In Situ Measurement of Elastic Constants and Thickness of Silicon–Underfill–Silicon Sandwiched Electronic Package Using Noncontact Laser Ultrasound Array

Quantifying Young’s modulus and Poisson’s ratio of underfill (UF) is critical in clarifying material failure mechanisms for better optimizing their performance in reducing thermal stresses on solder joints in flip-chip devices. Conventional mechanical methods are generally developed for the pure UF block and cannot directly measure thin UF layers sandwiched in heterogeneous multilayered flip-chip devices during service. Nondestructive ultrasonic methods are more promising but are challenged by complex wave propagation induced by multilayered, anisotropic acoustic properties. This work presents a noncontact laser ultrasonic (LU) method for in situ and simultaneous measurement of elastic constants and thickness of a thin UF layer in a silicon–UF–silicon sandwich structure. Longitudinal and transverse waves in divergent propagation directions are acquired by using a line-scan point laser transmitter and a fixed-point receiver on opposite surfaces. Young’s modulus, Poisson’s ratio, and thickness of the sandwiched UF layer are inversely determined by iteratively matching experimental LU travel times with theoretical predictions considering interface reflection, refraction, and mode conversion effects, with relative errors to ultrasonic measured reference values <3.722%. The in situ LU method is advantageous in continuously monitoring mechanical property evolution during various cyclic tests, advancing reliability assessment and fabrication optimization of UF materials in real-life electronic packaging applications.