Paste Deposition
Deposition of solder paste through stencil openings constitutes the primary step in establishing electrical connections during surface mount board assembly. During this sequence, aperture filling defines the percentage of the stencil cavity volume that receives paste during the squeegee stroke.
Solder Release
Stenciling performance depends heavily on the ratio of the aperture width to the stencil thickness. If this ratio falls below the critical threshold of 1.5, aperture filling suffers because the paste adheres more to the sidewalls than to the circuit board. Wall friction holds the material inside the opening when the board separates from the stencil.
Laser cut stencils with polished walls reduce this friction, ensuring that the paste transfers cleanly to the pads.
Print Optimization
Solder paste formulation also alters the effectiveness of the deposition cycle. Smaller solder powder sizes, such as type 4 or type 5, allow better packing of the paste within tiny openings. This material selection extends the limits of successful print runs.
High viscosity paste demands slower squeegee movements to allow sufficient time for the material to flow into the gaps. Consequently, optimized paste viscosity allows reliable deposit heights even on dense circuit designs. When squeegee angles remain constant, the speed of the print stroke must be dialed in to match the flow behavior of the flux vehicle.
If paste fails to enter the cavity, dry solder joints and subsequent electrical open defects occur during reflow soldering.
