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Thread: Why a good thermal path really matters

  1. #31

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by SemiMan View Post
    No, what it does is show the results of your particular and potentially poorly implemented solution.
    My isn't your glass half empty. I should think given the results quite the obvious is on display. But twist it if you must.


    Quote Originally Posted by SemiMan View Post
    To my point, there are many different metal core implementations, some very good, some very bad. Thickness and quality of your attachment method could have a major impact as well. Thickness of the copper layer. What LED you use (Rebel, Cree) all make a difference. You have just shown one method and one particular implementation of it that may have no basis for what someone else may do.
    Yeah some "good" some bad. So what. This will all become quite clear when the testing is done. I am not going to test every single LED out there and there is no reason to. What works with one LED will work with another. That being - decreasing the thermal resistance increases the performance of the LED. I am using what has started to become an industry standard for LED construction. Sure the K2, for instance, would not benefit as much from from some of these methods but those LEDs are dead or dieing. Welcome to the new world. It's not like some LEDs will see decreased performance from moving from a mcpcb to a heatpipe and there is no need to do calculations to figure that out. I am part of a clan here(Overdrivers) that likes to push things to the very boundaries. When doing this there is no need to do calculations. The solution is obvious. You simply do the absolute best you can thermally and call it a day. So while we would be done with our projects you'd still be tapping away at your calculator. Sure I get the need to do all those calculations when you are working with computers and designing boards and whatnot but what we do here is fairly simple and I feel the same way as Curt that I can do all those calculations in my head based on experience. In fact I designed and have built a light that has yet been made public trusting my experience alone to guide the way in designing the thermal path(and it is far from a normal solution) and after I built the light everything performed exactly as I expected it would.

    Quote Originally Posted by SemiMan View Post
    Show the way the thermal path is built up and hence understanding what does what, what can be improved, what makes little difference, etc. would be far more valuable to an experimenter. It also gives you something to work towards to see if you did it right.
    Seriously? That is the point of this thread. Granted I have yet to do all the testing of various boards but I have clearly and repeatedly indicated I was going to. I have also mentioned the fact that I am going to be testing the ablated ceramic vs stock to see if there really is any benefit. If you are suggesting I make this thread into a boring science book about numbers you can forget about that. If you've followed my other threads you will know I am more about practical information. I have learned how to distinguish what needs to be learned and what serves merely to impress others. I am trying to show people merely what needs to be learned. You may feel otherwise and you certainly are most welcome to do so and certainly free to start your own calculations thread.

    Quote Originally Posted by SemiMan View Post
    In terms of best metal core, Laird makes what I have found to be some of the best commonly available material but I am not sure who would make a star based on it. I use it for product stuff.
    OK now show me a link to a product the average consumer here can buy that is a "good" product. Not a substrate a purchasable star that either has LEDs on it or can simply have them baked on and be done. Where are these wonderful boards you say exist because we would all like to start buying them.



    Quote Originally Posted by SemiMan View Post
    The reason they have the switch is to deal with the inherent inaccuracies. Remember that refine is nothing more than a gain setting. It does not change the spectrum. Unfortunately (or fortunately), they do not have switches with a position for LEDS. They would not work if they did as there are so many different spectrums for LEDS. Even the fluorescent setting is really not right as there are a ton of spectrums for these as well. Depending on which LED you are using, the incandescent setting may be the best one to use. Unless the LED spectrum somewhat matches the spectrum of what you picked, then you may make the measurement better or worse compared to incandescent.
    So tell me. Does the switch adjust for the spectrum or adjust for color temp and how do you know?

  2. #32

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by Techjunkie View Post
    I've been curious to know something about the copper MCPCBs from DX... is the thermal pad of the XML soldered directly to the copper of the MCPCB through a window in the laminate layer? If not, is it possible to desolder the LED, srape off the laminate in the center, apply solder paste and reflow back onto the MCPCB? It seems to me that would eliminate the thermal insulation of the laminate layer.

    Q#2: which XML stars have the thinnest laminate layer? (Are the KD stars in the running?)
    No it does not have the LED direct to the bottom copper. It may be possible to do as you say. Why don't you do some calculations to find out because that will surely be far simpler than just trying it right? Seriously I think that isn't a half bad idea but I think you might find it a little difficult as it appears to be the pre-preg stuff. I haven't messed with it yet though.

