Re: [SI-LIST] : Traces isolation vs FCC68

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From: Georg Ramsch (georg.ramsch@po.uni-stuttgart.de)
Date: Mon Nov 15 1999 - 02:49:27 PST


Hello, Michael !

Do the tables specify, wether the given values are related to a
microstrip (mixed dielectric FR4/air/solder mask) or stripline (only FR4 as
dielectric)construction ?

Years ago we tested a telecom backplane with 2kV (2 layer microstrip)/ less than
40mils (~1mm) clearance
and it passed the test. The connectors had been pressed in, so no solder remains
could degrade
the breakdown voltage.
What also may have an influence is the water content of FR4; shall be around 1%
or better less.
For maintaining solderability, the stock for the pcbs should be air conditioned
with low humidity.

Regarding thickness: this 8-layer board has no impedance controlled traces?
(because otherwise the 115mils would be reasonable)

Regards

Georg

Michel Bazinet schrieb:
>
> To whom it may concern,
>
> I would like to know what you think about one of the assertions of Mr
> Lee Ritchey who said that the dielectric breakdown voltage for a
> standard
> FR-4 Epoxy Glass is about 1100Volts/mils. On one of our projects, to
> pass the FCC68 test, we needed 1500 Volts of isolation on some traces.
> At
> this
> time when I made the PCB I checked in the IPC-D-275 table (table 3-1,
> P11,
> september 91 edition) to find some very huge results (1500V ->
> 150mils). In
> reality yon cannot design a project and keep this clearance between 130
> different nets. So in this specific project I kept an isolation of
> 30mils
> between each of the concerned traces. And in the Z axis I increased the
> thickness of the PCB to 115mils (just to be able to solder every part
> on
> the pcb) on an 8 layer board. The problem with the tickness of this
> board
> is that we have a lot of trouble with the fabrication and the assembly.
> Furthermore, it is more expensive.
>
> When I spoke with Mr. Ritchey at the PCB Design Conference he assured
> me
> that the table he gave me is correct. I would like to know your opinion
> on
> this
> topic, because I don't have any sources other than IPC. If it's
> correct, we
> can
> go down to a 0.062" pcb thickness and reduce the size of the pcb.
>
> We are currently working on a new version of this board so I hope you
> will
> have some good news for us.
>
> Michel.
>
> __________________________________________________
> Michel Bazinet m.bazinet@nhc.com
> PCB Designer http://www.nhc.com
>
> NHC Communications Inc. (514) 735-2741 ext.: 262
> 5450 Cote de Liesse 1-800-361-1965
> Mont-Royal, Quebec, H4P 1A5 Fax: (514) 735-8057
>
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