Simple Temperature Rise question
standard copper foil --- 1 ounce per square foot 35 microns thick, or 1.4 mils thick --- has 70 degree C per watt thermal resistance per square of foil, the heat flowing from edge to opposite edge.

A via, with height/periphery of 1:1, if plated to 35 micron thickness, will also be 70 degree C per watt

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If there is no heat flow off the top of the via, the will be no temperature difference.