John Burkhert

Buried and blind vias solve most HDI routing studies.

A popular answer to a high density interconnect (HDI) problem is to start with a simple printed circuit board and then proceed to add on layer after layer. This is known as a sequential lamination process. For the sake of balance, the layers are always added to the top and bottom in pairs. A notation we use describes the sequence.

A typical example is a board that starts with N number of layers in the initial pressing and has three additional lamination steps after that. Each additional pressing adds two layers: one above and one below the previous step. The shorthand for that type of construction is 3+N+3 or simply a 3N3 stack-up.

We could get more detailed and substitute the actual number of layers in the first pressing for the N and call it, for instance, a 3+4+3 board for an even 10 layers. The fact is the fabricator is more concerned about how many layers are added afterward than how many are used in the first step (FIGURE 1).


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