For those people interested in last week's discussion of metric PCB layout, the 2nd part of the article has been published.
http://www.pcbdesign007.com/pages/zone.cgi?topic=0&artcatid=0&a=77209&artpg=1&artid=77209&pg=1 It is rather detailed, and mostly tables of "if this inch-based practice, then this metric practice." I have not checked in detail; I think it is the same table I linked to last week. The rules seem reasonable, but there is nothing wrong with "user defined environment." We developed our own pad rules for chip (surface mount) components and GREATLY reduced soldering defects. However, doing so involves design of experiments and optimizes results for a particular solder machine and setup. No one company would ever use all these rules, but whatever inch rules you used, it is a place to start your metric practice. However, your practices manual should be written in metric and should NOT continue to depend on these conversions. ----- Forwarded Message ---- From: John M. Steele <[email protected]> To: [email protected]; U.S. Metric Association <[email protected]> Cc: USMA Metric Association <[email protected]> Sent: Thu, June 9, 2011 8:57:17 AM Subject: Re: [USMA:50580] PCB chip design layout I agree with his remarks, metric PCB layout is better, particularly if you use primarily surface mount devices. Note that his dates are for his company, not the industry, which is still a mixed bag. We used metric layout even in the early 80's, primarily because we were a metric company. Leaded components (especially DIL) had lead spacings in inches and we did lots of conversion. We had to specify standard board and copper thicknesses, just converting the dimensions. Our suppliers grumbled, but bought metric drills for the holes. It made a lot more sense as we migrated primarily to SMD by the mid 80's. Some of our suppliers were actually grateful (later) we had made them be early adopters as other (smaller) customers asked for metric. (They weren't at the time though, it was very much of a "you WILL or my NEXT supplier will" discussion, but we were good at those.) I would have thought that "everybody" migrated to metric layout. However, as this forum shows, in 2002, most recommendations were thousandths of an inch. By 2005, about 50/50 support for metric. I am surprised by the support for thousandths, even from engineers in other countries. http://www.edaboard.com/thread2783-3.html It seems to me that converting one metric ball grid array to thousandths would make a metric advocate of anyone, but read the remarks. This is a useful table for people migrating from inch to metric layout. Note comments at bottom of page 3. It is a crutch to help you move from a current practice to a new practice, not eternally convert. http://electronica.ugr.es/~amroldan/pcb/2007/modulos/temas/InchtoMetricTransitionTables.pdf "The main idea is not to convert but to adapt. Conversion creates long unnecessary numbers. If a PC design is created using Inch units then it should always stay in Inch units. Conversion is against all standard practices and should be avoided. All EDA Standards Groups (JEDEC, IPC, IEC, EIA, NIST, IEEE and ANSI) promote the use of Metric Units for PCB Design Layout." I searched but could not find any comprehensive survey of industry practice, only more anecdotes. ________________________________ From: Pat Naughtin <[email protected]> To: U.S. Metric Association <[email protected]> Cc: USMA Metric Association <[email protected]> Sent: Thu, June 9, 2011 2:29:14 AM Subject: [USMA:50580] PCB chip design layout Dear Don, Here is another success story that you might like to add you your collection. For everyone else, you might like to give this man as much support as you can. He writes, in part: ## The metric unit system is one of the greatest secrets to PCB design perfection today. From 1974-1991, we used inch units for our PCB layout. From 1991-2001, we used mil units. From 2001-2011 we used millimeter units. I have to say that when we made the transition from mils to millimeters, our productivity levels slipped a bit during the learning curve. But after five or six PCB layouts, our productivity was back to normal. And after about 15 PCB layouts our productivity levels surpassed all previous results. If I were forced to go back to the mil measurement system, my productivity levels would reverse. There is no way in the world that anyone in 2011 using mil units can outperform the same talent using millimeter units, because most component pin pitches are on a millimeter grid system (like the 1 mm pitch BGA) and metric units are vastly superior to work within the PCB design space. All of the numbers are evenly divisible by 10 and there is no need for calculators for mathematical calculations. No designer I know of who has successfully transitioned to the metric unit system for PCB layout wants to go back to the imperial unit system. That statement alone says it all. ## See http://www.pcbdesign007.com/pages/zone.cgi?a=77073&artpg=1&topic=0 for the full text. Cheers, Pat Naughtin LCAMS Author of the ebook, Metrication Leaders Guide, see http://metricationmatters.com/MetricationLeadersGuideInfo.html Hear Pat speak at: http://www.youtube.com/watch?v=_lshRAPvPZY PO Box 305 Belmont 3216, Geelong, Australia Phone: 61 3 5241 2008 Metric system consultant, writer, and speaker, Pat Naughtin, has helped thousands of people and hundreds of companies upgrade to the modern metric system smoothly, quickly, and so economically that they now save thousands each year when buying, processing, or selling for their businesses. Pat provides services and resources for many different trades, crafts, and professions for commercial, industrial and government metrication leaders in Asia, Europe, and in the USA. Pat's clients include the Australian Government, Google, NASA, NIST, and the metric associations of Canada, the UK, and the USA. See http://www.metricationmatters.com/ to subscribe.
