Heat sink temperature ratings

Heat sinks seem to be given a number something like "12 degrees C/W".

What does the term mean exactly? And does 12 cool better than say 8?

Thanks,

Antknee.

That is saying that for every Watt that the heatsink tries to dissipate, it will heat up by 12 degrees Celsius. Hence, 12 degrees C per Watt. So, knowing this, obviously a lower number is better.

Remember, this number does not include the resistance of the thermal grease material that sits between the heatsink and the component, and the component case itself also has another thermal resistance between the case the die/junction case which is actually generating the heat. That means that the more thermal resistance you have between the heatsink and the die/junction (or whatever is actually generating the heat), then the temperature of the the die/junction becomes higher and higher above the heatsink temperature.

Example 1:
-a heatsink with 10C/W
-0C/W between the heatsink and junction/die
-junction/die dissipates 3W

Then the heatsink will increase to 10C/W * 3W = 30C above ambient temperature. Since there is 0C/W between heatsink and junction, then their temperatures are the same so the junction temperature is also 30C above ambient. This is the ideal case and never happens in real life.

Example 2:-a heatsink with 10C/W
-3C/W between the heatsink and junction/die
-junction/die dissipates 3W

Then the heatsink will increase to 10C/W * 3W = 30C above ambient temperature. But, now we have 3C/W between the heatsink and the junction. This means that the junction will be at a higher temperature than the heatsink. The junction temperature will be (10C/W+3C/W)*3W = 39C rise above ambient temperature.

I get it, thanks. My amp keeps cutting off, it has a 12 degrees /w heatsink so I will get a lower number.

I will keep the volts down while I order a better heatsink. I used some thermal glue, will I also have to get a new chip? I'm wondering if taking the chip and heatsink apart will damage it. I wouldn't know exactly what the thermal resistance of the glue was, I'll have to have a look.

Antknee.

The thermal adhesive I used was 120 degrees C cm/W.

Depending on the thermal paste used it should be soluble using isopropanol alcohol. Occasionally applying a little heat (hairdryer) helps if the device won't easily separate from the heat sink. Be sure that you throughly clean the surface of the new heat sink before applying any thermal grease.

Ron

I have some isopropyl alcohol, it sounds like the same stuff. The exact adhesive I used is below. I'm hoping I don't have to buy a new chip because they are £5 ($8) and all 4 suppliers on my usual website are out of stock so hunting around for a new one would take some time.

FISCHER ELEKTRONIK|WLK 5|ADHESIVE, THERMAL CONDUCTIVE | Farnell United Kingdom

Uh Oh, yep, it is two part epoxy glue. Not sure you will be able to save your chip. If the chip and heatsink can be removed you may get lucky soaking it but although the data sheets talk about application they don't seem to mention removal.

Ron

I was wondering if there was much difference between glue and grease. I think I will get some grease for future reference. I have thought of a work around, I have a spare heatsink from a board I don't use, so I will glue this to the first heatsink. It should work, but I don't think it'll be winning any design awards

difference really depends on the greese ... the thermal glue conductivity is around 0.8w/mk and this is much better then your standard thermal paste that offers around 0.5w/mk. You can find better quality paste that will go up to 0.9w/mk but that's really not a big difference (especially if you need to design a way to stick the heat sink to the chip). There are bit more expensive 3w/mk like this one but this is all pretty much in the same ball park as the thermal glue because if you initially wanted to use thermal glue it is because you were unable to make the heat sink press against the chip properly (no brackets etc..) like in 99% of real life cases - so thermal glue - good thing

Now if you really need that contact to be low resistance (high temp conductivity) you outta go with cpu paste like arctic silver that will offer 350w/mk but note - this requires 0.001" layer and heat sink that is pressed against the chip with a lot of force (think PC CPU and cooler)

The only difference between glue and grease is that with grease you need something to give positive pressure from the heatsink to the chip, the glue is it's own bond.