Evaluation of Thermal Crack Propagation in Die-attached Joints Due to Cyclic Energization by Synchrotron Radiation Laminography Monitoring

Abstract

In this study, in order to evaluate the fatigue crack propagation in solder joints of printed circuit boards (PCBs) under a condition close to the actual temperature change and distribution, a die-attached-type specimen that generates heat owing to energization was fabricated. Subsequently, synchrotron radiation X-ray laminography was employed to observe the fatigue crack propagation process. The specimens included a typical die-attached joint structure, in which five square Al2O3 ceramic dies of side 3 mm are mounted in a cross shape at intervals of 2 mm and in the center position of a square FR-4 substrate of side 40 mm, and subsequently joined by Sn-3.0Ag-0.5Cu solder layers. The following results were obtained. First, the image quality of laminography was evaluated by comparing the obtained laminography images with scanning electron microscope (SEM) images of the same cross-section of the specimen. In the specimen, it was possible to observe cracks with an opening over several micrometers using the laminography system. In addition, a verification test was conducted to determine whether in situ observation is possible while applying thermal load via energization. Consequently, through continuous monitoring of the same position in the same specimen, it was confirmed that the quality of the obtained laminography image in the specimen undergoing energization was equivalent to that in a non energization state, provided that the temperature of the specimen was stable after sufficient time had passed. Subsequently, to observe and quantify the thermal fatigue crack propagation process, a thermal cyclic loading was applied to the specimen, laminography images were obtained at arbitrary number of cycles, and the cross-sectional area of the cracks was measured. It was observed that the crosssectional area of the cracks increases linearly as the number of cycles increases, and that the average crack growth rate can also be calculated. This will make it possible to estimate the life time of the fatigue cracks generated in solder joints. Finally, for comparison, other die-attached specimens were loaded through thermal cycle tests under accelerated conditions using a thermal shock chamber, which are more severe than the energization test. However, even after the thermal cycle proceeded, there was hardly any change in the laminography images, and no evidence of fatigue crack propagation at the solder joint could be confirmed. The images suggested that interfacial delamination may occur at some boundaries, and this was confirmed through SEM observations of the same specimen.