Do you use Imperial or Metric units for PCB design?

If the French would have won the French & Indian War against the British (the 7 Year War from 1754 to 1763) Imperial Units or the English measurement system would not exist in society today. Even the British transitioned to the metric measurement system 46 years ago. America is the last stronghold for the Imperial measurement system and how much longer will it take for the world to become united under a single measurement system. This is what world standards and space age technology acceleration will require to fully automate all PCB processes.

One of the greatest secrets to PCB design perfection today in 2011 is the use of the metric unit system. From 1974 – 1991 we used Inch units for 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 5 or 6 PCB layouts our productivity was back to normal. After about 15 PCB layouts our productivity levels surpassed all previous results. If I was forced to go back to the Mil measurement system, my productivity levels would reverse backwards. 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 because all the numbers are evenly divisible by 10 and there is no use for calculators for mathematical calculations. There is no one that I know of that has successfully transitioned to the metric unit system for PCB layout that wants to go back to the Imperial unit system. That statement alone tells it all.

As a matter of fact, there would not be Imperial units in the world today if the United States government (congress) fulfilled the commitment that they signed at the Treaty of the Meter back in 1875. I hear it all the time from corporations who will not convert – “We’re American and we have our own measurement system. We are not part of the European Union or Russia or Japan. We’re proud to be Americans and we believe in our way of life and the system and values that we use”.  Well, let me shine a little light on all those proud Americans who obviously do not know the historical facts. So before I go into PCB design details of why metric units are superior, I need to explain the historical background to set the stage.

Most Americans think that our involvement with metric measurement is relatively new. In fact, the United States has been increasing its use of metric units for many years, and the pace has accelerated in the past four decades. In the early 1800’s, under the presidency of Thomas Jefferson, the U.S. Coast and Geodetic Survey (the government’s surveying and map-making agency) used meter and kilogram standards brought from France. Abraham Lincoln was a strong proponent of the metric unit system and in 1866 (just 1 year after his assassination), Congress authorized the use of the metric system in America and supplied every state with a set of standard metric weights and measures.

 In 1875, the United States solidified its commitment to the development of the internationally recognized metric system by becoming one of the original seventeen signatory nations to the Treaty of the Meter. The signing of this international agreement concluded five years of meetings in which the metric system was reformulated, refining the accuracy of its standards. The Treaty of the Meter, also known as the “Metric Convention” established the International Bureau of Weights and Measures (BIPM) in Sèvres, France, to provide standards of measurement for worldwide use.

 In 1893, metric standards, developed through international cooperation under the auspices of BIPM, were adopted as the fundamental standards for length and mass in the United States. Our customary measurements — the foot, pound, quart, etc. — have been defined in relation to the meter and the kilogram ever since. The General Conference of Weights and Measures, the governing body that has overall responsibility for the metric system, and which is made up of the signatory nations to the Treaty of the Meter, approved an updated version of the metric system in 1960. This modern system is called Le Système International d’Unités or the International System of Units, abbreviated SI.

The United Kingdom began a transition to the metric system in 1965 to more fully mesh its business and trade practices with those of the European Common Market. The conversion of the United Kingdom and the Commonwealth nations to SI created a new sense of urgency regarding the use of metric units in the United States.

In 1968, Congress authorized a three-year study of systems of measurement in the U.S., with particular emphasis on the feasibility of adopting SI. The detailed U.S. Metric Study was conducted by the Department of Commerce. A 45-member advisory panel consulted with and took testimony from hundreds of consumers, business organizations, labor groups, manufacturers, and state and local officials.

The final report of the study, “A Metric America: A Decision Whose Time Has Come” concluded that the U.S. would eventually join the rest of the world in the use of the metric system of measurement. The study found that measurement in the United States was already based on metric units in many areas and that it was becoming more so every day. The majority of study participants believed that conversion to the metric system was in the best interests of the Nation, particularly in view of the importance of foreign trade and the increasing influence of technology in American life.

The study recommended that the United States implement a carefully planned transition to predominant use of the metric system over a ten-year period. Note: In 1975, the Australian continent also implemented its metric conversion act and successfully transitioned.  The United States Congress passed the Metric Conversion Act of 1975 “to coordinate and plan the increasing use of the metric system in the United States.” The Act, however, did not require a ten-year conversion period. A process of voluntary conversion was initiated, and the U.S. Metric Board was established. The Board was charged with “devising and carrying out a broad program of planning, coordination, and public education, consistent with other national policy and interests, with the aim of implementing the policy set forth in this Act.” The efforts of the Metric Board were largely ignored by the American public, and, in 1981, the Board reported to Congress that it lacked the clear Congressional mandate necessary to bring about national conversion. Due to this apparent ineffectiveness, and in an effort to reduce Federal spending, the Metric Board was disestablished in the fall of 1982.

