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Chip Cap Naming

Printed From: PCB Libraries Forum
Category: PCB Footprint Expert
Forum Name: Questions & Answers
Forum Description: issues and technical support
URL: https://www.PCBLibraries.com/forum/forum_posts.asp?TID=949
Printed Date: 19 Nov 2024 at 6:41am


Topic: Chip Cap Naming
Posted By: JZsori
Subject: Chip Cap Naming
Date Posted: 28 May 2013 at 11:17am
Using 2012.53 to create a footprint for a Vishay VJ2225 series cap.  The dimensions I used from the datasheet are 5.59 X 6.35 L(D) /W(E).  But the tool gives me a 5563 name.  Shouldn't this be 5663?


Replies:
Posted By: Tom H
Date Posted: 28 May 2013 at 12:02pm
For the Land Pattern (Footprint) Name, the standards do not round-off, they drop numbers.
 
Example: EIA 1206 = 0.125" X 0.062"
One would think that you would round up 0.125" to 0.13 but they don't.
 
A better example is the 0805 or metric 2012 is 2.0 mm X 1.25 mm. One would think that 1.25 mm should be rounded up to 13 to create a 2013, but they don't, they just drop the 3rd number.
 

Posted By: JZsori
Date Posted: 28 May 2013 at 12:09pm
I can see where that might sense if you're sitting in the middle.  But when you are within 0.01 why would they want it rounded down?
 
Interestingly enough the question was posed to me by a client when I created that footprint for him.

Posted By: Tom H
Date Posted: 28 May 2013 at 12:12pm
Where do you start rounding? 7, 8 or 9? The Standards don't round and the component manufacturer's should not have 5.99 mm as a value it should be 6.00 mm
 

Posted By: JZsori
Date Posted: 28 May 2013 at 12:16pm
But, the Standards do round if 5.59 becomes 56.  To me, I would start rounding up at 6 so that, in this case, 5.59 would become 56.  No argument on the vendor dim - 5.59 should have been 5.6, esp when they show a tolerance of +/-0.25.

Posted By: Tom H
Date Posted: 28 May 2013 at 12:23pm
I will bring this issue up at our next IPC standards meeting scheduled for June 3.
 
Thanks for the info.
 

Posted By: SteveASmith
Date Posted: 28 May 2013 at 12:29pm
I believe that comes from an old engineering rule that if an even number is followed by a 5 then you don't round up.  I have also seen this in other technical disciplines.

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My regards,
Steve Smith


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