Thermal Gradient
Viscous flow behavior inside solder paste during high speed component placement governs how quickly fluid deforms under applied mechanical stress. Shear strain rate quantifies this velocity gradient across microscopic layers during stencil printing and syringe dispensing operations. High deformation speeds lower apparent viscosity through thixotropic breakdown, which allows paste to roll freely across apertures before snapping back onto copper pads.
Fluid deformation stops abruptly once squeegee travel ceases, leaving deposited material to hold its wall definition without slumping.
Rheological Bound
Solder paste manufacturers specify testing parameters using rotational rheometers equipped with parallel plate geometries to chart flow curves. Shear strain rate defines the horizontal axis in these logarithmic flow sweeps, mapping directly against measured apparent viscosity on the vertical axis. Boundary conditions apply strictly to unbonded, non-Newtonian fluids where recoverable elastic deformation remains negligible compared to permanent viscous flow.
Operators adjust printer stroke velocities to match optimal shear regimes, preventing bridging defects caused by incomplete structural recovery or insufficient paste release.
Paste Deformation
Fluid slippage along aperture walls introduces localized velocity variations that alter the effective deformation profile during paste extrusion. Shear strain rate concentrates heavily near solid boundaries where velocity gradients reach maximum values across narrow stencil openings. Friction between metal squeegee blades and stainless steel foils forces paste particles into tighter packing arrangements, modifying local resistance to flow.
Proper velocity tuning minimizes void formation under fine pitch ball grid array packages by maintaining stable laminar flow through every aperture edge.
