). Accordingly, the drop-induced stress in the solder joints was influenced by the tightness of the screw. The results reveal that for the WLCSP, the von Mises stress of the solder joint for loose screw binding condition is higher than tight screw binding condition because of less constraint at the PCB hole, the PCB board allowed a large deformation. Therefore, in an actual experiment, it is necessary to monitor screw tightness and re-tighten screws between drops to ensure the good repeatability of the experiment. In terms of design parameters, chip thickness reduction could also improve the package’s reliability due to less stretching occurring in the solder joint. The soft and thick dielectric layer can absorb the impact, thereby protecting the solder joint. Moreover, packages with enhancement balls can also share impact loading and help improve drop impact performance.