IPC-7351 Placement Tolerance Setting

 

We're working on the new IPC-7351C and we removed the manufacturing tolerances from the standard mathematical model. In our last committee meeting at IPC APEX is was voted to remove the manufacturing tolerances but we need to simulate the new math on a variety of components and compare the results.  

 

This update will be reflected in V2016 Library Expert after we run tests for the next few months.

 

But it's OK for you to change the manufacturing tolerance in the current Library Expert. It's a User Definable setting. I would also make the fabrication tolerance 0 because every fabrication shop swells the outer layer etch to compensate for their etching process tolerance. There is no need for double tolerance.

I reopen this threat because the new release of IPC-7352 changes the fabrication tolerances and the placement tolerances to 0.00. Due to this the negative values of the solder joints change  to 0.00, as well.

I can understand the reason to modify the fabrication tolerance value, it has a lot of sense. The pcb manufacturer applies a correction factor to the pcb due to the etching. Creating a footprint with a fabrication tolerance is to apply twice this tolerance.

My question is related to the placement tolerance. Untill now this value was 0.025mm and I cannot understand the reason to modify it to 0.00. 

For example if a P&P machine has an accuracy of +/-50um (3 sigma), assuming that the component placement is a normal distribution 

I think that it means that:

• the 68.2% of the components will be placed with a tolerance +/- 17.05 um
• the 95.4% of the components will be placed with a tolerance +/- 23.85 um
• the 99.7% of the components will be placed with a tolerance +/- 24.925 um

I am not a designer with so many years of experience like many of the members of Footprint Expert Family and this is the reason why I need your help. I need to understand this subject of tolerances.

Many thanks for help to all of you.

Regards.

David