PCB layout software

[Tony Clark]

   A few replies here...

> >It takes a hell lot of time to set up the system.  You have to define
> >object bodys, schematic views, simulation code, pinouts and a footprint
> >for EVERY single type of component you use.  Most probably you have to
> >define a lot of resistor and capacitor types because of varying wire
> >length (instead using wire bridges you often use resistors with long
> >wires to get over some pcb trakcks, whenever possible etc.).

   In a good package program, most of the work has been done for you.  
They usually come with a fairly extensive libraries so that you do not 
have to do more than pull in a component and assign a footprint.
   Also, unless you really need it, simulation code is unnecessary in PCB 
software.  It is nice to have when you are actually _designing_ circuits, 
I suppose, but for just schematic entry and PCB layout, not needed.

> >Most software packages I know have lots of 74LSXXX in their librarys,
> >and maybe a 741, but most of our analog stuff is not there.

   Well I remember when the Engineering dept. was trying out MicroSim as 
a possible schematic entry/PCB layout software for the PCB class I 
teach.  I had already been using windows OrCAD (learned on DOS OrCAD!) 
for a good year, so I told them it'd be easier to use what I knew instead 
of retraining myself on a new package.
   But I went ahead and evaluated the MicroSim package and was abhorred 
to learn that the libraries were vastly incomplete!  They were seriously 
lacking in analog component primitives (ie no transistor pinout 
variations) and completely missing interconnection components!

> >Not to mention all the debugging afterwards, because of wrong defined
> >pinouts etc. etc. I do this all day long in my day job, every cell I
> >design gets a schematic body, simulation code (hand coded if necessary)
> >etc etc, and very often I get netlist problems at first, because I've
> >forgotten something, or mixed it up.

   Again, a decent program should already have a library of parts.  
Although I will say that even OrCAD goofed in this regard!  When the 
Engineering dept. bought their copies of OrCAD for their lab, I made the 
awful discovery that all of the IC parts in the libraries had the power 
pins incorrectly assigned!  Funny that it wasn't goofed in the copy I 
have.  :/
   So maybe you really can't escape some amount of hardship, even with 
top of the line programs!

> A couple more reasons for not using PCB layout software:
> 
> 1. Most software cost money. A general drawing program is useful for 
> so many other things, that you want to have it in any case. But an extra 
> program for PCB layout means extra cost.

   This is true.  OrCAD tops in at 10 grand with the advanced PCB layout 
program!  But comes in much less for educational institutions!  And of 
course there are MUCH more expensive packages out there!  I know of a few 
that are in the 40-60 grand range!  The nice thing about OrCAD is that 
you can buy different levels of software so that you don't have to pay 
for extra features (such as autorouting - see below).

> 2. When I design the board manually, I can do sneaky things like 
> shaving off portions of pads to make room for a trace that would not 
> otherwise fit. The boards I have seen other people do are not as compact 
> and have more wire links than mine, that's a fact.

   Firstly, any decent PCB program should allow you to assign different 
kinds of pads!  With OrCAD you can either change pads on the fly or you 
can edit the footprint library to customize regularly used footprints.
   As far as wiring, this is an art, not a science.  Absolutely NO PCB 
program on the planet will save you from badly routed PCBs!  The most 
unreliable feature of any PCB program is the auto-router!  I have yet to 
see one handle high-density (compact) designs!

> 3. I can design the schematics symbols to look exactly like I want 
> them. I have emulated the style in Elektor magazine, for nostalgic reasons.

   Any good PCB program will let you edit or create your own part library.

> 5. It is good brain gymnastics to keep the routing in the head. 
> Occasionally I make a mistake, but it is necessary to build a prototype 
> anyway. Board layout errors are usually easy to correct on the prototype.

   Well everyone should be doing this!  I always prototype my boards 
before committing to pre-production prototype circuit boards!  It costs 
too much to continually run new batches of circuit boards if you discover 
flaws along the way.  Far better to hand-wire or vero-board a circuit and 
work out flaws first!
   Really, we can discuss what programs are better or what features they 
should have all day!  I think that we all have a method that suits our 
needs.  For those that are still looking, all I can say is get the best 
that you are willing to afford, because it makes a world of difference!
   To me, the most important thing to determine is whether or not you'll 
be doing your own circuit boards or if you will be farming them to 
outside sources.  If you are farming them out, then be sure it will 
support Gerber photoplotter files and use Excellon format.  These are 
universal with PCB fab houses (in the US, anyway).  This is also a good 
idea if you want to share your PCB files with others.  Anyone can then 
take the data and have circuit boards made from those files without any 
further editing!
   Anything else beyond file output format is purely up to your own 
personal preference!
   This is a great thread!

   Tony

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I can't drive (my Moog) 55!         |     The E-Music DIY Archive
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Tony Clark -- clark at andrews.edu     | aupe.phys.andrews.edu/diy_archive
http://aupe.phys.andrews.edu/~clark |     Contributions welcomed!
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