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Old 12th September 2011, 11:23 PM   #1
rjm is offline rjm  Japan
Richard Murdey
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Default not-so-newbie Eagle question : metric or imperial?

I've had it in mind to do an update of my PCB layouts for the Phonoclone and VSPS projects. No large physical changes, just a clean up of the board design to the current "state of the art" i.e. my improved Eagle skills in working with polygon fills, restrict areas, etc.

I hate imperial, but when I started pretty all he components were based on 0.1" hole spacing, so it made sense, even though the board size was defined in cm, to make the layout working from a 50 micron grid.

Looking it at now, pretty much everything is metric, or purports to be.

If I was starting over again today, should I work from a cm or inch based grid?

If the board is in cm, should the mounting holes also be spaced in cm?

Finally, if mixing a cm board with an inch grid, what is the best way to keep the grid "centered" on the board area?

Any suggestions welcome - Thanks in advance

Richard
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Old 13th September 2011, 11:42 AM   #2
marce is offline marce  United Kingdom
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METRIC
Most components these days (SMD) are metric, and if you use BGA's its almost mandatory.
Metric is the international standard.
Look up IPC-7351 fast becoming the standard to use, and if you are new or even a jaded old hand at PCB's its the way to go.
http://www.dnu.no/arkiv1/The%20CAD%2...e%20Future.pdf
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Old 13th September 2011, 11:47 AM   #3
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Most new components are metric, but there's a lot of SOIC parts around with a 0.05" lead pitch. Why not set the working units on the CAD program to metric, but set a 1.27mm grid? (That's what we do on CADSTAR here at work).
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Old 13th September 2011, 11:54 AM   #4
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I still mostly use through hole, so do the layout in imperial, then switch to metric for placing dimensions and mounting holes. But I could quite easily work in ether format, and frankly, unless you have to interface with, say, an enclosure or mechanical design, I don't see it as being a great issue.
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Old 13th September 2011, 02:22 PM   #5
marce is offline marce  United Kingdom
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Most of the new components and probably 80+% of the worlds designs are now metric based. Even the SOIC is fast being replaced by smaller pitch 0.65mm and 0.5mm devices. A lot of the new components are bottom terminated devices these days, which again are metric pitch. These devices are going to become more common as they provide numerous advantages over more tradiytional packaging, ie no lead frame to add parasitic capacitance and inductance for signal integrity, excellent heat transfer away from silicon, via the copper slug, that becomes the thermal pad on the bottom of the device.
As stated earlier most (if not all BGA's) used in modern designs are based on a metric pitch.
More and more companies are moving towards using IPC-7351.
Pinkmouse, I cant remember when I did a PCB that wasn't going in some sort of enclosure, nearly all will be in enclosures/.

Ouroborous, I aslo use Cadstar. I use the 0.05mm grid that again is becoming common practice in the wider world. I guess you are using quite a few op-amps and logic in the old soic packages.
I have also been using Metric now for quite a few years.

So I would go metric, IIts easy to translate and update the components to the IPC-7351 standard, I have done a few libraries both in Cadstar and Allegro. Look at the PCB and electronic standard organisations that are behing metrification of PCB design.

http://www.smtnet.com/library/files/...rid-System.pdf

Most of the new components and probably 80+% of the worlds designs are now metric based. Even the SOIC is fast being replaced by smaller pitch 0.65mm and 0.5mm devices. A lot of the new components are bottom terminated devices these days, which again are metric pitch. These devices are going to become more common as they provide numerous advantages over more tradiytional packaging, ie no lead frame to add parasitic capacitance and inductance for signal integrity, excellent heat transfer away from silicon, via the copper slug, that becomes the thermal pad on the bottom of the device.
As stated earlier most (if not all BGA's) used in modern designs are based on a metric pitch.
More and more companies are moving towards using IPC-7351.
Pinkmouse, I cant remember when I did a PCB that wasn't going in some sort of enclosure, most will be packaged.

Ouroborous, I aslo use Cadstar. I use the 0.05mm grid that again is becoming common practice in the wider world. I presume you are using quite a few op-amps and logic in the old soic packages.
I have been using Metric now for quite a few years, and when interfacing with the 3D world via IDF (again with packing densities all products are built up virtualy in 3D land).

So I would go metric, Its easy to translate and update the components to the IPC-7351 standard, I have done a few libraries both in Cadstar and Allegro. Look at the PCB and electronic standard organisations that are behind metrification of PCB design.

http://www.smtnet.com/library/files/...rid-System.pdf

Some excellent links to white papers from the Mentor site.
http://www.mentor.com/products/pcb-s...d-system-58117

Another excellent source of information for ALL things PCB is the IPC
http://www.ipc.org/Default.aspx

87% of ridgid PCBs are now produced in Aisa, the majority of these boards now are either 100% SMD (phones PC's, Apple products etc) or have a few connectors and instrusively reflowed through hole components. (6.7% North America, 5.2% Europe). China is top of the league with 40% of world production.
90% of flexis are produced in Aisa.
CEM basewd (very cheep) products account for 5% of world production, about 11% of boards are 2 layers PCB's, 13% are IC interposters (BGA / LGA devices etc), 10% Flexis, that leaves 61% of world wide PCB production in 2010 was multi-layer technology (included in here is HDI based designs, now reaching nearly 15%, (these are designs with 0.1mm laser drilled holes).
Above figure from the IPC World production report for 2010.

Last edited by marce; 13th September 2011 at 02:51 PM.
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Old 13th September 2011, 02:32 PM   #6
Eva is offline Eva  Spain
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One more vote for metric with a grid based in a multiple of 0.635mm (I use 0.3175mm for SMD)
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Old 13th September 2011, 03:16 PM   #7
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Marce.
Yes, a lot of my work is on Class-D Voice-Alarm/Public-Address amps. The analog side of these use SOIC8 op-amps, comparators and SOIC16 MOSFET gate drivers. The multi-channel amps have an embedded uP on the backplane board for control and monitoring. There's still quite a few SOIC logic chips on there as well as various TSSOP chips. As Eva said, a grid of 0.635mm (0.025") allows for most chips, with only a few dog-legs as the tracks connect to pins on the fine-pitch metric devices.
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Old 13th September 2011, 03:24 PM   #8
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Hi again, I pressed the wrong button during editing.
We use mainly the 0.65 and 0.5mm pitch devices, BGA's and QFNs etc, the soics are just to big for us now, our standard resistor is an (1005) 0402, as production wont let us go smaller. The boards I do are mainly 10-14 layer.
I have a set of IPC footprints in Cadstar Archive format if you want to look at some. I presume we will also have the same VAR, are you going to the engineering day at Telford?

Translation Table from Inch to Metric, I have a nicer PCB libraies version - via PM if anyone wantsit.
Inch to Metric Conversion Tables for PCB design Tom Hausherr's Blog
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Old 13th September 2011, 03:32 PM   #9
marce is offline marce  United Kingdom
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Oh sorry if I seem a bit zealus over this, but as the IPC figures show, we are loosing a lot of our manufacturing and design base in the West, and this upsets me, as over the last 25 years I have seen numerous firms I have worked for or done contracts for disapear. Interestingly Germay is still doing well with 2.1%, the UK is nowhere to be seen. Lack of investment, management, unions, a general undermining of Engineering in this country have all helped decimate our industrial heritage...
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Old 13th September 2011, 03:33 PM   #10
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0603 as the standard R and C size for us! ( we hand solder the prototype boards). Our Cadstar system is really the second system here, as the company uses Mentor as the standard package. (though our CAD pcb guys much prefer Cadstar).

4-layer boards are the norm as well, so a lot less dense than yours.
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