Simultaneous Nondestructive Thickness and Acoustic Parameter Determination of Sandwiched Thermal Interface Material Using Ultrasonic Reflection Coefficient Spectroscopy

Thermal interface material (TIM) is crucial for heat dissipation of microchips, and their mechanical performance evolution is a key factor for thermal performance and reliability degradation in long-term service. Conventional mechanical characterization methods for TIM are destructive or conducted on standalone bulk samples, thus inefficient or cannot provide a clear understanding of mechanical performance degradation process. This work presents a nondestructive characterization method for TIM in sandwiched structure of Si-TIM-Cu using a flat ultrasonic transducer based on the ultrasonic reflection coefficient spectroscopy. The bond line thickness (BLTs) and acoustic parameters, including longitudinal wave speed, mass density, and attenuation of TIM that are related to mechanical properties, are simultaneously determined by minimizing the difference between theoretically calculated and experimentally measured reflection coefficient spectra using the particle swarm optimization (PSO) algorithm. The acquired thickness, longitudinal wave speed, and mass density of the TIM are in good agreement with the reference values measured on bulk samples, with maximum relative errors less than 7.25% and averaged errors less than 5.42%. This ultrasonic method can be further combined with the scanning acoustic microscopy to reveal the relation between mechanical and morphological degradation for better understanding of failure mechanism during material development of TIM.