Employment of roll-offset printing for fabrication of solder bump arrays: Harnessing the rheological properties of lead-free solder pastes using particle size distribution

Abstract

This study aims to enable roll-offset (RO) printing for fabrication of solder bump arrays for microelectronic packaging by developing lead-free solder paste optimized for RO printing. Using lead-free solder pastes blended with particles of two different sizes (microparticles and nanoparticles, 2μm and 100nm in average diameter, respectively) in various proportions, we carefully characterized the effects of the particle size distribution on the rheological properties (viscosity, power law index, and loss/ storage shear moduli (G′ and G″)) of solder pastes. The solder paste formulated with only micro-particles showed a liquid-like viscous behavior dominant with a low viscosity and high power law index (n). Raising the nanoparticle proportion effectively increased the viscosity, the cross-over stress of G′ and G″ of solder pastes. Such a strong visco-elasticity imparted from the addition of nanoparticles did not originated only from the increase of the effective volume fraction by the particle size reduction, but also the microstructural effect of nanoparticle aggregation. RO printed solder bump arrays with a thickness of 8 ~ 38μm are fabricated using the solder pastes having power law index under 0.7 and viscosity below several tens of Pa·s.