Stenciling tape

First, in terms of their physical form, adhesives may be either pastes or preforms (also known as films or tapes). Pastes are semisolid materials, easily dispensed through a needle or applied by screen or stencil printing. Film adhesives are solid sheets of thermoplastic or partially cured (B-staged) thermosetting polymers that can be cut to size and generally used to attach large-area components, substrates, and lids. 

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. 

Stencil printing is very similar to Ihis process except that it is normally conducted with zero snap-off distance, i.e., with the stencil s bottom surface in contact with the tape layer. Also in the case of stencil printing, higher squeegee speeds are used in the via-filling process to provide increased ink shear forces, thereby facilitating the flow of ink into the vias. 

A low-volume prototype screen- and stencil-printing machine is shown in Figure 6.18. This machine is equipped with a porous stone vacuum chuck that is specifically designed for handling tape layers. Unlike a prefired ceramic substrate, the green sheets of LTCC, HTCC, or MTCC are fragile and must be handled with care however, adequate force must be applied... 

Vias are subsequently filled with a specially formulated conductive material applied through a stencil in a screen printing process. The stencil is made from 0.002- to 0.003-inthick brass or stainless steel containing etched or punched vias of the same pattern as the tape. The stencil and punched tape are typically optically aligned before printing the conductive pastes into the vias. Quality factors are completeness of fill, without overfill, and accuracy of placement of the fill material. 

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