Nominal With Uneven Tolerances

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   I've noted that Library Expert doesn't save the nominal, but calculates it as the mean of the minimum and maximum of a dimension. 

I've been working with a few parts where the tolerances are uneven, and this means that the naming doesn't match the datasheet nominal. Dimensions D and E from this DFN provide a quick example. 

Is using the mean value correct according to IPC, or is this a limitation of PCB libraries?

Cheers,
John

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ransonjd Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 15 Nov 2016 Status: Offline Points: 139	Post Options Post Reply Quote ransonjd Report Post Thanks(0) Quote Reply Topic: Nominal With Uneven Tolerances Posted: 27 Jan 2017 at 8:00am
	I've noted that Library Expert doesn't save the nominal, but calculates it as the mean of the minimum and maximum of a dimension. I've been working with a few parts where the tolerances are uneven, and this means that the naming doesn't match the datasheet nominal. Dimensions D and E from this DFN provide a quick example. Is using the mean value correct according to IPC, or is this a limitation of PCB libraries? Cheers, John



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: 27 Jan 2017 at 8:32am
	I the IPC mathematical model, various minimum and maximum tolerances have no affect or impact on the resulting land pattern. You can enter various minimum and maximum tolerances in Library Expert, but Library Expert will save the data as the nominal tolerance.
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