The Board’s demise increased doubts about the United States’ commitment to metrication. Public and private sector metric transition slowed at the same time that the very reasons for the United States to adopt the metric system — the increasing competitiveness of other nations and the demands of global marketplaces — made completing the conversion even more important.

Congress, recognizing the necessity of the United States’ conformance with international standards for trade, included new encouragement for U.S. industrial metrication in the Omnibus Trade and Competitiveness Act of 1988. This legislation amended the Metric Conversion Act of 1975 and designates the metric system as the preferred system of weights and measures for United States trade and commerce.” The legislation states that the Federal Government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement.

Federal agencies were required by this legislation, with certain exceptions, to use the metric system in their procurement, grants and other business-related activities by the end of 1992. While not mandating metric use in the private sector, the Federal Government has sought to serve as a catalyst in the metric conversion of the country’s trade, industry, and commerce.

The current effort toward national metrication is based on the conclusion that industrial and commercial productivity, mathematics and science education, and the competitiveness of American products and services in world markets, will be enhanced by completing the change to the metric system of units. Failure to complete the change will increasingly handicap the Nation’s industry and economy.

There is one thing that I would like to clarify to the reader that I’m not proposing that the American “way of life” change in our sports (football, baseball, golf, etc.) or cooking units in our kitchens, but rather our “industry” must change to increase our competitiveness with the rest of the world. However, America has an impact on other counties weights and measurement systems. The EU Metric Directive (80/181/EEC), that was scheduled to go into effect on January 1, 2010, has been modified to allow the continuation of both supplemental (U.S. customary, inch-pound) and metric units for consumer goods sold in the EU. The rule was published on May 7, 2009 in the Official Journal of the European Union.

The modified Directive instructs the European Commission to produce a report to the Parliament and Council regarding the smooth functioning of the internal market and international acceptance of SI units by December 31, 2019, including proposals where appropriate. Demonstrated progress will be important to achieve long-term acceptance of supplemental units in the EU. Modifying the U.S. Fair Package and Labeling Act (FPLA) to permit metric labeling is an example where greater international marketplace acceptance of SI units can be achieved. 

Next week I will present Imperial to Metric conversion charts as they apply to the PCB design industry. I will also post a short message on the proper terminology that I refer to as “Metric Etiquette”.

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5 comments on this post | ↓ Add Your Own

Commented on March 24, 2011 at 1:28 am
By James Head

Some historians have argued that the Americans and British supported Treaty of the Meter in 1875 to let the French have their way on this, so that they could then argue for the Greenwich Meridian later in 1884. The idea being the French have got the metre so it’s “someone else’s” turn to have the prime meridian. The French supported the Paris meridian on scientific grounds whilst the US and British pushed the Greenwich meridian on commercial grounds. To this day the French will refer to “the Paris meridian delimited by the offset from the Greenwich meridian.
I support metric units though and use them extensively.

Commented on March 24, 2011 at 10:52 am
By Tom Hausherr

I had no idea that compromises like this were being made between the French and British governments. Very interesting.

The Imperial measurment system origin – Prior to 1066 the English units included Anglo-Saxon measures (likely of ancient Germanic origin). After the Norman Conquest (Battle of Hastings in 1066), the Normans re-introduced Roman measures, resulting in a system including ancient Anglo-Saxon measures and Roman measures. These units were standardized by the Magna Carta of 1215 and were periodically reviewed and updated (notably in 1496, 1588 and 1758). The UK Weights and Measures Act of 1824 was one such review, which not only modified the types and sizes of the units, but renamed them from the “English Units” of measurement to the “Imperial System” of Measurement. This new standard was then introduced throughout the UK and its colonies at this time.

Commented on May 5, 2011 at 10:44 am
By Bob Roush

It amazes me how many PCB designers are stuck on mils! It is not that difficult to covert over to mm’s. i think an interesting conversation is how does one determine whether to build library parts using Metric or English units. If a librarian is careless, it is easy to get in trouble by using the wrong units.

Commented on August 2, 2011 at 7:22 am
By John Jacobson

When you present conversion from English to metric and the other way. You need to make sure everyone understands that 1/25.4 is not just 4 digits to the right of the decimal point but like 14! Rounding is what kills this function and gives you incorrect answers and results.

Commented on October 19, 2011 at 12:51 pm
By Eliot

But think of the loss of trade with all the other countries not using the metric system : Burma, Liberia and… well thats all actually…

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