Study of flip chip solder joint cracks under temperature cycling using a laser ultrasound inspection system

The current techniques for nondestructive quality evaluation of bumped solder joint connections in electronics packages are either incapable of detecting solder joint cracks, or unsuitable for in-line inspection due to cost and throughput reasons. As an alternative, a novel solder joint inspection system is being developed. The system projects short laser pulses onto specimen surface through optic fibers to induce structural vibration. The induced surface displacements are then measured by a laser Doppler vibrometer. A number of digital signal processing algorithms, such as error ratio and correlation coefficient, were used to analyze the vibration responses and identify defective specimens. This paper presents a systematic study of parameters such as error ratio, correlation coefficient, resonant frequency, electrical resistance, and the extension of solder joint cracks, to quantitatively characterize the relationships among them. These relationships verified that the laser ultrasound inspection system provides a reliable and efficient way to evaluate flip chip solder joint cracks nondestructively.