We have had the problem of internal layers being swapped
more than we care to think about, mostly because we number
our layers backwards from industry standard (don't ask why).

One of the checks we use is to put a short nub of a trace
at the edge of the board staggered on each layer so that,
after routing the  board edge, the copper is exposed on
the board edge and the layer stack-up can be checked
and also the dielectric thickness between layers can be visually
checked.  We also put the layer number (like 1,2,3,...) on each
layer in a place on the board where you could hold the board to the
light and see the stack-up through the board (relieved of soldermask).
We do not have a lot of layers, though.

Sorry this happened to you.

KK


At 11:07 AM 10/16/2001 -0600, you wrote:
>Good morning,
>My morning has not been so good.  I have a build of PCB's (over 300 boards)
>where two inner layers have been swapped.  The board fabricator constructed
>the board in the wrong order.  These boards are RF in nature and have
>frequencies approaching 2GHz on them.  In many places we put a signal trace
>on the inner layer furthest from the component side and then a wide grounded
>trace over it on the closer inner layer as shielding.  So not only is there
>no longer shielding over many traces, these signals are also about .03"
>closer to the component circuitry than they used to be.  Can you contemplate
>the nightmare of spurious issues that have been created?  We did not notice
>the problem until they were fully populated and almost thru our testing
>process and started failing one of the tests miserably.
>Needless to say, we will be revising our inspection techniques to try to
>catch occurrences like this much sooner.
>
>I was wondering if anyone knows of a way that layer order could be
>electrically tested for on a bare board, that we could require our
>fabricator to complete?
>
>Genny Gibbard (mailto:[log in to unmask])
>
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