Electronic – SOT-223 Thermal Pad and Vias

heatsinkpcb-designsurface-mountthermalvia

I am using a LDO with a SOT-223 footprint and since it might get hot, I wanted to make a nice thermal pad under it to dissipate that heat.
I googled and I only found thermal pads, but i wanted some guidelines on thermal vias to dissipate the heat to other layers. Could some one please give me some reading material?
I want to know how far to place the vias from each other, how many vias to use and the size of them.

EDIT:
The part is the MCP1703 but I think this question is more related to the footprint than the part itself

Best Answer

First, a couple of the answers (at least on the first draft) seem to have confused SOT-223 with SOT-23. SOT-23 is a very small packaged designed more for small size than for heat dissipation. SOT-223 is also quite small, but does have a substantial thermal tab:

enter image description here

Sources differ on the actual thermal properties of SOT-223. The TI app note AN-1028 cited by Garrett gives a junction-to-ambient thermal resistance (\$\theta_{JC}\$) of 12 C/W. The Microchip app note AN792 also cited in Garrett's answer gives 57 C/W. Another TI datasheet, for the TLV1117, gives 104 C/W.

The main reason for this discrepency is that the thermal resistance depends not just on the package, but on the size of the copper pads available to serve as a heat sink for the part, as shown in this graph taken from the TI app note:

enter image description here

The 12 C/W number is apparently the asymptotic limit of this curve. Note that it requires 2 oz copper and probably 2 in2 or more of copper area to achieve that value.

To finally get to your question, how to lay out the heat sink pad, in roughly decreasing importance:

  • The larger the pad you can fit in your design the better.
  • Heavier copper is better (e.g., 2 oz rather than 1 oz copper).
  • When connecting through to a thermal pad on the opposite side of the board, use many vias. As a rule of thumb, I'd recommend spacing vias on a 50 mil grid or so, over the whole pad area.
  • Use vias larger than the minimum size. As a rule of thumb I'd try to use at least 8 mil via diameter and use 10 to 18 mil by preference. Extremely large vias, of course, end up reducing the pad area, so there's a limit to how large you want to go.
  • Place the heat-generating part as close to the center of the thermal pad as possible.

Finally, in contrast to the suggestion in another answer, I would do my design this way:

  • Determine the input and output voltages of your regulator, and the operating current. From this determine the power requirement.

  • Determine the maximum ambient temperature where your circuit will operate.

  • Determine the maximum junction temperature you can operate at. Typically this is 125 C in the datasheet, but you may want to de-rate by 25 C or more to give design margin and improve reliability.

  • Now choose a package and design a layout that allows you to meet your maximum operating junction temperature.

In particular, it is not possible to determine the temperature rise until after you've chosen a package.

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