I'm looking at using a D Series heatsink from Ohmite. The land patterns on the last page of the product PDF are very lightly labeled and I'm not entirely sure how to interpret them https://www.ohmite.com/assets/docs/sink_d.pdf?r=false. From what I've seen elsewhere I think the first rectangle is the actual copper area under the back pad of the device and the second trio of rectangles are the solder mask openings. I think the dashed outline rectangles under both of those are supposed to be the pads for the leads but they don't seem to be spaced right, at least not on the first diagram.
Best Answer
If you look at the picture of the heat sink, you will notice there are (relatively) narrow pads at the sides:
Looking at the front view, we can see that clearly:
So the land patterns are for either a single pad (where the transistor and heat sink have been mated prior to soldering) or for two pads (where the transistor and heat sink have not necessarily been mated prior to soldering, noticing that the overall dimensions of the pads are almost the same:
So there are two possible pads, depending on process flow. A single pad would make it difficult to solder the heat sink after the transistor has been soldered.
The best heat sink performance is for a single pad obviously; these devices are designed such that the top mating part will actually be in contact with the transistor (see datasheet); clearly separate pads will eliminate one heat path, but will make soldering the heat sink alone much easier.
In production, it is not unusual to pre-mate the parts using thermally conductive adhesives of which there are many (and then they can be handled as a single part).