A novel method and device for solder joint quality inspection by using laser ultrasound

A novel, non-contact, non-destructive, on-line approach for nip chip, ball gird array (BGA), chip scale and micro EGA 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. Changes of solder joint quality produce different vibration responses. A signal-processing algorithm was developed to measure the difference between a good chip and chip with defects, both in time domain and frequency domain. The inspection system was characterized and results are presented for two cases of flip chips with missing solder balls. Results indicate that this laser ultrasonic/interferometeric system offers great promise for solder bump inspection in flip chip, EGA, chip scale and micro EGA packages.