An experimental study of failure and fatigue life of a stacked CSP subjected to cyclic bending

During the entire service life of portable electronic products such as cellular phone, game station, and PDA, the printed circuit board (PCB) is subjected to a low frequency random vibration as a result of the keypad depression. It is not sure how this kind of PCB flexing will degrade the structural performance of second level interconnects. Therefore, reliability evaluation of the solder joints becomes very critical as the application of packages into products. In this study, second level reliability of the on-board stacked chip scale package (SCSP) under cyclic bending is conducted to evaluate the structural integrity of interconnects. To understand the impact of keypad depression on the solder joints, the test vehicle is simply supported and subjected to repetitive deflection in the middle. Various cyclic deformation histories such as sinusoidal, triangular, and square types are considered. External loading factors like maximum applied deflection and excitation frequencies are explored to examine their influence on joint reliability as well. It is observed that interconnect fatigue life is strongly degraded by the input deflection level. Furthermore, the effects of different ball size, substrate, and PCB thickness on the fatigue behavior of eutectic solder joints are also probed. The observed fatigue life is characterized by two-parameter Weibull model with cumulative damage plot for each test condition. In addition to the comparisons of characteristic fatigue life of different stand-off/substrate/PCB configuration, and external loading conditions, failure analysis is also employed with consideration given to identify failure sites and mechanisms such as crack initiation and continuous growth to the complete failure of test vehicles