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Old 03-21-2012, 01:47 AM #1
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Default I want copper copper copper! (well... maybe not)

I have a running theory that there is a major misunderstanding of what we're actually doing when we create "heatsinks" for our builds. There's no doubt that the common wisdom is that copper is the ideal choice (within the realm of affordable metals) for heatsinks that need to dissipate heat away from sensitive electronics - things like CPUs, GPUs, RAM chips, etc. I agree with this common wisdom, but I propose that it does not translate into copper being a better choice for the majority the host-heatsinks that we use in laser builds. Even ignoring price entirely, I'm going to suggest that steel, iron, or nickel would be better options. Think I'm crazy?

There are two primary functions for the heatsinks we use inside our builds.
Function (A) is to transfer heat from the diode to the host walls, where the host can then dissipate the heat off to the air.
Function (B) is to absorb a whole lot of heat from the diode during your duty cycle.
For the vast majority of hosts we build in, Function (B) is more of a priority than Function (A). While transfering diode heat to the outside air is important, the amount of heat you can actually dissipate this way is limited:
  • Many of our hosts are themselves made of metals that are HORRIBLE conductors like steel. So once the heat reaches the edge of your heatsink, it hits a major bottle neck.
  • Even when a host is aluminum (and thus able to conduct fairly well), there's generally not that much surface area for it to use in transferring the heat into air.
  • The hosts themselves are generally fairly thin, and don't add substantially to the heat absorbing mass of your build. It's not rare for a heatsink to itself weigh more than your host!
In other words, given a choice between a heatsink that can conduct the heat to the host walls more quickly, and a heatsink that can absorb a whole lot more heat, in most builds you would prioritize the heatsink that could absorb more heat. Exceptions might be hosts made of highly finned copper (or maybe really really highly finned aluminum), where you realistically COULD actually dissipate lots of heat from host walls to outside air rapidly. Here's why it is ridiculous that people will create TINY builds, and then prioritize using "copper! copper! copper!" for the heatsink:

Thermal conductivity is a completely different property from a metal's specific heat capacity!

Sometimes metals will do really well or really poorly at both, but the two properties are not necessarily linked. While copper is known to a fantastic choice for finned heatsinks (for CPUs, etc), that knowledge does not automatically translate into our application, where what we really need is the ability to RETAIN or ABSORB a whole lot of heat!
  • Copper has a relatively crappy specific heat capacity, at 0.39 kJ/kg K
  • Aluminum actually does much better, at 0.91 kJ/kg K

Now, what makes up for copper's poor specific heat capacity, is that it is a really dense metal. So adjusted for volume, copper still performs about half again as well as aluminum, but it's nothing like the 2x thermal conductivity advantage that many people point to.

Here's the real kicker though. Neither copper nor aluminum are the ideal choices (even within what is reasonably affordable) for heatsinks where specific heat capacity is the real issue. I selected a handful of reasonably attainable metals (and a few that I was just curious about) and I created a chart. I researched their specific heat capacity per kg, and then adjusted based on their typical density to get their specific heat capacity per unit of volume instead of mass.



I've highlighted, in blue, the clear winners. Not copper, not aluminum. Steel, iron, and nickel. Long story short, we're doing this backwards. We should be using copper for all of our Aixiz modules, and we should be using steel, iron, or nickel for our heatsinks. The exception, being in builds where you have a host made of material that conducts really well (basically aluminum or copper) and has some sort of substantial fin structure to increase the surface area with the air. Short of that, we should be waging war on "heat absorption" rather than "heat transfer".

Spending money on Aixiz's copper 3.8mm, 5.6mm and 9mm diode modules (made by forum vet PontiacG5) to carry the heat TO your heatsink makes a whole lot more sense than trying to make the heatsink itself out of an expensive metal that can hold less heat than steel


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Old 03-21-2012, 01:51 AM #2
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Default Re: I want copper copper copper! (well... maybe not)

Wow, very interesting. +rep if I can.

what about stainless steel? is that just as good as steel?

Nickel is expensive. http://www.mcmaster.com/#nickel-rods/=gr31ew

Iron is much cheaper, but IIRC, it rusts, and is quite brittle http://www.mcmaster.com/#iron/=gr32b6

Steel comes in different types, but seems more expensive on average then iron. It also rusts. http://www.mcmaster.com/#steel/=gr333s

Stainless doesn't appear to be hugely more expensive then steel, and doesn't rust. I would be curious about the specific heat of it. http://www.mcmaster.com/#stainless-steel/=gr344l

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Old 03-21-2012, 01:54 AM #3
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Default Re: I want copper copper copper! (well... maybe not)

Great explanation. I have tried to explain this to people with my limited knowledge many times but you put it much better than I.

I like to think of a heatsink like a glass of water. If the heatsink is insulated inside the host it will fill up with the water(heat) just as quick if it is copper or aluminum unless it has somewhere to go.
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Old 03-21-2012, 02:19 AM #4
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Default Re: I want copper copper copper! (well... maybe not)

But what about the ability of the material to transfer heat FROM the module TO the heatsink? I think thermal conductivity works both ways - if you have a copper module but a steel heatsink, then you're going to have a harder time conducting heat from the module to the heatsink, even if the heatsink can store more heat. That's why I like to use copper (and for the weight!). Copper guarantees a maximal heat transfer TO the heatsink so it can absorb it as well as having a higher heat capacity per cubic meter than aluminum.

