Jeff Fries wrote:
>
> I have a .062" thick, 10 layer PCB that I am working with.  My first
> round of prototypes came back a bit thin at .052" thick.  Between each
> of the inner layers of copper is only one sheet of prepreg, .0025"
> thick.  I have a couple of long runs that are on separate inner
> layers, but they run right on top of each other.  The runs are about
> 5" long, 1oz. copper, .04" thick.  The only separation between these
> runs is through the prepreg layer.  Usually, we do not have a problem
> with doing this, but we usually have more than one layer of prepreg
> between conductor layers.
>
> My problem is this:  during testing of the assembly, somewhere during
> surge, eft, or isolation verification, a few of the runs that run on
> top of each other, but on different inner layers have shorted together
> internally in the PCB.  I don't know if it was during surge, eft, or
> isolation testing.  Is there a way to quantify, calculate, or is there
> a general rule of thumb for determining the isolation characteristics
> between inner layers of a PCB?  We are ready to do a second production
> run of the board where the prepreg thickness between the inner layers
> is doubled compared to the first prototypes, but I would like to get a
> good feeling if it will pass my testing before we sign off to produce
> the boards.  I would appreciate anyone's comments or insight.  I have
> listed some of the test requirements below:
>
> Surge Testing:  3kV peak voltage (IEC waveform of 50usec duration)
> EFT Testing: 2kV peak voltage (IEC waveform of 2.5kHz frequency)
> Isolation Testing: 2kV ACrms, 60Hz, no more than 1mA conduction,
> sustained for at least 1 minute
>
> Thank you for your time,
> Jeff Fries
> Design Engineer
> Harmon Industries, Inc.
> [log in to unmask]
There is something wrong with your design rules.  Predominant routings
on adjacent layers should be in different directions - i.e., x & y - not
in the same direction.  Routings through a multilayer structure should
be x; y x; y etc.,; then crossovers are only one conductor width and not
several inches.

Phil Dudley

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