Laser-Induced Forward Transfer of Pre-Patterned Solder Paste for High-Aspect-Ratio Deposits

Area of Science:

• Materials Science
• Mechanical Engineering
• Electrical Engineering

Background:

• Precision solder deposition for 3D and flexible substrates is a significant challenge in advanced electronic packaging.
• Existing methods struggle with contactless, high-aspect-ratio solder placement.

Purpose of the Study:

• To develop and characterize a hybrid stencil printing and laser-induced forward transfer (LIFT) process for high-aspect-ratio solder deposition.
• To investigate the transfer dynamics and influencing factors for optimized solder paste transfer.

Main Methods:

• A hybrid process combining stencil printing with laser-induced forward transfer (LIFT) using a line-scan trajectory.
• High-speed imaging to analyze solder paste transfer dynamics under large-gap, contactless conditions.
• Systematic variation of initial aspect ratio and laser energy to identify transfer regimes.

Main Results:

• Identified three transfer regimes: stable, unstable, and no transfer, with stable transfer governed by vaporization pressure and viscoelasticity.
• Initial aspect ratio critically affects separation morphology, with AR = 0.3 marking a transition.
• Achieved final aspect ratios near 0.7, with low-AR patterns showing a 2.2-fold height increase.

Conclusions:

• The hybrid LIFT process offers a viable solution for high-aspect-ratio solder deposition on challenging substrates.
• Understanding transfer dynamics provides practical guidance for optimizing solder paste deposition in advanced packaging.
• The process demonstrates a robust energy window suitable for reliable fabrication.