Electronic – Impact of increased pad length in PCB

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:

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:

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.

1. I think that you are not gaining a lot oft space using 0805. Of course it's depend on the size of your circuit. if it'really big board then may be.
2. For the power 1206(1/4W), 0805(1/8W). if accuracy in design is well done just check for Rpower =(3/2)*PowerCalculate. Must of time it's really enough to be 150%. some people choose 200% of the power dissipation.
3. If you are doing Hand Soldering, i suggest your board is not as big as many as i saw in my live. this say that 1206 can be good if you haven't good experience in SMD soldering. For information companies must of time are using 0603 (other 0805) . There are the must use for standard board. For PCB with BGA they go for 0402 because of board size constraint and routing complexity.
4. About prices. I think there are no price fixed for any component. There are many companies that make component, and sometime can be different by companies.