Characterizing Solder Paste Rheological Breakdown under Squeegee Forces
Excessive squeegee speed and downforce over-shear solder paste, reducing yield stress and causing severe deposit slump and bridging on fine-pitch components.
Pseudoplastic Behavior
Viscosity Change
Fluid deformation describes the property of a non-Newtonian material where viscosity decreases as the applied shear rate increases. In stencil printing processes, pseudoplastic behavior allows solder paste to flow easily through fine stencil apertures when under the force of the squeegee blade. This characteristic prevents the paste from spilling or running after the squeegee has passed.
Flow Dynamic
Squeegee movement applies a continuous shear force that temporarily breaks down the internal gel structure of the solder paste. This pseudoplastic behavior ensures that the paste behaves like a low-viscosity liquid during the print stroke, allowing it to fill the micro-apertures completely. Once the shear force is removed, the viscosity must rise rapidly to prevent the printed paste from slumping before component placement.
Solder paste formulations combine metal alloy powder with specialized flux vehicles to achieve this precise flow dynamic. Modifying the polymer additives within the flux allows chemists to control the rate of viscosity change across different printing speeds.
Performance Metric
Rheological testing evaluates the flow properties by measuring viscosity across a wide range of shear rates using a rotational viscometer. Maintaining the correct shear-thinning index ensures that the solder paste delivers consistent print definition on high-density circuit boards. This control is essential for preventing bridging on fine-pitch designs.