PCB's

Started by church_art October 15, 2005
I am doing my first 2 sided PCB using expresspcb ....

What is the advice on which side to place traces ? Their tips section
say power and ground on the bottom - but show a red trace example
which is on the top - for their software.

Is it better to have the majority of traces if possible on the top
(component side) or bottom ? Their software defaults to top (red) and
so I did most of them there. Then I started looking at other boards
and they seem to be mostly on the bottom (perhaps from one sided
days ?)

Maybe it doesn't really matter but any advice would be appreciated.

Art


--- In basicx@basi..., "church_art" <achurch@m...> wrote:
>
> I am doing my first 2 sided PCB using expresspcb ....
>
It is best to put all you can on the bottom so that solder joints
make to the parts. Green is the bottom and Red is the top.

Ken Smith


Art,

Use both sides as neccessary. However, most of the time the
bottom size is better. That way you can get at the trace should
you ever need to "cut & resolder" when you've discovered you've
made a mistake or when any change is required. Usually a little study
will yield which traces can go on top and which must go on the
bottom. I've been using ExpressPCB of five years and wouldn't
use anyone else! Concerning power and ground traces. To help
reduce noise it is best to try and run them along the same path.
When that is not possible, try running power on the bottom and
ground on top along the same path. Don't be afraid to use vias
to make layout easier. They don't cost any extra and can help your
layout look "cleaner". Good luck.

Chuck



--- In basicx@basi..., "church_art" <achurch@m...> wrote:
>
> I am doing my first 2 sided PCB using expresspcb ....

Art, They also have a group if you want some more info on this.
http://groups.yahoo.com/group/expresspcb
There you can find extra componets.

Ken Smith


--- In basicx@basi..., "church_art" <achurch@m...> wrote:
>What is the advice on which side to place traces?

If you want to stay with a two layer board I'd suggest putting power
and ground busses on the top layer. Consider running a heavy trace
for power down one edge of the board and another one for ground down
the other edge. Then, extend "fingers" from each laterally toward
the other side (but not connecting, of course) as necessary for your
component rows. You'll want to have the power/ground traces as
large as possible. If you start with them big you can narrow them
down, or do so only in critical places, as necessary.

Then, try to place most of the signal runs on the bottom layer
generally running them perpendicular to the traces on the top side.
When you need to put a signal run on the top, try to keep it running
parallel with the power/ground traces. This strategy allows you to
route signals by changing layers as necessary with vias.

An alternate strategy is to place the power bus on the top layer and
the ground bus on the bottom layer, perpendicular as before. As
described earlier, you'd want to have a larger main trace with
fingers extending off of it at intervals.

Depending on what you're doing, it may be advisable to go with a 4-
layer board with power and ground on the middle two layers. Yes,
it's more expensive but it solves two problems. Firstly, it
provides an excellent ground plane that helps reduce noise. Also,
if your project has an analog section it will likely benefit even
more from a good ground plane. Secondly, getting power and ground
to each device is a cinch - just make connections to the appropriate
inner layer. This leaves the entirety of the top and bottom layers
for signals. Again, keeping the runs on the top layer generally
perpendicular to those on the bottom helps keep things orderly.

One drawback to the 4-layer boards is that you generally don't get
the special "prototype price" that outfits like ExpressPCB offer.

Don