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IPC7351 & IPCA610 Side Overhang

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MSM_KOPF View Drop Down
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    Posted: 21 Apr 2022 at 1:31am
Footprints design based upon IPC7351B
PCB part placement IPC-A-610 Acceptance Class 3

Example CAPC3216X190
Body width E nom 1.6mm & max 1.9mm from all datasheets taken into account
leads to a pad width W 1.91mm for density level N
Courtyard excess 0.25mm 

IPC-A-610 Acceptance Class 3 for chip components
Side overhang is less or equal to 25% width of component termination area W or 25% of land whichever is less
Note. Class 1 or 2 is 50%, to strive for is no side overhang

Now 25% of land (Pad) = 1.91mm is 0.475mm accepted side overhang

0.475mm > 0.25mm means accepted side overhang > Courtyard excess
the component can be placed to be outside of the courtyard and fullfills the acceptance class 3

for a capc1005 the 25% leads to max 0.15mm what is equal to the courtyard excess of 0.15mm for density level N

Am i right?
How is this usually handled?
Any information in case of high volume automotive ems?



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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 21 Apr 2022 at 9:12am
Yes, the IPC-A-610 Acceptance Class 3 for chip components 
Side overhang is less or equal to 25% width of component termination area W or 25% of land whichever is less
Note. Class 1 or 2 is 50%, to strive for is no side overhang

But, 99.999% of the time the chip is centered on the pad during reflow and in reality there is no overhang. The standard just says what is acceptable. 

However, if you're concerned change the Courtyard to Pad or Body here: 

Tools > Options > Terminals > Surface Mount > Rectangular End Cap 

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MSM_KOPF View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MSM_KOPF Quote  Post ReplyReply Direct Link To This Post Posted: 21 Apr 2022 at 10:33pm
The footprints should be designed acc.to IPC7351B whenever possible that prevents any discussions with EMS.

Extending the courtyard would lead to much bigger courtyard area. imagine a CAPC2020 extended 25% only for those 0.0001% and this happens in huge mass production.

What more concerns is the extended required clearance around the pad due to 25% side overhang as solder mask is not an isolation and the overhanging pad distance to copper of a different net underneath the component pad would be a clearance error.

The IPC 610 has a "Note 1" underneath the table for almost all component type.
something like "the minimum electrical insulation distance is not violated" I guess that is the escape path here :-)

Now the IPC also says something like copper tolerances round about 20% if the pad width is reduced by 20% then the component will have a side overhang and that is accepted but the nominal clearance between the pad / component and the surrounding copper of a different net is not violated.

I currently assume that the IPC 610 25% value takes into account the IPC copper tolerances of 20%
a component completely moved 25% is only accepted when isolation is not violated.

This IPC note 1 could mean if isolation is violated, side hangover is not accepted.

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