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Incorrect Rounding - Bug or Feature? |
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jgregoire 
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Joined: 19 Jan 2017 Status: Offline Points: 31 |
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 Topic: Incorrect Rounding - Bug or Feature?Posted: 27 Mar 2017 at 7:37am |
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I have noticed a few cases of part dimensions being truncated instead of correctly rounded up or down. I'm not so worried about a -0.01mm rounding error on Length or Width dimensions, but we are very strict about package Max Height.
I'm not sure if this is a bug, or behavior specified by the IPC standard, but it seems obvious that Max Height should never be rounded down. For example, Vishay resistor WSL2512R0100DEA (Datasheet: http://www.mouser.com/ds/2/427/wsl-101914.pdf). According to the datasheet, the max height for the 2512 package is 0.889mm. This should be rounded to 0.89mm. Instead, Library Expert rounds it down to 0.88mm in the footprint name (RESC710X420X88L157). The actual height of the exported footprint in OrCAD is 0.89mm, so it's fine there.
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Tom H
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Joined: 05 Jan 2012 Location: San Diego, CA Status: Offline Points: 5717 |
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Posted: 27 Mar 2017 at 12:11pm |
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Library Expert Pro rounds to 2 decimal places (3 if necessary) for millimeters.
We opened the datasheet and the dimensions for a 2512 don’t have a reference height of 0.889 mm.
When built using these dimensions we don’t see any rounding other as described above. |
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jgregoire
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Posted: 04 Apr 2017 at 8:57am |
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0.635 + 0.254 = 0.889 Max height.
Instead of rounding the height in the footprint name, Library Expert seems to floor() it.
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Tom H
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Posted: 04 Apr 2017 at 9:10am |
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Library Expert only uses dimensional data that the user inserts. So if the package Height was 0.635 +/- 0.254 = 0.889 Max height, that is the value the user would enter. The 3D STEP model is created using the exact dimensions. |
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jgregoire
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Posted: 07 Apr 2017 at 11:23am |
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We're not using the 3D Step feature. The height I entered is "0.889". Once again, here is the name of the footprint generated by Library Expert: RESC710X420X88L157 If I enter "0.889" for max height, the generated name should be RESC710X420X89L157. For a design with height-constrained sections, if I export a BOM from OrCAD that lists the part height as entered in our parts database, and the footprint name, the two will disagree. Whoever is reviewing the design will have to waste time investigating what appears to be an error.
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Tom H
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Posted: 07 Apr 2017 at 11:31am |
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The Height entered into the Calculator should be 0.89 mm The EIA / IPC / JEDEC naming conventions do not round-off values, rather they drop values. Example of the EIA 1206 Chip: The body dimensions are 0.125" X 0.062" Example of the IPC 2012 (0805) Chip: The body dimensions are 2.00 mm X 1.25 mm In both cases, the package names should be 1306 and 2013 respectively if the numbers rounded up. |
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