Solder paste printing is the process of depositing solder paste onto a printed circuit board (PCB) through a stencil. The stencil is a metal foil with numerous apertures cut into it that correspond to where paste should go on the PCB.
The paste is essentially powdered solder suspended in a thick medium called flux that acts as a temporary adhesive, holding the components until the soldering process melts the solder and forms the electrical/mechanical connection. The stencil delivers a bead of paste to the apertures on the PCB.
The stencil’s design affects the quantity and quality of deposited solder paste. In general, a higher area ratio yields better solder deposition.
Size of the Stencil Apertures:
The size of the apertures in a stencil determines the amount of paste that can be deposited into them during the printing process. Smaller apertures can cause problems such as clogged stencils or a lack of adhesion between the stencil and the PCB.
The separation speed of the PCB and stencil is a critical parameter for delivering a high-quality printed circuit board. It is typically selected in millimeters per second and depends on the size of the stencil’s apertures.
During the printing process, the squeegee blade must be held at a consistent pressure to ensure an effective wipe of the stencil and transfer of paste to the PCB. Too little pressure can result in “smearing” of the paste on the stencil and poor deposition; too much pressure can cause excess wear on the squeegee blade and stencil, and can cause bleeding between the stencil and the PCB.