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Home > Archive > Electrical Engineering > May 2006 > "3 layer" pcb
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| a7yvm109gf5d1@netzero.com 2006-05-27, 8:21 pm |
| I'm wondering if it's viable to lay out high speed CMOS logic ("high
speed" meaning 32MHz clock, 1-5nS edges) on two layer FR-4 62 mils
thick as usual, but omitting the ground plane. You give priority to
power and signals, then you glue self-adhesive copper sheet to the
bottom of the PCB, driving small brass nails through the ground vias
and soldering them in place.
I would expect the soldermask + adhesive to provide insulation, if I
tent the vias.
I'd route signals on top as usual, use the bottom layer for power, and
then you'd have a higher plane to plane capacitance, and I'd assume the
signals would "see" the ground plane and the power plane on more or
less the same height and use both as the reference plane.
Reason: 2 layer PCBs are much cheaper to get built than 4 layer!
Make sense, or it's lunacy?
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| In article <1148769122.143389.221920@j33g2000cwa.googlegroups.com>,
a7yvm109gf5d1@netzero.com says...
> I'm wondering if it's viable to lay out high speed CMOS logic ("high
> speed" meaning 32MHz clock, 1-5nS edges) on two layer FR-4 62 mils
> thick as usual, but omitting the ground plane.
I wouldn't even try it. Even though your timings aren't all that
bad, power planes are too cheap. Omitting the ground plane, of all
things, is just nuts!
> You give priority to power and signals, then you glue self-adhesive copper sheet to the
> bottom of the PCB, driving small brass nails through the ground vias
> and soldering them in place.
What *are* you hacking? Do the opposite. The ground (reference)
plane is the most important.
> I would expect the soldermask + adhesive to provide insulation, if I
> tent the vias.
Why?
> I'd route signals on top as usual, use the bottom layer for power, and
> then you'd have a higher plane to plane capacitance, and I'd assume the
> signals would "see" the ground plane and the power plane on more or
> less the same height and use both as the reference plane.
*BAD* idea.
> Reason: 2 layer PCBs are much cheaper to get built than 4 layer!
Not that much.
> Make sense, or it's lunacy?
Yes.
--
Keith
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| a7yvm109gf5d1@netzero.com 2006-05-28, 12:21 am |
| " Omitting the ground plane, of all things, is just nuts! "
I'm not omitting it...
"What *are* you hacking? Do the opposite. The ground (reference)
plane is the most important. "
Yes, I like this idea. The board goes into a Hammond 1455 enclosure
with the ridges in the metal extrusion. All I have to do is reduce the
copper area of the power plane so there's no shorts. Good insight
there, friend.
"> I would expect the soldermask + adhesive to provide insulation, if I
> tent the vias.
Why?"
Vias are not generally tented. That's exposed copper.
"> Reason: 2 layer PCBs are much cheaper to get built than 4 layer!
Not that much. "
Oh yes they are. At least a 2X factor. Anyways this isn't for a
production item, just a personal prototype. I have enough money to get
a 4 layer board done. But I enjoy the challenge of thinking of loony
alternatives. I mean I could just buy what I'm building.
"> Make sense, or it's lunacy?
Yes. "
I'll take that as a yes to the former!  Hm, I think building it is
the way to go to see if it would work in my case. I'm worried that
soldering the nails will melt the adhesive on the copper foil. I got it
at a stained glass store though, but I don't know what they use it for.
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