The semiconductor packaging industry is spending a great deal of time and effort developing thinner and thinner packages. Ultrasound works by measuring the amount of signal reflected from material interfaces. Thin packages create a potential problem because the resultant reflected signal from the die top and die attach layers can be nearly superimposed in the ultrasonic waveform. This makes it difficult to inspect for delaminations at the die attach adhesive interfaces. Generally a preconditioning test is performed to evaluate reliability of semiconductor packages. This includes utilizing a Scanning Acoustic Microscope (SAM) at the beginning and end of the test to aid in the discovery of the existence of physical defects, such as like delaminations or package cracks. In particular and of primary concern are package cracks and delaminations caused by moisture being absorbed under ambient conditions. This paper discusses the kind of failure mechanism associated with moisture absorption in plastic encapsulated semiconductors and the methodology of using SAMs to find failures such as cracks or delaminations grown during the preconditioning test.