Tim Hutcheson wrote:

> Has anyone any experience building small (4x7 inch) 7-layer .063" boards?  I
> would like to have a certain stack up impedance that is best met with
> 4,9,9,9,9,4 for the 5-mil trace widths I am using but I know from what I
> have read that warping can be a problem.

I haven't actually done this, but I think the more layers you get, the less the
odd layer count is a factor in warpage.  In other words, if they add a single
layer
to one side of a 2-layer core, you have really unbalanced forces at work.
I'm not sure anyone can build such a board, though.  There is NO core to
support the whole thing.  They would likely have to make several cores
with 9-mil laminate, and resist and etch them.  Depending on what those
layers look like, they may not hold dimensions during the lamination step.
I would approach your board fabricator before even starting such a design.
They may have techniques, like several registration pins within the board
dimensions, that will help the 'book' stay in registration during laminating.
The military fabricators may be more familiar with this sort of design.

>  On the other hand, a conventional
> an 8-layer board would thin the layers and upset the impedance and a 6-layer
> board adds impedance calculation problems and more layout issues.
>
> Do vendors even build odd layer boards?

Their process often requires them to charge extra for it, as if it were the
next higher even # of layers.

Jon

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