I recently learned about the best way to apply Thermal Compound. Previously I thought spreading the thermal compound on the entire bottom of the LED MCPCB (surface spread method) would be best, but I learned that using a small dot of thermal compound (middle dot method) is better since air is not trapped with that method.
I modified a Convoy C8 not too long ago and it bugged me that I applied the thermal compound the wrong way. I didn’t want to have any issues with the flashlight so I thought it would be prudent to re-apply the thermal compound the correct way.
Before I re-applied the thermal compound, I took some temperature measurements with a fully charged Samsung 30Q battery:
Start - room/flashlight temperature at 67 degrees Fahrenheit
1 minute - 84F
2 minutes - 95F
3 minutes - 104F
4 minutes - 113F
I disconnected the wires and removed the LED MCPCB. It was really stuck in there, but eventually it started moving and I was able to pull it out.
I cleaned-up the thermal compound with Q-tips and rubbing alcohol. I then used the middle dot method by putting a small amount of thermal compound on the flashlight shelf.
I put the LED MCPCB back on the shelf, pressed it firmly and twisted it slightly back and forth. As you can see the thermal compound oozed out the sides affirming that I used enough. I then screwed the LED MCPCB back into place and re-soldered the wires.
I charged the battery to full and took the temperature measurements again:
Start - room/flashlight temperature at 67 degrees Fahrenheit
1 minute -
2 minutes -
3 minutes -
4 minutes - ???
Your predictions? What do you think the ending temperature will be at 4 minutes?
if you measure the body temperature no difference as the power dissapation is the same
on the MCPCB no real difference as with spread or place it in the middle
the contact surface is >90% even with bubbles when you spread it
you would need a good IR camera to measure the close to the junction of the LED to get thje difference that matters
30K junction difference can be from DTP to non DTP star, even if all other mesasurements are the same
Look at the top right picture. The distribution of the thermal compound is uneven. Seems like your MCPCB is a bit convex. I always flatten my MCPCB on a lapping stone before I put it in. No lapping stone? Use fine sandpaper on glass pane.
When your MCPCB is flat, you will have trouble removing it from the shelf. Because of the vacuum under it.
The LED MCPCB was really stuck on the shelf. At first I wasn’t able to move it at all. I used a popsicle stick on the edges and with a lot of force I was able to gradually move the MCPCB. And then removing it was equally as hard. I didn’t notice any unevenness between the MCPCB and the shelf.
Thermal compound is intended to fill-in the microscopic valleys in which there is no metal-on-metal contact—less is actually better, as too much will act as a thermal barrier. As mentioned above the best fit is when the two surfaces are dead flat which maximizes the metal to metal area.
Looking at the cleaned-up photo above you can see the center lathe tit—that needs to be removed with a file or grinding stone at the very least, then lap both surfaces with a whetstone. Then apply a drop of thermal grease to both surfaces, spread around and then wipe if off with a flat straight edge—this will leave grease in the microscopic valleys and expose the metal tops.
Good luck with the testing it is very interesting.
No, because when the thermal path is good, the LED will get less hot and have better efficiency and thus produce less heat.
The heat that it produces either way will eventually end up in the body, so with a bad thermal path the body will get hotter anyway.
If he is measuring the body temperature of the flashlight then the temperature should be greater because more energy is being transferred to the body.
The better the thermal path, the higher the body temperature should be and the lower the MCPCB temperature should be.
It is not possible to have the two be the same temperature, there will always be a delta T between them.
Better thermal path = lower delta = less energy in MCPCB = more energy in body.
More energy in body => higher temperature => larger delta T between body and ambient air temperature => more heat dissipation.
Ditto the rest, there should be no measurable difference in the body temperature. The exact voltage that the cells are at and the exact resistance that the switch had that time around, among other things will play a bigger role if I had to guess.
When I tested thermal paste, I found that as long as it is there and squished reasonably well,the performance is goning be too close to notice.
You’re right that the delta T will be higher with the poorly applied thermal compound. But so will the thermal resistance.
Those that predicted 113 degrees know that the flashlight as a whole will eventually drift towards thermal equilibrium, and they are right. Energy into the light HAS to equal energy shed. That energy is the sum of heat and the energy of photons emitted.
Take it a step further and as the LED gets super hot due to the poorly applied compound, it will draw MORE current. Also as it gets hotter it will become less efficient. An even greater percentage of the energy input will be heat.
As I type this I can’t remember if the driver is constant current or DD.
For the better thermal path, my prediction would be temperature goes down, slightly.
Assuming it is DD, the energy will be the same in both cases, regardless of good or bad thermal path.
The difference is that when it has a bad thermal path then more heat remains in the MCPCB and less transfers to the body.
The total energy dissipated is still the same, but there is more energy stuck in the MCPBC than in the body (compared to a good thermal path, where the energy can easily transfer from the PCB to the body.)
Since the MCPCB has a lot more energy and doesn’t transfer as much to the body, that results in a higher temperature at the PCB.
In the opposite case where the thermal path is good, the temperature difference becomes less, so the PCB and body are closer to the same degrees.
That means the MCPCB is cooler and the body is hotter. (but the MCPCB will always be at a hotter temperature than the body)