    Quote Originally Posted by MikeAusC View Post
    The DX copper MCPCBs have such thin copper it seems pointless.

    If you're going to make the effort to solder an LED to copper, you may as well use a decent thickness of copper to spread the heat from the tiny area under the LED, to a much larger interface area with the heatsink.
    I agree. I was a little bit underwhelmed when I pulled it out of the package and realized just how much they scrimped on the copper. That said my intuition is that it will still be far far better than the Cutter boards. We shall see.

  3. #33
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    Default Re: Why a good thermal path really matters

    Quote Originally Posted by saabluster View Post
    No it does not have the LED direct to the bottom copper. It may be possible to do as you say. Why don't you do some calculations to find out because that will surely be far simpler than just trying it right? Seriously I think that isn't a half bad idea but I think you might find it a little difficult as it appears to be the pre-preg stuff. I haven't messed with it yet though.


    I agree. I was a little bit underwhelmed when I pulled it out of the package and realized just how much they scrimped on the copper. That said my intuition is that it will still be far far better than the Cutter boards. We shall see.
    Not familiar with the term "prepreg"... are you referring to the laminate layer? Is it different than the layer you peeled up from the cutter board in the pic above? If I were to attempt creating a window through it for the thermal pad, my first approach would be something abrasive, like a stone bit on my dremel. That will surely wouldn't create a perfectly flat surface, but solder should replace whatever is scraped away without creating a void between the emitter's center pad and the star. It can only be an improvement over the laminate, thermally.

    Regarding the thinness of the 20mm copper star, the star's mass is less important than how fast a conduit it is to the heatsink beneath it, right? My expectation is that eliminating the thermally insulating laminate layer between the LED's center pad and the star, by soldering (the center pad only) directly to the star should make for less thermal resistance between the LED and the heatsink than any other 20mm star where laminate sits between it and all of the emitter.


    Quote Originally Posted by MikeAusC View Post
    The DX copper MCPCBs have such thin copper it seems pointless.

    If you're going to make the effort to solder an LED to copper, you may as well use a decent thickness of copper to spread the heat from the tiny area under the LED, to a much larger interface area with the heatsink.
    Regardless of how thick a 20mm star is, the interface area with the heatsink beneath it will be the same. If my existing application calls for a 20mm star, and re-soldering the LED to a thin copper star after removing the laminate under the center thermal pad outperforms a conventional Aluminum star, or even the same Copper star with all the laminate in tact, then that doesn't seem pointless to me at all, and requires very little effort compared to a more custom solution.

    If I were designing (and capable of manufacturing) everything from scratch, then obviously, there are more effective designs than the star, however, that requires far, far more effort than a simple de-solder, scrape and re-solder operation. I know my limitations. Tweaking the existing design is my forte, custom manufacturing something entirely new is not. I've tried my hand at creating completely custom parts with what tools I have at my disposal, and the results are seldom more rewarding than the results with the easy tweaks.

  4. #34

    Default Re: Why a good thermal path really matters

    Laird Theramgon Tlam SS 1KA04 or DS 1KA06
    http://www.lairdtech.com/Products/Th...PCB-Materials/

    That said I have seen the data sheet for the Opulent thermal prepreg (many years ago) and I remember it being very good.

    Before I started using any volume, I did my first ones the old fashioned way. I asked nicely for a bare board sample, and like any good hobbyist I rolled my own using my trusty printer and etched my own.

    "What works with one LED will work with another." .... not true. I regularly use FR4 with vias for Luxeon Rebels and I used to use it for Cree XRE. I never use it for Cree XPE/XPG. Rebels and the XRE have a large thermal path that I can take advantage on on an FR4 board. On the XPE/XPG, there is almost no way I can spread the heat out on the surface, so I need to have a low thermal resistance downwards first and foremost.

    "I am using what has started to become an industry standard for LED construction." ... and what would that be? Since last time I checked, LED on FR4, metal core, and direct thermal connection to the heat sink are all regularly practiced though you run into more patent issues with direct thermal contact unfortunately.

    How do I know what those meters do? I asked! No no yesterday, but when I was looking at meters. They do not adjust for spectrum or color temperature. They simply adjust the gain (in s/w) based on a calibration done with standard sources .... which is great when you are measuring standard sources but no so great when you are doing other things.

