Oscillatory Rheometry Qualification for Fine Pitch High Speed SMT Printing
Oscillatory rheometry strain and frequency sweeps qualify solder paste viscoelasticity for fine pitch SMT printing where static viscometry fails.
Dynamic Yield Stress
Rheological Threshold
Rheological resistance defines the critical shear stress required to keep a structured fluid flowing once its internal gel matrix has been disrupted. Dynamic yield stress helps engineers predict the flow behavior of solder paste after it enters a state of motion.
Process Control
Continuous shear during stencil printing forces the material into a state of fluid motion as the squeegee travels along the board. Proper management of dynamic yield stress ensures that the paste flows smoothly into the apertures without sticking to the squeegee blade. When the blade halts, the material must transition quickly back to a solid-like state to prevent the printed deposits from spreading.
Measuring this property involves rotational viscometry or capillary rheometer scans that ramp the shear rate down from a high value.
Release Performance
Aperture emptying during stencil separation relies heavily on the behavior of the paste under low-shear conditions. If the dynamic yield stress of the formulation is elevated, the paste will cling to the aperture walls and cause insufficient deposit volumes on the PCB. Adjusting the flux chemistry or changing the metal powder volume fraction permits fine-tuning of this rheological property to optimize release performance.
Correct rheology results in sharp, well-defined print volumes that match the pad dimensions.