Solder Paste Viscosity and Area Ratio Print Performance
Matching paste particle size and viscosity to stencil area ratio maintains stable transfer efficiency above 80 percent on fine-pitch components.
Solder Paste Viscosity
Flow Resistance
Internal friction of a solder paste mixture determines how the material flows under the pressure of a squeegee and how well it maintains its shape after printing. Correct solder paste viscosity allows the material to roll smoothly across the stencil and fill small apertures without sticking.
Printing Influence
Shear thinning properties describe how the paste becomes less viscous when force is applied during the print stroke. As the squeegee moves, the paste must liquefy enough to enter the apertures but then thicken rapidly once the stencil is lifted. This recovery prevents the deposits from sagging or slumping into their neighbors.
Formulations with high metal content generally exhibit higher resistance to flow and require more force to print correctly.
Temperature Sensitivity
Ambient conditions on the factory floor greatly alter the behavior of the chemicals. Warm air reduces the resistance, causing the paste to become too thin and prone to bleeding under the stencil. Conversely, a cold environment makes the material stiff and difficult to transfer into fine-pitch openings.
Manufacturers specify a narrow range of operating temperatures to ensure the consistency of the print. Regular testing with a viscometer confirms that the paste remains within the allowed limits during long production runs.