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I want to cool this bridge rectifier from Diotec in my circuit to minimize radiated heat by placing a small copper area directly underneath it and connect it to a larger copper area on the opposite side of my 2-layer PCB through vias.

Is this technique useful for cooling the component even though it does not have a dedicated thermal pad?

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    \$\begingroup\$ Every little bit helps, but having a large heat sink on the same side as the component is probably far more effective. This also seems like a component that you should easily be able to find some thermal pad equivalent of (in D2PAK or similar). \$\endgroup\$ – Lundin Dec 16 '20 at 13:47
  • \$\begingroup\$ A dab of thermal grease will improve conductivity between that pad and the epoxy case of the chip. \$\endgroup\$ – user_1818839 Dec 16 '20 at 14:39
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Take note of what the data sheet says about thermal resistance: -

enter image description here

The main point is that heat is transferred via the terminals about 2.5 times better than it is transferred via the body. Either way, the data sheet is suggesting that you use 5 mm x 5 mm copper areas for each pin and, if you bring them on vias to the underside to make 4 bigger pads you will be doing your self a bigger favour than just having an unconnected copper area beneath the chip taken on vias to another copper are on the underside. Every bit helps though but concentrate on the pins is my advice.

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