Hi all, let's get science-y!
There is an ongoing debate over the superiority of thermal paste vs thermal tape for securing an MCPCB to a heat sink. The majority of opinions I've seen say (hands down) that a "screw down" configuration with thermal paste is far superior to using pressure sensitive thermal tape...in terms of overall thermal performance.
Now if you look at the numbers on paper, thermal paste is the clear winner. It (in theory) conducts heat much more efficiently than thermal tape. I knew those figures once, and I'm in a bit of a hurry, so I don't have them here. Feel free to look them up and post them here.
Before we look at the data:
One thing that sets my lights apart from most others is the head design. I do not use a pill. I use a one-piece, solid head. This means the best possible thermal transfer away from the LED because there is no threaded connection between the pill and the head. In other words, most other lights use the pill as the primary heat sink. I use the entire head and the 3/8" thick piece of aluminum directly behind the LED, as you can see in the photo below.
The driver is mounted to a thick walled copper cylinder (this can be seen both above and below) with sufficient thermal mass to cool the driver without transferring as much heat to the head itself. The copper cylinder is press fit into the head only at the very bottom, otherwise it "floats" inside the body (see below). This is very different from mounting the driver directly to the head of the light.
In the next image below you can see what that looks like in practice. On the left is the prototype copper sleeve and on the right, the production sleeve. The production version has twice the thickness/mass. The prototype sleeve is depicted in the cutaway (first photo above).
I've had a few critics say that if I had any clue what I was doing (and if I wanted to be taken seriously) I would ditch the tape and get with the paste. I wanted to find out if this was true. I've been in the business of making things long enough to know that what looks good on paper doesn't always pan out in the real world. I'm not an engineer that specializes in thermal design. Given that, my best recourse to understand a given situation is through applied research: make something and test it in the real world. Blah blah blah, let's get down to it. Here is what I found...
The Contenders: Arctic Silver AS5 thermal paste vs Bergquist Bond-Ply 100 thermal tape
- The key for me was to compare the two materials on the relative performance of thermal transfer, not the absolute performance in terms of lumens. In plain english I'm asking, "Which thermal material moves heat more efficiently by observing the trend of output over time." (not sure that was plain english). As the LED heats up it will become less efficient and hence less bright.
- If thermal paste is a better conductor of heat, the output graph (over time) should be relatively flat. Thermal tape (being a worse conductor) should show a relatively more downward sloping line over time.
- Arctic Silver AS5 is the undisputed king of thermal paste. I won't go into it here. It's good.
- Bond-Ply 100 is specifically designed for LED interface and Berquist (the manufacturer) specialized in thermal interface materials. It's also best in class.
- Both materials were applied per the manufacturer specifications.
- Both LEDs were installed in "open" heads with no optic or reflector. All I need is a reference number, not actual lumen output.
- LEDs were selected from the same batch and bin.
- Fully charged batteries were used for each test.
- Over a period of days, five samples were taken with each light. Each day I would measure the AS5 light once and the Thermal Tape light once.
- The test began with each light at room temperature.
- The output was measured every thirty seconds (including turn on) for the course of 3 minutes
- The five values, for each configuration, were averaged and put into an excel chart.
So each data point is the average of 5 values taken over 5 days. At first, it looks like AS-5 is the clear winner because it shows higher values. However, we are not looking at absolute brightness because LED and every other component are a little different. No two LEDs will be the same, and no two flashlights will be the same. What we are looking for is a difference in the slope of the two lines. Flatter for AS-5 and steeper for Bond-Ply. Let's move on to the next chart below.
The first step was to remove the lumen values for "turn on" measured at time zero. The remaining data in the chart above begins at 30 seconds. The next step was to "correct" the values by moving the entire blue line up so it matches at the first value at 30 seconds. Now we can directly compare the slope of the two lines and the relative performance of the two thermal interface materials.
Anecdotally, I have another data set (using and XML instead of MCE) where the thermal tape appears to perform better than thermal paste.
The only conclusion I can draw from the data is that (over 3 minutes) Thermal Paste and Thermal Tape have almost exactly the same performance. Remarkably identical in fact. This (apparently) "busts" the myth that thermal paste is far superior to thermal tape. There is some hint, at time 180 seconds, that thermal paste might be gaining an advantage, but it is incredibly slight. A longer study, maybe out to 10 minutes, would need to be conducted.
I don't think this closes the book on the issue, but IMHO the data clearly debunks the thermal paste trumps thermal tape myth.