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Rounded Rectangle Pad Shape

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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 Topic: Rounded Rectangle Pad Shape
    Posted: 05 Nov 2014 at 10:11am
IPC Land Pattern Standards for SMD technology IPC-SM-782, IPC-SM-782A, IPC-7351, IPC-7351A and IPC-7351B have always promoted Rounded Corner pad shape as the preferred pad stack. However, Most CAD tools did not support Rounded Rectangle pad shape until recently both PADS Layout and Altium Designer added this feature to their PCB library editor.

 

Here is a picture from the IPC-SM-782A that describes the default pad shape:




However, all component manufacturer recommended footprint patterns recommend a rectangular pad shape. So the great compromise between the world standards and component manufacturers is “Rounded Rectangle” as the default. In the Library Expert program, the user can easily switch pad shape to Oblong, Rectangle or D-Shape but the default installation is now Rounded Rectangle and this will be the recommendation in the upcoming IPC-7351C standard due for release in Q2 2016. It should also be noted that most fabrication and assembly shops prefer Rounded Rectangle pad shape and some say that it is better for lead-free solder attachment.



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matthelm View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote matthelm Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2014 at 1:21pm
What should be done with larger pads where full radius pads make no sense?  (Slugs, large metal parts, etc.)  Is there a recommended radius for those?
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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: 07 Nov 2014 at 1:38pm
Full Radius = Oblong

IPC only preferred Oblong because all CAD tools supported it.

Now the standard is Rounded Rectangle instead of Oblong. All SMD pad shapes the used to be Rectangle and now Rounded Rectangle.

Here are the Rounded Rectangle default rules (that you can change in User Preferences)

  1. Corner Radius = 25% of Pad Width (pad width is the narrowest part of the SMD pad)
  2. Maximum Radius = 0.25 mm, so regardless of how wide the pad is the corner radius will never exceed 0.25 mm (10 mils)
  3. Radius Corner Round-off value is 0.01 mm

The controls for Rounded Rectangle pad shape are here -

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5not4 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 5not4 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 2014 at 9:31am
Hi Tom.
 
You mention IPC-SM-782(a) as a reference but I can't find anything at IPC for this document.
 
Also, where in the 7351B document does it state that rounded corners are preferred?
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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: 18 Nov 2014 at 10:45am

The IPC-SM-782 committee formed in 1985 and released the standard in March 1987 and it lasted 18 years until 2005 when IPC-7351 replaced it.

PCB manufacturer's have long recommended Rounded Corner SMD pad shape for several reasons. Better stencil release and paste mask stencil apertures are normally corner radius.


J-STD-001 has illustrations of solder joints of rectangle pads and there is no solder in the rectangle corners after reflow. 


IPC-7351 contradicted itself regarding pad shape. The Land Pattern committee always confirmed the preferred use of a rounded pad shape but every picture in the standard illustrated rectangular pad shape. Oblong (full radius) was the preference because many CAD tools did not natively support Rounded Rectangle pad shape in 2005 when the standard was released.

This contradiction was finally brought to the attention of the committee after IPC-7351A was released. So IPC had their graphic illustration department change every rectangle pad shape to rounded pad shape in the IPC-7351B release. Here are two of the graphic pictures that were changed, QFP and SOP.

QFP -

SOP -


Most of the CAD vendors introduced "Rounded Rectangle" pad shape in their PCB library editors. This was to meet the demand of their customer base who got their information from the manufacturer's that Rounded Rectangle was better for their manufacturing process.

Of course this is a decision that every PCB designer needs to discuss with their fabrication and assembly manufacturer's because this is a very controversial subject with various opinions. An example of the controversy is that All component manufacturer datasheets that include a mfr. recommended pattern in the datasheet always recommend a Rectangular pad shape. I have never seen a mfr. recommended pattern with oblong or rounded rectangle pad shape even though that's all IPC recommends. So we recommend that if you do use a mfr. recommended pattern that you add a corner radius to the pad.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 5not4 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 2014 at 10:45am
Found the answer to #1. SM-782 is the precursor to 7351 standard and no longer in print.
 
But I would still like to know where rounded rectangles stated to be preferred in the 7351.
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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: 20 Nov 2014 at 6:22am

I just got word from IPC that "Rounded Rectangle" pad shape strengthens the solder joint.

This information will be published in the upcoming IPC-7351C standard.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote 5not4 Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2014 at 6:46am

Thanks, Tom

This is all good information.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AGONZ67 Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2014 at 3:36pm
I suppose if there was ample solder that argument could be made. However in the example pictures it appears even with oblong pads there would not be enough solder to fully cover the pads. Six of one half a dozen of the other. Smile
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