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.
Deposit Slump
Printed Deformation
Dimensional deformation of printed solder paste occurs when the printed bricks spread beyond their defined pad boundaries prior to the reflow process. Uncontrolled deposit slump leads to solder bridging and short circuits.
Slump Mechanism
Viscous flow driven by gravity dominates the behavior of the printed brick at room temperature before the assembly moves to the oven. Insufficient powder loading or low vehicle viscosity accelerates deposit slump by reducing the internal resistance to shearing forces. When the squeegee shears the material, the temporary breakdown of the gel structure requires a rapid recovery phase to prevent lateral spreading.
Testing this behavior involves printing on standard pattern cards, measuring the increase in deposit width after a set time, and verifying the minimum gap between adjacent pads that remains clear of paste.
Thermal Influence
Elevated temperatures during the initial stages of reflow cause a drastic reduction in the viscosity of the organic binder. This preheat phase triggers hot deposit slump as the rosin softens and the activator chemistry begins to flow. If the activation profile heats the assembly too slowly, the paste spreads across the maskless areas of the board before the alloy reaches its liquidus state.
Selecting a formulation with a high thermal stability prevents these premature bridging defects.