Cleaning Intervals
Squeegee strokes during solder paste printing can leave behind small amounts of solder paste on the underside of the stencil, which must be removed to prevent print defects. Adjusting the under stencil wipe frequency sets the number of print cycles that can occur before the printer’s automated cleaning mechanism wipes the bottom of the stencil. This parameter is critical for preventing paste from smearing onto the solder mask or transferring to the next board.
A typical setting requires a wipe every three to five print cycles, depending on the complexity of the design. For instance, a high-density board with micro-BGA components might require a wipe after every single print to maintain aperture cleanliness. This frequent cleaning increases the cycle time and solvent consumption but minimizes print defects.
Wipe Dynamics
Automated cleaning systems use a roll of lint-free paper and a solvent, such as isopropyl alcohol, to dissolve and wipe away dried paste residue. The cycle can include wet wipes, vacuum strokes, and dry wipes to ensure the stencil is dry and clean. The effectiveness of this process depends on the solvent type and the wiping speed.
Defect Prevention
Infrequent cleaning leads to paste buildup around the edges of the stencil apertures, which causes the printed solder deposits to smear and bridge. Smeared paste results in electrical short circuits after reflow, which are costly to repair. Optimizing the wiping interval reduces these defects while maintaining high production throughput.
