Rheological Limit
Rheological behavior in solder pastes dictates how well a material resists flow under low forces during stencil storage and squeegee pauses. Stencil printing requires that the batch yield stress threshold is high enough to prevent pre-dispense slump and dripping from the apertures. This physical boundary marks the point where the internal microstructure of the suspended metal particles and flux vehicle breaks down to allow liquid-like behavior.
Process Performance
Paste deposition relies on a sharp transition from an elastic solid to a flowing fluid when the printer squeegee exerts dynamic pressure. If the batch yield stress threshold sits too high, the squeegee fails to roll the paste across the stencil surface evenly, causing incomplete aperture filling and skipped pads. Conversely, an inadequate threshold leads to sag during pauses in the assembly line.
The measurement employs controlled-stress shear scans on sample batches prior to releasing them to the factory floor.
Acceptance Quality
Standard operating procedures for solder paste inspection evaluate deposits directly after stencil separation to verify deposit heights. Batches falling outside the specified batch yield stress threshold range generate immediate defects like bridging or insufficient volume. Print quality stays consistent when the limit is regularly checked under standardized temperatures since heat alters paste viscosity.
