Solder paste printing is central to the mass reflow soldering process for surface mount technology. The miniaturization of components has put an increased demand on the printing process and requires it to successfully print 75 μm-100 μm(3 to 4 mil) wide apertures. The amount of solder paste deposited is a matter of concern. This study presents results from experiments on printing with apertures having circular, rectangular, square and triangular geometries. The ratio of the printed volume to the aperture volume has been used as a definition of print quality. It was observed that acceptable prints were obtained when the ratio of aperture area to the aperture wall area was more than 0.8. A simple analytical model is also presented for the release of solder paste as the stencil separates from the substrate board assuming that the apertures were filled. The solder is currently treated as a single phase material with Newtonian behavior. The motion of solder paste in the stencil aperture is modelled as the developing viscous flow with velocity boundary layers developing along the walls. The shear strength of the paste is used to determine the area sticking to the wall, thus making it possible to get an estimate of the print quality. The model incorporates the effect of paste properties like viscosity, density, tack and shear strength, and other process variables like aperture dimensions and separation (lift off) velocities between the stencil and the board in predicting the print quality. The model predicts the effect of shear to tack strength of the paste, stencil thickness, and the ratio of aperture to wall area ratio on print quality.

Issue Section:
Technical Papers
Topics:
Printing, Solders, Shear strength, Boundary layers, Density, Dimensions, Reflow soldering, Separation (Technology), Shear (Mechanics), Surface mount technology, Viscosity, Viscous flow
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