Transition Threshold
Material deformation analysis identifies the force required to break down the internal structure of a structured fluid and initiate flow. The viscoelastic crossover stress defines the point during rheological testing where the storage modulus and the loss modulus are equal. Below this stress level, the material behaves as an elastic solid, maintaining its shape under gravity.
Above this threshold, the viscous behavior dominates, allowing the material to flow like a liquid when pushed by a squeegee.
Print Consistency
Stencil printing performance depends on selecting paste with a suitable yield point to ensure clean roll and aperture release. If the viscoelastic crossover stress is too high, the squeegee blade cannot apply enough force to initiate flow, causing the paste to skip over apertures rather than fill them. This results in insufficient solder joints and assembly defects after reflow.
Conversely, a paste with too low a crossover stress will flow too easily, slumping on the board before the reflow stage and causing solder bridges. Rheometers measure this crossover point using amplitude sweeps to monitor the breakdown of the paste’s internal network.
Temperature Dependence
Environmental temperature shifts directly alter the viscoelastic behavior of the flux gel. When assembly rooms are not temperature controlled, a rise in ambient heat lowers the crossover stress, making the paste more susceptible to premature flow and stencil bleeding.
