Flow Resistance
Rheological properties determine the force required to initiate flow in a solder alloy suspension. Solder paste yield stress defines the specific pressure threshold at which the material transitions from a quasi solid state to a viscous liquid. This transition is critical during the stencil printing phase where the squeegee must overcome the internal resistance of the paste to fill small apertures.
Stencil Release
High values for this property prevent the material from slumping or spreading after the stencil is removed from the board. If the stress threshold is too low, the solder bricks will lose their shape and cause bridges between adjacent pads. Conversely, a value that is too high will result in incomplete filling of the apertures and insufficient solder on the pads.
The measurement is taken by a rheometer that monitors the deformation of the paste under a controlled increase in shear.
Slump Prevention
Maintaining a consistent yield stress is necessary for high yield in fine pitch assembly. It ensures that the paste remains on the pad until the component is placed and the board enters the reflow oven. Temperature and humidity in the assembly room must be controlled because they directly influence the rheology of the flux system.
