Correlating solder paste composition with stencil printing performance

Soldering technologies continue to evolve, particularly in the area of solder pastes used in SMD reflow soldering. Solder pastes typically consist of solder alloy powder, flux, viscosity control agents, and a solvent system. By varying solder particle size, distribution and shape, as well as the other constituent materials, solder paste rheology and printing performance can be controlled. As PCB pad and component lead sizes reduce to meet ultra-fine pitch and flip-chip assembly requirements, most paste suppliers have opted for smaller powder particle sizes and new rheology modifier formulations. Successful assembly at these small-scale geometries requires deposition of small and consistent paste deposits from pad to pad, and from board to board. Two primary mechanisms dominate paste printing at this chip-scale geometry: paste transfer into stencil apertures, and paste release from the apertures on to PCB pads, i.e. paste flow behaviour is crucial in defining printing performance. Solder paste flow properties primarily depend on their chemical and physical structure, and hence composition. In this paper, we study the rheological profile of various paste formulations, and how this can be used to provide better understanding of composition effects on stencil printing performance. Pastes for ultra-fine pitch and flip chip applications were studied via a wide range of rheological tests, including the creep recovery test to determine paste slump characteristics and the oscillatory test to determine visco-elastic behaviour required for two critical sub-processes: aperture filling and paste withdrawal