Computational study of the dynamics of a liquid droplet rolling over the edge of a flipped chip die

Flexible-hybrid electronics aims to integrate conventional rigid circuit components such as Integrated Circuits (ICs) with printed conductive traces to manufacture wearable electronics. Flipping an IC to expose its contact pads and printing conductive traces onto them is one such attractive but challenging manufacturing technology. A ramp like underfill is required to support the conductive traces running between the circuit substrate and the die of the IC. In this regard, the current study computationally investigated rolling of a liquid droplet over the edge of a flipped die to form a smooth ramp like underfill. The dynamics of a liquid droplet with properties similar to DuPont 5036 UV curable dielectric ink was simulated using OpenFoam CFD toolbox. Droplets with diameters 1.54 mm, 0.84 mm, and 0.41 mm that are experimentally feasible were numerically dropped from a 0.08 mm height onto the top surface of a 0.8 mm thick flipped chip. The horizontal distances between centres of the droplets and the edge of the die were 0.77 mm, 0.42 mm, and 0.205 mm, respectively. The rolling of the 0.84 mm diameter droplet and subsequent settling after 0.75 s formed the ideal ramp with a slope of 60.6°. Further settling after 4 s formed a ramp with a slope of 46.3°. These observations suggest that selective solidification of the droplet at designated times yield ramps with desired slope which would act as an underfill in printing conductive traces on to flipped chip dies to form conductive interconnects.