From: Luis Moreira <luis.moreira1575@googlemail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sun, 13 Sep 2020 10:57:53 -0700
Subject: Re: [EE] Four Layer Routing Strategy
Thread-Topic: [EE] Four Layer Routing Strategy
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Hi Alan,

I can see for through-hole components in essence it will be really strait
forward.

I'm using a traco power DC to DC converter with  +15V/ -15V output which is
surface mount. It will be on the top layer. After dealing with it's layout
requirements, I need then to distribute power as required. so my question
was since I'm on the top layer should I try to route power on the top or
just use a via to connect into the internal power layer and distribut that
way.
Also if you have two or 3 voltages on the circuit do you use the power
layer to route all three.

The 500KHz is the bandwidth of the analogue signal which will be
transmitted as a pulse width modulation. This is actually based on an old
silabs technical note. Instead of using one of their isolation ICs I'm
using fibre optics.
I have it working but picked up some noise from the DC to DC converter, do
to my not so good layout around it. I need to redesign the board and I'm
looking for best options.

Thank you.
Best Regards
              Luis

On Sun, 13 Sep 2020, 18:28 Alan Pearce, <kiwiantipodean@googlemail.com>
wrote:

> > - What do you do with through hole components?
>
> well, they still have holes through the PCB as done in the past. If
> one end is connected to power or ground then the via will connect to
> the appropriate plane with thermal reliefs.
>
> > - how do you deal with a surface mode power supply for the circuit boar=
d?
> > You will have to place it on the top routing layer  but do you
> immediately
> > get vias to the power layers?
>
> Are you talking a prebuilt module here or are you looking at regulator
> IC? Either way the manufacturer will generally have a recommended PCB
> layout.
>
> > - The vias crossing from the routing layers through the power layers,
> will
> > this  not create other issues?
>
> Depends on what you regard as issues. Yes on a high speed circuit such
> as gigabit video then a via becomes a stub that imparts some extra
> inductance that may have to be balanced out in some way, but such
> signals are normally run as differential signals as they will be
> impedance controlled. You talk of doing 500kHz bandwidth, but what is
> your effective carrier frequency, or are you talking in terms of a
> 500kbaud signal? In the latter case i wouldn't expect vias to be a
> problem, but if you are working with a higher frequency carrier then
> you may need to be more careful with routing.
>
> The thing with vias going through power layers is to make sure you set
> up your constraints so that there is sufficient clearance around the
> via that production tolerances don't produce a short to the plane due
> to drilling tolerances. And ALWAYS use thermal reliefs to any plane or
> fill on any layer.
>
>
> On Sun, 13 Sep 2020 at 13:29, Luis Moreira
> <luis.moreira1575@googlemail.com> wrote:
> >
> > Hi All,
> > I've been doing a bit of PCB layout lately, I'm actually doing a 500KHZ
> > bandwidth analogue optical link at the moment. Obviously noise issues a=
re
> > very high on my list of issues to avoid.
> > The layout strategy you are describing seems very good, but I gave a fe=
w
> > questions:
> >
> > - What do you do with through hole components?
> >
> > - how do you deal with a surface mode power supply for the circuit boar=
d?
> > You will have to place it on the top routing layer  but do you
> immediately
> > get vias to the power layers?
> >
> > - The vias crossing from the routing layers through the power layers,
> will
> > this  not create other issues?
> >
> > On a sightly different subject, on a two layer board I tend to run powe=
r
> > and routing on the top layer and ground layer mostly as a copper poor o=
n
> > the bottom layer, is this best practice?
> >
> > Thank you.
> > Best Regards
> >                Luis
> >
> > On Sun, 13 Sep 2020, 12:57 Alan Pearce, <kiwiantipodean@googlemail.com>
> > wrote:
> >
> > > > Ok, sounds like keeping the inner layers as ground and 3.3V
> separately
> > > > will be the best. No problems there.
> > >
> > > That is the 'normal' way to do it. And using manhattan routing gives
> > > you a good starting point. Once you have the board routed you can loo=
k
> > > to see where the routing can be tweaked to minimise vias.
> > >
> > > > One thing about planes though, I once read an article about being
> > > > careful using planes, that essentially small ground loops could for=
m
> > > > in them.
> > >
> > > Opinion on this has gone back and forth like a leaf in the wind, but
> > > one of the most authoritative texts I have seen was an appnote or
> > > article from ADI which advocated having one complete ground plane wit=
h
> > > minimal distinction between digital and analogue ground. It doesn't
> > > mean you could mix the digital and analogue parts willy nilly, some
> > > care is still needed with relative parts placement.
> > >
> > > On Sun, 13 Sep 2020 at 02:57, Josh Koffman <joshybear@gmail.com>
> wrote:
> > > >
> > > > Thank you all for the advice!
> > > >
> > > > Ok, sounds like keeping the inner layers as ground and 3.3V
> separately
> > > > will be the best. No problems there.
> > > >
> > > > One thing about planes though, I once read an article about being
> > > > careful using planes, that essentially small ground loops could for=
m
> > > > in them. Since then I've always had my planes as a separate net (ie
> > > > not ground), and I connected them to the ground traces strategicall=
y.
> > > > Each area of pour had only one connection to ground.
> > > >
> > > > Should I be using the pours a bit more liberally and letting them
> join
> > > > all the pins on that actual net? That's easily done (actually easie=
r
> > > > than the way I have been doing it). I'd add thermals in to try to
> help
> > > > make it easier to solder.
> > > >
> > > > The board service I've been using lately doesn't allow for
> > > > blind/buried vias without a hefty upcharge. I am definitely not at
> > > > that level, that's for sure!
> > > >
> > > > Thank you!
> > > >
> > > > Josh
> > > > --
> > > > A common mistake that people make when trying to design something
> > > > completely foolproof is to underestimate the ingenuity of complete
> > > > fools.
> > > >         -Douglas Adams
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