Possible LED attachment improvements

It’s probably a bit related. Just got an IC Graphite thermal pad kindly shipped to me by zak.wilson. Here’s a quick test comparing it to the MX-4 thermal paste. No TIM for reference. It seems to suck heat off the MCPCB just as well as normal paste. It’s reusable too, and no cleaning required so perfect for LED testing.

Thanks for the test! That is indeed very interesting for lights which are modded vers often.
Could you repeat the test, but measure the brightness of the LED?

OK, so I got some Indium off AE (advertised as 99.99% pure….) and tried soldering it. While it’s very very soft, about as soft as pure lead, joint strength was surprisingly good. Two strips of FR4 PCB soldered together in an area of ~1cm² wouldn’t come apart until the FR4 itself was severely deformed. Wetting, as expected, was very good (I used some standard rosin flux).

I found one possible downside to pure indium though:

This could be an issue with ceramic packages, though I doubt it applies to any but the most expensive sources.

(This paper also somewhat relieved my concerns about creep failure though I’d take extra precaution not to stress the emitter via the centering ring, just to be sure.)

Nice! The strength shouldn’t be a problem because you can use normal solder for the plus and minus connections.

I have also been researching this quite a bit and will post something here soon.

Optimizing the solder connection is the most fundamental thing we can improve with a given LED. I don’t know why nobody started doing this earlier.

I just remembered an additional material upgrade. Normal dtp pcbs are made out of copper or aluminium. There is another possible material onto which the LED can be directly soldered which transfers heat even better than copper: silver. Basti in the German TLF forum made such a pcb a few years ago. He made it just for the looks though.

The thermal conductivity of silver is 7% higher than that of the best possible (most pure) copper. Most common copper alloys are not 99.99% pure though, making the difference for pronounced.

I asked him if he would make one for me for my big thrower when I was planning the build, but he is not an active flashahlic anymore.

I wonder if this would be of any use…

Yes, but ONLY if the pressure is applied evenly.

This is a variant of Panasonic’s PGS, and most importantly, soft PGS.

The two disadvantages of using PGS are higher contact thermal impedance compared to thermal paste if applied without enough pressure, and electrical conductivity.

Soft-PGS is better in this regard than pure PGS, since while it features much higher thermal resistance(20W-28W/k vs 300-1800W/k), PC users have tested that thermal performance is much better with soft PGS because of much lower contact thermal resistance as specified above.

It’s also because soft PGS has much better thermal transfer in the Z-Axis compared to the X/Y-Axis, meaning thermal transfer of high power density products is much better than regular PGS.

Link:

TLDR: Soft-PGS is better in almost all cases with high power density electronics, such as CPUs/GPUs and VHP LEDs such as XHP70.2s overdriven, CFT-90s up to 100W power levels without starting to fall behind thermal paste because of heat spots without very high pressure mounting. The best combination for massive thermal transfer capabilities are liquid metal below the soft PGS, and the soft PGS under the MCPCB itself.

Found this regarding the topic: Graphite Thermal Pads: IC's product is a ripoff @ reddit

So I made this search on Mouser for Thermal Graphite Sheet in Thermal Interface Products: https://www2.mouser.com/Thermal-Management/Thermal-Interface-Products/_/N-71x7n?P=1yzraj9

Most flashlights allow providing some decent MCPCB to pill/shelf pressure. Does this mean a piece of 0.2mm soft-PGS under the emitter board is nice enough? Is it electrically conductive?

Cheers ^:)

Yes, it is electrically conductive. Just be great for almost all LED builds, except that CFT-90 LED. That thing has huge power density, and needs liquid metal.

In other words:
PCB-soft PGS-liquid metal-host
Right?

Why is there the asymmetry with no liquid metal between PCB and PGS?
Why is PGS + liquid metal better than liquid metal alone?

Another thing which I suspect won’t work but I’ll throw it anyway. I wonder if improvements to radiation either directly from the LED package or from the MCPCB could bring measurable performance increase?

I think you’re mixing up two different products.

-The one with 28 W/m·K is the EYGS series which comes in 0.2mm thickness and is electrically conductive.

-The one with high thermal conductivity in the z-axis (vs. x/y) is the EYGT series. This is graphite in a silicone matrix , it looks like this is NOT conductive (4*10^5 Ω·cm), but comes only in >0.5mm thick sheets and has a conductivity of 5 to 10 W/m·K (strangely, it goes up with thickness?). Has some sort of separator but the datasheet is not clear what its properties are or whether it needs to be removed before use.

This doesn’t make much sense to me. The LED and pcb radiate into the air inside the flashligth head. Air is a very bad conductor of heat. After this the heat would still need to go through the metal of the head or through the glass lens (glass is somewhat good at conducting heat). You would need to replace the air inside the head with something else that doesn’t transmit light any worse, but has a noticeably better thermal conductivity.

Would air absorb significant part of the radiation? I assumed no, but maybe incorrectly.
Otherwise goes towards the optics or towards the head sides or both, depending on what radiates it and what’s the optic.
Head can absorb it, conduct it towards the edges and remove.

No, not in such a short distance.

Maybe a solid glass TIR lens which touches the PCB with as large of an area as possible might improve the heat transfer a bit.

Opinion for this wire?

Thermal conductivity of 65.73 K/W. Better than SnPb or SAC305, worse than Indium. Melting temp of 227C, slightly higher than SAC305.

Soldering with Sn99.3Pb0.7 is like sailing with a headwind, and it also increases stress in the soldering iron's tips. Check here: Why do tips easily oxidize when they are used with lead-free solder? @ hakko.com

I recommend standard eutectic Sn63Pb37 or Sn60Pb40 classic solder alloys, namely if newbie.

Cheers ^:)

P.S.: editing Niko's above post to reduce image size would be nice, somehow messes up with the page view on mobile or small screen devices.

How is S-Bi57sn43 ?
I also have this.

…sorry for extra big photo

I remember it’s worse than SnPb.