Characterizing Solder Paste Rheological Breakdown under Squeegee Forces
Excessive squeegee speed and downforce over-shear solder paste, reducing yield stress and causing severe deposit slump and bridging on fine-pitch components.
Underside Smearing
Paste Accumulation
Material contamination refers to the unwanted transfer of solder paste from the stencil apertures to the bottom surface of the stencil foil during printing. In surface mount technology lines, underside smearing occurs when paste squeezed between the stencil and the printed circuit board is not wiped clean, leading to solder balls and bridging on the next printed board. This accumulation is a primary cause of print defects.
Defect Genesis
Several factors contribute to the transfer of solder paste to the bottom of the stencil, including incorrect squeegee pressure and warped circuit boards. Once underside smearing is present, the contaminated stencil foil transfers solder paste directly onto the solder mask of subsequent boards during the print cycle. This misplaced solder paste does not align with the pads and remains on the board surface, creating stray solder balls after reflow soldering.
Implementing a frequent under-stencil wipe cycle using solvent-wetted lint-free paper is necessary to remove the build-up before it can ruin subsequent prints. Production lines adjust the wipe frequency based on the complexity and pad pitch of the board design.
Inspection Technique
Automated solder paste inspection systems detect this defect by identifying misplaced paste deposits on the board solder mask between the metal pads. Detecting the smear early allows operators to wash the misprinted boards and clean the stencil before starting another cycle. Regular inspection reduces scrap rates and maintains high assembly yields.