# MCPCB



## frenzee (Jan 11, 2008)

I received a large sheet of 1/16" copper-core Bergquist T-Clad MCPCB and am going to have a few round stars and Rebel protype pads precision cut with abrasive waterjet tomorrow. If anyone wants a sample cut, feel free to send me your pattern and I will incorporate it in my Autocad file. Sorry for the short notice, but I just found a place to get this done cheaply today and want to get it done before the guy changes his mind.

Mods: Feel free to move my post if you think this is the wrong place.


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## Low_Rider (Jan 12, 2008)

PM Sent!


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## LEDite (Jan 12, 2008)

frenzee;

PM sent.

Copper MCPCB is a great idea. 

Heat dissipation is a big factor in multi-LED design.

They would be great to show at our DFW LightFest on Jan. 26.

Thanks for taking the initative.

Larry


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## frenzee (Jan 12, 2008)

A copper core MCPCB is second only to IRC's Anotherm technology - in terms of K value. Anotherm basically consists of an aluminum substrate coated by a micron thick layer of Aluminum Oxide which is then coverd by a chemically deposited ("printed") gold layer which forms the trace pattern. The thermal resistance of an Anotherm heat sink is almost zero, but the problem with Anotherm, as far as us hobbyists are concerned, is that it cannot be etched using conventional etching methods. The circuit layer has to be printed at the manufacturing stage, so you can't just design a custom circuit board. Bergquist's copper core MCPCB are the second best material available, as far as my research shows, for high-density, high thermal conductivity applications and is routinely used in the aerospace industry, so I have high hopes for how this turns out. I am hoping to do a side by side comparison test between this and an aluminum core board, e.g. LEDlightingsupply's stars and a 1/64" FR4 board with thermal vias. (ref., 2, 3)

For those who responded, thank you and sorry I couldn't give more notice, but I already sent the board to the waterjet guy. If this turns out well, and if there is enough interest, I might do a group buy or passaround or something to that effect, as I will still have some stock left over.


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## VanIsleDSM (Jan 13, 2008)

If your project requires SMT to a board this looks great, but for ultimate heat extraction is it not better to just thermal epoxy the LED right to the heat sink?


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## saabluster (Jan 13, 2008)

VanIsleDSM said:


> for ultimate heat extraction is it not better to just thermal epoxy the LED right to the heat sink?


Not exactly "ultimate". With the Crees ultimate would be soldering it directly to a heat-pipe(which I plan to do soon) or to a lesser degree a chunk of copper. Thermal epoxy is just adequate.


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## frenzee (Jan 13, 2008)

Absolutely. Actualy soldering the heat pad directly to the heatsink is even better (much, much better in my experience). The problem is if you have more than one emitter, and especially if you are working with an SMT emitter with contacts on the bottom only, e.g. the Rebel, then you don't have any choice but to have some kind of a circuit board directly below the emitter. How else can you run the current?

Another problem with epoxying is that if one of your emitters goes bad (assuming you have more than one), then you have to pretty much throw the whole thing away (ever try to remove an epoxied emtter?) whereas a MCPCB can be easily repaied.



VanIsleDSM said:


> If your project requires SMT to a board this looks great, but for ultimate heat extraction is it not better to just thermal epoxy the LED right to the heat sink?


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## Oznog (Jan 13, 2008)

frenzee said:


> Absolutely. Actualy soldering the heat pad directly to the heatsink is even better (much, much better in my experience). The problem is if you have more than one emitter, and especially if you are working with an SMT emitter with contacts on the bottom only, e.g. the Rebel, then you don't have any choice but to have some kind of a circuit board directly below the emitter. How else can you run the current?



Overhang the power pads off the heatsink. Could cut a depression or hole for a PCB that the power pads hang over and solder onto. Or just drill a hole all the way through and either mount the PCB inside the hole or solder wires to the pads and drop through the hole (kinda susceptible to shorts and getting ripped out). I did this with an aluminum heatsink and was predictably having problems dipping the soldering iron tip-first into the hole to attach a wire and accidentally globbed the solder from the pad onto the aluminum wall of the hole. No prob, solder doesn't stick to aluminum, right? Well, the solder sure didn't want to melt OFF the aluminum either due to heat conductivity and the glob was also attached to the Rebel's power pad so I couldn't just knock it off the aluminum either.


