[sdiy] PCB files

[Dave Manley]

Charles Bisaillon wrote:
> So my team mates have finished the PCB files with only few thing to fix. But
> they followed only half the PCB design guidelines from Joe Grisso that I've
> posted for them. The trace size is alright, the PCB size is alright but the
> component placement is horrible (for trace length) and they finished them
> off with autotrace and 2 layers. Check it out.
>
> http://www.oveloe.com/490/VCOpcb.JPG
>
> I have never done this before but, according to the guidelines, this PCB has
> major problems. But on the other hand, the breadboard is surely worst with
> all those stray capacitances, but experimentally, it was a bit noisy but
> really not that bad (on my 10$ cpu speaker).
>
> I am asking those of you who have experienced this before. Is this
> acceptable? Would any of you consider this good work? I really am new to
> this and thanks a lot for your help.
>
> Charles
I don't want to be too discouraging, but in over 25 years of working as 
an engineer, that is absolutely the worst layout I've ever seen. :-(

One key to good pcb design is spending the time up front in placement.  
The people doing the placement need to study the schematics, determine 
what components form sub-circuits.  Think about how the sub-circuits 
relate.  What is the overall signal flow on the board.  Then, turn the 
"rats nest" feature on and start moving related components together, 
rotating components as required (*see comment below), and spending time 
optimizing the layout of each sub-circuit to minimize route length and 
number of vias.  I'm sure a good placement could result in a total route 
length that is half or even a quarter of what has been done here.  
Putting all the like components together as was done here is a really 
poor design practice.

Power and ground should not be treated like any other net.  I would 
pre-route these before the other traces with thicker traces. 

In the future, before routing the board, I'd recommend a placement 
review be done.  Get a print of the placement and the schematic and a 
couple of different color high-lighters.  Work your way through the 
design and see what is ok and what needs improvement.  Once you have a 
good placement, then start routing.

Take a look here: http://www.wiseguysynth.com/larry/motm/420/part7.jpg 
or here http://www.wiseguysynth.com/larry/motm/300/part9.jpg (lots of 
good pics at this site).  Just looking at the assembled boards you can 
almost see the sub-circuits.  Notice the ground plane on top, the 
capacitors placed at the ends of the ICs, etc.

-Dave

*It is a good idea to pick one orientation for components, especially  
ICs and polarized parts like electrolytic capacitors and diodes.  It 
makes assembly errors less likely if all components face the same 
direction.  Of course, there are many other rules to follow for 
manufacturability that you don't necessarily need to follow for your needs.