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42 PCB007 MAGAZINE I AUGUST 2019 • Better thermal management • Density, density, density • Increased I/O number for packaging applications • Minimized signal delays and to avoid defects associated with electromigration • Enables stacked microvia structures (often seen in smartphones' board technology, Figure 2) Via-in-pad reduces the footprint as well as increases the density. This design concept places the via directly below the component contacts and reduces the footprint when com- pared to fan-out. When via-in-pad is used in a design, there will be the call-out for via filling or plugging process (more on the process op- tions for via filling in a future column). Filling the via that is in the pad will improve the bond strength of the component when mounted over the filled via (Figure 2). The concern with the issue shown in Figure 2 is that air inclusions during the lamination process may reduce long-term reliability. An additional concern with air inclusions is that, in effect, air is an insulator. Thus, air reduces both electrical and thermal conductance. While it is acceptable to endure very small voids in the via simply due to processing and material properties, it is desirable to minimize air voids through material property selection, via plug- ging techniques, and equipment designs [1] . In the next few columns, I will present over- views of the different via fill technologies avail- able. Meanwhile, one must first understand the definitions used for via fill and/or via plug- ging. While this distinction may seem trivial, it must nonetheless be clearly communicated between the board supplier and end user (as agreed between user and supplier, or AABUS). Figure 3 shows a schematic of an HDI struc- ture containing through-holes, blind and buried vias, and microvias. Via hole filling is used for Figure 2: Component mounted on unfilled vias (note the air pockets). Figure 3: HDI structure. (Source: Happy Holden, used here with permission)

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