Various definitions of plugged vias / via-in-pad can be found in IPC-4671. Typically, via in pad refers to a via that is plated through and then filled with a conductive (or non-conductive) epoxy plugging ink. The ink is then planarized to create a flat surface and plated over. Most commonly, this via will eventually be patterned into an SMT or BGA pad for soldering.
Via-in-Pad conserves real estate and also provides a direct conduit for heat transfer. Current fine pitch BGA and flip-chip component footprints prohibit running traces and vias (typical BGA configuration). Instead, the BGA pad, trace, and via are combined into one single feature allowing for 50% space savings. Conductive inks (electrical and thermal) transfer heat from critical components.
HDI via fill platingprocess dramatically reduces the number of steps required while at the same time allowing for smaller circuit line and spacing.
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