Solder joint transfers heat.
It is a metallic connection.

Joyce's attachment describes most of the component PCB heat transfer
possibilities.

For serious heat loads there are other alternatives.

The design of the assembly determines the best path.
Metallic PCB's offer the most efficient transfer of heat.
Voiding less than 5% can be achieved, at a cost.
The determining factor in these assemblies is the thermal connection of PCA
to the heatsink.
Melted the component off the PCB in less than 60 seconds without a heatsink
attached and with a less than 5% void solder joint.

Extremely high current designs adhere to the best method of heat
dissipation rather than most efficient routing.
Includes heatsinks mounted to top of componets that have a metal lid
attached directly to die.
Vias under device filled with solder, can be done, takes process/design
development.
Metal PCB's to transfer heat from device to PCB to heatsink.
All this movement comes at a cost and serious design consideration of the
entire system.

Question is how many watts?
1, 2, 5, 20, 50, 75?

article is very good for less than 5 watts.