Why a good thermal path really matters

[SemiMan]

Vaska, good thermal materials for boards are available up to 7-8 W (m*K). Your board suppliers should be able to order in for you.

Semiman

 

[SemiMan]

Flashlight designers? .. no .... LED Luminaire circuit board designers ... YES.

I never promised to post a link to a star ... but I did show where YOU could get board material and I assume from your posts you are capable of making your own boards. That said, if there was enough interest, having some truly high performance stars made up would be a worthy pursuit, though if I did that, I would likely not use board material at all but direct go direct trace on copper with Anotherm. Not sure how much people would be willing to pay for high performance bare stars though.

 

[vaska]

good thermal materials for boards are available up to 7-8 W (m*K). Your board suppliers should be able to order in for you.

I put the wrong figure in my previous post: my boards include 5 W/(m*K) insulator, and total figure is 2 K/W of course, not W/K
Manufacturers here do not order material with better parameters because they suppose it expensive but not much in demand.

 

[saabluster]

I am not aware of any other manufacturer using this type of method as it is more costly than
just bolting down a MCPCB. As those of you making your own flashlights, this is one method
that you might consider. The circuit board that we use is designed only for electrical connections.

Curt

One thing I meant to mention Curt is that I designed the DEFT heatsink with an interference fit three years ago. Just for the record.

 

[ma_sha1]

Just to report back that I received the DX copper star XML a short while ago, just got some time to play with it.

The led was visibly lifted from the board, there's a visible gap on one end, bad news if one want to use it "As is", may perform worse than a well done alu. star due to the "gap". Cutter leds are usually re-flowed on to the star with better quality job.

So I proceeded to a re-re-flow, added a little bid of solder paste to the gap side,
used a heat gun & managed to "close the gap".

Tested it on a big copper heat sink running between 5-5.5 Amp, so far so good.

 

[saabluster]

Just to report back that I received the DX copper star XML a short while ago, just got some time to play with it.

The led was visibly lifted from the board, there's a visible gap on one end, bad news if one want to use it "As is", may perform worse than a well done alu. star due to the "gap". Cutter leds are usually re-flowed on to the star with better quality job.

So I proceeded to a re-re-flow, added a little bid of solder paste to the gap side,
used a heat gun & managed to "close the gap".

Tested it on a big copper heat sink running between 5-5.5 Amp, so far so good.

Good thing you noticed the bad mount. One thing to keep in mind when doing your own reflow work is to make sure you don't have too much solder under the LED. You can tell if you do by pushing down on the LED while the solder is still molten. If there is too much it will squirt out as a tiny ball. Any more than necessary will increase the thermal resistance. Just keep it held down until it solidifies and then knock the ball away.

 

[ma_sha1]

Good thing you noticed the bad mount. One thing to keep in mind when doing your own reflow work is to make sure you don't have too much solder under the LED. You can tell if you do by pushing down on the LED while the solder is still molten. If there is too much it will squirt out as a tiny ball. Any more than necessary will increase the thermal resistance. Just keep it held down until it solidifies and then knock the ball away.

Thanks. I use needle nose to grab the star with one hand & the other hand blow from under with my heat gun, so my operation won't allow fingering from above . But it still managed to spit out a tiny solder bead, I watched the youtube video a while ago a guy was showing the heat gun method. looks like the surface tension is squeezing the excess out when solder melted down to liquid form. When I was done, it looks pretty tight, lot better than before.

BTW, this wasn't my first time, my 1st time using this technique was trying to mount
triple SST-90 onto copper, I ended up killing 2 out of 3 SST-90s.

 

[ergotelis]

The best way for me is a oven like this
http://image.made-in-china.com/2f0j00BMQTnzyanikm/Toaster-Oven-2801H-Series.jpg
, using the top side , i don't know how do we call this in english.
On the lowest setting, it is getting around 150-200 degrees, depending on the point you are measuring.
I used an IR thermometer to measure this.
Is is the best way to reflow a led. You have complete control and stable temperature, using two nice and long needles it is very easy.



To add, finding the right way to reflow, i have managed to destroy 2 mce,1 sst90,2 sst50, one xm-l and a few xr-e, fortunately no xp-g xp-e!

 

[Techjunkie]

I use a copper-clad stainless steel frying pan and a natural gas range/stovetop on med-low.

 

[saabluster]

Thanks. I use needle nose to grab the star with one hand & the other hand blow from under with my heat gun, so my operation won't allow fingering from above . But it still managed to spit out a tiny solder bead, I watched the youtube video a while ago a guy was showing the heat gun method. looks like the surface tension is squeezing the excess out when solder melted down to liquid form. When I was done, it looks pretty tight, lot better than before.

BTW, this wasn't my first time, my 1st time using this technique was trying to mount
triple SST-90 onto copper, I ended up killing 2 out of 3 SST-90s.

I am aware of surface tension pulling the LED into place on the x&y axis but am not sure if that happens(to the degree I am talking about) on the z. You may be right but I have had many experiences where it did not happen. It may be what you saw was simply due to the natural process where the metal liquefies and pushes the flux out so that there is now a reduction in the amount of material remaining between the two devices combined with a contribution from surface tension drawing down the LED. So although I see some role for surface tension in the fact that the two parts draw to one another I have not seen this be of sufficient strength to overcome the application of too much solder. I'm sure someone here who has specific knowledge to this can explain all the forces at play here. I just know that all too often you can get balls to squirt out even when the LED looks perfectly mounted.

I too use the heatgun and needlenose method at times and I have no problem getting it down and pushing the LED down before it sets up. For most production work I use the lab oven as is specified in the data sheets.

