Acoustic analysis of ultrasonic assisted soldering for enhanced adhesion

Ultrasonic soldering utilizes high-intensity acoustic fields to induce cavitation in the solder melt in order to (i) bond dissimilar materials and (ii) improve solder joint strength. The acoustic energy transfer from the piezoelectric transducer (PZT) into the liquid solder pool is critical in both understanding and optimizing this process. We use finite element analysis of the acoustics and compare with experiment. Our finite-element modeling approach is two-pronged; (i) we develop a one-dimensional model that is used as a design tool to optimize the solder stack geometry to match the transducer frequency for maximal acoustic energy transfer and (ii) we use a three-dimensional model to compute the frequency response in the solder stack assembly (solid acoustics) and the acoustic pressure in the liquid solder pool (solid-fluid interaction). The acoustic pressure is a proxy for cavitation and therefore bond strength. Our simulations show the acoustic pressure rapidly decreases as the height of the solder tip above the substrate surface increases, which correlates with controlled experiments that show the solder bond quality also decreases with increasing tip height.