Paste Deformation
Deposit stability describes the physical spreading of a printed deposit before the application of heat. Once a stencil is removed, cold slump mechanics dictate how well the solder brick retains its height and width at room temperature. The force of gravity acts upon the suspension of alloy particles and liquid flux.
Surface tension within the flux vehicle resists this downward movement to prevent bridging between adjacent pads.
Structural Integrity
Viscosity remains the primary defense against the widening of a deposit during the delay between printing and component placement. High ambient temperatures or low metal content often accelerate cold slump mechanics by reducing the internal friction of the paste. If the paste spreads too far, it connects with neighboring pads and creates electrical shorts.
Precision printing relies on a formulation that maintains its shape for several hours. This property is measured by observing the change in diameter of a test pattern over a fixed time interval.
Inspection Threshold
Automated solder paste inspection systems identify deposits that have exceeded their volumetric boundaries. Excessive cold slump mechanics trigger an immediate rejection of the board to avoid permanent rework after reflow. Consistent environmental control in the assembly room mitigates these flow issues.
Proper stencil thickness and aperture design also compensate for minor paste movement.
