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enter image description here

I see similar W and L for both Cu and FR4.

Why is the Cu thermal resistance shown higher than FR4? Is it because of Ounz difference?

1 Oz Copper= 34.8um. FR4 seems to have more thickness-> How would it reduce the thermal resistnce?

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Note the direction of the resistors!

The FR4 resistance is THROUGH the 0.3mm FR4 layer to the next plane of copper. Because of the huge area involved it is relatively low.

The CU resistance is ALONG the board (through a wide but very thin sliver of copper).

The message here is that heat spreads easily through the PCB to inner copper planes and the back plane, thus allowing multiple planes to spread heat across the PCB more effectively than a single plane heatsink on the component side.

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    \$\begingroup\$ In such case, I get the same result 71.4 K/W. \$\endgroup\$ 2 days ago
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Could be a typo. I have calculated it as \$8.7\cdot10^{-4} K/W\$.

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