SOT223 with copper pour to TAB?

[Faringdon]

Hi,
Supposing you have a SOT223 and you want to maximise the copper pour to its TAB pad and yet still allow it to be well soldered in the SMD bath.
How big could it be made? (the coper pour to the tab pad)
20z copper and 4 layer pcb and no thermal reliefs (ie direct connected tab pad to copper pour)

PBSS4540Z
https://assets.nexperia.com/documents/data-sheet/PBSS4540Z.pdf

[jpanhalt]

Does page 7 here help: https://www.st.com/resource/en/application_note/an1703-guidelines-for-using-sts-mosfet-smd-packages-stmicroelectronics.pdf

[Faringdon]

Thanks but no, it just shows Rth(j-a) with various copper areas

[SMTech]

I've seen plenty of boards where the tab "pad" is nothing more than a mask defined area on a copper pour covering a large area. It works just fine in reflow, where it would fail would be if the exposed&pasted tab pad area was long enough to pull the sot223 away from the pads for the other feet. I have seen this happen quite often with a D2PAK footprint that was the default in a version of Proteus. I don't think you could tombstone a SOT223, although it would be interesting to see someone explicitly try to...

[jpanhalt]

Thanks but no, it just shows Rth(j-a) with various copper areas

There is no "maximum" size.  It's asymptotic  There is a minimum size recommended for 2oz copper and a formula is given for calculating maximum watts based on the size of the pad. 

I suspect it will be difficult to find an exact answer for your situation, but it seems an approximation could be calculated with the information provided by someone normally skilled in the trade.