Inch to Metric Conversion Tables for PCB design
Inch to Metric Conversion Tables for PCB design
Here are some tips about Metric Speak that all PCB designers need to know. “Metric” is not a unit of measure. Metric is a term that describes a measurement system. You use either millimeters or microns for your PCB design units. The proper terminology to describe your working units when using the metric measurement system is millimeters or microns, not metric. Example: When doing PCB layout in Inches or Mils you never refer to working in “Imperial Units”.
Millimeters allow finer (and greater) granularity in the PCB design grid system to optimize board real-estate, part placement, via fanout and routing trace/space features and snap grids. This will be very important in the future of PCB RF Micro-technology. PCB impedance measurements are more accurate in Micron units than “Ounces of Copper” and Mil core/Prepreg dielectric. Use Micron Units to achieve the highest level of accuracy for impedance calculations.
Unfortunately, PCB manufacturers are directly responsible for holding back the progress of the transition to metrication of our industry. When the PCB fabrication companies transitions to the metric system, the entire electronics industry will achieve the peak of “electronic product development automation”. Until then, we’ll plod along using dual units in the land of chaos.
Here is an example of the chaos in the Chip Component family. All Chip names refer to their body length and width. When EIAJ introduced the standard Chip and Molded body component dimensions, only millimeter units were used. A 3216 was 3.2 mm long and 1.6 mm wide. It was very simple. When the data was passed on to EIA in America, they changed all the chip names from millimeters to Inches and a 3216 was renamed 1206 or 0.125” length and 0.062” width (just drop the 3rd place number). Today most component manufacturers dimension all there component packages in millimeters see Table 1 that illustrates Metric vs. Imperial names. You can easily see the confusion in the dual measurement system.
Let’s start the transition process. 99% of all PCB layouts use vias. See Table 2 for an Inch to Millimeter chart for common via sizes starting with a 0.15 mm hole and growing in 0.05 mm increments. I’ll provide the entire padstack conversion. I intentionally did not add thermal relief data because vias should have a direct plane connection (no thermal relief is necessary). When transitioning from Imperial units to Metric units, always round-off the millimeter values in 0.05mm increments for normal resolution. If you’re working on extremely dense hand held device technology, round-off to the nearest 0.01 mm. For PCB design, there is no reason to go more than 2 places to the right of the decimal point for the present. 0.01 mm = 0.0003937”
Table 3 illustrates 4 common inch based part placement grids and their millimeter equivalent. The common rule in placing parts in millimeters is to always stay one place to the right of the decimal or 0.1 mm increments.
Table 4 provides all the common trace/space technology and routing snap grids. The common rule when working in millimeters is to always use a 0.05 mm routing grid. Most component lead pin pitches are 0.05 mm increments and IPC-7351B land (pad) sizes and snap grids are in 0.05 mm increments. This totally optimizes trace routing and eliminates wasted PCB real-estate. Everything fits together tighter than Lego building blocks. Notice that in the inch units, a gridless shape-based option is used, but in millimeters all objects can easily snap to a grid and still achieve maximum density solutions. I provide 3 various route snap grid solutions for the various trace/space rules.
Note: Inch based routing grids are evenly divisible into 0.100” while millimeter based routing grids are evenly divisible into 1 mm.
Table 5 provides the PCB material equivalents. Note that the various columns are not related to each other. Each column describes a specific PCB feature. In the first column “Board Thickness” is common PCB finished material thicknesses and the metric equivalent rounded off to the nearest 0.1 mm. The second column is copper weight in ounces and their micron equivalent. Using microns to describe copper thickness is better than using weight. The third and forth columns go together. Column 3 defines the type of hole and column 4 provides the PCB fabrication tolerance for each different hole type in the chart.
Table 6 is common plated through-hole padstacks for component leads and their inch to millimeter conversion. All hole, pad and plane clearance values are in 0.05mm increments. The Solder Mask is the same value as the outer layer pads. This padstack information was taken from the proportional padstack table and you can download it here under “Appnote 10836: Proportional Through-hole Padstacks” – http://www.mentor.com/products/pcb-system-design/library-tools/lp-wizard/import-docs
Note: this downloadable chart only contains millimeter values and not the inch equivalents in Table 6.
Table 7 is common non-plated through-hole padstacks and their inch to millimeter conversion. All hole, pad and plane clearance values are in 0.05mm increments. The Solder Mask is the same value as the hole size to allow the PCB manufacturer to oversize it per their specific fabrication tolerances. Notice that the pad size for every padstack is 1.00 mm. Because the holes are not plated, the hole size is typically larger than the hole size. Also, there is no reason to have multiple pad sizes when the pad is eventually drilled away. The only reason for having a pad in a non-plated padstack is display a marker as a guide for the hole location. The PCB manufacturer does not need the pad in the padstack, but sometimes when there is no pad (but there is a drill hole) the manufacturer might question if the hole is valid. Of course there is no thermal relief required in non-plated hole padstacks.
