Vitaliy wrote:
> Can someone point me to a resource, be it a book, a webpage, or
> whatever, that explains the manufacturing process for ML boards and
> specifically goes into design for manufacturability and the tradeoffs?
>
> Seat-of-the-pants type feedback is also welcome.

This is more seat of the pants.  Multilayer boards are built up from "cores"
and "prepregs".  A core is a lot like a double sided circuit board, but
thinner.  It is the fiberglass (or whatever) structural and insulating
material the board is made from plus copper laminated on both sides.  A
prepreg is just the insulating material, or a core without the copper.
Multi-layer boards are stacks of these two basic building blocks.

Obviously it's easier to drill all the holes after the board has been
assembled into a single sheet.  Blind and buried vias require drilling and
plating some of the cores and prepregs before assembly, adding extra process
steps and therefore cost.  There are restrictions too, since cores are
either drilled thru or not drilled at all.  That means holes will be thru
multiples of two layers, and always together in a consecutive odd/even pair
of layers.  For example, you can't have a hole only in layers 2 and 3 of a
conventional 4 layer board because those are halves of separate cores.
(Actually you can build boards differently to get this, but that gets
expensive).

Basically, the cost goes up with the number of extra steps the manufacturer
has to go thru.  The best rule is to stay away from blind and buried vias
unless you really need the extra routing space they can give you.


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