Stencil filled vias

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. 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

Fill the vias by screen printing or stencil printing sufficient silver-filled epoxy to form bumps or leave the vias open for wire bonding. 

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. 

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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