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## frenzee (Jan 13, 2008)

Oznog said:


> Overhang the power pads off the heatsink...


 
I tried something like that (see below), but as you mentioned, this is not a reliable solution. I didn't have shorting problems, but even the smallest flexing of the substrate was causing the solder connections to come loose, typically right after you assemble the piece and you can't go back (very aggravating!). Bottom line, I don't think there is any susbstiture for a PCB, metal or otherwise.


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## Oznog (Jan 13, 2008)

Before you die, you see the ring...

How did you solder those? One difficulty that occurred to me was placing these tiny devices on a hot piece of copper when the solder all melts at the same time. Do you just use solder paste and rely on them to retain their initial placement? (don't bump the oven!!)

Perhaps a substrate thick enough that it does not flex?

I have been working with gluing the Rebels onto a thick piece of aluminum. Making a sink out of copper isn't going to be practical. The aluminum is surely more than enough dissipation but the C/W of the Arctic Silver interface may be a problem. Well, I switched to 6 devices for my app run at 350mA (1W) each so really we're not asking a lot of the emitter anyways. Also it's of note that for the reds I'm using the pad-to-die is 12C/W anyways, so while it's always helpful to reduce that pad-to-sink thermal resistance the pad-to-die's still going to dominate. Reducing the interface by 2C/W or 4C/W with soldering isn't going to mean it can be run all that much higher.


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## frenzee (Jan 13, 2008)

Yes, I agree with your theory. Especially with just one emitter, pad to sink T delta won't make that much of a difference, as most of the temperature gradient will be from junction to pad and within the heatsink itself, assuming you have adequate heat sinking. But for higher density applications, every little bit counts. Copper being almost twice as thermally conductive as aluminum does make a huge difference as far as the heat sink is concerned.

The way I see it, thermal management is the biggest challenge in high-brightness LED applications, but technlogy is gradually moving forward to meet these challenges, Anotherm mentioned above being but one example.

Re ring above, I had to make a jig that held all the emitters in place while I heated the ring from below with an 80W iron and while holding everything steady with a thrid hand (below). The jig basically consisted of a thin flat copper plate on the bottom, a round center plug which fit into the donut, and a weighted silicone sheet which put pressure on the emitters from the top.

I experimented with all kinds of solder including lead-free, silver-bearing and even low temp (192 F) TIX solder, but eventually settled on ordinary 60/40 (Kester brand) and a good coating of no-clean flux:





I also made a jig for soldeing this contraption. Let me know if you want see a picture of it.




My "third hand":


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## SemiMan (Jan 13, 2008)

frenzee said:


> A copper core MCPCB is second only to IRC's Anotherm technology - in terms of K value. Anotherm basically consists of an aluminum substrate coated by a micron thick layer of Aluminum Oxide which is then coverd by a chemically deposited ("printed") gold layer which forms the trace pattern. The thermal resistance of an Anotherm heat sink is almost zero, but the problem with Anotherm, as far as us hobbyists are concerned, is that it cannot be etched using conventional etching methods. The circuit layer has to be printed at the manufacturing stage, so you can't just design a custom circuit board. Bergquist's copper core MCPCB are the second best material available, as far as my research shows, for high-density, high thermal conductivity applications and is routinely used in the aerospace industry, so I have high hopes for how this turns out.



I agree that Anotherm appears to be the best thermal solution. However, the aluminum oxide layer is actually 35-40 micrometers thick, or about 1.5 thousands of an inch.

I am wondering for the rebels with their small thermal pads that I could not achieve almost as good of results with copper core board. I am looking at a competing product with higher thermal conductivity substrate. I am coupling that with 4 ounce copper. The thick copper coupled with a larger pad will essentially increase the effective area for heat transfer. I am thinking of getting my mechanical guys to run some simulations.

