Ultrasonic inspection of package internal defects considering multiple interface effects

Scanning acoustic microscopy (SAM) is the most commonly used method to inspect internal defects of electronic packages. However, in practice, the multiple interface effect will bring many difficulties for ultrasonic inspection. Thus, understanding the physics is the key to indicate defects correctly. In this paper, quad flat no-lead (QFN) package is chosen as the test vehicle. The SAM working principle and pulse analysis are introduced with details. For comparison, the QFN package is also scanned by Sonix SAM after three times reflow with MSL-1 preconditioning. From the scanning results, it is found that the multiple interface effect can be avoided completely using the combination method with C-scan and T-scan modes.