Studies have shown that over 80% of all defects can be traced back to the printing of the solder paste, consequently printing can be considered the most important step in the assembly process. The quality and repeatability are key to a successful product. The paste must have sufficient tack that it adheres lightly to the stencil and squeegee and the viscosity must be low enough to cause the solder paste to roll across the stencil and fill the apertures. Paste that does not roll will not fill the apertures and there will be insufficient solder paste on the pads. The paste must not be too tacky, otherwise it will not release cleanly from the apertures when the stencil is raised. Finally, the paste must maintain high enough viscosity when it is released from the apertures to provide excellent print definition and prevent slumping. These competing requirements result in material with a complex rheological profile where the viscosity varies with different shear conditions.