 

[MikeAusC]

. . . So although I see some role for surface tension in the fact that the two parts draw to one another I have not seen this be of sufficient strength to overcome the application of two much solder. . . .

I agree. Surface tension pulls the chip into position horizontally because it will maximise the metal to metal contact area - but only because the metal pattern on the underside of the LED has the same pattern as the metal on the star.

There is NO surface tension force that will naturally squeeze out any excess solder from between the two surfaces.

Since the best solder has a Thermal Conductivity far worse than Copper, it's essential that the two Copper faces are in as close a contact as possible, with solder only filling gaps cause by surface roughness. This will ONLY happen if press the two parts together.

 

[SemiMan]

If you follow EBAY, you can find surplus temp controlled hot plates that are somewhat accurate and work quite well. Now If I could just find a really good way to put down thin paste reliably for prototyping, then I would not need to press out the solder which seems almost standard practice when using paste manually. I sometimes find that just tinning the pad and using flux gives me better control.

Semiman

 

[MikeAusC]

Do you have a Temperature Controlled Soldering Iron - can you make a plate to attach to the barrel that the Star can sit on.

 

[Neondiod]

Do someone perheps know how the metall heat contact plate (it's grind away in saablusters thinning process) are attached to the ceramic base of these kind of emitters? Are they glued on whith something? In that case that glue must have some thermal resistans that you get rid of too.

 

[bshanahan14rulz]

Now, saabluster, you just need to sand an XR-E down to the reverse-polarity diode and test again!

@vaska (and lux-rc/serge), wow, I hope your implementation of that metal heat spreader makes it to production!

@TJ, now with copper metal stars, this idea seems plausible enough to try! I say go for it!

 

[saabluster]

Now, saabluster, you just need to sand an XR-E down to the reverse-polarity diode and test again!

That's part of the plan.

 

[beerwax]

hi guys.
i can no longer do prac but am still engrossed by these threads.
its a shame that cree dont supply the led on a larger piece of ceramic.

anyway when you mount the ceramic (what you guys call reflow ), which is just under 1 mm thick? , would it help to build a bead of heat conducting goo around the edge the ceramic tile, effectively conducting some heat thu the sides of the tile ? being pretty close to epicenter and all. if that tile is 4mm by 4mm by 1mm thats 16 mm sqared on the base and 16 mm squared on the vertical border surface.

gee i hope this isnt already in this thread somewhere .

cheers

 

[saabluster]

hi guys.
i can no longer do prac but am still engrossed by these threads.
its a shame that cree dont supply the led on a larger piece of ceramic.

anyway when you mount the ceramic (what you guys call reflow ), which is just under 1 mm thick? , would it help to build a bead of heat conducting goo around the edge the ceramic tile, effectively conducting some heat thu the sides of the tile ? being pretty close to epicenter and all. if that tile is 4mm by 4mm by 1mm thats 16 mm sqared on the base and 16 mm squared on the vertical border surface.

gee i hope this isnt already in this thread somewhere .

cheers

No that will not have any appreciable benefit as the heat does not really travel laterally through the ceramic.

 

[ma_sha1]

Just want to provide some feedback on the DX copper star XML, it didn't work,
When I put a light together, it wouldn't lit up, some times emit a blink of light.

I ordered & received another one, same thing, initial test with a battery fine, after soldering
leads onto the star, it no longer work, again some rimes emit a blink of light.

I am down two for two on this Copper start XML, so I am out.

 

[CKOD]

Speaking of Cutter vs other MCPCBs for the XM-L

Did a comparison of the Cutter XM-L boards and KD XM-L boards. Mounted a U2 2S XM-L to one of each PCB (reflowing onto pre-tinned pads with RMA flux)

Screwed the pcb to a heatsink I pulled from an old mobo for the mosfets, using thermal compound(one at a time of course). Ran each LED at 3.73A for 10 minutes to reach a somewhat steady equilibrium. I recorded the Vf at the power supply and light output on a lux-meter 6.5" away, neither the clamp for the heatsink nor the lux-meter moved between tests.

Even soldering the wire leads to the PCBs with them mounted to the heatsink, I noticed the KD board was harder to solder. I also noticed the heatsink became hotter much faster with the KD compared to the cutter PCB (hotter faster is good!)

Results:

                KD     Cutter
Initial lux    11.65k   9.17k
T+10 Lux       10.31k   7.47k
Lux drop       11.5%    18.5%
Initial Vf*    3.78v    3.73v
T+10 Vf*       3.70v    3.63
Vf drop        .08v     .1v

*The Vf was at the power supply, not at the LED, so there is ~6ft of wire that is probably a touch small for 4A between the LED and the voltage measurement.
I think the results pretty much speak for themselves, the KD boards are the better of the two

from a comparison I did.

I'm not sure how much difference the boards made on the initial lux readings, and I'd say the % drop would be more meaningful then the initial lux difference, but FWIW the LEDs were right next to each other in the cut-tape from the same bin, so I couldnt see them varying that much.


EDIT:

for those looking to do more then a little bit of reflow work,
http://www.circuitspecialists.com/prod.itml/icOid/8010 may be of interest, very nicely priced at $89, powerful enough for doing a star on its own, and regulated temperature so you dont burn your PCBs or LEDs by leaving them on for just second too long. I use one for SMD work at home also...

http://www.amazon.com/dp/B002NKM1MK/?tag=cpf0b6-20 if youre feeling more spendy, and what I use at work for bigger stuff, and would work fine for LEDs and smaller stuff alike, other then it being a bit large. Ive reflowed LEDs onto MCPCBs with this, while they were attached to the heatsink.