Fluid Resistance
Viscous forces acting on suspended metallic spheres in a fluid medium determine the rheological flow properties of solder paste during stencil printing. Within these concentrated suspensions, inter-particle hydrodynamic drag governs how individual solder particles move past each other when subjected to squeegee shear. The drag force increases as the clearance between adjacent particles decreases.
This fluid interaction influences the transfer efficiency of the paste through small stencil apertures.
Viscosity Influence
Evaluation of this drag is performed using high shear capillary rheometers to simulate the high speeds of the stencil printing process. The inter-particle hydrodynamic drag depends on both the viscosity of the liquid flux medium and the packing density of the solder powder. High drag forces can cause paste clotting or incomplete aperture filling, which leads to insufficient solder defects on the printed circuit board.
Low drag forces can result in paste slump, where the printed deposit loses its shape and causes bridging defects between adjacent pads. Print quality is optimized when these viscous interactions are maintained within a specific range.
Print Performance
Control of paste flow during the deposition step determines the final volume of solder delivered to the circuit pads. When the inter-particle hydrodynamic drag is too high, the paste cannot release properly from the stencil walls. This behavior results in printed deposits with uneven heights and insufficient volume.
SMT line operators adjust squeegee speed to control these hydrodynamic forces and ensure uniform paste deposits.
