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97,841 Views 8 Replies Last post: May 12, 2011 11:30 AM by Tom_Hausherr RSS
Michael_Karp Contributor 21 posts since
Aug 25, 2008
Currently Being Moderated

May 11, 2011 6:05 AM

Rectangular vs Oblong Pads

Tom,

 

What is the determining factor for using oblong pads instead of rectangular pads?

IPC-7351B uses both!

If the calculator defines a rectangular pad can you replace it with an oblong pad with the same x and y dimensions?

You briefly mentioned square pads with rounded corners in one of your blogs, is this something that should be implemented?

 

Thank You in advance,

Mike

Aficionado 131 posts since
Mar 23, 2010
Currently Being Moderated
May 11, 2011 8:39 AM in response to: Michael_Karp
Re: Rectangular vs Oblong Pads

Mike,

 

The IPC-7351B publication recommends Oblong pad shape for J-Lead and Gull Wing component leads becasue it was released prior to several CAD vendors introducing Rounded Rectangle Pad Shape. Like PADS just introduced Rounded Rectangle Pad Shape in V9.0 and beyond. Expedition can already handle Rounded Rectangle Pad Shape.

 

However, due to low profile component height, manufacturer's are in a race to create the lowest profile height and the space between the PCB and the component bottom is shrinking as part of the compensation to reduce overall height. This aspect introduces Land (Pad) trimming when the land is under the component body. Land (Pad) trimming starts when the space between the PCB and component bottom is less than the paste mask stencil thickness. The last thing you want is a full 100% paste mask on the pad when it's under the component body of a low profile component. As a result of Land (Pad) trimming, the LP Calculator turns the default Oblong Land shape to Rectangular Land shape to add additional copper to the Land to compensate for Land trimming.

 

However, Rounded Rectangle Pad Shape is superior to Rectangle Pad Shape because the aperture openings in the paste mask stencil are rounded corners anyway.

 

There is a XML file in the Preferences menu called "RoHS Rounded Land Shape" (attached) that you can load into the LP Calculator and it will convert all Rectangle pad shapes to Rounded Rectangle Pad Shape. All rectangular pad shapes should have rounded corners. Rounded Rectangle Pad Shape is better than both Oblong and Rectangular.

 

Hopefully in the near future, the LP Calculator will introduce a new feature that will automatically generate Rounded Rectangle Pad Shape using calculations of the Land width. Here is an example of how it will work.

 

The LP Calculator will start rounding each corner at 15% of the Land width. The round-off radius will be in 0.05 mm increments. The maximum corner radius will be 0.25 mm. This will auto-calculate perfect Rounded Rectangle Pad Shape. Of course the LP Calculator will have user preferences to allow the user to change the values of the Land Width Percentage, the Corner Radius Round-off and the Maximum Radius. And of course the user will be able to override the Global Preferences at each individual component family calculator.

 

So I am saying that Rounded Rectangle Pad Shape is the IPC preference for all Lands (except BGA, CGA and LGA) including the Chip components. Oblong is the secondary preference for CAD tools that do not support Rounded Rectangle Pad Shape. And Rectangle is only preferred for SMT land shape for flat bottom component leads that have a "Periphery" solder joint goal used in LGA's and Pull-back lead QFN and SON. The most popular lead shape for Pull-back lead PQFN and PSON is the D-Shape lead and the next release of LP Calculator will support the D-Shape Land for those 2 component families.

 

The attached RoHS Rounded Land Shape.xml file goes into the folder main installation folder of LP Calculator or LP Wizard. You load it into the program in the top of the Preferences menu. It does not auto-calculate Rounded Rectangle Pad Shape, but it contains hard coded values that can be a starting point until this feature can be added to the LP Calculator program.

 

The only graphic pictures of land shapes that you will see in my blog - http://blogs.mentor.com/tom-hausherr/ will be Rounded Rectangle Pad Shape, because that is the future preference.

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Aficionado 131 posts since
Mar 23, 2010
Currently Being Moderated
May 11, 2011 12:09 PM in response to: Michael_Karp
Re: Rectangular vs Oblong Pads

All of the LP Suite tools including LP Viewer, LP Calculator and LP Wizard automatically notifies the user that an updated version is available.

 

Every time you open the program it checks the website for an updated version.

 

This is why it takes 20 seconds or so (depending on your bandwidth speed) to open the program.

Aficionado 131 posts since
Mar 23, 2010
Currently Being Moderated
May 12, 2011 8:59 AM in response to: Michael_Karp
Re: Rectangular vs Oblong Pads

The component manufacturer's are rounding the corners of the thermal tab and adding a chamfer by Pin 1.

 

Here is a Linear Technologies QFN package. Note the 0.1 mm corner radius.

 

QFN Pkg w-Rounded Thermal Tab Corners.png

 

Here is the LP Calculator V10.3 for the same part -

 

LP Calculator QFN w-rounded Thermal Tab.png

 

We introduced rounded corner thermal tabs for QFN packages last year (2010).

Aficionado 131 posts since
Mar 23, 2010
Currently Being Moderated
May 12, 2011 11:30 AM in response to: Michael_Karp
Re: Rectangular vs Oblong Pads

I would only recommend rounding any thermal pad corners if the component thermal tab has rounded corners.

 

As far as D-Shaped pad style, a good example of its use is when the "Pull-back" lead is D-Shaped to have a higher pin quanity with a smaller package outline. If the land (pad) style in this example below was rectangle, the corner pads would short circuit.

 

LP Calculator PQFN w-D-Shape Pads.png

 

It's the rounded pad style that allows for increased pin density and still maintain pad to pad spacing rules.

 

Note: The QFN pull-back D-Shape component lead solder joint goal is "Periphery" i.e.: the same land oversize on all sides of the component lead and not a Toe, Heel or Side solder joint calculation.

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