Metric VS: Imperial Measurement Systems
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?
Posted July 8th, 2010, by Tom Hausherr
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