Semiman


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## frenzee (Jan 13, 2008)

I have been trying to get some Anotherm modules to do a side-by-side comparison, but unfortunately the company is not nearly as resoponsive or accomodating as Bergquist.

Regarding thermal resistance, I have no hard data to prove it, which is why I wanted to do this experiment in the first place, but at least in the published literature, Anotherm beats IMS (=Insulated Metal Substrate=MCPCB) by an order of magnitude (See this paper). SemiMan, the dielectric (Al2O3) layer is actually 0.0017" or 4.3 mocrons thick , but that is still much, much thinner than the Polyimide layer that most MCPCB manufacurers use and on top of that, the Alumina itself has a lower thermal resistanace than the polyimide which is basically an organic material.

IRC claims to have done a comparison test between the two technolgies here, but I would really like to confirm their results.

Yes, Rebels's heat pad is small, very small, but that is what we have to work with, and IMO, making the pad larger (a al Cree) won't make much of a difference since the marginal added area is not contributing to the heat tranasfer as much as the area directly below the die. And the distance between the die and that margial area is ceramic, which to me, could not possibly perform as well as or better than copper anyway.

Having 4 oz. copper or 1 oz. or whatever, at least in theory, should not make any material difference either. Since we are interested in the temperature gradient, adding thickness, and assuming copper's resistance is at or near zero in these small scales, should not make any difference in how well heat is conducted away. You might as well replace the added thickness with the heatsink itself. Please feel free to correct me here, but that's what comes to mind.

I really look forward to seeing you simulation results. Also, if you know of a product with higher thermal conductivity susbstrate (than copper? are you serious?:thinking: Composites/Silver/amorphous Diamond/nanotubes?), please do let us know.


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## saabluster (Jan 14, 2008)

frenzee said:


> Also, if you know of a product with higher thermal conductivity susbstrate (than copper? are you serious?:thinking: Composites/Silver/amorphous Diamond/nanotubes?), please do let us know.


Many heatpipes have thermal resistances 2,000 times lower than copper and 4,000 times lower than aluminum. Thats just a little better wouldn't you say?


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## Oznog (Jan 14, 2008)

Well, I can say this...

Running my red-orange Rebels @350mA, 3x on a big sink. Letting it run for a minute or two and the sink's not really warming yet but the devices have had time to warm up over the sink temp if they're going to, yet I really can't feel the devices being any warmer. I know the silicone coating's an insulator and this isn't a very scientific test so make what you will of it. I'm of the opinion that maybe the sink-pad thermal resistance with Arctic Silver may already be down to the insignificant level.

The way we're supposed to be able to measure this is feed a precision constant current, get an exact voltage reading from the cold die in the first instant, and read the change in voltage as the die temp rises. However, the voltage only changes -3mv/deg C. A 2mA change in input current will produce a 3mV dV too BTW so it's gotta be really well regulated.

I'm of the theory that the die heats up REAL fast, right? The thermal mass is miniscule. How fast? Tenth of a second or what? I'd need to make some hardware to ensure the constant current is stable within +/-2mA and a fast-responding voltmeter can read that voltage within +/-3mV in the first split second of turning on.

It's possible, and it'll give definitive measurements, but it'll require special equipment. Now a PIC's ADC with a 3.3V Vref will have 3mV steps, offset error isn't a prob, but noise may be. Still, I'm kinda guesstimating the thermal resistance of the pad-to-sink may be under 5 C/W. So at 1W input, that's probably around 0.8W of die heat (200mW of light output), well that's only 4C difference so a +/-1C or 2C error is way high. It makes sense to bring the current up to 700mA to make the measurement bigger. And now that I think of it the red is a bad choice, the other colors have a lower die-to-pad thermal resistance so more of the measurement should be pad-to-sink.

I saw the photometric power output tables so if we know the current and die temp, I can look up the radiated lumens at that die temp in the rebel data sheet, use the photometric table to translate lumens at that wavelength to milliwatts, use the (V * I) - radiated mW to calculate heat flow from die-to-sink, subtract the 10 C/W specified for the die-to-pad and hopefully the remainder's the pad-to-sink.


