Quality Evaluation of Solder Bump in PBGA Package for Commercial Product Application Using Laser Ultrasonic Technique

A laser-ultrasound inspection (LUI) system has been successfully applied to detect solder bump defects including missing, misaligned, open, and cracked solder bumps in flip chips, land grid array packages and chip capacitors. This system uses a pulsed 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 amplitudes of the out-of-plane displacement are usually in the order of nanometer. The quality of solder bumps is evaluated by analyzing the transient responses. In this paper, this system is used to evaluate quality of solder bumps in plastic ball grid array (PBGA) package on a commercial product. Each chip was divided into 4 quadrants during testing and signal processing. Test results showed that LUI technique is capable of separating good chips from defective chips. Furthermore, LUI technique is able to reveal defect severity at each quadrant for each chip. Finally, chips were cross-sectioned and defects such as open and cracked solder bumps were observed. The cross-section results correlated well with LUI test results. This study demonstrates the feasibility and capability of this system on evaluating the solder bump quality on commercial products.