Characterizing Solder Paste Rheological Breakdown under Squeegee Forces
Excessive squeegee speed and downforce over-shear solder paste, reducing yield stress and causing severe deposit slump and bridging on fine-pitch components.
Shear-Thinning Matrix
Viscosity Behavior
The reduction of fluid viscosity under mechanical stress allows solder paste to flow easily during squeegee movements. Inside the shear-thinning matrix of the paste, temporary polymer bonds break apart as the squeegee applies pressure. This change converts the paste from a thick paste to a fluid that fills the stencil apertures.
Once the squeegee passes, the paste recovers its high viscosity.
Aperture Clearance
Clean release from the stencil requires rapid viscosity recovery after deposition. Solder paste must flow easily into the aperture, but it must quickly solidify once the stencil lifts to prevent the deposit from slumping. When the recovery time is too slow, the printed paste slumps and spreads beyond the pad boundaries.
This behavior increases the risk of bridging during the reflow process.
Temperature Control
Ambient temperature fluctuations alter the behavior of the paste matrix. High room temperatures reduce the starting viscosity, which can cause the paste to slump before any shear force is applied. Low temperatures make the paste too stiff to roll properly across the stencil, leading to insufficient aperture fill.
Fabricators must maintain strict environmental controls in the print room to ensure consistent flow behavior during continuous printing operations, preventing variations that disrupt the printing process on high-volume lines.