Application of experimental and finite element modal analysis in development of a novel solder joint inspection systems

Inspection of flip chip solder joints has been a crucial process in the electronics manufacturing industry to reduce manufacturing cost, improve yield, and ensure product quality andreliability. New inspection techniques are urgently needed to fill in the gap between available inspection capabilities and industry requirements of low-cost, fast-speed, and highly reliable inspection systems. The laser ultrasound inspection system under development aims to provide a solution that can overcome some of the limitations of current inspection techniques. This research project is based on laser ultrasound and interferometrie techniques. A pulsed laser generates ultrasound on the chip's surface, and the entire chip is excited into structural vibration. An interferometer is used to measure the vibration displacement of the chip's surface at several points, and solder joints with different qualities cause different vibration responses. By analyzing the surface vibration responses, defects in solder joints may be detected andor classified. In this paper, a broadband laser pulse is used for dynamic excitation of the chip before its vibration signature is analyzed. Experimental modal analysis extracts and estimates the modal information (mode frequencies and mode shapes) from raw vibration signals. Multiple-point surface responses are scanned with an interferometer under repeated laser pulse excitation. Spectral estimation on the raw vibration signals gives dominant-mode frequencies. The corresponding vibrational patterns are obtained by reconstructing the scanned grids using the decomposed signals at each mode frequency. Because we are interested only in the transverse vibration on the chip's surface, and because the chip's length and width are much larger than its thickness, it can be modeled as a rectangular plate with pin supports. Modal analysis by finite element modeling is applied to devices with and without defects. The objectives are to identify the differences of mode frequencies and mode shapes between good chips and chips with defects and to identify measurement points that show maximum difference in the presence and absence of defects.