    You are contradicting yourself. You claim to be trying to achieve "the best possible". So why even discuss circuit boards? Best is direct solder and/or ultra thin high thermal conductivity glue direct to copper or even better a graphite/copper hybrid.

    Yes seriously! Showing experimental results without understanding what they truly mean is useless. I did work with someone who had ruled out FR4 as being no good. What was really no good was his implementation.

    Will there be an improvement between ablated ceramic and non ablated ceramic? ... of course there will be! Yes you can shave off the ceramic, hopefully you do not damage the part, hopefully you create the exact same thermal interface conditions with the non shaved and shaved version, etc. OR ... you could find out the material (or assume it is some aluminum/beryllium oxide derivative), find out the characteristics for that material, estimate the width of the thermal path, and then calculate fairly accurately how much improvement you will get thermally based on how much material is removed. That may sound complicated, but it really is not and it would not be susceptible to measurement error such as not having consistent mounting to the heat sink. The calculations will tell you it is worth doing (or not) and then you test it out. If it is not better in the test, then it is likely the test implementation, not the concept. However, if you just did tests, you could assume it was no better.

    My glass is niether half full nor half empty.

    I have worked with many good "feel" engineers and they are great when things are simple, easily understood and when you were not truly pushing the boundaries. However, their lack of proper methodology ususually was their (and ultimately the companies) downfall when things got complex and/or difficult.

    Trust your experience ... I do too. But I am not arrogant enough to think that I do not make mistakes and/or I can't find better ways of doing things .... just like my GPS tells me often of routes I had not considered .... yet my experience sometimes knows that traffic will make certain of those routes unviable.

    What was that assinine comment about the K2? In fact, older gen LEDs would benefit more from proper heat sinking that many new ones. Older LEDs generally had lower max Tjunction and they had worse output response w.r.t. temperature. The real change with new LEDs is that they are on small packages not much bigger than the die itself and hence you no longer have what is essentially a copper heat spreader. So yes now, more than ever it is important to get the LED connected to something that can spread the heat out as quickly as possible since your contact surface area is so small.

  5. #35

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by Techjunkie View Post
    Regarding the thinness of the 20mm copper star, the star's mass is less important than how fast a conduit it is to the heatsink beneath it, right? My expectation is that eliminating the thermally insulating laminate layer between the LED's center pad and the star, by soldering (the center pad only) directly to the star should make for less thermal resistance between the LED and the heatsink than any other 20mm star where laminate sits between it and all of the emitter.
    Techjunkie,

    I agree, the thickness of the copper is pretty meaningless for what you are doing. It's all about reducing the thermal resistance between the LED and primary heatsink and the air. \

    That said, I think that SAA's complaint was about the copper circuit layer. Thicker surface copper will spread the heat more sideways so there is less resistance downwards into the metal core. For some LEDS, that can be highly advantageous, for some it does not make as much of a difference.

    YES ... let's do some calculations!

    I don't know the material, but I will assume it is relatively good, 0.5 C/cm2/watt.

    Assume Cree XPG/XPE which has a thermal pad 4.4mm2. I am going to assume some reasonable heat spreading and say the effective heat path is 8mm2. If I knew the copper thickness I could make a better calculation.

    So the thermal resistance is going to be 0.5 * 100/8 = 6C / watt. I would say that is best case and it could be as bad as 0.8 * 100 / 6 = 13C / watt. Most likely it is in the middle. If you are pumping 5 watts into LED, a 30-50C decrease in LED temp may be possible.

  6. #36

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by Techjunkie View Post
    Not familiar with the term "prepreg"... are you referring to the laminate layer? Is it different than the layer you peeled up from the cutter board in the pic above? If I were to attempt creating a window through it for the thermal pad, my first approach would be something abrasive, like a stone bit on my dremel. That will surely wouldn't create a perfectly flat surface, but solder should replace whatever is scraped away without creating a void between the emitter's center pad and the star. It can only be an improvement over the laminate, thermally.

