Why Metric?

[Tom]

What is the American Government and our Standards committees doing toward transitioning us to the metric weights & measurement system?

Here are two websites.

NIST = National Institute for Standards & Technology

http://www.nist.gov/metric

ANSI = American National Standards Institute

http://www.ansi.org/public/news/1999apr/measure_2.html

Both of these are American Government institutions that explain the metric transition prosess in our country.

If you're not doing PCB design using metric units yet, you will be soon or you will be alone. Let's get on with the process instead of holding it back. Metric PCB design layout is truly better than Inches. I bet that after anyone does 5 designs in metric, that they will never go back to Inches.

Tom Hausherr
Wind River Systems
Tom.Hausherr@WindRiver.com

"Vision without execution is Hallucination"... (Steve Case)

[Lameris]

Here Here, I tried to do that 10 years ago. I hate having to convert all the time. (I also hate having to get my companies devices approved in both Avoirdupois and Metric units)

re http://www.nist.gov/metric
Gerard Iannelli, Metric Program Director kind of puts one to sleep in his presentations...

[TonyQ]

It's a long battle.
I do all my designs (from circuit design to PCB layout) strickly in metric, within a company that does all its CAD documentation in inches. I get weird looks, but all I tell them is : 1 inch=25.4mm.
By the way, I use Cadence's Layout for my PCB layouts. I don't know if this is a valid assertion but I think Layout was designed in inches. I say this because if I enter, say 1mm for a via drill size, my drill table always displays 0.999mm. I think Layout does a double conversion; from what I enter in mm to inches and back to mm for display. Is this the same for PowerPCB and the other PCB layout tools?
Tony

[Tom]

Tony,

Yes, Cadence Allegro software code was written using the English measurement system. This is also true for PowerPCB. However, Innoveda is trying to transition PowerPCB to the metric system. I was told 5 months ago that Innoveda was in the process of revising the PowerPCB training manuals to metric. The introduction to metric technology is supposed to start soon (Q2 2002). This means that when you sign up to take a 3 day PowerPCB class, they teach you PCB design layout and library construction strickly in metric units.

This is a very smart thing to do because all component manufacturers are now only using the metric measurement system for their production lines. (there is one exception - the 1.27mm pitch BGA is still being produced by some component manufacturers) They in turn are creating metric parts because every standard organization in the world has established metric as the primary measurement system and they are all in the process of eradicating the English weights & measurement system over the next 8 years.

I can see the day when English measurements will be a thing of the past and we will tell our grandchildren "Once upon a time we used a different measurement system". Basically, we will not need to refer to millimeters as being metric because all weights and measurements will be metric and there will not be a need to refer to it as a measurement system, because it will be "The System" (the only system).

We need to despirately standardize the PCB design industry to acheive the highest possible productivity levels, reduce PCB layout time, increase quality and eliminate errors and duplication of effort. It will be impossible to acheive this goal with two measurement systems.

[Mark Larson]

Tom:

Is there a common grid in the Metric library? I think I read a while back that everything was being placed on a .0125 mm grid, or was it some other number?

[Tom]

Mark,

The common metric grid is 0.05mm. This means many things.

1. Disregarding all "Inch Based" components (PLCC, SOIC, or any part with a pin picth of 0.050" which are currently being "Phased Out"), all the new metric based parts have their solder pad snap to a 0.05mm grid.
Note: All SMT Chip Capacitors, Inductors, Resistors, Tantalum Caps, etc. are metric by design. The USA converted them to inches for convienience. The name 1206 is really 3216.

2. 0.05mm is the common routing grid for routing the following trace widths: 0.1mm, 0.125mm, 0.15mm, 0.2mm, 0.25mm, etc.

3. 0.05mm in library construction is the snap grid used for all Placement Courtyards in the 3-Tier library system.

4. All through hole and pad and drill sizes are in increments of 0.05mm.

5. In SMT library construction formulas, 0.05mm is the common round-off factor for all pad lengths and pad widths.

[Mark Larson]

That is SUPER! the .05 mm grid should work great.

I have not checked the inch parts in the pcb standards library, but if the pins are not rounded to the closest .05 grid I will modify them so they do before I use them. That way I can set the fannout grid in Blaze to .05mm rather than turning snap to grid off. My suspicion is the reason snap to grid off yields better results for most people is they are mixing inch and metric parts, the only common gid matrix is then the PowerPCB base unit (or in other words snap to grid off). most of us have found what a problem it is to try to edit a board with routing not on a grid.

The .05 grid should be an improvement for placement too, currently I do the initial placement on a 10 mil grid and then zoom in and fine tune with a 1 mil grid. My hand and eyes just aren't sharp enough. being able to double the grid will effectively allow me to zoom out twice as far.

[Ross Bailey]

OK Tom, you win! We’re going to Metric. I see the May 2002 issue of PCD has an article. Do you know of any additional references specifically related to layout? Any words of wisdom for the transition?

[Tom]

Ross,

The only thing I know is this.....

About 4 years ago IPC and JEDEC made a joint agreement never to publish any technical documents using English Measurements. They standardized on the Metric System. When they made that decision, it affected every component manufacturer. Now all the component manufacturers are producing metric based parts and it's impossible to place and route them using English measurements and get a good clean solution. So we PCB designers have to re-think the way we layout boards. This in turn is affecting the PCB manufacturers who have to deal with the data that we produce.

This entire scenario is totally backward to the way past technology breakthroughs have happened. Normally we could only design what the manufacturers were able to build. So we always got our technology from the manufacturers. It's called DFM. This is the first time that the PCB designers are dictating to the manufacturers what we need to solve today's technology issues.

So, where are the educational resources coming from? IPC. They are the ones who started this Metric Standard and they are the ones that must lead us through it.

I'll be at the Dallas IPC Designer Learning Symposium on May 21 & 22, trying to bang out some of the issues that we face. The transition period is a struggle because our PCB designs are temporarily split down the middle with 1/2 the part pins on an inch grid and 1/2 the parts pins on a metric grid. Not a pretty site. But the fact remains that we are heading toward a metric measurement system and those who transition first will be tomorrow’s leaders.

There is a ton of documentation available for free on pcbstandards.com. I recommend that you start downloading or purchase a $25 CD-ROM of the entire website and start reading.

Big Tip: Do Not Convert! If you have a PCB design laid out in inches, keep it in inches. Find a good breaking point in your workload to start the metrication process with new PCB designs. Start by creating your Metric working environment piece by piece in your spare time. Once you have a stable environment created, go for it. Read - "Increasing Productivity!". That will explain all the elements you will need to create your working environment.

Good Luck and if you need help, I'm right here.

[Mark Larson]

I have loaded the metric environment CD on my system at home and am using the metric library rather than developing my own metric based library. in addition to enjoying the bubble gum I won I have found the color manual to be a big help, in the past I printed out individual sheets to black and white. Color does make a difference, and having a bound manual is alot nicer than a bunch of sheets laying around. If you have the time and a color printer do it yourself, but if your company will pay for the manual I gotta say it is worth it, I'm sure printing yourself could cost close to $50 just in materials, and I'm sure would take at least an hour+

It all makes alot more sense and is easier to navigate with a manual.