How to cool ICs?

Question

I've read this question and in its comments it is said:

LDO and IC heatsinks will generally have a very different answer then computer motherboard heat sink. This question really doesn't belong here

What I'm asking is how to use coolers with IC packages. For example let's say I have a device in TO220 package which according to my calculations needs cooling. How would I cool it? Most obvious answer is of course using a cooler, but that part isn't very clear to me.

I've seen that sometimes heatsink is directly connected to the package by a screw but sometimes insulator is used to prevent direct contact between screw and package. Some other times, heat conductive insulator is used together with insulator for screw to prevent direct contact between package and heatsink.

Sometimes silicon paste is used and sometimes it isn't. How would I determine when it is needed and when it isn't needed? My experience with computers tells me to always use it.

I've also seen silicon pastes marketed as for use in electronics. How are they different that ones used in computers? Would thermal pastes for computers work well with ICs?

Answer 1

When you see a heatsink with a screw, that is because the chip it is mounted to has a hole for a screw mount.

Sometimes the plate that you have to heatsink is going to be a different voltage then the surrounding board, so you need to use something that is electrically isolating, but thermally conductive. These replace the need for conductive paste.

If you have the area of the regulator/IC that generates heat at a voltage like ground, and the other connections for your heat-sink will be ground, you can connect them directly, and you normally want to use a form of thermal paste. I have attached a heatsink without thermal paste and still had a device temperate at almost 90 degrees C. After adding thermal paste it measured at 5 degrees C above room temperature. Often being able to connect your heatsink to ground via solder helps dissipation as it dissipates to the ground plane.

In computers you have a very specific task, cooling the Processor. In electronics it can be a very large range of tasks, and often you are willing to pay more to cool something because you design calls for it, or you are willing to pay less because your design does not need some very nice thermal paste.

In general, you are going to just want to use thermal paste, you do not have to worry about insulating your heat-sink if you leave it floating in the air, or ensure where it mounts to the board there is not a voltage connection. This keeps things simpler for a tinkerer. For any chip you get that you think needs heat-sinking, read what they suggest to do (on the datasheet), and follow it.

Last but not least, thermal paste for computer processors will work for ICs.

Answer 2

Cooling is pretty tricky to get right. In most cases, a heatsink is all that is necessary, however, for higher loads, you will need a fan. Look at a computer power supply. They all use fans, but many will run without them. The heatsinks are usually quite big because the supplies are not very efficient (it turns out it's cheaper to make an inefficient power supply with a big fan than an efficient power supply with a smaller fan.) The fan helps circulate air inside the case, and prevent the other components in the computer from overheating.

The first question you need to ask is whether or not the IC you are using can actually dissipate enough power to require a heatsink and/or fan. Many cannot. For example, the LM7805 has a 5°C/W J-CC rating, which means at best (assuming an infinitely large heatsink attached to it), running at the maximum junction temperature of 150°C, at room temperature, the power dissipation is limited to 25W. In most cases you can get away with just a heatsink with this amount of power, but if you're wasting this much power in a linear regulator, you have more problems than cooling!

Dave Jones has an interesting video on this. He discusses heatsinks and fans.

Answer 3

Just like cooling mosfets, you bolt them to a heatsink with some form of thermal paste that does not conduct electricity. The bigger the heatsink, the better. All thermal paste does is it fills in all the little holes and dents in the surface of the heatsink, and the component, thermal paste is thermal paste regardless of what application it is used for. Yes thermal paste used in computers would be good to use.

(If someone disagrees with me, say why you do, not just that you do.)