    Regarding the thinness of the 20mm copper star, the star's mass is less important than how fast a conduit it is to the heatsink beneath it, right? My expectation is that eliminating the thermally insulating laminate layer between the LED's center pad and the star, by soldering (the center pad only) directly to the star should make for less thermal resistance between the LED and the heatsink than any other 20mm star where laminate sits between it and all of the emitter.
    I actually took a little closer look at it and scraped the top away and it appears to be some sort of ceramic dielectric. Alumina probably but I couldn't tell you what exactly. It should be far better than the Cutter boards at any rate.

    You are correct that mass has nothing to do with it. A heatpipe has very little mass but can transfer heat exceedingly efficiently. I say go right ahead and try your idea. If you can do it right and not gouge the underlying copper you will definitely see improved performance. I just don't think it will be very easy.

  7. #37
    Enlightened vaska's Avatar
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    Default Re: Why a good thermal path really matters

    It's a pity that a thread, where a discussion on different aluminium PC boards took place, is lost. There I wrote about what level of thermal conductivity one can achieve if he bothers to make PCB design of his own. Here in Moscow local manufacturers offer different materials for PCB design, and the best combination available is like this: Al1100 alloy with 222 W/(m*K) thermal coductivity, 0,05 mm thin insulation layer with 1,8 W/(m*K) conductivity and copper layer 0,07 mm thin. Thickness of copper layer matters when you are trying to enlarge effective area for heat dissipation through insulation layer which has the least thermal conductivity among named materials. This picture illustrates difference between poor (left) and competent (right) design.
    I'm too lazy to make calculations again, but I remember that total conductivity estimation was near 2 W/K for XM-L on 2 mm thick PCB 22 mm in diameter. It could be much less if I ordered PCBs with 0,140 mm or even 0,35 mm copper layer, but there were some technological difficulties.
    So, looking at Lux-rc's attempts of optimising thermal qualities of PCB, I thought that they are not worth charges.
    Last edited by vaska; 04-02-2011 at 02:03 AM.

  8. #38

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by SemiMan View Post
    Laird Theramgon Tlam SS 1KA04 or DS 1KA06
    http://www.lairdtech.com/Products/Th...PCB-Materials/
    Hey nice link to some stars there.

    Quote Originally Posted by SemiMan View Post
    That said I have seen the data sheet for the Opulent thermal prepreg (many years ago) and I remember it being very good.
    Yes I suspect they will do quite well. For an mcpcb.

    Quote Originally Posted by SemiMan View Post
    Before I started using any volume, I did my first ones the old fashioned way. I asked nicely for a bare board sample, and like any good hobbyist I rolled my own using my trusty printer and etched my own.
    I suppose it is within the realm of possibility for people here to make their own boards but it is out of the reach of most everyone here to do. Of course so is some of what's on display in this thread.


    Quote Originally Posted by SemiMan View Post
    "What works with one LED will work with another." .... not true.
    That's some rather selective quoting going on there. Why don't we add back in what came directly after it.

    " That being - decreasing the thermal resistance increases the performance of the LED. "

    But then you can't argue with that now can you.

    Quote Originally Posted by SemiMan View Post
    "I am using what has started to become an industry standard for LED construction." ... and what would that be? Since last time I checked, LED on FR4, metal core, and direct thermal connection to the heat sink are all regularly practiced though you run into more patent issues with direct thermal contact unfortunately.
    I am not referring to the boards. I am referring to the construction of the LED itself. With the LED die directly attached to ceramic unlike what was the old standard of being mounted on a chunk of copper. Standardizing on one LED for these tests is the proper and sensible thing to do. I am also using what is one of the most common LEDs seen in these parts so the results are quite relevant.

    Quote Originally Posted by SemiMan View Post
    How do I know what those meters do? I asked! No no yesterday, but when I was looking at meters. They do not adjust for spectrum or color temperature. They simply adjust the gain (in s/w) based on a calibration done with standard sources .... which is great when you are measuring standard sources but no so great when you are doing other things.
    Should be interesting to see if they tell me the same thing. Thanks for info.

    Quote Originally Posted by SemiMan View Post
    You are contradicting yourself. You claim to be trying to achieve "the best possible". So why even discuss circuit boards? Best is direct solder and/or ultra thin high thermal conductivity glue direct to copper or even better a graphite/copper hybrid.
    Multiple reasons. Stars are the most common way to mount LEDs here. So much so that some people(annoyingly so) simply refer to the whole assembly as an "LED". Just showing the results of mounting an LED on a heatpipe and calling it a day gives people no sense of where that stands in relation to other solutions. So it is therefore useless information. However testing the various solutions out there and posting the data will help people to clearly see where each solution lies relative to one another.

