Solder-joint inspection using dry-contact ultrasonic device

A dry-contact ultrasonic device is developed for inspection of solder joints. The device enables us to inspect packages in a non-wetting manner by the aid of the interface formed by inserting a plastic film between water and the package. The jointing points in a wafer level package, which contained open circuit between the metal posts and the solder balls, was imaged by the device. The open points were detected clearly, and effective transmission of higher frequency ultrasound over the immersion device was achieved, where the inserted film performed as an acoustic matching layer. It was confirmed by both theoretical calculation and the experiment that the device realizes high lateral resolution similarly to the immersion one.