Static Fluidity
A rheological property defines the internal resistance of a structured fluid when it is in a state of rest. Solder paste formulations require a high zero shear viscosity to prevent the heavy metal particles from settling out of the flux suspension during storage. High rest-state viscosity is achieved by incorporating thixotropic gelling agents into the liquid flux.
When a squeegee applies force, this internal structure breaks down, and the viscosity drops. Rest-state stability ensures that the paste behaves like a solid until it experiences a shear force that exceeds its yield point. Rheometers measure this behavior by applying tiny, non-destructive oscillations to the material.
Slump Resistance
Printed deposits must maintain their shape on the circuit board pads prior to the reflow process. Solder pastes with low zero shear viscosity suffer from cold slump, which allows the deposits to spread and bridge adjacent pads. This structural collapse is especially critical for fine-pitch components where the spacing is minimal.
High rest-state viscosity resists the forces of gravity and placement pressure.
Inspection Check
Automated 3D optical inspection measures the height profile of the printed paste to ensure it does not sag. Measuring zero shear viscosity in the laboratory involves using a stress-controlled rheometer at very low shear rates. This test verifies that the paste maintains its structure over time after printing.
Formulations that fall below the viscosity threshold are rejected to avoid soldering defects on the assembly line.
