Defect Detection of Flip Chip Solder Bumps With Wavelet Analysis of Laser Ultrasound Signals

Microelectronics packaging technology has evolved from through-hole and bulk configuration to surface-mount and small-profile ones. In surface mount packaging, such as flip chips, chip scale packages, and ball grid arrays, chips/packages are attached to the substrates/printed wiring board (PWB) using solder bump interconnections. Solder bumps hidden between the chips/packages and the substrate/board are no longer visible for inspection. A novel solder bump inspection system has been developed using laser ultrasound and interferometer techniques. This system has been successfully applied to detect solder bump defects including missing, misaligned, open, and cracked solder bumps in flip chip packages, chip scale packages and land grid arrays. The system uses a pulsed Nd:YAG laser to induce ultrasound in the thermoelastic regime and the transient out-of-plane displacement response in nanometer scale on the package surface is measured using the interferometer technique. In this paper, wavelet analysis of laser ultrasound signals is presented and compared to previous signal processing methods, such as error ratio and correlation coefficient. The results show that wavelet analysis increases measurement sensitivity for inspecting solder bumps in electronic packages. Laser ultrasound inspection results are also compared to X-ray results. In particular, this paper discusses defect detection for a 6.35 mm × 6.35 mm × 0.6 mm PB18 flip chip package and flip chip package (¿SiMAF¿) with 24 lead-free solder bumps. These two types of flip chip specimens are both nonunderfilled.