Packaging of large and low-pitch size 2D ultrasonic transducer arrays

The successful packaging and electronics integration of large 2D array devices with small pitch-sizes, such as fully populated 2D ultrasonic transducer arrays, require a flexible, simple, and reliable integration approach. One example for such electronics integration is based on through silicon vias (TSVs) with under-bump metallization (UBM) stack for solder bumping. In this paper, we demonstrate such an approach by successfully integrating a fully populated 2D ultrasonic transducer array. Our integration is based on a previously reported TSV technology (trench-frame technology), based on trench-isolated interconnects with supporting frame. We successfully combined the trench-frame technology with a simple UBM preparation technique - electro plating or chemical plating techniques with passivation layers for UBM pad definition are not required. Our results show high shear strength (26.5 g) of the UBM, which is essential for successful flip-chip bonding. The yield of the interconnections is 100% with excellent solder-ball-height uniformity (¿ = 0.9 ¿m). As demonstrated in this paper, this allows for a large-scale assembly of a tiled array by using an interposer. A design guideline for finer element-pitch design was developed suggesting that fusion bonding strength and the length of pillars are the main design parameters.