I. Introduction
Flip-Chip technology in combination with ball-grid-array interconnections is applied increasingly in electronic device manufacturing. For the reason of ensuring the connection-reliability appropriate failure analysis methods for nondestructively inspecting the electrical connectors are needed. Due to power dissipation thermally induced mechanical stress is a major issue for the reliability of bonded and soldered interfaces (Omathuna et al. 1994; Reichl, 1994; White 1985). Several techniques for inspecting interconnections in electronic circuits including optical systems, X-ray microscopy, scanning electron microscopy, strength testing and microsection analysis are available. Scanning acoustic microscopy is of increasing interest due to its unique ability to illuminate optically opaque materials, providing depth-specific information and to locally evaluate mechanical properties. Moreover, acoustic microscopy is working non-destructively, which allows the application of several tests at a single device (Briggs et al. 1995).