A novel laser ultrasound and interferometer inspection system has been successfully applied to detect solder joint defects including missing, misaligned, open, and cracked solder bumps in flip chips, land grid array packages and chip capacitors. This system uses a pulsed Nd:YAG laser to induce ultrasound in the chip packages in the thermoelastic regime; it then measures the transient out-of-plane displacement response on the package surface using a laser interferometer. The quality of solder bumps is evaluated by analyzing the transient responses. In this paper, the application of this system is expanded to evaluate quality of lead-free solder bumps in ball grid array (BGA) packages; specifically BGA packages with poor wetting are used as test vehicles. Poor wetting not only decreases the mechanical strength of interconnection at the interface between the solder bumps and substrate, but also increases electrical resistance, which is a reliability issue. Causes of poor wetting vary from materials themselves to manufacturing process. Here, poor wetting of solder bumps were intentionally created by using an improper reflow profile. The transient out-of-plane displacement responses from these packages were compared with the responses from defect-free samples. Solder bumps with poor wetting were distinguished from the normal solder bumps by unusual correlation coefficient. Then, laser ultrasound inspection results are also compared with results from X-ray inspection and continuity test. Finally, the cross-section images were used to further confirm the existence of the poor wetting in samples with unusual correlation coefficient. It can be concluded that this laser-ultrasound system is capable of identifying the presence of poor wetting in BGA packages.

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