Acoustic imaging of bump defects in flip-chip devices using split spectrum analysis

In this paper the performance of multi-narrow-band spectral analysis was evaluated concerning defect detection in microelectronic components with flip-chip contacts. Today, flip-chip technology is widely applied for interconnecting silicon dies to a substrate within high-end semiconductor packaging technologies. The integrity of the bump solder interconnection is of major concern for the reliability in this technology. Non-destructive defect localization and analysis of the flip-chip interconnections operating in a semi-automated mode is strongly desired. Scanning acoustic microscopy (SAM) combined with subsequent signal analysis has high potential for non-destructive localization of defective flip-chip interconnects. Analyzing multiple narrow spectral bands of signals acquired by a scanning acoustic microscope enabled the identification and localization of defective flip-chip interconnects. In the current study a 180 MHz transducer with 8 mm focal length was employed for acoustic data acquisition by SAM. Those data were then analyzed off-line by discrete Fourier transformation, chirp z-transform and cosine transform using custom made MATLAB software. Through multi-narrow band spectral analysis, defective flip-chip interconnects that have not been revealed by standard acoustical imaging methods have been detected successfully. Acoustically found defects have been confirmed by subsequent FIB-cross sectioning and SEM imaging. The high resolution SEM imaging revealed complete and partial delamination at the interface between the die and the bump.