Nondestructive Evaluation of Poor-Wetted Lead-Free Solder Bumps in Ball Grid Array Packages Using Laser Ultrasound and Interferometric Technique

A laser ultrasound and interferometer inspection 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 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 amplitudes of the out-of-plane displacement responses 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 the quality of lead-free solder bumps in ball grid array (BGA) packages; specifically, BGA packages with poor wetted solder bumps 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 deteriorates electrical and thermal performance because of the unreliable connections between the package and the printed circuit board (PCB). Causes of poor wetting vary from the solder materials themselves to the manufacturing process. Here, poor wetting of solder bumps were intentionally created by using an improper reflow profile. The transient out-of-plane displacement signal responses from these packages are compared to a hybrid reference signal generated by signal responses from defect-free samples. Packages with poor wetted solder bumps are distinguished from the normal packages by using the correlation coefficient signal analysis method. Then, laser ultrasound inspection results are also compared with results from continuity test, X-ray inspection and scanning acoustic microscope (SAM). Finally, the cross-section images are further used to confirm the existence of the poor wetting in samples with unusual correlation coefficient. It can be concluded that this laser-ultrasound system is - apable of identifying the presence of poor wetting in BGA packages.