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?
Best Answer
Here's a somewhat general rule. When a SMT transistor or IC has got a thermal pad at the bottom, it's designed to heatsink to the PCB. (The components without thermal pads heatsink to the PCB through pins.) The thermal resistance from the die to the thermal pad is much lower than to the top of the IC. In case of ICs, the thermal pad is as the ground potential, and you can connect it to a ground plane which is a good heat sink. In case of transistors, the thermal pad often can't be connected to a ground or power plane.
So we heat sunk the IC to the PCB. Then how do we dissipate the heat from the PCB?
A PCB can dissipate some power by itself. If the PCB is large enough, and the power which it has to dissipate is not too large, and the ambient temperature is not too high, then the PCB itself may be enough. If the design is dense, and the PCB can't dissipate enough power by itself, then additional heat sinking is needed.
You can heat sink the PCB to a metal enclosure. You can add SMT heat sinks. You can bolt-on heat sinks. There are different ways to do this, which depend on different requirements and situations.
This is where it becomes tough to speak about general rules.
additional reading:
How does power dissipation for surface mount components work?
Optimize heat sink design - connect cooling pad on PCB backside by vias