IPC-4761 reflects IPC's work towards standardizing the via plugging process. To summarize, this document classifies 7 different types of via plugs. Two of these are dedicated to the use of dry film soldermask, which now has only limited usage and applications. From what we know, this usage is primarily limited to older military applications. The remainder, we would separate between via plugging and Via-in-Pad as thse two types of via plugs serve very different purposes.
Historically, and even continuing to today, the requirement for via plugging in designs has simply been called out as "via must be plugged", with some diligent designers calling out that they must be plugged with an epoxy. Overall, this is a very ambiguous callout that IPC-4761 serves to lend discipline and clarity to.
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