Non-Symmetric Tolerance Question?

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   Hello, 
I'm Calculating a tantalum cap with non symmetric tolerances , eg. "+20 , -10 "  but when i save the part it converts it to " +/- 15" .

how can i make the program not do that?

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Eng. Jesus Mora Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 11 Jul 2013 Status: Offline Points: 56	Post Options Post Reply Quote Eng. Jesus Mora Report Post Thanks(0) Quote Reply Topic: Non-Symmetric Tolerance Question? Posted: 20 Jun 2018 at 4:38pm
	Hello, I'm Calculating a tantalum cap with non symmetric tolerances , eg. "+20 , -10 " but when i save the part it converts it to " +/- 15" . how can i make the program not do that?



Tom H Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 05 Jan 2012 Location: San Diego, CA Status: Offline Points: 5716	Post Options Post Reply Quote Tom H Report Post Thanks(0) Quote Reply Posted: 20 Jun 2018 at 4:56pm
	The Nominal dimensions are used for the Footprint Name and the 3D STEP model. Different + and - tolerances affect the Minimum and Maximum dimensions. The Minimum and Maximum dimensions auto-calculate the Nominal dimension. Rather than entering a Nominal value with an offset tolerance, enter the Minimum and Maximum dimensions instead. Also, entering different + and - tolerances has no affect on the resulting footprint because the IPC mathematical model uses the Minimum and Maximum dimensions to calculate the pad size and location, not the Nominal dimension with offset tolerances.
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Eng. Jesus Mora Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 11 Jul 2013 Status: Offline Points: 56	Post Options Post Reply Quote Eng. Jesus Mora Report Post Thanks(0) Quote Reply Posted: 21 Jun 2018 at 9:27am
	Hello Tom, thanks for the quick reply So if I understand correctly what you are saying, if, for example, I have a datasheet that says my nominal is 3.2 with a +.20 / - .10, I should just enter a min of 3.1 and max of 3.4 right? That would give me a calculated nominal of 3.25 (using the calculated +/- .15 that the program creates on its own), so when importing the footprint, the Body Outline mechanical drawing will be done using that, instead of the 3.20 that the datasheet specifically says. This can throw off the person that verifies the footprint later on because when he goes to measure the body outline and compare it to what the datasheet says it will be different. Is there any way to maintain the dimensions and tolerances the same as what the datasheet says? Regards

Tom H Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 05 Jan 2012 Location: San Diego, CA Status: Offline Points: 5716	Post Options Post Reply Quote Tom H Report Post Thanks(0) Quote Reply Posted: 21 Jun 2018 at 9:49am
	The mathematical model for footprint calculation comes from the Minimum and Maximum dimensions. When you get a datasheet that has different + & - tolerances, do not enter the Nominal + tolerances, enter the Minimum & Maximum dimensions. Teach your checkers and QC to look at the Minimum & Maximum values, (not the tolerances). We will see if the new V2019 code can be written to allow for different + & - tolerances. But the current Library Expert V2018 cannot be updated to accommodate this feature.
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