Abstract:
Objective Flip-chip technology is widely used in advanced microelectronic packaging due to its high signal transmission speed, low interconnection loss, and compact structure. However, with the continuous miniaturization of solder bumps and their operation under harsh conditions, internal defects such as missing solder bumps seriously threaten packaging reliability. Laser ultrasonic testing based on an optical microphone is an emerging non-contact nondestructive technique with high sensitivity and wide bandwidth. This study evaluates the feasibility of applying all-optical laser ultrasonic testing for high-resolution detection of solder bump defects in flip-chip structures.
Methods A three-dimensional thermo-mechanical coupled finite element model containing missing solder bumps is established to investigate the effects of defects on temperature and displacement fields. Simulated flip-chip specimens with missing solder balls (600 μm diameter and 400 μm height) are fabricated and inspected using an all-optical laser ultrasonic system based on an optical microphone. Laser ultrasonic A-, and C-scan imaging experiments are conducted. A polynomial fitting coefficient method is proposed for C-scan data processing.
Results and Discussions Simulation results show that missing solder bumps cause significant variations in temperature and displacement fields, which affect laser-induced ultrasonic responses. Experimental results demonstrate that the proposed system achieves high-resolution imaging of missing solder bumps with a diameter of 600 μm. Clear distinctions between normal solder bump regions and defect regions are observed in A- and C-scan images (Fig.6 and Fig.7). The proposed polynomial fitting method provides the highest signal-to-noise ratio, achieving a 5.26-fold improvement over the original signal and outperforming PCA and ICA methods (Fig.8).
Conclusions Both simulation and experimental results demonstrate that all-optical laser ultrasonic testing based on an optical microphone enables non-contact, high-resolution detection of missing solder bump defects in flip-chip packaging, providing a promising approach for reliability inspection of advanced microelectronic interconnections.
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