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Simulation of the stencil printing process [solder pastes]

This paper describes the application of advanced computational fluid dynamics (CFD) methods to model the stencil printing process at both macroscopic and microscopic length scales. The solder paste flow is simulated on a macroscopic scale to study flow characteristics of the bulk motion of the whole paste roll as it travels ahead of a squeegee blade. The studies are performed using finite volume CFD methods, treating the paste as a continuum. Constitutive relationships are used with experimentally obtained parameters to represent the nonNewtonian properties of solder pastes. Shear rate and pressure distributions are obtained and applied as boundary conditions for microscopic studies, in which lattice-Boltzmann methods are used to couple the simultaneous solution of the solder particle dynamics and flux flow during aperture filling and stencil withdrawal. Discrete element method (DEM) type algorithms are used to control particle collisions. The simulation results are intended to provide insight into the phenomena which occur during the printing of solder paste in fine-pitch applications.