Flow Characterization
Viscous behavior wherein the flow resistance of a material changes with the rate of applied shear force governs the behavior of critical liquids during circuit board assembly. Non-Newtonian fluid dynamics explain how solder pastes, underfill resins and conductive adhesives behave when subjected to stress. This study of shear-thinning behavior dictates the speed and force settings of squeegees and dispensing needles.
It applies to materials that are heavily filled with solid particles.
Stencil Deposition
During the stencil printing process, solder paste must flow easily through the tiny laser-cut apertures and then remain standing as a defined brick on the board surface. The high shear force applied by the squeegee blade reduces the viscosity of the solder paste, allowing it to pass through the stencil openings with minimal friction. Once the squeegee has passed and the shear force drops to zero, the paste’s viscosity must recover instantly to prevent the deposit from slumping and causing solder bridges.
Correctly formulated non-Newtonian behavior is necessary to achieve high-resolution print deposits.
Dispensing Control
Capillary underfills and glob-top encapsulants use these flow properties to flow under mounted silicon dies. The material’s viscosity drops when heated, enabling it to travel through tiny gaps under the component via capillary forces. Once in place, the material remains stable during the thermal cure process.
