Imaging flaws in soldered joints of power integrated circuits using electronic scanning techniques of ultrasound

An electronic scanning c-scopic 25-MHz ultrasonic imaging system has been developed for high-speed precise testing of electronic parts such as ICs. Two-dimensional scanning is performed, one axis being electronic, and the other mechanical. A focused beam is formed so as to be a longitudinal wave in solid samples. Voids equal to or larger than 0.2 mm in diameter in the solder are reliably detected and their sizing data agree well within 0.1 mm with those measured visually through an optical microscope after destruction. The imaging performance is also examined for fatigue cracks in soldered joints. In 1000 heat cycles between -55 and +150°C, cracks are caused in the soldered joints of most samples, and crack growth is clearly visible. By comparing the images with those observed by a scanning electron microscope, it is confirmed that minimum cracks of 1-μm separation are detected