The rounded rectangle and the oval pad shape suit the solder paste process better since the solder paste when reflowing won't necessarily flow out to the corners.
These pad styles also let you route tracks closer to the pads when the tracks are running at 45 degrees next to the pad's radius corner.
A rectangle with 90 degree corners isn't strictly achiveable in fabrication anyway.

I use a rounded rectangle to denote pin 1, and an oval for the remaining pad styles on SOICs, SOPs, QFPs, QFNs, SOT-23 etc.  The difference between the two pad styles probably isn't very noticeable to the assembler on the fabricated PCB so doesn't do anything here, but on screen when I'm placing components and we're reviewing layouts and circuit diagrams, it's a little extra indication for the eye to make out the orientation and pin 1 position, as well as the assembly diagram.  We may turn off the assembly drawing layer to make the view on the screen less cluttered for instance when reviewing a layout and circuit.

James

________________________________________
From: DesignerCouncil <[log in to unmask]> on behalf of Silicon Valley Chapter IPC Designers Council <[log in to unmask]>
Sent: 12 January 2016 16:09
To: [log in to unmask]
Subject: [DC] IPC-7351? (Pad Shape Rectangular/Rounded/Oblong/Oval?)

Hello Fellow Designers,
Riddle me this, what is the status of IPC-7351C, released or still pending?
Drilling further, what are the pros/cons of these four pad shapes:
RectangleRounded RectangleOblongOval
Let the merriment begin  :-)
Bob McCreight, CIDSilicon Valley Chapter

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