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## chris_m (Jan 14, 2008)

I did something similar to that ring with my Crees, though soldering each Cree to its own thin piece of copper, such that the electrical contacts overhang. Since there is one electrical contact either side, and less overhang, it shouldn't have so much problem with flexing - in any case the light has been going strong for over 6 months, including some fairly rough handling (rough enough that I've had to replace the cracked glass outer lens!)


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## SemiMan (Jan 14, 2008)

I checked my math (and the Anotherm data sheet to be sure), the dielectric thickness is 0.0014 inches or 35 microns nominal.

The thickness of the copper could make a significant difference on a Rebel metal core board. Here are some things to consider:

- The thermal resistance between the chip pad and the core is a function of (thermal resistance/area). Keep in mind though, just like putting resistors in parallel, you can put heat paths in parallel.

- Copper is a much much better conductor of heat than the insulating layer used in metal core boards even though that material is actually pretty good (more on that later).

- As copper is a much better conductor of heat, as opposed to heat only being transferred downwards through the copper, through the laminate and into the metal core, it will also travel along the copper on the surface, then down through the laminate into the core. The thicker that copper on the surface, the more that will happen. Effectively, this creates a greater area for heat transfer between the Rebel and the underlying metal core board. As the Rebel thermal pad is quite small, the advantage provided by this could actually be significant, but certainly more advantageous versus the larger pad of the Cree XR-E. If you look at the suggested pad layout for the Rebel, the copper area under the Rebel is much larger than just the thermal pad on the Rebel itself. The reason for this is to maximize thermal transfer to the underlying material.

When I was referring to better thermal conductivity I meant the laminate used in the construction. There are better ones than what Bergquist is using. In terms of construction, the laminate is not pure polyimide but is impregnated with thermal conductive material, i.e. aluminum oxide or boron nitride. Send me a private note and I will suggest some alternatives. 

In addition to heat pipes, there are solid graphite materials that have better conductivity than copper, however, they have a preference to what axis they conduct on. That can be good or bad depending on what you are trying to do.

Semiman




frenzee said:


> I have been trying to get some Anotherm modules to do a side-by-side comparison, but unfortunately the company is not nearly as resoponsive or accomodating as Bergquist.
> 
> Regarding thermal resistance, I have no hard data to prove it, which is why I wanted to do this experiment in the first place, but at least in the published literature, Anotherm beats IMS (=Insulated Metal Substrate=MCPCB) by an order of magnitude (See this paper). SemiMan, the dielectric (Al2O3) layer is actually 0.0017" or 4.3 mocrons thick , but that is still much, much thinner than the Polyimide layer that most MCPCB manufacurers use and on top of that, the Alumina itself has a lower thermal resistanace than the polyimide which is basically an organic material.
> 
> ...


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## SemiMan (Jan 14, 2008)

I would not say Arctic Silver thermal epoxy is perfect. It has a thermal conductivity of 7.5 W/mK. This is about twice the thermal conductivity of the best metal core board laminate and about 3 times what Bergquist uses I believe. Metal core laminate is about 75-100 microns (3-4 thousands of an inch). I think you could get an Arctic Silver layer as thin as this. 

Back to my last argument though, you get substantial conductivity along the "surface" copper layer on a metal core board which will increase the effective area conducting area through the laminate. You do not get this with thermal epoxy. I would expect in the end, a high quality soldered metal core board is going to be better most of the time though with luck you may get as good with Arctic Silver Epoxy.

Unless you have a very good light meter, the lumens measurement is likely not going to be the best way to measure I expect. You get a shift in the blue die wavelength and other spectrum effects that may come into play. If you have a spectrophotometer though, you may get extremely good results this way.

The die temperature method should work. It may be a good "device" to have available for pass-around made with high end components that are stable.

