Solder pastes are a very complex "fluid" of high viscosity. Their behavior, when experiencing shear stresses, is "non-Newtonian," meaning that their viscosity is not constant as the shear stress varies. The viscosity of solder pastes is high when there is little or no shear stress and low when shear stresses are high such as when the paste is forced through a stencil aperture. This property is called thixotropy. The solder paste being thixotropic is ideal, as it enables the stencil printed "brick" of solder paste to retain its shape after it is printed, yet the low viscosity, when stressed, allows good filling of the stencil aperture.
Many people might assume that this relatively complex phenomenon is the end of the story, however there is at least one other well known property of solder pastes during printing that is important: response to pause. A solder paste with a poor response to pause, will stiffen up when allowed to sit idle for as little as 15 minutes. When this occurs, the first print will likely have insufficient solder paste for effective assembly. Hence, response to pause is a critical variable to measure when evaluating a potential solder paste.
Another important solder paste property has only recently become well known: shear thinning. My Indium Corporation colleague Tim Jensen, was one of the first people to point this out. Shear thinning is a property of some solder pastes in which the viscosity becomes lower and lower as the paste is repeatedly printed. See the figure above. The x axis is number of prints, the y axis is the viscosity. It is normal for the viscosity to go down during the print, but the viscosity should recover as the "good paste" does, not have a downward trend as the "bad paste." The resulting drop in viscosity, that the bad paste exhibits, will often result in too much paste being printed and potentially lead to defects such as shorts or solder balls. Unfortunately, if not tested for, shear thinning might first be observed after a paste has been implemented on the line.
If you are interested in a method to test a paste for its resistance to shear thinning, send me a note and I will send you a test protocol.