    Second, "the best possible" is different with each build. Different lights impose different design restraints and they may be forced to use something other than a heatpipe. If it is the case that they will be beholden to the use of mcpcbs then they need to know what is the "best possible" for that situation.

    Quote Originally Posted by SemiMan View Post
    Yes seriously! Showing experimental results without understanding what they truly mean is useless. I did work with someone who had ruled out FR4 as being no good. What was really no good was his implementation.
    You can call my tests useless but that is one opinion. One I do not share.


    Quote Originally Posted by SemiMan View Post
    Will there be an improvement between ablated ceramic and non ablated ceramic? ... of course there will be! Yes you can shave off the ceramic, hopefully you do not damage the part, hopefully you create the exact same thermal interface conditions with the non shaved and shaved version, etc. OR ... you could find out the material (or assume it is some aluminum/beryllium oxide derivative), find out the characteristics for that material, estimate the width of the thermal path, and then calculate fairly accurately how much improvement you will get thermally based on how much material is removed.
    OR you could just do it and find out. You have your way and I have mine.

    Quote Originally Posted by SemiMan View Post
    That may sound complicated, but it really is not and it would not be susceptible to measurement error such as not having consistent mounting to the heat sink. The calculations will tell you it is worth doing (or not) and then you test it out. If it is not better in the test, then it is likely the test implementation, not the concept. However, if you just did tests, you could assume it was no better.
    Well I say it is more complicated than just testing it in real life. If the other works for you great. It will not work for me and frankly I don't want to go that route. Actual testing is far far more interesting. I enjoy that side of the hobby at least as much as I do the lights themselves.

    I would agree with you if we were designing flashlights for a virtual world but we aren't. You say calculating first would be superior because you remove the possible error or deviation that can occur in real life application but ignore that the world we live in and the products we use are based on actual implementation where there can and are variances in our results. Therefore the best way to know real world results is to make real world tests.

    I get what your saying about the fact that calculations can show potentially bad implementation but I really think you are presuming that I don't know proper implementation. You'd be wrong. I also think you are severely over thinking our hobby. Again, on an industrial level dealing with very complex components sure. Then I would agree with everything you are saying. Not here however.

    Quote Originally Posted by SemiMan View Post
    My glass is niether half full nor half empty.

    I have worked with many good "feel" engineers and they are great when things are simple, easily understood and when you were not truly pushing the boundaries. However, their lack of proper methodology ususually was their (and ultimately the companies) downfall when things got complex and/or difficult.
    So how many were flashlight designers?

    Quote Originally Posted by SemiMan View Post
    Trust your experience ... I do too. But I am not arrogant enough to think that I do not make mistakes and/or I can't find better ways of doing things .... just like my GPS tells me often of routes I had not considered .... yet my experience sometimes knows that traffic will make certain of those routes unviable.
    I have never claimed infallibility. So thank you so much for opening my eyes to the wonderful world of theoretical calculations. Since you are not arrogant then you will realize there is often more than one way to skin a cat.

    Quote Originally Posted by SemiMan View Post
    What was that assinine comment about the K2? In fact, older gen LEDs would benefit more from proper heat sinking that many new ones. Older LEDs generally had lower max Tjunction and they had worse output response w.r.t. temperature. The real change with new LEDs is that they are on small packages not much bigger than the die itself and hence you no longer have what is essentially a copper heat spreader. So yes now, more than ever it is important to get the LED connected to something that can spread the heat out as quickly as possible since your contact surface area is so small.
    That was in response to your saying that what LED you use makes a difference. As if my use of a K2 for instance would result in entirely different results. It was just used as an example. Sure the results would be different but the underlying principle is always the same. The cooler the LED the better it performs.