Semiman



Oznog said:


> Well, I can say this...
> 
> Running my red-orange Rebels @350mA, 3x on a big sink. Letting it run for a minute or two and the sink's not really warming yet but the devices have had time to warm up over the sink temp if they're going to, yet I really can't feel the devices being any warmer. I know the silicone coating's an insulator and this isn't a very scientific test so make what you will of it. I'm of the opinion that maybe the sink-pad thermal resistance with Arctic Silver may already be down to the insignificant level.
> 
> ...


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## frenzee (Jan 14, 2008)

SemiMan said:


> ...As copper is a much better conductor of heat, as opposed to heat only being transferred downwards through the copper, through the laminate and into the metal core, it will also travel along the copper on the surface, then down through the laminate into the core. The thicker that copper on the surface, the more that will happen. Effectively, this creates a greater area for heat transfer between the Rebel and the underlying metal core board. As the Rebel thermal pad is quite small, the advantage provided by this could actually be significant, but certainly more advantageous versus the larger pad of the Cree XR-E. If you look at the suggested pad layout for the Rebel, the copper area under the Rebel is much larger than just the thermal pad on the Rebel itself. The reason for this is to maximize thermal transfer to the underlying material.


 
Thanks for the explanations SemiMan. OK, now I see what you were saying. I thought you were talking about the substrate thickness. Yes, absolutely. Your explanation of how heat can spread sideways through the foil and then down through the dielectric and then the substrate is pefectly accurate - the thicker the foil, up to the limits of precticality, the better - and I think it is best when designing your pattern to leave all areas of the trace layer that don't interfere with other components in place (i.e. not etched). The sample I have has a 2 oz. foil and a 3 MIL (76 microns) dielectric. I did ask for 4 oz., but they didn't have any in stock. They had aluminum core with 4 oz., but I went with the copper core.

The Arctic Silver comments reminds me, a while back I tried making my own MCPCB using Arctic Silver, but the bonding strength is not high enough to hold the copper foil in place. As soon as the molten solder touches it, the trace layer comes off  I had prepped both sides really well, but it didn't make any difference. I put the assembled foil, substrate, and an ultrathin AS-coated nylon mesh in between to maintain electrical isolation under 4000 psi of pressure in a large wise until it cured, but the thinnest I was able to get the Arctic Silver/nylon mesh was about .1mm or 100microns. Perhaps someone else has had better luck.

Oznog, have you tried using a scope with a max hold and a short pulse to make your measurements?

(P.S. I need to correct myself above. 0.0017" is 43 microns, not 4.3.)


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## Oznog (Jan 14, 2008)

SemiMan said:


> I would not say Arctic Silver thermal epoxy is perfect. It has a thermal conductivity of 7.5 W/mK. This is about twice the thermal conductivity of the best metal core board laminate and about 3 times what Bergquist uses I believe. Metal core laminate is about 75-100 microns (3-4 thousands of an inch). I think you could get an Arctic Silver layer as thin as this.



Yeah the MCPCB is actually looking bad relative to the Arctic Silver onto aluminum. I looked it up. The most thermally conductive dielectric used in MPCB is 2.2W/mK, 3.4x worse than AS. You can get 75um or 150um dielectric board, let's assume 75um. 
The thickness of Arctic Silver is harder to predict. However, in their thermal design guides Luxeon mentions 70um as being typical for "adhesive". AS literature says that for LARGE processors, the thickness may need to be 100um-200um due to the convex/concave deviation from flatness, and that smaller stuff is less. Rebel is MUCH smaller, but it is also difficult to clamp evenly (much less clamping force is required to get the same PSI on a small bottom).

So far the dielectric thickness seems comparable but the AS is 3.4x more conductive that the MPCB. But the MPCB has the top foil layer acting as a heat spreader to use more of the dielectric to conduct heat.

How significant? Well, lemme see if I can get a picture of this. Say we have the thickest foil show, 350um. That's brutally thick 10oz stuff! Copper is 401W/meter*C. So let's look at the thermal resistance to say a line 1mm outside the perimeter of the Rebel's pad. The Rebel's got 9.1mm of perimeter (and the corner area is a mess to calculate so I'm gonna neglect it for now!). I can now model the foil as a copper block 9.1mm x 0.350mm x 1mm, and I'm trying to get heat from one 9.1mm x 0.350mm face to the other. Textbook problem!

