Ic Pad Shapes

[PaulBoston]

I have spent a bit of time browsing the PCBSTANDARDS site
and notice that pcb padstacks all seem to be oblong or oval
in shape and NOT rectangular.

Is there any good reason for this preference?

[Tom]

Paul,

Yes, all "Gullwing & J-Lead" IC's get an oval pad shape.

IPC has performed studies over the past 10 years regarding this subject and they have concluded that the Oval shape is easier of manufacture and easier to assemble.

1. PCB manufacturers have a difficult time producing objects with sharp corners.

2. PCB Assembly shops have an easier time with the Paste Mask. It doesn't wick out to the pad corners. Also, it prevents solder bridging in wave solder.

3. Using rounded corners provides greater routing density.

4. We used to have a theory that if you use rectangle pads that you actually get more copper pad and that helps rework because the pad wouldn't lift off the board when heated with an iron. Pads lifting off the board have nothing to do with the shape. If the resin under the pad gets too hot the pad comes off regardless of the shape.

[PaulBoston]

THANX, question answered.