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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.

Land patterns from PDF

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If you look at the picture of the heat sink, you will notice there are (relatively) narrow pads at the sides: Heat Sink Picture

Looking at the front view, we can see that clearly: Heat Sink front view

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: Land patterns

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).

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  • \$\begingroup\$ My understanding of these heat sinks (which matches the picture of them) is that they don't actually mate with the component directly and all heat flow has to go through the PCB to get to the heatsink. In that case having separate pads would be counter productive as there would be a huge amount of thermal conductivity loss in the gap between pads. \$\endgroup\$
    – Jonathan
    Apr 11, 2020 at 14:51
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    \$\begingroup\$ In some cases you may not want the heatsink at the same voltage as the pad. \$\endgroup\$
    – scorpdaddy
    Apr 11, 2020 at 14:58
  • \$\begingroup\$ Updated to address the thermal performance; clearly a single pad is best but may not be suitable for all applications. \$\endgroup\$ Apr 11, 2020 at 15:28
  • \$\begingroup\$ I'm still not seeing how you get "these devices are designed such that the top mating part will actually be in contact with the transistor (see datasheet)". Looking at the heat sinks themselves in the first image in your answer there are fins above the IC itself. I really doubt an IC would mount directly to fins. From what I see the heat sinks straddle the IC but don't mate with it directly, hence the need to have a heat path across the PCB. \$\endgroup\$
    – Jonathan
    Apr 13, 2020 at 20:59
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I have another interpretation of the two patterns. I think the simple pattern is the minimum copper under the heatsink. The more complex pattern is for the solder paste. The larger center solder paste goes on the tab of the TO-2xx part, the two narrow solder paste areas adhere to the "rollers" of the heat sink. You'll have to determine the location of pins 1 and 3 pads based on where the TO-2xx part centers up. But for thermal effectivity, the PCB copper must be continuous, no gaps.I also believe that the simple rectangle pattern could be much larger if your PCB allows it. All that heat sinking will make reflow a tricky proposition, but most CMs can handle it.

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