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Footprint / Land Pattern Naming Convention

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matthelm View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote matthelm Quote  Post ReplyReply Direct Link To This Post Posted: 10 Jul 2012 at 2:37pm
My 2 cents on this issue.   These are my opinions only, and you can take or leave them as needed.

First, design the part in the units the part was designed in.  MUCH less likely to have errors in the part.  Computers are GREAT at math, let them do all the conversions for you.  If your tool can not handle the conversions, get a new tool, that one is out of date.

Second, do not round!  I worked at a place that said to use 100% metric, but only show 1 place decimal.  One of the first parts I worked on was a "inch" based part, and had the pins at .1 inch, but the drawing showed 2.5mm.  2.5 is NOT equal to 2.54, and NEVER will be!!!  DO NOT ROUND!!!  If they would have left the drawing alone (in inches), build the part in inches, the part would have worked perfect!  (tool would have put the pins at 2.54 with ease)  Luckily I had the old drawing, and an ECO was created to fix the drawing.

Third, do not use old rules, as most of them no longer apply.  Example when generating Gerbers:  The old rule of 2.4 (inches, can you tell where I'm from?) data format.  This was mainly done to keep file sizes small.  Does it matter if a file is 10K or 1M any more?  Not really.  Plus if all you add is a bunch of zeros, zipping will take care of almost all of the extra.

I do think we need to switch to 100% metric, but 1 inch does equal 25.4mm which equals 1000 mils, so you can convert all you want, but I'm going to let the computer do the work!
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adaptive View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adaptive Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 2012 at 1:42pm

Hello to All,

This is very interesting topic. I support metric PCB design 100% but I struggle to find a single PCB shop in US or Canada that is truly geared up for metric PCB fabrication.
If anyone knows a metric PCB shop please let me know.
 
Thank you,
Ed.
 
 
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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 2012 at 2:11pm
Ed, I'm with you 100%. I do all my PCB layout in Metric but I can't find 1 PCB manufacturer in the USA who is metric compliant. All layer stack-ups, impedance control data, differential pair demensions (EVERYTHING) that comes out of USA manufacturer's is in Mil Units.
 
I attend IPC Designer's Council Meetings and IPC Symposiums and PCB West/East conferences, IPC APEX conferences, DesignCon, SMTA and ALL the speakers from ALL the manufacturer's speak and teach in Mil Units. I love to hear them teach PCB designers how to solve complex metric fine pitch BGA solutions using Mil Units. I just got an email from the Orange County IPC Designers Council that Charles Pfeil from Mentor Graphics is the next speaker on fine pitch BGA's 0.65 mm, 0.5 mm and 0.4 mm. I worked with Charles and all he talks about is Mil Units. So if you want to learn how to solve Metric Pitch BGA's using Mil Units, this meeting is for you.
 
When I was in China talking to employees of PCB manufacturer's they told me that they just learned a NEW measurement system "Mil Units". They told me that American educators taught them. I can't believe it!
 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jameshead Quote  Post ReplyReply Direct Link To This Post Posted: 05 Oct 2012 at 12:23am
UK schools and Universities teach and use the SI system and metric.  In the real world when I joined Graphic in the mid nineties there was a mix with most UK companies giving us gerber data in "thou" (as we call "mil" over on this side of the pond),  excellon data which could either be also in thou (good) or metric (not ideal but the CAM software could snap drill hits back to pad centres) and the fabrication drawing which was typically output in HPGL from Autocad or similar, all in metric with metric drill sizes.

European and Japanese companies used metric exclusively and North American ones used imperial exclusively.

To be honest it didn't matter one jot what the customer supplied us with - we'd accept both, work with it, and if we needed to contact the customer we'd talk to them in the units they'd use.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adaptive Quote  Post ReplyReply Direct Link To This Post Posted: 05 Oct 2012 at 5:29am

Tom,

Thank you for a quick reply. I find it sad that we are embracing a less superior system onto Chinese fabrication houses.I have been desiging in  "mil" system for years and always struggle with the grid when it comes to metric BGA parts. Now I am trying switch over to metric because I see how easy it is to stay on grid when it comes to BGA fanout. However, your point from earlier posts is well taken, DFM is what is most critical. What is design good for if you can't manufacture it?
 
Can you briefly explain to me when you layout the PCB designs in metric, are you using just metric routing grid and imperial drill holes (for domestic US fab shops) , or are your design fully in metric inlcuding the drill holes?
 
