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By changing surface's emissivity factor, you can change the surface temperature of the same-area component dissipating same power without breaking any laws of physics:
P = emissivity factor * Stefan-Boltzmann constant * A * T^4
You can also achieve exactly the same by reducing thermal resistance of the leg bonding and the legs itself. -
Thank you for explaining why TH resistors are God-forsaken dark blue - to obtain higher emissivity, lol. I'd say it's a half-space on a pc board.
I note the SFR25H 0.5W is dark red-brown body vs SFR25 light green, 0.4W I guess it's the dark lacquer giving 25% more? Usually it's due to a change from steel to copper leads.
But the smaller SFR16S isn't the darker lacquer, has thinner leads- which is why I smell a rat.
I find OP got burned because resistor manufacturers are specifying power rating P70 at different film temperatures. Considering the parts have 100 years of history, and their standards are European, Asian, American with different material tech, it's not a standard it seems.
Same size MELF SSM0204 is given three different "operation modes": precision 0.07W, standard 0.25W, power 0.4W, all for the same part.
This is very important here, especially noobs that don't know how much or when to derate film resistors, and that their rated power is specified at any one of those three modes, and for different temperatures.
Digi-Key parameter for the part is 0.25W which is the standard mode, OP's fake SFH16S is 0.5W power mode (no other manufacturer is doing this for a similar part) and surely a typo anyway, as if it's 0.5W at 155°C beating the larger parts.
Other manufacturers are using standard mode on their datasheets, unless precision parts. Some are 55-85°C at their spec. So the datasheets are all over the place.
I'm avoiding going into haggling over thermodynamics here in a Beginner's thread.
Thermal Management in Surface-Mounted Resistor Applications is a good read. They're using an infinite heatsink.
edit: forgot clip
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So, I put some testing in on these.
The first ones I ordered SFR16S0004700JA500. It looks like a 1/8W resistor. I put 1/2W in them and their temp rose on the thermal camera to 120 deg C.
Then I ordered FMP100JR-52-470R, a 1W 470ohm resistor. It looks like a 1/4W resistor. It got up to 100 deg C.
Then I got some 1W 470 ohm resistors from a friend that are a decent size, at least what I'm used to 1/2W size or even a little larger. They got to 95 deg C. -
Comparing and testing these is difficult because of convection air currents, and heatsinking due to the test clips, both of which make a big difference in cooling the part.
Are you using a thermal cam? It would be nice to see a picture. IR spot thermometers do not do well with something so small and focusing.
The Yageo is a power resistor, so it's rated at 155°C for 1W in 70°C ambient, or 110°C on the bench. On a circuit board, it can go either way because you lose convection cooling and PCB pads can add conduction cooling, but PCB ground-pour reflects radiation. The 6.3mm length league used to be known for 1/4W resistors, back in the day.
At 1/2W, the FMP100JR rise is only 43°C above ambient, so 68°C (at 25°C ambient) in theory which is cool and what I think is more practical.
A different way is to look at things is from the other side- what power level would you run these at? How hot do you want them? -
Back in the day, the old Oz "Dept of Civil Aviation" would specify what everybody else called "one watt" resistors, as " half watt"---------"one watt" to them, were hulking great "two watt" resistors to everybody else.
This was a "trap for young players" for those suppliers who didn't know this particular bit of information.
Why they did this?
Well, the hint is the "Civil-Aviation" in their name!
As we all know, aviation is well known for overkill on specs.
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Also note you can't assess emissivity factor by eye very well because your eye is limited to visible spectrum only.
If you look at black clothing with near-IR (surveillance) camera for example, you'll notice some look completely white some are black. You can guess which ones feel hot on a sunny day even when they both look exactly the same to naked eye.