The graphic below shows the main structures – type I, type II and type III as defined in IPC-2226.
Type I. Defines a single microvia layer on either one or both sides of core. Uses both plated microvia and PTH for interconnection, employing blind, but not buried vias.
Type II. Defines a single microvia layer on either one or both sides of core. Uses both plated microvia and PTH for interconnection. Employs blind and buried vias.
Type III. Defines at least two layers of microvia on either one or both sides of core. Uses both plated microvia and PTH for interconnection. Employs blind and buried vias.
Construction terminology to define the degree of HDI construction:
1+n+1 = single layer of microvia (as per the type I and type II examples above)
2+n+2 = 2 layers of microvia (as per the type III example above)
3+n+3 = 3 layers of microvia
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