Fluid Friction
Mechanical force exerted by a moving liquid parallel to the surface of a solid boundary describes the intensity of the drag at the interface. In the context of wet chemical processing for circuit boards, wall shear stress influences the removal of debris and the replenishment of fresh reagents in narrow holes. The value depends on the viscosity of the fluid and the velocity gradient near the wall.
Higher stress levels are often required to ensure that high aspect ratio vias are cleaned properly.
Cleaning Efficiency
Fluid flow inside small apertures often becomes stagnant, preventing the etching or plating chemicals from reaching the center of the hole. Increasing the wall shear stress through ultrasonic agitation or high pressure spray systems overcomes this stagnation. This mechanical action physically strips away the spent boundary layer.
If the stress is too low, the reaction slows down or stops, leading to incomplete plating or residual drill smear. Conversely, excessive force can damage fragile photoresist patterns or delicate surface features.
Geometric Constraint
The diameter and depth of a via heavily limit the achievable force at the surface. As the aspect ratio increases, the wall shear stress drops rapidly toward the center of the structure while the fluid velocity at the boundary reaches its minimum. This limitation necessitates the use of specialized chemistries with high diffusion rates.
