# Metric VS: Imperial Measurement Systems

Posted Jul 8, 2010, by Tom Hausherr

I'm curious as how do PCB designers route metric pitch BGA's on a mil grid system or gridless (which consumes memory and CPU)? I can't figure why anyone would use a mil grid system for any PCB CAD library construction or PCB design layout when all the SMT component manufacturers are only producing metric pin pitch packages. This is where we note that all chip resistors and capacitors are dimensioned using whole metric numbers. So by design all of them are considered metric based components. i.e.: 1206 = 0.125" x 0.062" or 3216 = 3.2mm x 1.6mm for the same component. Which is easier to understand? To me it's interesting why some PCB designers still use the Imperial measurement system. I'd like to hear from designers and engineers who use the technology combination of metric pin pitch components and mil based part placement and trace sizes & routing grids. How do you do it?

My big secret was to teach all the mechanical engineers and EE engineers the metric measurement system by providing equivalent charts. Even purchasing had to have a metric to Imperial chart for chip components because the BOM had all metric land pattern names but the buyers had to have the inch equivalent names to place an order.

So we had to do for Chips. Notice that 0603 and 0402 appear in both columns -
Metric = Inch
3216 = 1206
2012 = 0805
1608 = 0603
1005 = 0402
0603 = 0201
0402 = 01005

Here is what I'm talking about. This is a table from a component manufacturer that lists the dimensions of a series of resistors in hard metric but all of the component names in the far left column are Imperial units (Type Inches). i.e. the first part is a metric 0402 (0.4mm x 0.2mm) but Panasonic calls it a 01005 (the inch equivalent 0.4mm = 0.0015748" x 0.2mm = 0.007874). Why doesn't Panasonic call it for what it really is, 0402?

After successfully transitioned all the engineers then the PCB design process got really easy, faster and simpler. Then when I give the PCB manufacturer all my nice clean metric drawings and metric Gerber and Drill data the first thing they do is convert all units to Imperial to panelize and CAM the job with their mil based DRC rules. Wow, it took longer to CAM my job because of the translation.

When I dig deeper to find out "Why do they do that", I discover that all the materials like core, copper and prepreg all come in mil based thicknesses and shapes. So that's where the buck stops! I mean that if overnight all the material providers only produced metric thickness and metric sizes that all of the PCB fabrication shops would be doing metric based manufacturing. The next thing you know they'd be recommending all their customers to produce metric based Gerber and drill data for their new machines made in Europe. (The reason why they're made in Europe is because there's no way in this lifetime that any USA manufacturer is going to create a metric based machine. Maybe in another lifetime...) and I digress.

When the PCB fabrication shop starts recommending metric units preferred to their customers that's when true electronic product development automation will really kick in and maybe we'll start creating faster, better, more accurate, cheaper products or rather products that today cost \$100,000 will only cost \$1000. I hope to see the revelation in my lifetime where all design units are the same regardless if it's Imperial or metric, just pick one and make life easy. 99 nations voted for metric. USA voted to gradually adopt with full conversion by 2015. At least that's the 1985 Omnibus Act signed by Ronald Regan defines to extend Jimmy Carter's 1975 Metric Conversion Act which was to convert USA in 10 years and adds another 30 years for the gradual metric conversion one industry at a time. Note that the EU Metric Directive went into effect January 1, 2010 for all weights & measures throughout Europe. The worldwide PCB design industry is 50% converted on the front end and we're waiting on the back end. The PCB component manufacturers have semi successfully converted to metric units. At least they dimension everything in metric units but some still refer to the Imperial (Inch) name to order the component (like the Panasonic chart above). Very confusing! /

Come on PCB suppliers. The electronics industry needs all of you to transition to hard metric to complete the entire loop in the PCB design engineering and manufacturing industry. You are directly responsible for holding the electronics industry back from achieving the highest productivity levels obtainable and standardizing on a single unit system. I do not think the PCB material suppliers have a clue on how much of an impact they have on every electronic product being developed today. I beg you; please make our job easier and put an end to the dual measurement system chaos that EE engineers, mechanical engineers and PCB designers have to deal with every day until you transition. It's time to stop using the Imperial unit system in the electronics industry and use one measurement system. Every Standards organization in the world has referred to the metric unit system as the "vastly superior" alternative. So why isn't everyone listening?

