Re: gEDA-user: six-layer stack

[Larry Doolittle <ldoolitt@xxxxxxxxxxxxxxx>]

Stuart -

On Wed, Feb 01, 2006 at 02:12:08PM -0500, Stuart Brorson wrote:
> 
> The thicknesses you get are usually determined by the thickness of the
> raw materials stocked by your board vendor.  You may or may not have
> much flexibility there.

Except by choosing a proto-vendor that publishes a default
set of dielectric thicknesses that is consistent with my
electrical goals.

> It sounds like you are trying to create a controlled impedance board,
> with some traces 50ohms, or somethign like that.

Not really.  At 100 MHz, and a 4x5" board, the concept of
line impedance is, err, a stretch.  The concepts of minimizing
antenna loop area, and keeping power supply inductances low
and capacitances high, are still valid.

> [chop] I suggest that you just spec the impedance of the tracks you need
> to control, and let the board house vary the track width and laminate
> thickness to match their process while still getting you your desired
> impedance.  That's how I usually see it done.  

OK, but that sounds excessive, and I doubt I can find a proto
house to do that for a "reasonable" charge.

> [chop] (Charles Grasso) is a frequent contributor
> to the SI-list, which is a good resource for learning about this kind
> of stuff.  If you're doing high speed stuff, it pays to subscribe to
> that list.

Well, lurk through its archives anyway.
I'm on too many lists already.

     - Larry