Coarse-grained molecular dynamics simulation of wettability and bleed out of capillary underfill

Hiroshi Ito,Hiroshi Moriya,Tomohisa Suzuki,Takeshi Terasaki,Taisuke Sugii,Shigenori Matsumoto

doi:10.1063/5.0037400

Abstract

Coarse-grained molecular dynamics simulations were performed to elucidate the capillary flow process of liquid state capillary underfill (CUF), a sealing resin material. First, we ran a wettability simulation with the CUF consisting of a monomer with small and large fillers. We observed that a certain amount of the monomer spreads ahead on the substrate, while many fillers are left inside the droplet. This was confirmed by subsequent mean square deviation (MSD), which showed that the monomer had a higher MSD, 25–45 σ2, than the small and large fillers, which were 0.4–1.4 σ2 and 0.02–0.2 σ2, respectively. When one part of large fillers was replaced with small fillers, small fillers helped accelerate the wetting dynamics because they could move fast. However, when the small filler ratio was high (20%), the MSD of small fillers decreased. Next, we performed a capillary flow simulation in which the CUF flowed between parallel walls and observed that it formed a ridgeline at the upper wall edge. Small fillers contributed to a decreased flow time. However, when the small filler ratio was even higher, the flow time increased. Then, the small fillers slowed themselves down, as shown in the MSD. This is due to an increase in monomer interactions and less space to move. We also found that the bleed length decreased with an increase in the small filler ratio. This study clarified the effects of filler usage on the flow time and bleed length and contributed to new insight into the capillary actions and material design relevant to CUF.

Highlights

The wettability of curable resin materials is significant in the development of various materials because it affects several different processes and the final products
We used coarse-grained molecular dynamics (CGMD) simulations to elucidate the capillary flow of a capillary underfill (CUF)
Results showed that a certain amount of the monomer is spread on the substrate, and many fillers were left inside the droplet. This was numerically confirmed by the mean square deviation (MSD) calculation showing that monomers had a higher MSD, 25–45 σ2, than those of small and large fillers, which were 0.4–1.4 σ2 and 0.02–0.2 σ2, respectively

Summary

INTRODUCTION

The wettability of curable resin materials is significant in the development of various materials because it affects several different processes and the final products. Shiomoto et al observed the precursor thin film of water on line-patterned substrates with polymer brushes by using an upright transmitted light microscope and cameras and revealed the mechanisms of growing and stopping of the precursor thin film stemming from the interfacial energies These studies on the precursor thin film contributed to our understanding of nm-scale liquid behaviors. The molecular dynamics approach is suitable for representing the wettability and related phenomena for a liquid material, and the CGMD simulation can be adapted to show the dynamic behaviors of fillers. Different behaviors of resin and fillers in a CUF are still unclear, and effects of material composition on the flow time and bleed out of the capillary flow are unrevealed in the molecular scale. Material compositions are suggested for decreasing the flow time and bleed length

METHOD

Droplet wettability simulations

Capillary flow simulations

CONCLUSION