The meeting on 0.4mm BGA pitch using "mil" would we be something worth seeing.
Can you tell me date and time for the meeting and is it possible to join online?
 
Thank you,
Ed.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 05 Oct 2012 at 8:25am

Mixing PCB Design Layout units will compromise perfection every time. PCB Design perfection starts with building CAD library parts and quickly moves to part placement, via fanout and trace routing challenges. Outputting data for machine production can be extremely complex or very simple based on the PCB Design Layout units that were used throughout the PCB design process. One of the single most important, but sometimes overlooked or taken for granted, aspects of the electronics industry is the PCB Design Grid System.

 

From the 1960’s through the 80’s the primary PCB design grid system used Imperial units. All PCB design features and grid layouts were in 0.001” (1 mil) increments and everything was symmetrical and evenly balanced. Then in 1988 the world standards organizations banded together to agree that the metric unit system was superior for solving PCB design development. The first signs of this transition started appearing in the 1990’s in component manufacturer’s datasheets and the JEDEC component packaging dimensional datasheets, which were once entirely based on Imperial “inch” units, where slowly converted to metric units.

  

IPC, a world standards organization, proposed a base value of 0.05mm for the “PCB Design Grid System”. And the process of getting all features in the PCB design back “On-Grid” was started. However, this was met by great resistance in the USA. Some American PCB designers, manufacturing companies, mechanical engineers and EE engineers are still fighting the transition process.

  

The transition from one unit system to another introduced chaos in the PCB design industry because PCB designers were forced into using two different unit systems during the transition period. The CAD vendor’s way of coping with the transition was to introduce a “Gridless Shape Based” auto-routing feature that provided the PCB designer a solution for working with both metric and imperial unit pin pitched land patterns. New technical terms were introduced like “Off-Grid” or “Gridless” and “Shape Based” routing solutions. This concept was entirely based on the fact that PCB design rules are the primary factor and the PCB designer’s objective goal was to adhere to the rules regardless of how irregular the land pattern features were. Some CAD library parts have an inch based pin pitch and some have a metric pin pitch. The PCB design grid system was chaotic and working in a gridless environment presented new challenges for PCB designers as well as CAD vendors.

  

The main impact of the gridless system for PCB layout is the fact that trace routing computations is so granular that it consumes far more memory and CPU processing. The gridless system has tens of thousands of additional options to commutate and actually slows the auto-routing tools down. It also makes it extremely difficult to cleanly manually route traces in-between the center of two component leads or vias. The "Enterprise CAD Tools" such as Mentor's Expedition, Zuken's CR-500 or Cadence Allegro are an exception to this rule as they handle gridless computations extremely effienct. All the trace / space rules are defined and that's the only thing that matters for PCB manufacturing. And Expedition has trace / space centering too to add the "Manual Route" look to a PCB layout. However, 90% of all PCB layout companies cannot afford the cost and learning curve of an Enterprise CAD tool. As a matter of fact, most PCB designs are done in CAD tools that cost less than $4,000 USD. These low end CAD tools absolutely need a grid system and most of the trace routing is done manually.

  

The “Universal PCB Design Grid System” impacts everything from CAD library creation, part placement, via fanout to trace routing while at the same time consuming far less computer memory and CPU processing. It also centers traces between pins and vias increasing manufacturing yields. It also improves the overall aesthetic look of the part placement and trace routing.

 

The ultimate goal for IPC Standards and designing a perfect PCB is to have all element feature sizes in the PCB design rounded off in 0.05 mm increments and snapped to a 0.05mm grid system. Note: 0.05mm = 0.0019685” or almost 2 mils

 

Due to microminiature component packages and smaller tighter PCB layouts, IPC is now considering a shift from the 50um grid system to 10um (high resolution). The new PCB Footprint Expert uses a 10um grid sytem for all PCB library creation to increase the accuracy. 

The Universal PCB Design Grid System is based on the 0.05mm unit. All shapes and sizes for every aspect of the PCB layout should be in increments of 0.05mm. Transitioning to the metric system for PCB layout is necessary to achieve Electronic Product Development Automation.

 

The United States is now the only industrialized country in the world that does not use the metric system as its predominant system of measurement. However, PCB design worldwide has been driven historically by the component manufacturers and CAD vendors to use the Imperial measurement system.

