I often do designing power modules and always wonder if there is some kind of rule how to properly cool SMD devices. I always look at final products from other companies and some are placing heatsink on bottom of PCB, while others pleace them on top - directly on SMD components with plastic packages.
Many power IC and MOSFETs do have large thermal pads on bottom, and thermal transfer should be really good to the bottom. But adding about eg 2mm FR4 of PCB is also good thermal insulator... Comparing to like 1 mm or less of epoxy resin on IO package, I dont think it could be worse...
To be more confusing, some power IC do not even have proper thermal pad - e.g. TPS63070 that I am working with right now.
What is better? Adding heatsink on bottom or on top? Does it depend on how thick Cu layers are?