Viscosity stencil printing

Screen or stencil printing can be used for adhesives as well as solder paste. The low viscosity of most flux solutions precludes their successful dispensing by this technique. The adhesive or solder paste is deposited through openings in the stencil or screen called apertures. The apertures are located over the locations on the circuit board where the adhesive or solder paste is required. The placement of material is performed by a squeegee pushing a quantity of adhesive or paste ahead of it, over the screen or stencil, as illustrated by Fig. 40.16a. 

Solder Paste Measurements. Typical solder paste measurements, shown in Fig. 53.8(a), are volume, area of pad covered, height, and misalignment with the pad. These quantitative measurements provide information about the paste viscosity, stencil registration, cleanhness, snap-off, and squeegee speed and pressure that can lead to improvement in the paste printing process. 

Figure 6.15 illustrates a typical set of filled vias after this process. The quality of the fill is important because an imder-filled via may result in an incomplete contact between the printed ink on that layer and the via metal. This results in open interconnects within the substrate. At the other extreme, over-filled vias may result in smearing of the ink across the surface of the tape during subsequent processing, which generally results in undesired short-circuit connections between interconnects. In both via-fill processes the ink viscosity, solids content, stencil openings, and process settings can be adjusted to accomplish the desired fill. 

The viscosity of a solder paste can be an indication of the ability of the paste to be printed and transferred properly through apertures of a solder paste stencil. If the viscosity is low, the paste... 

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