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Rounded Rectangular Pad Shape
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bnoel View Drop Down
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    Posted: 08 Jun 2017 at 6:13am
Hello - We have a library including almost 6000 parts.  Around 90% of our footprints have been created using rectangular pads.  We are considering migrating to rounded rectangular pads based on some of the recommendations I've seen on this forum.  

We use OrCAD Padstack Editor version 17.2-2016 for our padstack creation.  The Padstack Editor includes the option to convert rectangular pads to rounded rectangular pads.  

By default the editor calculates the radius of the rounded corners to be one third of the smaller of the 2 rectangle dimensions.  

For instance a rectangular pad of 1.20 mm X 0.80 mm when converted to a rounded rectangular pad would use 0.267 mm for the radius (0.80 / 3)  Is this an acceptable approach?
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Tom H View Drop Down
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Points: 5718 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 2017 at 7:53am
In Library Expert Pro there are Rule Settings for -

- Corner Radius % of pad width - the default is 25%
- Corner Radius Limit - the default is 0.25 mm
- Corner Radius Round-off - the default is 0.01

But these values are all user definable.

Rounded Rectangle pad shape conforms to the paste mask stencil aperture opening.
The stencil maker uses a laser to cut the aperture openings and rounds the corners of rectangular shapes to increase the stencil release yield.

Typical Stencil -

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bnoel View Drop Down
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Points: 14 		Post Options Post Options   Thanks (0) Thanks(0)   Quote bnoel Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 2017 at 11:56am
Two additional questions about rounded rectangular pads:
1. If a mfr recommends rectangular pads should we change them to rounded rectangular pads?
2. What about large components?  For example, we have a 10.3mm X 10.3mm X 10.3mm Aluminum Electrolytic Cap with 4.3mm X 2.1mm pads.  Should we change these to rounded rectangles?
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Tom H View Drop Down
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Points: 5718 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 2017 at 12:27pm
Texas Instruments is starting to promote 0.05 mm radius corners on their new parts. Other mfr.'s are starting to recommend rounded rectangle pad shape too, but even if they don't we still do.

All rectangular pad get a corner radius except DFN & LGA where the lead is BTC (Bottom Termination Component) and the pad size is equal to the Terminal Lead size or a 0.05 mm periphery.
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robmeyer View Drop Down
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Hello,

in Version 2017.22 and 2018.1 Library expert using other rules for the exception: Here it is per default rounded padshape under LGA. Also the Tab under QFN is now per default rounded. I have in mind, that in older Versions per default this was a square. Why this change?

Thanks
Robert
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Tom H View Drop Down
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Points: 5718 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 02 Feb 2018 at 8:21am
In order for any Thermal Tab to have Rounded Corners, the user must enter a value for "tr" in the Calculator. I have never used Rounded Corners for Thermal Pads. Can you please send us a FPX file so we can prove it. 

The LGA has always had radius corners on square Leaded parts but that can easily be changed in Preferences. 
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Points: 113 		Post Options Post Options   Thanks (0) Thanks(0)   Quote robmeyer Quote  Post ReplyReply Direct Link To This Post Posted: 05 Feb 2018 at 7:45am
Hi,

you are right, the tr value I have overseen in the Demopart.

And that the default rounded square for LGA could be changed to square I know. But in the earlier post it sounds that it should be per default square and not Rounded square. For DFN it was done but not for LGA. So what is correct?

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Robert
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Tom H View Drop Down
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Points: 5718 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 05 Feb 2018 at 8:18am
We can change the default LGA pad shape from Rounded Rectangle to Square. 

I think that's a good recommendation. 

Thanks!
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