Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!ginosko!uunet!ingr!phil
From: phil@ingr.com (Phil Johnson)
Newsgroups: sci.electronics
Subject: Re: PC Boards: Gerber formats
Message-ID: <6564@ingr.com>
Date: 25 Sep 89 15:21:24 GMT
References: <6261@pt.cs.cmu.edu>
Reply-To: phil@ingr.UUCP (Phil Johnson)
Organization: Intergraph Corp. Huntsville, Al
Lines: 87

In article <6261@pt.cs.cmu.edu> nivek@rover.ri.cmu.edu (Kevin Dowling) writes:
>

>(Postscript) so we can preview mask layouts.  Reprographics places that
>generate artwork for PCB production usually use 'Gerber' formats however.
>McCadd does supply a Gerber translator for their product as do other PC
>layout programs on the market.
>
You will find that all of the PC houses use the Gerber Photoplot format.  The
Gerber format is the industry accepted standard for photoplotting.  It was 
originally developed by Gerber Scientic, who produced the first photoplotters 
for PCB generation.  The format defines a specific aperature wheel, aperature,
coordinates, and flash.  The output of the photoplotter is the slicks for each
signal, plane, drill, solder mask, silkscreen layers, and any other user
defined layers.

Any printed circuit board layout software that does not provide the ability to
produce a Gerber tape is at best a "toy" system (toy system: a drawing package
the claims to provide PCB layout capabilities).

>
>My questions are: Have there been proposed new formats for describing PC
>board layouts? Could postscript or PICT or some subset of these be used to
>generate layouts? Is Gerber good enough for the forseeable future?
>

The IPC-D-350B exchange file format that has been defined by the industry 
association for transfering PCB definition, You are fairly safe with the Gerber
format.  The PC houses are still in the transition phase of implementing the 
IPC-D-350B standard.  If your software does not support the Gerber format and 
can not give you an availablity date for IPC-D-350B, then I recommend you find
a real PCB layout package.

"NO ONE" would want to pay the cost for photoplotting in postscript and PICT
has limited platform support and is inferior to Gerber or 350B.

>Since masks for PC production are just dark and light areas you can just
>'paint' a board using a single small aperature and lots of time, but are

An aperature wheel contains more than various sizes of "dots".  An aperature
wheel contains various size dots for plotting traces and areas. Also, there
are aperatures for pads, vias, and other special symbols as define by the
user.  Rather than "painting the fill area between the inner circle and the
outer circle of a pad" as you purpose the correct pad aperature is flashed.
Your method compared to the aperature method is about a factor of 10 to 20  
time slower.  Multiply that by the number of pads, holes, and vias that you
have on a normal board and you could end up plotting a board for days.

>there printers of high-resolution that can plot 1:1 masks on mylar directly?
>How about a system that could write on boards directly?  (A PCB printer!)
>

The resolution needed is determined by the density of the PC layout.  If you
are laying out a PCB using a 0.1 inch grid then a 300 dpi laser printer should
be sufficient for your needs.  If you are using a finer grid and doing layouts
such as multiple traces between pads you will need a 1200 or 2400 dpi laser
printer.

Most PCB houses should take your slicks for use in producing the PCBs, but 
NOT guarantee the boards.  This is because they do not have control over the
quality and registration of the slicks.

I have seen a board prototyping system the "routers" the board. Routering 
actually uses a router bit to remove copper plate form the board.  I don't
remember the name of the company who sells it though.

>There has been a proliferation of PCB generation programs, there are several
>for the PC, a couple for the Mac, and other workstation based.  Do all these
>generate Gerber formats?

Every "REAL" (real is defined as production quality) PCB layout package that I
have seen provides a Gerber format output.

I appologize if I sound somewhat condesending, but I have dealt with this
same situation many times as an Application Analysis for electronic design at
Intergraph Corp.  I have seen many users buy a "CAD" system that "provides"
PCB layout capabilities.  In reality, layout is only one facet of PCB design.
If your package does not output industry accepted or standard formats it is
difficult to produce the board and a picture on a computer screen IS NOT a 
printed circuit board.



-- 
Philip E. Johnson                    UUCP:  usenet!ingr!b3!sys_7a!phil
MY words,                           VOICE:  (205) 772-2497
MY opinion!