Quantifying Solder Paste Transfer Efficiency across High Density Stencil Apertures
Optimizing stencil area ratios above 0.55 using electropolished foils and Type 5 paste maximizes volume transfer efficiency while minimizing SMT assembly defects.
Shear Rate Dynamics
Fluid Behavior
Fluid behavior describes how the internal resistance of a substance changes when it is forced to flow by an external force. In stencil printing, the shear rate dynamics control the thinning of solder paste as the squeegee pushes it across the metal foil. This thinning is essential for the paste to roll and fill the apertures correctly.
The Velocity
The velocity of the blade directly determines the rate at which the material is deformed. Managing the shear rate dynamics ensures the paste remains liquid enough to enter the holes but regains its shape once the stencil is lifted. If the speed is too low, the paste may skip over apertures rather than filling them.
Recovery Time
Once the shearing force is removed, the material must recover its viscosity quickly to prevent slumping. The shear rate dynamics of a specific flux and powder blend dictate how long the paste remains stable on the board before component placement. Proper recovery ensures the solder brick maintains its sharp edges and volume during the assembly process.
Advanced formulations are tested to withstand multiple print strokes without a permanent loss of viscosity. This resilience allows for a wide process window even in high volume manufacturing environments where the paste stays on the stencil for several hours.