A new concept of acoustic microscopy for ball grid array assembly is reported. Commonly, the acoustic microscopy has to be performed with immersing the package in the coupling liquid. On the other hand, the present technique does not require the immersion of the package in the liquid, because the transduction of high frequency ultrasound is performed thorough a thin solid layer. A theoretical model is shown to perform the transduction of ultrasound more effective than usual immersion, and an acoustic matching layer, which realizes the signal amplification and the modulation, is shown. The acoustic imaging of the solder joints of a wafer level package is carried out by the present dry-contact and usual immersion techniques. The dry-contact acoustic images show the defective joint clearly, and the detectability of the defective joint is improved remarkably as compared with the usual immersion images.

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