  9. #39

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by vaska View Post
    It's a pity that a thread, where a discussion on different aluminium PC boards took place, is lost. There I wrote about what level of thermal conductivity one can achieve if he bothers to make PCB design of his own. Here in Moscow local manufacturers offer different materials for PCB design, and the best combination available is like this: Al1100 alloy with 222 W/(m*K) thermal coductivity, 0,05 mm thin insulation layer with 1,8 W/(m*K) conductivity and copper layer 0,07 mm thin. Thickness of copper layer matters when you are trying to enlarge effective area for heat dissipation through insulation layer which has the least thermal conductivity among named materials. This picture illustrates difference between poor (left) and competent (right) design.
    I'm too lasy to make calculations again, but I remember that total conductivity estimation was near 2 W/K for XM-L on 2 mm thick PCB 22 mm in diameter. It could be much less if I ordered PCBs with 0,140 mm or even 0,35 mm copper layer, but there were some technological difficulties.
    So, looking at Lux-rc's attempts of optimising thermal qualities of PCB, I thought that they are not worth charges.
    Do you have a link to where these fancy boards could be bought? I'd love to test them.

    If Lux-rc is putting that chunk of solid metal in salable product with a direct attach point then I have a lot of respect for them.

  10. #40
    Enlightened vaska's Avatar
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    Default Re: Why a good thermal path really matters

    Quote Originally Posted by saabluster View Post
    Do you have a link to where these fancy boards could be bought? I'd love to test them.
    As far as I know, it's not finished yet: in plans is thin PCB with holes for chunks stuck to the top of the "pill".

    Quote Originally Posted by saabluster View Post
    If Lux-rc is putting that chunk of solid metal in salable product with a direct attach point then I have a lot of respect for them.
    He's going to. His name is Serge and he is a good engineer, very good technologist and a nice guy who'll surely give you his patterns for experiments free of charge
    His site

  11. #41

    Default Re: Why a good thermal path really matters

    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

  12. #42

    Default Re: Why a good thermal path really matters

    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.

  13. #43
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    Default Re: Why a good thermal path really matters

    Quote Originally Posted by SemiMan View Post
    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.

  14. #44

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by Curt R View Post
    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.

  15. #45
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    Default Re: Why a good thermal path really matters

    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.
    Last edited by ma_sha1; 04-04-2011 at 01:13 PM.
    My Mods.. http://www.candlepowerforums.com/vb/...5&postcount=78
    Hobby only, I don't do custom mods as a service, thanks for understanding.

  16. #46

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by ma_sha1 View Post
    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.

  17. #47
    Flashaholic* ma_sha1's Avatar
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    Default Re: Why a good thermal path really matters

    Quote Originally Posted by saabluster View Post
    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.
    Last edited by ma_sha1; 04-04-2011 at 03:03 PM.
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    Hobby only, I don't do custom mods as a service, thanks for understanding.

  18. #48
    Flashaholic*
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    Default Re: Why a good thermal path really matters

    The best way for me is a oven like this
    http://image.made-in-china.com/2f0j0...01H-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!

  19. #49
    Flashaholic* Techjunkie's Avatar
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    Default Re: Why a good thermal path really matters

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

  20. #50

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by ma_sha1 View Post
    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.
    Last edited by saabluster; 04-05-2011 at 01:07 AM. Reason: spelling..grr

  21. #51
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    Default Re: Why a good thermal path really matters

    Quote Originally Posted by saabluster View Post
    . . . 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.

  22. #52

    Default Re: Why a good thermal path really matters

    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

  23. #53
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    Default Re: Why a good thermal path really matters

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

  24. #54
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    Default Re: Why a good thermal path really matters

    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.
    Last edited by Neondiod; 04-07-2011 at 02:39 AM.

  25. #55
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    Default Re: Why a good thermal path really matters

    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!

  26. #56

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by bshanahan14rulz View Post
    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.

  27. #57

    Default Re: Why a good thermal path really matters

    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

  28. #58

    Default Re: Why a good thermal path really matters

    Quote Originally Posted by beerwax View Post
    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.

  29. #59
    Flashaholic* ma_sha1's Avatar
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    Default Re: Why a good thermal path really matters

    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.
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    Hobby only, I don't do custom mods as a service, thanks for understanding.

  30. #60

    Default Re: Why a good thermal path really matters

    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:
    Code:
                    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/Steinel-34890-.../dp/B002NKM1MK 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.
    Last edited by CKOD; 04-27-2011 at 10:21 AM.

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