Rheological Behavior
Viscosity decreases as the applied stress rate increases within specific polymer-based fluids. Non Newtonian shear thinning governs the dispensing characteristics of solder pastes and conductive adhesives utilized in circuit board assembly. High deformation rates force the internal structure to align along the flow path, which reduces resistance to movement during the stencil printing process.
This temporary drop in viscosity allows material to pass through fine apertures with consistent volume delivery.
Flow Mechanism
Molecular chains within the medium begin to disentangle when the material experiences sufficient force. The rate of this alignment dictates how quickly the fluid regains its high viscosity state after the shear stress is removed. Stencil apertures require a drop in resistance to ensure complete cavity filling across high density board layouts.
Inadequate recovery rates lead to slumping, while excessive viscosity at high shear leads to incomplete release from the stencil mesh.
Production Quality
Verification of these characteristics occurs during the characterization of the material rheology under defined shear profiles. Manufacturers utilize a viscometer to map viscosity against varying rotation speeds to determine if the batch meets printability requirements. A variance from the target curve results in insufficient solder deposition on the pad or bridging across narrow component pitches.
Stability under shear determines the consistency of the printing operation over long production cycles.
