[SI-LIST] Re: Layout service recommendations
- From: "Bill Reams" <breams@xxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 1 Aug 2002 08:51:47 -0500
A few added items and some extra emphasis on others...
* Find a layout person you can deal with. A colleague of mine recently =
had a layout done by an outside vendor. In his words, the final layout =
was one of the best he ever got but the vendor was so hard to deal with =
(arguing about whether they even needed to follow the layout =
instructions for example) that he won't be using that layout person =
again. In short, their personalities clashed. Try to avoid the same =
situation.
* Definitely document your layout instructions. This does two things. =
First it lets the layout person know what you expect. More importantly, =
it makes you think through every detail of your design and what you want =
(and aren't you the one on the line to make the board work?).
* Consider the layout document to be a project deliverable. If you just =
throw out some random details, it will be hard for anyone to understand =
and follow. If you document the instructions in a detailed, =
professional, and organized manner, you know everything is there. Now =
your layout person can follow the instructions and can find details when =
working specific areas. And future projects have a chance to learn from =
what you did right (or wrong).
* Don't be afraid to get very detailed, after all, the layout person =
probably cannot read your mind. For example, if a part must be in a =
specific location write "backplane connector P1 must be at location (x", =
y")." If you want something but it isn't a hard requirement write "place =
decoupling cap C1 near U1 pin 1".
* Again, don't expect your layout person to read your mind. If you say =
"match the length of the bus traces" does that mean that they all must =
be identical to the angstrom and does (s)he know what all the bus =
signals are? Better to say something like "the signals listed in table 1 =
shall have their lengths matched to within 0.25" and . . . ." so that =
there is no misunderstanding on what is expected.=20
* Don't over constrain. In the above example, there was a note on =
exactly matching signal lengths. Is the constraint really needed and is =
it worth the cost in time to meet the constraint? Could the constraint =
be loosened and still produce a working board? Ask yourself that =
question every time you put down a rule.
* I prefer to use local layout people. Why? Because there are times when =
it is much easier to get together and draw on paper or a white board or =
to drag stuff across a computer screen. The times that I've been =
involved with non-local layout, the interaction has been more difficult. =
Someday, things like video teleconferencing will make this a non-issue.
-----Original Message-----
From: Bob Perlman [mailto:bobperl@xxxxxxxxxxxxx]
Sent: Wednesday, 31 July, 2002 06:02 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Layout service recommendations
Hi -=20
Here's what's worked for me:
1) Create a PCB layout guidelines document describing stackup, net=20
classes, spacing rules, etc. At a minimum, it'll give you a clear=20
idea what you're aiming for, and it's a lot better than trying to=20
come up with the rules on the fly. Best case, the PCB designer will=20
actually internalize some or all of the rules, and follow them. (My=20
impression is that most PCB designers don't like more than a bare=20
minimum of guidelines, because it plays against their ability to turn=20
a board quickly. But what they like even less is working for a=20
designer who makes up or modifies rules daily.)
2) Try to incorporate as many of those rules as possible in the=20
actual netlist/board design tool. In the past I've written scripts=20
that add net class properties to the netlist; then I entered=20
constraints for net spacings, etc., for each class, and between=20
classes, in the router. I've found that even the best PCB designer=20
has trouble internalizing a large number of rules. But if violating=20
a rule produces a DRC flag on the screen, your chances of actually=20
getting a board routed to your requirements increases greatly.
3) As a previous poster mentioned, plan to spend a lot of time at the=20
PCB designer's side as the design progresses.
If you follow only one of these rules, make it (3). Good luck with=20
your board.
Bob Perlman=20
--=20
Cambrian Design Works
Consulting on digital design, signal integrity
http://www.cambriandesign.com
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