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

Precision solder deposition for 3D or flexible substrates remains a persistent challenge in electronic packaging. This study introduces a hybrid process that integrates stencil printing with laser-induced forward transfer (LIFT), employing a customized line-scan trajectory to fabricate high-aspect-ratio solder deposits under large-gap, contactless conditions. Solder paste patterns were first printed on a glass carrier and subsequently transferred using pulsed laser scanning, with high-speed imaging employed to resolve the transfer dynamics. Three transfer regimes-stable, unstable, and no transfer-were identified, with the stable regime exhibiting sequential stages governed by vaporization-induced pressure and the viscoelastic response of the solder paste. The initial aspect ratio (AR) was found to critically influence separation behavior, with AR = 0.3 marking the transition between bridging and cantilevered morphologies. Transferred deposits consistently achieved final aspect ratios approaching 0.7; notably, low-AR (<0.15) patterns showed a 2.2-fold height increase. The process maintains a robust energy window (0.937-1.112 J/cm²), offering both mechanistic insight into transfer stability and practical guidance for optimizing solder paste deposition in advanced packaging applications.