Well if I calc'ed this right I get 0.7829C/W to transport heat to the perimeter 1mm away (not counting corners) for 350um foil. Yes, this spreading effect sounds quite significant! But for the more common 1oz 35um foil, it's 7.829C/W which throws the whole heat spreading effect into a much less significant role. So the foil thickness IS critical.

Note the heat spreading effect's significance is proportionally higher with small devices like the Rebel, and less significant with larger devices because the ratio of perimeter area over the straight-down thermal path area is less with large devices. 

Both the MPCB and aluminum block will dissipate heat out of the top surface. However, the MPCB has some consequences of that thermal spreading calculation. The area far away from the LED pad's perimeter, more than a few mm even for 10oz foil, starts becoming too high a thermal resistance along this path to be significant. There IS a second thermal path though, where the copper or aluminum board underneath received heat from that low thermal resistance sweet spot right under and slightly to the side of the LED pad and then has to deal with the thermal resistance of the the dielectric to get heat from the BACK of the board out the front. A solid aluminum block though has great sink-to-air coefficients on the front AND back surfaces.

I'm trying to give a fair case either way, but it's looking like Arctic Silver on the Rebel on an aluminum block may still win out (remember, 3.4x better conductivity). That's considering of course you can even build it right; the whole thing about the MPCB is being able to provide the PCB board connections. Most mfgs would far rather work with MPCB than mill out tiny aluminum blocks to the desired shape and wait for the epoxy to dry on each one!


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## Oznog (Jan 14, 2008)

Oh hey what am I doin. If I'm willing to accept the estimate of 70um for adhesive thickness of a small device, I can calculate thermal resistance with 7.5W/mK Arctic silver.

And the pad-to-sink resistance is....

1.88C/W! Yay! It's... a number! Backed up by other numbers!

What might be worth mentioning is that for extremely small pads the "normal" C/W of an aluminum heatsink can be greater than predicted for that sink because the limited heat spreading around the pad itself is a factor.


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## frenzee (Jan 14, 2008)

There is a fun-to-read article here that talks about thermal conductivity and selecting the right interface material. I keep coming back to it from time to time as I find it very educational. Here is a quote:

_"Should you go with the alumina-filled, the silver-filled, or the diamond-filled epoxy between the component and the cold plate? The temptation is to pick the one with the highest value of thermal conductivity (k)...._

_But conduction through the epoxy layer may be the smallest thermal resistance in the joint. There are two other thermal resistances that can't be ignored, (even though you can't evaluate them from the epoxy vendor's data sheet): the contact resistance between the component and the epoxy, and the contact resistance between the cold plate and the epoxy..."_

BTW, I think the author means to say highest, not "smallest".

PCB manufacturers take great pains in prepping the mating surfaces of the foil and the substrate (by electro-chemical etching) to create microscopic peaks, valleys and pores and they cure the whole assemble under immense pressure and heat to eliminate gaps and to create an efficient thermal path. I just don't think that that mechanism can be re-created by the average hobbyist. Sorry Oznog, but I just think there might be more to this enigma than the K value alone.


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## SemiMan (Jan 15, 2008)

Good analysis Oznog. I must say this has been one of the more enjoyable threads of late.....for me at least.

I think you are on to something in your post below. Big copper heat sinks are just about impossible to find (and expensive). Perhaps bonding a thick layer of copper to the surface of the aluminum heatsink would solve this issue?

I would not worry about your analysis losing the corners. Realistically you are going to lose space at the electrical connections anyway. However, what does come into play is you get increasing cross-section as you move outwards which tends to increase the effective area for transfer downwards. If you are using a copper backing on metal core, you also have the advantage over localized heating of the aluminum heat sink....well essentially a small thermal path in the heat sink.

