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Inch to Metric Conversion Tables for PCB design

Tom Hausherr

Tom Hausherr

Posted Mar 31, 2011
1 Comment

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.

Table 1 - Chip Component Names

Table 1 - Chip Component Names

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 2 - Via Padstack Technology

Table 2 - Via Padstack Technology

 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 3 - Component Placement

Table 3 - Component Placement

 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 4 - Trace Widths & Optimum Routing Grids

Table 4 - Trace Widths & Optimum Routing Grids

 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 5 - PC Board Criteria

Table 5 - PC Board Criteria

 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 6 - Common Plated Through-hole Padstacks

Table 6 - Common Plated Through-hole Padstacks

 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.

Table 7 - Common Non-Plated Through-hole Padstacks

Table 7 - Common Non-Plated Through-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 –

  1. Proportional Environment
  2. IPC-7251 Most Environment
  3. IPC-7251 Nominal Environment
  4. IPC-7251 Least Environment
  5. 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.

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Comments 1

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Hi Tom I hope you can just clarify something for me. EIA package type 0805 is sometimes called 2013 or 2012 in metric, I'm assuming that this difference is simply due to "decimal rounding". My question is: "Which one should I be using or are both considered acceptable?" Best regards Alan Premier Solutions, UK

Alan Johnson
3:10 PM Aug 30, 2011

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