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SMD Heatsink Schematic Symbol and PCB Footprint

r.gibson

I am using a DPAK power component and an SMD heatsink intended for use with a DPAK device (i.e. DigiKey: HS407-ND). I am curious what the best practice is for creating a schematic symbol and PCB footprint for this? Should the heat sink symbol / footprint simply be part of the electrical component, or it's own component? I would prefer the heatsink to appear as a separate item on the BOM, to make sure that it isn't missed in the procurement process.

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  • 0

    Keeping the device and heatsink as separate components would make sense, they are separate items and this way they would both appear as separate BOM entries. Also gives you more flexibility in future to upsize/downsize the heatsink if needed for subsequent designs.

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  • 0

    Keeping the device and heatsink as separate components would make sense, they are separate items and this way they would both appear as separate BOM entries. Also gives you more flexibility in future to upsize/downsize the heatsink if needed for subsequent designs.

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  • 0 in reply to e14softwareuk

    Ok, but how do I handle PCB components that are stacked vertically and share an electrically hot pad, separated only by soldermask?

  • 0 in reply to r.gibson

    Would it work to define the heatsink as a dumb item without any active pads/drill holes if it is sharing with the active device?

  • 0 in reply to e14softwareuk

    So I think what you're saying is to include the heatsink pads in the electrical component footprint and then define a HS schematic symbol, so it is populated in the BOM and can be located near the associated component graphically for assembly purposes. Then create a blank footprint for the heatsink that only includes reference designator and silkscreen outline? That sounds like it will work. Are there any design rules that will be broken by stacking components like that though?

  • 0 in reply to r.gibson

    Just tested and it will work without breaking any design rules, I created a HS 3D body that 'floats' higher than the active device and did not receive any violations.

     

    Another option is to design a combined component of active device and heatsink that will be used for PCB layout. You would then also define a 'mechanical' symbol that is heatsink. This will show on the schematic but not be transferred to the PCB so you will get a heatsink in the BOM. Disadvantage is you don't get an automatic designator appearing on the PCB. If you are using isolating washers for the heatsink then these can be defined as mechanical symbols so that are in the BOM but not on the PCB.

  • 0 in reply to e14softwareuk

    I was also able to successfully create and place them this morning as well. Thank you for your help Peter!

    I ended up adding both a Top Overlay (silk) and Top 3D to the heatsink footprint, while the electrical pads and Top Solder[mask] were added to the DPAK footprint.

     

    Here is a screenshot of the result:

    image