John, I'll share with you what I know about Staggered and Spiral vias.



We do some MCM-C (Multi-chip-Modules Ceramic) designs.
Sometimes Staggered and Spiral vias are required. Ceramic
Multi-chip-Modules are built different from pwbs. The holes
are punched or laser drilled in each layer prior to printing
the conductors on them. After the holes are made and gold
conductors are printed, the layers are stacked. This is were
the Staggered and Spiral vias come in. If holes stack up
like thru hole vias, bumps appear on the outer surfaces of
the ceramic MCM. This is caused by the gold fill in each hole
being stacked up on top of each other. To avoid this, you can
stagger the vias (blind) in different patterns. One method is
to arrange them in a spiral staircase type of structure allowing
you to travel to different layers. Although it appeared to
solve manufacturing problems, routing the design is much harder.
Auto routers don't like it either, some routers handle it better
than others but setup is still complex. I believe some alternatives
have been found to avoid the need for the via patterns but I'll leave
it at that.
Also, micro vias in PWBs may use common patterns but I haven't been
there yet.


TWO

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