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Commented on 6:46 AM, Jul 9, 2010
By Robin Bornoff

Despite the US's gradual adoption of SI it is in the less than enviable company of Liberia and Burma as the only countries not to have adopted it as standard. A bit farcical really.

Commented on 2:08 AM, Jul 14, 2010
By Keith Ackermann

Metric vs. imperial... I can only speak for myself but the resistance to metric... or more accurately, the appeal of imperial was ingrained years ago. I intuitively knew how big 0.025 inches was, but not 0.635 millimeters. It sounds stupid, but that's the way it was. One thing I did to solve this in my system was to allow a suffix on the end of numbers, so the system might be set to display in metric, but it would allow 0.025in to be entered, but that value would be converted to internal units and redisplayed as 0.635mm. In fact, all numbers could be entered as expressions. The internal units were 1/10,000,000th of an inch which made the irrational rounding insignificant over the entire physical domain.

Commented on 2:46 AM, Jul 14, 2010
By Tom Hausherr

Keith, Did you look at the component dimension numbers in the Panasonic Chip Resistor chart? Most of the values are rounded off to 0.1mm increments and the only variances are 0.05mm which is the global Universal Grid System increments for every day PCB layout in 2010. Why doesn't Panasonic provide alternate Imperial units in their latest component datasheets? You can't live in the past any longer. The metric unit system is calling on all members of the electronic industry to join together to form a single standard that will greatly simplify the processes for everyone in the supply chain. I guarantee that if you did 5 PCB layouts using a metric PCB library and a metric placement and route grid system that you would never go back to Imperial units. I cannot say the same for metric only people (try the Imperial unit system, you'll like it). There's no turning back once you learn metric units. Metric units are such a superior measurement system that I can't figure out why some people are still lingering in the Imperial world. I just don't get it and I wish for anyone who has converted to metric to please speak up and give us your testimony.

Commented on 4:58 AM, Jul 14, 2010
By Keith Ackermann

Oh, I agree. I eventually got used to thinking in both terms. My point was that I did my best to accomodate both units. There are terrible inconsistencies even within the same part vendor's datasheets. There are some terrible parts out there in general. I'll speak up for metric right now and say I love it. How can you not? The question is, why the resistance to change? I think it's because some people have imprinted on a certain unit, and unless they have to use an alternative unit constantly, they find the mapping difficult. This whole business should be run as a dictatorship. Even writing code... I've wasted so much time porting stuff at the margin that I don't even want to think about it.

Commented on 5:03 AM, Jul 14, 2010
By Keith Ackermann

Also, you touched upon PCB fab as a sorce of imperial usage, and I can tell that is very much true. I would guess that most of the domestic (US) manufacturers you call to ask about minimum geometries will answer in mils... maybe inches if you are a bit luckier. I'm just glad there are not 1200 mils in an inch :-)

Commented on 5:09 AM, Jul 14, 2010
By Keith Ackermann

1000 mils to an inch 12 inches to a foot 3 feet to a yard 1760 yards to a mile Such a smooth progression. Such nice round numbers for those unencumbered by the confines of sanity.

Commented on 3:17 PM, Jul 14, 2010
By Tom Hausherr

Commented on 5:28 PM, Jul 14, 2010
By Keith Ackermann

It's obvious you are very passionate about the subject, and that will help your goal. That same passion can be a source of frustration when others don't "get it". One can start brushing up against irrational resistance and, in this case, even nationalistic pride that can blind. I remember the push in the 70's was greeted with "we don't need no stinking metric..." or some such thing. There are many things that are killing us right now. We are being beat over the head with the free market mantra even as we see companies such as Applied Materials moving to China to get closer to their customers. That, of course, is a byproduct of labor arbitrage that sent their customers overseas, as if labor arbitrage is trade. TSMC has a low-key service that encourages companies here to fire their engineering departments and use them for all phases of engineering because they know the process, and they have the cheap labor - they outsource it themselves. I've got successful products in the field, and my technology is everywhere, but my old company only dribbles me work, and I can't even get anyone to look at what is certainly the fastest autorouter in the world, but then again, I haven't tried overseas yet.

Commented on 4:44 PM, Jul 16, 2010
By Tom Hausherr

Commented on 3:32 AM, Jul 28, 2010
By Tom Hausherr