Frenzee, I agree with your statements w.r.t. not being able to apply pressure easily during curing to remove voids and to ensure good thermal contact with the surfaces. With Luxes and K2s, I move them a bit and then hold them down with a "jig" .... ok it is a block of maple with appropriate sized holes in it that I use to keep pressure on the LEDs.... keep in mind this is for prototyping only. I tried plastic, but they slip too much. I generally run my LEDS at the max current to speed up the curing. For Rebels, I have only soldered onto MCPCB and for XRE I have done both.

Perhaps we should have a "sticky" thread on do-it-yourself manufacturing? .... it is a popular topic.





Oznog said:


> Oh hey what am I doin. If I'm willing to accept the estimate of 70um for adhesive thickness of a small device, I can calculate thermal resistance with 7.5W/mK Arctic silver.
> 
> And the pad-to-sink resistance is....
> 
> ...


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## SemiMan (Jan 15, 2008)

Accidental double post..


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## Oznog (Jan 15, 2008)

SemiMan said:


> I think you are on to something in your post below. Big copper heat sinks are just about impossible to find (and expensive). Perhaps bonding a thick layer of copper to the surface of the aluminum heatsink would solve this issue?
> 
> I would not worry about your analysis losing the corners. Realistically you are going to lose space at the electrical connections anyway.



The problem remains how to bond copper to an aluminum surface. If Arctic silver is used, then it won't survive soldering temps if you attach the LED later. Also soldering to a heatsink may not be healthy for the LED. By its nature the sink's not going to allow you to heat up just the LED pad area, the whole heat sink must be brought up to temp which is probably a much slower ramp up/cooldown period than prescribed for reflow. How serious a prob this is I have no idea, depends on the emitter. I could be wrong too, given the conductivity and surface area, a sink will heat up and cool down pretty fast in a oven, right?

Soldering aluminum is a pretty exotic process- there are some products for aluminum-to-aluminum but fewer for copper-to-aluminum. And of course it is essential to avoid the bringing the sink up the interface's melting temp during the reflow for the LEDs.

You could always solder to copper first, then thermal epoxy the copper to the aluminum, which seems to make sense.


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## SemiMan (Jan 15, 2008)

The Anotherm product is aluminum oxide with a thin silver layer on an aluminum heat sink. It is intended to be soldered on directly. I have not looked at it since the original Lumileds which were not reflow based, but I know you could solder power semiconductors and the reflow profile is similar. I would imagine it would cool relatively quickly. It may take some playing around with the reflow profile, but I imagine it can be done. 

Semiman




Oznog said:


> The problem remains how to bond copper to an aluminum surface. If Arctic silver is used, then it won't survive soldering temps if you attach the LED later. Also soldering to a heatsink may not be healthy for the LED. By its nature the sink's not going to allow you to heat up just the LED pad area, the whole heat sink must be brought up to temp which is probably a much slower ramp up/cooldown period than prescribed for reflow. How serious a prob this is I have no idea, depends on the emitter. I could be wrong too, given the conductivity and surface area, a sink will heat up and cool down pretty fast in a oven, right?
> 
> Soldering aluminum is a pretty exotic process- there are some products for aluminum-to-aluminum but fewer for copper-to-aluminum. And of course it is essential to avoid the bringing the sink up the interface's melting temp during the reflow for the LEDs.
> 
> You could always solder to copper first, then thermal epoxy the copper to the aluminum, which seems to make sense.


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## frenzee (Jan 17, 2008)

Ok, I just got these back. Quality is not quite what I was expexcting (3 MIL tolerance). Edges need to be de-burred and chamfered to eliminate possiblity of shorts. I am guessing the cutter used too wide a nozzle. Here is what they look like:

Original stock:




Pieces cut:



As you can see, some parts didn't make it. The water pressure peeled some of the foil layer edges right off. I guess now I know which side should be facing up.

