Rheological Analysis and Numerical Modeling of Viscoelastic Buffer Receiving Layer for Micro-LED Laser Mass Transfer

This study introduces an innovative numerical model to simulate the dynamic impact of Micro-LED chips on viscoelastic buffer receiving materials during the Laser-Induced Forward Transfer (LIFT) process. We developed a novel rheological computational framework to analyze the behavior of Matrix 50–Thixotropic 07 (M50-T07) and Matrix 50–Thixotropic 13 (M50-T13) under chip impact, validated by LIFT experiments. Rheological characterization showed M50-T13's higher viscosity (3620 Pa·s) at near-zero shear rates compared to M50-T07 (126 Pa·s), with a broader linear viscoelastic range and higher yield stresses (295 Pa vs. 133 Pa). Oscillatory tests indicated M50-T13's superior damping factor, reflecting enhanced energy dissipation. Using ANSYS Fluent, the model simulated material responses, revealing M50-T13's uniform static pressure distribution (peak <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$7.96 \times 10^{4} \text{Pa}$</tex>) and controlled strain rates (peak <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$6.52 \times 10^{6} \mathrm{s}^{-1}$</tex>), while M50-T07 exhibited rapid deformation (strain rates up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.92 \times 10^{6} \mathrm{s}^{-1}$</tex>). LIFT experiments confirmed simulations: M50-T13 achieved precise chip placement with deviations of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.32 \mu \mathrm{m}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.12 \mu \mathrm{m}$</tex>, whereas M50-T07 failed, showing deviations of 65.3 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$76.1 \mu \mathrm{m}$</tex> with chip inversion. We propose a new LIFT transfer accuracy metric, actual to target landing area ratio, with M50-T13 reaching 98.49%, surpassing the industry threshold of 88.54%. This validated model optimizes receiving materials for Micro-LED assembly, highlighting viscosity and damping factor as critical to transfer success, and lays a foundation for material design in precision chip transfer applications.