Laser ultrasonic technique for evaluating solder bump defects in flip chip packages using modal and signal analysis methods

Current techniques for nondestructive quality evaluation of solder bumps in microelectronic packages have their own limitations and are unsuitable for in-line inspection because of high cost and low throughput. In order to meet requirements of quality inspection of electronic packages, a nondestructive and noncontact solder bump inspection system is being developed using laser ultrasound and interferometer techniques. This system uses a pulsed Nd:YAG laser to induce ultrasound in electronic packages in the thermoelastic regime; it then measures the transient out-of-plane displacement responses with an ultrasonic arrival on the surface of the packages using laser interferometer technique. This paper presents a systematic study of defect detection in flip chip solder bumps using a combined modal and signal analysis method. The correlation between the finite element (FE) modal analysis and wavelet analysis of laser ultrasound signals has been studied. With the help of FE modal analysis, the modes of flip chip packages sensitive to open bumps were predicted for further signal processing before conducting any experiments. The decomposed signal components sensitive to specific defects were extracted and analyzed with discrete wavelet transform. The results show that wavelet analysis increases the measurement sensitivity of the flip chip solder bump inspection compared with existing methods.