12mm vs. 21.5mm stars:




Cross section:




Magnified cross section:


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## VanIsleDSM (Mar 7, 2008)

Well, I think the straight thermal epoxy is good for CREEs.. but with the Rebels I think MCPCB is probably better.. I tried scraping off the top enamel layer and soldering on to the top of the rebels and then thermal epoxying them right to the heatsink.. (after I dremeled off the contacts on the bottom) but it's too delicate that way, I ended up pulling up one of the top traces from moving around the wire.. had to epoxy down the wires too so there's zero movement or stress on the solder joint.. even then, I don't think it'd last too long that way unless it was totally fixed.

So.. how big of a sheet did you get frenzee, and how much was it?

You can just etch this stuff like normal PCB boards?


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## frenzee (Mar 8, 2008)

VanIsleDSM said:


> ...So.. how big of a sheet did you get frenzee, and how much was it?
> 
> You can just etch this stuff like normal PCB boards?


 
I have a couple square feet left. Yes, you can etch it just like a nornal PCB, however you do have to cover the backing from being attacked by the etchant. I used conformal coating, but nailpolish or a dip in wax should also work. The performance is really excellent, but I don't have any numbers to post yet. Hopefully after the new batch of 100's comes out, hopefully in a couple of weeks, I will try to run some tests against 1/64" FR4 with vias. The toughest part of working with MCPCBs is cutting it. Copper is an amazingly difficult material to cut or machine. The only way I figure it can be done cleanly is either waterjet or die punch.

In my experience, there is just no comparison between epoxying and soldering directly to the heatsink, both performance-wise and ease of attachment and troubleshooting, and monkeying around with the contacts on the bottom or top is just too risky in my opinion. I mean Rebels are tempermental creatures under best of circumstances. Why add another layer of complexity?


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## VanIsleDSM (Mar 8, 2008)

Do you mind telling me how much you paid for the sheet originally, and how large it was?


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## frenzee (Mar 8, 2008)

Not at all. I got it as a prototyping sample, thanks to Bergquist's generosity (They did ask a lot of questons though). This stock is about 2 feet by 1.5 feet (see pic above). I think that is the standard size.


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## VanIsleDSM (Mar 19, 2008)

I'm undergoing question bombardment right now. I hope their generosity extends to me aswell 

Which dielectric did you go with? LTI, HT, or MP?

I picked Low Thermal Impedance, I imagine you did too.. under the selection menu there was "heavy copper" aswell, but that didn't seem to fit with the other dielectrics? I was confused by this.. I just picked the LTI and mentioned that I was interested in a copper base material.


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## frenzee (Mar 19, 2008)

I asked for HT dielectric and the heaviest copper they had which I think is 4oz. I got HT and .060 copper, but that was not exactly by choice. They just sent me what they had, which was close enough for what I needed. Obviously you want the heaviest copper trace you can get as this will will contribute significantly to the heat distribution "before" it passes through the dielectric layer in a high-component-density application.

The salesrep gendteman I dealt with was extremely professional and technically savvy, however I got the distinct impression that if he had the slightest impressions that you did not know what you are talking about or that you don't have experience in this field, that he would brush you off in a heartbeat. Keep in mind they are used to dealing with R&D engineers, not hobbyists.

If you are really interested in this stuff, I can send you one (or a few) samples, but I would have to have somebody cut it for you. I wish you had contacted me before I went to the waterjet guy. Trust me, you can't cut this suff with a hacksaw (cleanly, that is). it's either waterjet or plasma (which is much more expensive). A CO2 Laser won't cut copper and CNC would be very tricky and difficult wihtout breaking or fowling the bit. Most places I called had a $250-500 minimum setup fee for a watrjet pattern. I got to know a guy who will do a waterjet cut at a more reasonable cost for small porjects. He's in SoCal as am I. If you're interested let me know..


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## VanIsleDSM (Mar 22, 2008)

I'm going to try and cut it myself, either by milling it on my lathe, or creating steel punches and using a press. This is actually berquist's recommended way (auto machine punching of course, not my homemade truck jack press) It saves the most material.. and I can't imaging it being too rough.

Aluminum loves to foul stuff up.. but a 2 flute end mill is pretty hard to foul, though I've never tried on copper.

I'll give both methods I shot I suppose..


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