do I need thermal paste when changing an led star.
Yes. It improves the transfer of heat to the heat sink.
Bob
I use Fujik from Fasttech. It is only paste, not glue but that works for me. screw holds down mcpcb.
thanks guys.
I use this stuff:
https://www.fasttech.com/product/1341200
It’s like the Fujik, but in a smaller tube, so when it hardens in the tube and you have to throw it away, not as much loss. I usually add on a spare tube or two whenever I make a Fasttech order.
Just don’t overdo it on the paste, a little is good, too much is not.
Also pressure > paste
The harder the mcpsb is clamped down, the better the heat transfer. Obviously don’t strip anything.
Thermal paste still transfers heat worse than direct contact, but better than air, so use only a little to fill the gaps that would otherwise be empty, but not too much.
Make sure mating surfaces are flat before hand, most are not. 
This is not true, there is far worse thermal transfer with just pressure than with paste.
Also, adding too much paste does not negatively affect temperature.
I would suggest you both go look at tests done of no thermal paste vs thermal paste vs too much thermal paste for computer CPU and GPUs.
I never said to not use any paste, I said that pressure is more important then paste. In my computer hot rodding days we did tons of tests to get every last bit of cooling.
Basically the type and application of paste could make a ~10 degree difference in temps on a good day (assuming you didn’t glob it on stupid thick). Pressure on the other hand could easily do 10-15 degrees if it was not clamped down hard enough.
A little cheap paste with high clamping force will out preform top of the line paste with little clamping force.
Adding too much paste most assuredly will reduce thermal transfer as well, paste does not transfer heat nearly as well as direct metal to metal contact. This is another part of the reason that high clamping force was so important, it would squeeze out all the excess paste and improve the heat transfer.
It is paste, when you put pressure it will squish out around the edges.
If you put a lot it will end up being the same thickness layer as if you put a little.
If you don’t put enough then that is when you start seeing a big temp difference because metal on metal gets worse temps than metal on metal with paste in between.
I can post some benchmarks if you still don’t believe me.
Putting two pieces of metal together does not transfer heat very well. Thermal paste transfers heat better than air, which is why it is used.
This is assuming that in all cases the force is the same. There is no reason why something shouldn’t be clamped down, but there is also no reason why in one case it would be clamped down more than in another.
The best thermal transfer you can get is 2 perfectly smooth metal surfaces mated together with nothing in between. Thermal paste has MUCH MUCH worse heat transfer then metal on metal (on the order of 50-100 times worse).
The only issue is air gaps, as long as those are filled then any more will reduce heat transfer.
More paste will be harder to squeeze out, it will reach a certain thickness but you generally want it to be less then just globbing it on and clamping it down will get you.
All of that said, it is a rare flashlight that it will matter if you follow proper thermal paste procedure. Simply applying it and screw everything down tight is plenty for the vast majority of flashlights to work fine. They can vary by 20-30+ degrees and not have a problem.
A CPU on the other hand can have problems with just 5 degrees of temp change when running on the edge.
i use a smear of thick silicone grease
This will work, just be careful to not get it on the LED obviously.
Really for an LED anything that is not air will work to fill the voids. LED’s are not that picky about the temps they run at (besides reduced output). Heck 95% of handheld flashlights don’t even run LED’s up to their rated temperatures, much less beyond them.
You will never get two perfectly smooth metal surfaces unless you get some special piece from nasa or something.
Thermal paste is used because it fills in all the gaps between two very smooth metal surfaces.
Take a look at some electron microscope images of flat metal, it is not as flat as you think it is.
People lap CPUs and CPU heatsinks, and it does not offer a temperature improvement. Thermal paste must ALWAYS be used even if the two surfaces are extremely flat.
This is less of an issue for LEDs which produce 10x less heat than CPUs or GPUs, but if you want proper heat transfer then you must use thermal paste.
No amount of lapping and pressure will produce better temperatures than using good thermal paste.
Simply pushing two pieces of metal together is the worst method of thermal transfer.
The ONLY thing better than thermal paste is liquid metal or soldering/welding the surfaces together.
When did I ever say to not use thermal paste?
All I said is that the pressure that you clamp the LED down is more important to what paste you use or how you apply it.
I stand by that.
There are test using toothpaste and also red lipstick on cpu’s. :smiling_imp:
While at the bottom of the list, they work better than using nothing at all. 
In fact only 8 degrees difference from the best rated thermal compound!
It is not though, because you can clamp it with 1N or 10N and the only thing that will do is decrease the distance between metals by a few microns.
The thermal paste will barely make a temperature difference whether the layer is 10 microns or 50 microns thick. Maybe like half a degree.
Using a good thermal paste like MX-4 can make a several degree difference compared to other cheap pastes, and as long as you apply enough to cover the surface when pressed there will be no problem if you add a bit too much.
The problem is not using thermal paste. That’s the worst thing you can do.
If you look at the chart right above your post you see why I state that the type and application of thermal paste doesn’t matter much.
Clamping it does indeed make a difference, I have tested this myself. I had a CPU that was overheating for some reason. It turned out that the bottom side of the heatsink clamp had broke and was not clamping like it should (even though I could not feel any play by hand).
After fixing that and clamping it down harder it stopped overheating and drooped the temps by 10 degrees. There have been other tests to show the same thing, although now days it is well known so all CPU coolers come with good clamping force.
LED flashlights on the other hand do not. From the factory TONS of them have little or even no clamping force at all on the LED. This leads to massive globs of thermal paste built up under the star. This is far worse then having less paste.
Hence my statements which I still stand behind.
I think the basic idea is that the metal must touch.
The paste just fills in the tiny scratches.
Too much paste will keep the metal from touching.
So too much paste is definitely a bad thing.
That’s why every instruction you can find for applying the paste
tell you to apply the thinnest layer possible. Scrape off any excess with a card.
Decreasing the distance between the metal is number one goal.
Filling in the scratches is secondary to this.