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.
Viscoelastic Relaxation Time
Rheological Recovery
The duration required for a structured fluid to transition from a flowing liquid-like state back to a stable solid-like state determines how well it retains its printed shape after being sheared. A long viscoelastic relaxation time allows the printed paste to slump before the gel structure recovers.
Printing Impact
Squeegee strokes force the material to shear thin rapidly so that it can flow into the stencil apertures. If the viscoelastic relaxation time is too short, the paste recovers its elasticity while still in the aperture, leading to incomplete release and high volume variability. Conversely, if it is too long, the paste flows too much after printing, causing bridging and short circuits.
Proper formulation of the flux gel ensures that the recovery rate is matched to the cycle time of the screen printer.
Rheological Measurement
Oscillatory shear tests are used to measure the storage and loss moduli of the material as a function of time. This testing determines the viscoelastic relaxation time by observing the phase angle shift after a high shear strain step is removed. If the paste shows a rapid return to elasticity, it will retain its brick geometry on the board pads.
Technicians use this data to screen paste formulations for high speed printing lines where short cycle times are required.