Package-on-Package solder paste is a relatively new concept in the solder industry. With limited material sets, we need to be able to capitalize on the flexibility of the few solder pastes that are currently optimized for this process. Of course a solder paste has a set of specifications, but characteristics like tack and viscosity can be modified by simple process variations. This can eliminate the need for a material change by dialing in the process you use.
Shear rate is one variable that can be adjusted to modify the viscosity of a Package-on-Package solder paste (or other thixotropic material – such as a ball attach flux). Whether using a circular or linear flux/paste doctor blade, a higher frequency of leveling will lower the viscosity of the flux or solder paste. This impacts the amount of solder that can be transferred by the component for a given reservoir depth. By simply adjusting shear rate, you can make one paste act like it has many different viscosities.