Application of Synchrotron Radiation X-Ray Micro-Tomography to Nondestructive Evaluation of Thermal Fatigue Damage in Flip Chip Interconnects

Abstract

A synchrotron radiation X-ray micro-tomography system called SP-μCT with a spatial resolution of about 1μm has been developed in SPring-8, the largest synchrotron radiation facility in Japan. In this work, SP-μCT was applied to the nondestructive evaluation of microstructure evolution; that is phase growth, and micro-crack propagation appearing as thermal fatigue damage in solder micro-bumps of flip chip interconnects. The observed specimens have a flip chip structure joined by Sn-37wt%Pb eutectic solder bumps 100μm in diameter. A thermal cycle test was carried out, and the specimens were picked up at any number of cycles. The solder bumps were observed by using SP-μCT at the beamlines BL47XU and BL20XU in SPring-8. An X-ray energy of 29.0 keV was selected to obtain absorption images with a high contrast between the Sn-rich and the Pb-rich phases. Additionally, a refraction-contrast imaging technique was applied to visualize fatigue cracks in the solder bumps. The obtained CT (Computed Tomography) images clearly show the process of phase growth and crack propagation due to the thermal cyclic loading of the same solder bump; such information has not been obtained at all by industrially-used X-ray CT systems. In the initial state, the Pb-rich phase was dispersed with characteristic shape, which appears in reflow soldering process. Remarkable phase growth was also observed clearly as the thermal cycle test proceeded. When the loading reached 300 cycles, fatigue cracks appeared in the corners of the interfaces between the solder bump and the Cu pad. The CT images enabled us to evaluate the lifetime of the bumps to the initiation of fatigue cracks by estimating the increase in a phase growth parameter, which corresponds to the accumulation of fatigue damage in the solder joints. The results showed that the estimated lifetime strongly agreed with the average value, which was determined by SEM (Scanning Electron Microscope) destructive observations. As the thermal cycle proceeded, the cracks propagated gradually to the inner region of the solder bump. From the CT images, the average propagation rate was calculated, and the mean of the total fatigue lifetime was estimated to be less than 1800 cycles. These results show the possibility that nondestructive testing by a synchrotron radiation X-ray micro CT system is useful for evaluating the thermal fatigue lifetime in micro-joints.