Specified vs calculated nominal sizes

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   Usually lower and upper deviations from nominal sizes are specified to be the same. However, that is not always the case. For example, nominal size can be specified to be 1.0 with lower deviation of -0.1 and upper deviation of +0.3. In this case your application recalculates nominal size to 1.1. This issue becomes relevant in case I select silkscreen outline to be drawn to nominal body size - I expect to have manufacturer specified width and length, not recalculated ones. What is also affected is 3D models.
Why have you chose to recalculate manufacturer specified nominal sizes?

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JJonas Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 21 Apr 2014 Status: Offline Points: 113	Post Options Post Reply Quote JJonas Report Post Thanks(0) Quote Reply Topic: Specified vs calculated nominal sizes Posted: 13 Apr 2015 at 3:54am
	Usually lower and upper deviations from nominal sizes are specified to be the same. However, that is not always the case. For example, nominal size can be specified to be 1.0 with lower deviation of -0.1 and upper deviation of +0.3. In this case your application recalculates nominal size to 1.1. This issue becomes relevant in case I select silkscreen outline to be drawn to nominal body size - I expect to have manufacturer specified width and length, not recalculated ones. What is also affected is 3D models. Why have you chose to recalculate manufacturer specified nominal sizes?



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: 5717	Post Options Post Reply Quote Tom H Report Post Thanks(0) Quote Reply Posted: 13 Apr 2015 at 7:00am
	The Tolerance Deviation of -0.10 mm to +0.30 mm does not play into the mathematical model of the IPC-7351 land pattern calculation for the resulting pad size. You can enter -0.10 mm to +0.30 mm or +/- 0.20 mm and get the same results. Also, it makes no difference if a component manufacturer only provides Nominal package dimensions. There still is a minimum of 0.10 mm +/- tolerance on every dimension. No manufacturer can produce component packages that are 100% perfect every time. i.e.: no one should ever put 0.00 for a tolerance. We've been creating land pattern calculators since 2001 and while we really appreciate users to challenge the standard to find flaws or suggest a better solution. We know that some mfr. tolerances are uneven and you can put them in the calculator uneven, but the program will make them even. Maybe this is something we need to look into for the V2016 Library Expert rewrite. The code is getting old and we need to create a brand new tool to make it look like 2016 to work with Windows 10 coming this October.
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