Re: Speccing 4-layer boards
From: Spehro Pefhany (speffSNIP_at_interlogDOTyou.knowwhat)
Date: 03/07/05
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Date: Mon, 07 Mar 2005 00:55:52 -0500
On Sun, 06 Mar 2005 20:54:09 -0800, the renowned John Larkin
<jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:
>On Sun, 06 Mar 2005 22:48:33 -0500, Spehro Pefhany
><speffSNIP@interlogDOTyou.knowwhat> wrote:
>
>>Trying to finish up some FR4 microstrip boards, the PCB maker tells me
>>that they can make them with various thicknesses and to ask for what I
>>want, but the trace widths in the gerbers depend on my assumptions for
>>the Er, dielectric thickness (and copper thickness).
>>
>>What *should* I be asking for to get maximum flexibility in sourcing?
>>Looking for 75 ohms with signal/gnd/pwr/signal layers.
>>
>>
>
>FR-4 has Er around 4.6, and 1 oz copper is 1.4 mils, so I use those
>numbers and impedances seem to come out close. It seems to me that the
>cheapie multilayer houses (in the back of EE Times and such) often
>prefer 12 mil dielectrics on the outer layers, about 30 for the inner,
>which is sort of silly but will give you 75 ohms microstrip with an 8
>mil trace. Could get lossy with such skinny traces.
That's about what I got from googling for information 12 mils with Er
4.6 (though some suggest 4.2), from which I get about a 9.1-9.2 mil
trace width).
>Probably calling out 20+20+20 with a few mils tolerance on the
>dielectrics is safe for multisourcing.
Which yields about a 16 mil trace width. Quite a difference. 8-(
>Lots of board houses will tweak
>your art to a target impedance, but I don't like board houses messing
>with my dimensions.
Yes.
>I'm just finishing up a board right now. 2 layers, 0.062 FR-4, 0.72
>square inches total. It's a tiny 50 MHz oscillator. The reference
>designators are 50 mils high, which means most nobody can screen them
>legibly, so I guess I'll skip the silkscreen step.
With 10 mil line widths it should be legible at 0.05" high, even
though it's a bit less than is usually recommended. They should only
care about the line width.
> Last week we did a board that must be about 0.1 square inch.
>
>John
I was just looking at a design that uses three or four boards like
that in a tiny awful box assembly to fit mechanically in a very tight
electro-mechanical assembly. The PCB house will supply them in panels,
with internal routed slots and with V-groove, so they are pretty easy
to handle until they are depanelized.
Thanks for the suggestions!
Best regards,
Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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