Leon,

Thanks for the reply.  That is what I already knew :o) .  I guess I should
have been more specific and asked for the formula.  Oh well, your reply did
bring a smile - which we all need now and then.

Regards
John

> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Leon Heller
> Sent: October 31, 2002 2:07 PM
> To: [log in to unmask]
> Subject: Re: [TN] FAB - Controlled Capacitance
>
>
> >From: John Parsons <[log in to unmask]>
> >Reply-To: "TechNet E-Mail Forum." <[log in to unmask]>,              John
> >Parsons <[log in to unmask]>
> >To: [log in to unmask]
> >Subject: [TN] FAB - Controlled Capacitance
> >Date: Thu, 31 Oct 2002 11:39:38 -0800
> >
> >This is a first for us.  Our customer has specified
> >
> >"Capacitance between layers 4 and 5, and layers 6 and 7, shall be greater
> >than 250pf per square inch, assuming unperforated planes".
> >
> >Layers 4 5 6 and 7 are all plane layers (ground/voltage pairs each).
> >
> >Now, I understand the concept of capacitance but I have not come across,
> >nor
> >can I seem to source any info telling me how to ensure that we are
> >manufacturing to this specification.  I have the IPC-2141 which makes
> >reference to controlled capacitance lines (not planes) but does
> not seem to
> >provide any useful info from a manufacturing perspective.
>
> >From the distance between the planes, and the dielectric
> constant of the PCB
> material, you can calculate the capacitance using the appropriate formula,
> which I've forgotten. 8-(
>
>
> Leon

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