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.
