Stirring mechanism for viscous-material printer and method of printing

In manufacturing surface-mount printed circuit boards, a stencil printer can be used to print solder paste onto the circuit board. Typically, a circuit board having a pattern of pads or some other, usually conductive, surface onto which solder paste will be deposited is automatically fed into the stencil printer; and one or more small holes or marks (known as "fiducials") on the circuit board are used to properly align the circuit board with the stencil or screen of the stencil printer prior to printing solder paste onto the circuit board. In some systems, an optical alignment system is used to align the circuit board with the stencil. A printer includes a dispensing head for dispensing viscous material, such as solder paste. The dispensing head defines a chamber and includes at least one source port and a dispensing slot, wherein the viscous material flows from a supply mounted to the source port, into the chamber and out the dispensing slot. Within the chamber, a stirring mechanism is mounted. The stirring mechanism is coupled to a drive mechanism that displaces the stirring mechanism through the chamber, thereby stirring the viscous material. The stirring mechanism is particularly effective in preventing or reducing compaction of solder paste within the chamber.