A novel approach for flip chip solder joint quality inspection: laser ultrasound and interferometric system

A novel, noncontact, nondestructive approach for flip chip solder joint quality inspection is presented. In this technique, a pulsed laser generates ultrasound on the chip's surface, exciting the whole chip into a vibration motion. An interferometer was used to measure the vibration displacement of the chip's surface. Because changes in solder joint quality produce a different vibration response, a value, "error ratio," is used to measure the difference between a good chip and a chip with defects. An automatic signal-processing algorithm to calculate the error ratio was developed and implemented, as well as a frequency analysis algorithm. The inspection system was characterized, and results are presented for two cases of flip chips with missing solder balls. Results indicate that a laser ultrasonic/interferometeric system offers great promise for solder bump inspection in flip chip, BGA, chip scale, and micro BGA packages.