I want to note that the LP Calculator automatically performs all of these through-hole padstack calculations for you and provides 5 different options –
IPC-7251 Most Environment
IPC-7251 Nominal Environment
IPC-7251 Least Environment
User Defined Environment Rules
You can get a free LP Calculator by signing up for a 10-day evaluation of LP Wizard here – http://www.mentor.com/products/pcb-system-design/library-tools/lp-wizard/lp-wizard-eval
After the LP Wizard 10-day evaluation is over, the LP Wizard program will run in “Demo Mode” as LP Calculator.
More Blog Posts
- PADS Tips and Tricks: Building a PCB Decal with Polar Patterns
- Interactive Routing in the PADS ES Suite
- Schematic Capture in the PADS ES Suite video release
- PADS Evaluation Now on the Cloud!
- Do Your Designs Require Simulation and Analysis?
- PADS Tips and Tricks: Printing a Color Image of a Decal
- FPGAs are Still Cool
- PADS Tips and Tricks: Creating Split Planes and Manipulating Thermal and Antipads
- Is there an Engineering Talent Crises in the US?
- Schematics with Pizzazz using DxDesigner
- May, 2013
- April, 2013
- March, 2013
- February, 2013
- January, 2013
- Introducing HyperLynx 9.0: Fastest time to accurate results
- DxDesigner With PADS Layout
- PADS Tips and Tricks - Assigning a Shortcut Key to a Macro
- Creating a Cover Sheet with Links
- High Speed Serial — Differential Pairs Done Right
- How much stitching do I need?
- Pick a layer and stick with it
- Manage reference plane changes for quiet boards
- PADS Tips and Tricks - Changing Trace Width While Routing
- December, 2012
- November, 2012
- October, 2012
- Is it Time to “Bring the Power” to your Front-End Design?
- Fabrication Capability Update from PCB West 2012
- New Webinar Series: Supercharge Your PCB Design Creation Environment with DxDesigner Beginning October 30th
- PADS Announces New PCB YouTube Video Channel
- Fabrication Capability Update from PCB West 2012
- Modern DxDesigner: Taking the Next Steps in Schematic Design Efficiency
- PADS 9.5 Now Available
- Space Jump
- Try DxDesigner Virtually
- Return current on a stripline
- Is it ever okay to cross a plane split?
- EMI problems are easier to fix than you might think
- Chromatic Color Schemes Within DxDesigner
- September, 2012
- August, 2012
- July, 2012
- June, 2012
- May, 2012
- April, 2012
- March, 2012
- Running at 6GHz with your eyes closed can be scary
- PADS Tips and Tricks: Sense Lines
- It's never too late
- PADS Tips and Tricks - Downloading New License File
- PADS Tips and Tricks: Differential Pairs
- Great Things in Small Packages
- PADS Tips and Tricks – Installation and Licensing
- PADS 9.4 Released with Unique Technology
- The Parallel Pain
- Put the Pieces in Place for SERDES Success
- Know your limits
- PADS Tips and Tricks - Cross Probing
- February, 2012
- January, 2012
- November, 2011
- August, 2011
- June, 2011
- May, 2011
- April, 2011
- March, 2011
- January, 2011
- PCB Design Perfection Starts in the CAD Library - Part 12
- The length of your terminator doesn't matter
- Vias are longer than their length
- How do you manage your trace lengths?
- PCB Design Perfection Starts in the CAD Library - Part 11
- PCB Design Perfection Starts in the CAD Library - Part 10
- S-parameters are for more than just packages
- Making SERDES sims faster with IBIS-AMI
- Tired of waiting for your SPICE to finish?
- PCB Design Perfection Starts in the CAD Library - Part 9: BGA Components
- December, 2010
- November, 2010
- October, 2010
- September, 2010
- July, 2010
- June, 2010
- May, 2010
- April, 2010
- March, 2010
- February, 2010
- January, 2010
- December, 2009
- November, 2009
- October, 2009
- September, 2009
- July, 2009
- June, 2009
- Power to the People!
- “Desktop” versus “Enterprise”. Which is for you?
- Observations from the Road
- You Are Special!!!!!
- Innovate like Edison..... or Not....
- PCB Designers are from Venus, Mechanical Designers are from Mars!
- PCB Developers are the Unsung Heroes of Innovation!
- A Passage to India: Questions & Answers for the Introduction to PADS 9.0 Webinar
- January, 1970