I foresee no discernible difference within error margin. 
Cheers ^:)
113°F
114 
Probably very little or none unless you’re running that LED at 6A+.
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.
110°F
Just a small improvement.
If it’s an improvement the temperature should be higher
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.
It’s better to put a blob in the middle that will spread out when you put the parts together.
This avoids air pockets.
Indicative of a good situation.
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.
When the LED produces more heat, the body will get warmer too, because the heat has nowhere else to go, albeit through a bad thermal path.
Of course.
The higher the delta T, the more heat will be transferred.
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.
neutralfan, did you press down on the star and gently twist side to side (45 ish degrees,) when applying the new set of thermal paste?
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)
Enderman,
IF it is assumed that the energy input is the same for each case, and we disregard the decrease in efficiency of the LED at higher temperatures, then the body of the flashlight will be the same.
You are right, there will be a higher delta T, but there will also be a higher thermal resistance, so the heat flow will be the same! (Double the Voltage and Double the Resistance in a simple circuit, and electron flow is the same)