Flow Resistance
Mechanical threshold measurements determine the minimum force required to induce flow in structured fluids used in circuit board assembly. The yield stress tau zero represents the critical boundary where solder paste transitions from an elastic solid to a flowing liquid. Below this stress limit, the paste resists deformation, which is necessary for the printed bricks to hold their shape on the pads.
When the squeegee exceeds this value, the paste flows into the stencil apertures to complete the printing cycle.
Deposition Stability
Solder pad bridging is prevented when the deposited material has sufficient strength to resist gravity and surface tension. If the yield stress tau zero is too low, the paste will slump immediately after the stencil is lifted, causing adjacent deposits to merge. This collapse is particularly problematic for fine-pitch components where the gap between pads is minimal.
Rheological testing is used to verify that the paste maintains a high enough threshold to prevent this behavior while remaining low enough for the printer to easily shear the material. Technicians use this metric to evaluate incoming paste batches for shelf-life degradation.
Rheology Control
Chemical additives within the flux system establish the temporary networks that give the paste its structured behavior. Formulators adjust the concentration of these rheological agents to achieve the target threshold for each powder size.
