Abstract
In this paper, we show an effective transmission technique of higher frequency ultrasound into a wafer level package. The high quality acoustic image of an interface in the package can be acquired by the technique, and the package is not wetted by water during the acoustic imaging. The technique transmits the ultrasound through the polyvinyl chloride layer inserted between water and the package, and the pressure of about 0.1 MPa is applied to the interface between the layer and the package. Furthermore, by selecting the layer thickness as the layer can work as the acoustic matching layer between water and the silicon chip, the signal amplification for improving the signal-to-noise ratio and the signal modulation toward the higher frequency components for improving the spatial resolution are realized. In practice, we carry out the solder joint inspection of a wafer level package, and the detectability of the defective joint is improved remarkably by the present dry-contact technique as compared with the usual water immersion technique.
