With quality and reliability improvements, plastic devices are widely used in aviation, aerospace, and military industries fields. Package defects are common reliability issues of plastic encapsulated semiconductor devices. Scanning acoustic inspection is an effective nondestructive testing method for plastic device package defects, which is based on ultrasonic reflection imaging technology. The material reliability, interface delamination, and corresponding failure mechanisms are introduced in this paper. The scanning acoustic inspections of exposedthermalpad construction, nonwirebonding construction, and BGA package are discussed in the paper. The author presents the application precautions of scanning acoustic inspection technology for plastic package defects detection. The differences between GJB standards and MIL standards are also discussed. The author makes proper suggestion for package defect criteria.