The application of through-transmission acoustic inspection to packaging strategies such as ball grid array (BGA) circumvents many of the problems encountered applying conventional pulse-echo inspection to BGAs. A unique feature of pulse-echo acoustic inspection is detection of delaminations in internal interfaces, which is based on the inversion of the pressure pulse (1800 phase shift) at the delamination. This article describes the advantages and applications of pulse-echo acoustic inspection and presents an experimental comparison of pulse-echo and through-transmission methods. It discusses the difficulties that BGA packages present to pulse-echo inspection, such as the complexity of the pulse overlap problem in pulse-echo inspection of the laminate, the problem of multiple echo signals overlapping in time is the possibility of interference effects, and that phase inversion cannot always be relied upon to assist in delamination detection. Practical considerations for through-transmission inspection are also covered.

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