Library footprint with pad on top of four pads

Layout Footprints
1

Footprint TO_SOT_Packages_SMD:TO-263-3_TabPin2 is for big surface mount power components such as MOSFETS and regulators with three pins and a huge square pad. In the library footprint, the pad appears both as one big square pad, and as four smaller squares underneath it. All are marked as Pin 2.

DRC is happy with this, but FreeRouter considers it a violation. Is there some reason for that? Is there a pad size limitation somewhere? I created a similar symbol with one big pad, and it seems to work fine. Somebody was trying to do something special, but what?

(Interestingly, it is a clearance violation in FreeRouter to have two pads of the same footprint with the same number touching. KiCAD itself allows that. However, FreeRouter will connect two pads with the same number that don’t touch.)

2

The four smaller pads are to split up the paste stencil, so that you get four smaller areas of solder paste rather than one large one.

Having one large area of paste can cause the component to float on the molten solder rather than settling down onto the pad.

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3

The big pad is responsible for copper and mask layer, the 4 smaller once are responsible for solder paste.

This is done to remove solder paste form certain areas. This helps in the re-flow process.
Quote from http://www.nxp.com/docs/en/application-note/AN1902.pdf:

The segmented PCB design facilitates the solder paste flux outgassing during reflow, thereby promoting a lower voiding level of the completed solder joint. At the same time, the maximum size of a single solder void is limited by the dimensions of a single matrix segment.

4

Ah. Thanks. That’s a good reason. You need edges for the surface tension thing that makes components self-align.

5

Freerouting needs some maintenance as the original developer was blocked. The source code is out there.