Chip Cap Naming

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   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?

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JZsori Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 01 Mar 2012 Status: Offline Points: 65	Post Options Post Reply Quote JZsori Report Post Thanks(0) Quote Reply Topic: Chip Cap Naming 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?



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: 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.

JZsori Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 01 Mar 2012 Status: Offline Points: 65	Post Options Post Reply Quote JZsori Report Post Thanks(0) Quote Reply 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.

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: 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

JZsori Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 01 Mar 2012 Status: Offline Points: 65	Post Options Post Reply Quote JZsori Report Post Thanks(0) Quote Reply 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.

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: 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.

SteveASmith Members Profile Send Private Message Find Members Posts Add to Buddy List Active User Joined: 20 Mar 2012 Location: Dayton, Ohio Status: Offline Points: 44	Post Options Post Reply Quote SteveASmith Report Post Thanks(0) Quote Reply 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.
	My regards, Steve Smith