 

Clearly, U.S. companies that do not produce products or services to metric specifications will risk being increasingly noncompetitive in world markets. Japan has identified the U.S. lack of metric usage as a strategic impediment to access of U.S. products to the Japanese home market. In addition, consolidation of the European market product standards will make sales of non-metric products increasingly difficult and uncertain. Most U.S. companies understand that using metric units is essential to future economic success. Their hesitation may be due to uncertainty as to when and how to convert.

 

Through their actions, U.S. federal agencies are demonstrating an increasing determination to use the metric system of units in business-related activities. Example: Most component manufacturers have converted their component dimensional datasheets to millimeter units. Many of the results are not yet very visible to the public, which is not a direct target of current federal transition activities. Most veterinary and medical institutions have completed the transition to metric units however, industry is the main target, and is becoming increasingly aware of and generally welcomes the government's progress. However, in the past 4 years USA government progress in the area for standards and metric conversion for the electronics industry has been dramatically reduced. And this is why PCB fabrication is stranded in the mil measurement system. There is no incentive for any USA manufacturer to transition and in an economy that's stagnant; no one has revenue for new equipment. As a matter of fact, over 2,000 PCB manufacturers have closed their operations in the USA and all of their machines and equipment went to auction for pennies on the dollar to existing manufacturer's.

 

Industry acceptance of the wisdom of proceeding with the metric transition is due partly to the realization that producing to metric specifications and surviving in tomorrow's economic environment are synonymous. Most companies today export their products to a global market where metric based products are expected.

 
It's interesting to note the component manufacturers, world standards organizations, assembly shops and many PCB designers have already transitioned to the metric unit system. When the PCB Fabrication companies in the USA transition to the metric unit system, then the global electronics industry will complete the full transition to a standard grid system. Micrometer units are definitely the future. I believe that 10 micrometers is better to express than 0.01 millimeters. It's the same thing as switching from Inches to Mils to remove the periods from the numbers. During the course of an average day, a PCB designer enters lots of numbers into a CAD tool and especially into the PCB Footprint Expert for PCB library creation. Typing in period's "." (or commas "," in Europe) slow progress down. Today, 80% of PCB desginers can get away with using a 0.05 mm grid system. However, due to the rise of micro-miniature component packages, by 2015 it will be common place for PCB designers to use a 10 micrometer grid system for everything from PCB library creation, part placement, trace routing and all PCB design features for pad sizes, trace widths, hole sizes, text heights, via sizes and board outlines. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adaptive Quote  Post ReplyReply Direct Link To This Post Posted: 05 Oct 2012 at 10:14am
Tom,
 
I think I got it! I will create all of my footprints and drill holes in metric. The nature of our company is that unfortunately we will be always using mixed technology parts (.100" spacing on connectors, etc..)  with our metric BGA's. However, I will convert all "imperial parts" to metric before saving it to library.
 
I guess I will have to trust our PCB fab shops to do the conversion from metric to imperial during the their fabrication proccess.
 
Thank you again,
Ed. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mattylad Quote  Post ReplyReply Direct Link To This Post Posted: 05 Oct 2012 at 12:58pm
I find that components always need to be created in mm, however for track/gap spacings, track widths etc thou is fine and there are no problems. Using mils (or thou as we use) is a much easier number to remember as I do not need to remember something point something, just something.

And for schematics it does not matter a jot. It is however what the original symbol libraries used to create the symbols so adding metric pitch symbols in with imperial ones never works as they will not line up.

At the end of the day its only a measurement unit and the user is free to use whichever unit they are most comfortable with to achieve their goal without making errors.

Or in other words, using both is not a problem.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote jameshead Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 2012 at 12:26am
The nature of our company is that unfortunately we will be always using mixed technology parts (.100" spacing on connectors, etc..)  with our metric BGA's. However, I will convert all "imperial parts" to metric before saving it to library.

Using the 10 um or 0.01 mm grid then 0.1" being 2.54 mm means you don't have any problem here as long as you output your CAM data in the same units as your design and with one more degree of precision, i.e. mm 3.3.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adaptive Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 2012 at 6:21am

If I understand correctly, I should always create my footprints in metric. In case of a 2.54mm pin spacing (converted parts) I should use a routing grid of 0.01mm to match the pin spacing resolution. Finally, I need to generate my gerber files in metric units with 0.01mm resolution as well.

Thank you for all of your feedback on my questions.
Ed.

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