This has been a great discussion. I'd thought I'd throw in my $.02. Generally we use inches. I don't know why other than that's just the way it's done. Boards are placed and routed in inches. When I create a library pattern (that's what we call it from our PCAD tools) I layout the land in metric if the data sheet is in metric and make the rest (silk, courtyard, etc.) in inches to make it easier for placement. Our mechanical engineers use inches and out vendors use inches.
To me they are all just units of measurement. All that matters is DFM and DFT. If the board can be assembled start to finish and works then I'm happy.
Allen Maddox
________________________________
From: Tom Hausherr <[log in to unmask]>
To: [log in to unmask]
Sent: Monday, March 19, 2012 12:51 PM
Subject: [DC] IPC APEX Designer's Day
Hi Everyone,
I posted my IPC APEX "Designers Day" Power Point presentation on "PCB Design
Optimization Starts in the CAD Library" here -
http://www.pcblibraries.com/Forum/ipc-apex-expo-2012_topic6.html
This presentation contains recommendations for adding PCB library drafting
elements such as Silkscreen and Assembly Outlines, Polarity marking,
Reference Designator sizes, Local Fiducials, round-off resolution update and
the implementation of Mil Units for the upcoming IPC-7351C standard.
One note on the Mil Unit subject. As PCB Designers, we have been taught DFM
(Design For Manufacturing) rules during our entire career. For the past 10
years I have been all over the world promoting PCB Designers to transition
to the metric measurement system and revealing why the metric system is
superior for PCB design layout. However, our PCB fabrication shops prefer
Mil unit Gerber or ODB++ data, drill data, fabrication drawings and layer
stack-ups, panel sizes, material thicknesses, etc. The global electronics
industry including component manufacturer's, CAD vendors, fabrication,
assembly, mechanical and PCB design is split down the middle on this
subject. I believe it will remain this way until the PCB fabrication process
completely transitions to metric.
I've been on the open market contracting PCB layout for DOD companies and
they're ALL doing mil unit PCB design. Most PCB design service bureaus are
doing mil (inch) based PCB layout because that's what their customers are
asking for. I've submitted employment resumes to firms all over the USA to
seek employment and the vast majority of them are doing mil unit PCB layout.
At this year's IPC APEX conference there was a large crowd at the Designer's
Day event and when I asked the question "How many of you are doing metric
PCB layout?" only a couple people raised their hand. When I asked "How many
of you are doing PCB layout in mil units?" the vast majority of the
attendees raised their hands. When I asked people why they still do PCB
layout in mil units the majority say "because our fabrication shops prefer
mil units and we are big on Design For Manufacturing (DFM)".
Most people in our industry refer to chip capacitors and resistors after
their inch based names 1206, 0805, 0603, 0402 and not the IPC metric names
3216, 2012, 1608, 1005. All CAD vendors create their default software
installation in mil or inch units (not metric). Popular PCB fabrication
shops default to mil unit presentations in their private seminars to their
customers or at IPC symposiums. Are we being brainwashed or are we in a
state of chaos?
I applaud IPC, JEDEC, NIST and the component manufacturing industry for
fully transitioning to the metric unit system, but I fear that things are
more messed up than ever before in our industry. Being split into two
measurement systems is not exactly optimizing the electronics industry. So I
propose that IPC go back and add mil dimensions in all of their standards to
eliminate the confusion in dealing with dual measurement systems. IPC needs
to recognize that a vast majority of the PCB industry is not transitioning
to the metric unit system and that we need standards that are applicable to
our profession today. Download my Power Point presentation from "Designer's
Day" and you'll get a good taste of the metric / mil state of the PCB
library industry.
That's my opinion, what's yours?
Best Regards,
Tom
Tom Hausherr
President
PCB Libraries, Inc.
13730 Sorbonne Court
San Diego, CA 92128
858.592.4826 Office
858.859.5371 Cell
[log in to unmask]
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