But, I am not sure: is net thermal resistance the sum of the two interfacing materials? Or is it the highest thermal resistance? If the former, then copper still makes sense - it reduces the junction bottleneck (due to its high thermal conductivity). However, if the latter, then it makes more sense to just use something with a thermal resistance slightly lower than that of the chrome and the highest specific heat that still has a higher thermal conductivity than chrome (because the Aixiz modules are chrome plated).
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Old 03-21-2012, 02:29 AM #5
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Default Re: I want copper copper copper! (well... maybe not)

I haven't read in depth yet, but doesn't the fact that copper has a great deal more mass by volume come into play? We are able to get way, way more kilos of copper in the same limited host space as Aluminum.
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Old 03-21-2012, 02:53 AM #6
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Default Re: I want copper copper copper! (well... maybe not)

Quote:
Originally Posted by tsteele93 View Post
I haven't read in depth yet, but doesn't the fact that copper has a great deal more mass by volume come into play? We are able to get way, way more kilos of copper in the same limited host space as Aluminum.
That's the adjustment for density that RHD did, it is taken in to account.

Thanks for posting this, RHD, I've been thinking it for a while. However, it should be emphasised again that it all really depends on how the host performs as either a heatsink or radiator.

For hosts which are an excellent sink, like the Phobos or MagicStick, a copper "heatsink" would do better than most other metals (except silver) because that host is able to both sink and radiate a lot more heat than a typical flashlight host. Since the entire host body is solid aluminium of considerable mass and surface area, it is able to both absorb a lot of heat and radiate it well.

For most other common hosts the bottle neck happens between the heatsink and the host, as RHD said. Very little of the heat that the heatsink absorbs gets transfered to the host, and thus, very little gets radiated. In this case a copper module + a nickel heatsink would be best.

This all just explains and highlights another facet of the "choosing a host" quandry.

It should also be noted that when your sink capability exceeds your radiative capability you have to extend the OFF time of your duty cycle. For a set up with exceptional radiative power but poor sinking you might only have a run time of 30seconds before the host gets very warm. It only has to stay off for maybe 30seconds though before it is cool again. If you have a copper module + nickel heatsink + poorly radiative host setup you might be able to have an ON time of 180seconds, but you'll need and OFF time that fits with how much heat energy is stored in your setup, which would be quite large. The result, probably 3min ON / 8min OFF. (just estimates for values)


Oh, and thanks for the giggle about including Berylium and Sodium on the table, RHD! I pictured someone turning some sodium... hahah!

(Beryllium is very low density, but an exceptional inhalation hazard. Minute amounts of dust or powder of beryllium can cause severe permanent disease.)
(Sodium is so soft you can carve it with a dull butterknife, and it is exceptionally reactive, usually exploding when in contact with water. It also forms explosive oxides when exposed to air.)

You should look in to Inconel alloy. I can't remember it's conductivity or specific heat off hand but it is a nickel alloy which is very interesting to work with. I've had the pleasure of welding it a couple times.
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Old 03-21-2012, 03:00 AM #7
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Default Re: I want copper copper copper! (well... maybe not)

I'm still hedging my bets on this, I've got some reading to do... I will say in my defense to RHD (and in complete agreement with him here) that I have been buying copper diode holders or modules where possible because of precisely what he is saying. I've never understood using pot metal or brass to mount the diode and then using a good metal for the sink.

It absolutely makes sense to mont the diode to a copper module or "holder" or at least aluminum.

In my mind, most of our sinking is dumping heat into the metal and not so much into the air. Maybe if you had a finned head on your host...

I'm going to go read up on heat sinks, I still say copper is better for our needs but I can't prove it yet.
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Old 03-21-2012, 03:06 AM #8
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Default Re: I want copper copper copper! (well... maybe not)

I guess it depends right? We can think of the inner copper housing as a heat sink that transfers the heat to the greater mass of material outside (whether air, metal, water, etc.). Heat sinks on computers and such work because they can ultimately transfer their heat to a mass with "infinite" heat capacity, the atmosphere. In a heat sink for a laser, maybe there isn't enough air movement to make that viable, so we expect the metal itself to absorb it and hope the surface area of the module is enough to prevent heat build-up.

Ultimately, I think copper still wins out as a metal of choice, given a choice. Even in the table, its volumetric SHC is still right up there with the highlighted materials, and it has some of the best thermal conductivity to boot. Where the copper loses out on is price. So while you could make a giant heatsink out of aluminum, or steel, or some cheap material, it would be very costly for copper.

The best would be to make a copper heat spreader that transfers the heat to a reservoir of water. Water has some of the highest SHC of all common materials. Water's thermal conductivity may not be so high, but if you want to store heat, use that.

Finally, the very best is to use something with a high thermal conductivity, and active heat removal using mechanical means. Without heat actually being removed somehow, any material will eventually build up too much heat and be useless as a heat reservoir.
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Old 03-21-2012, 03:13 AM #9
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Default Re: I want copper copper copper! (well... maybe not)

lol... Sodium.

Don't get that bitch wet...
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Old 03-21-2012, 03:16 AM #10
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Default Re: I want copper copper copper! (well... maybe not)

It would be ideal to have a copper module coupled with an aluminium heatsink due to the fact that copper absorbs heat faster but dissipates it slower than aluminium.

Pretty much why high-end computer heatsinks have a copper base coupled with aluminium fins.
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Old 03-21-2012, 03:18 AM #11
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Default Re: I want copper copper copper! (well... maybe not)

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I've never understood using pot metal or brass to mount the diode and then using a good metal for the sink.
You have to remember that aixiz modules were never designed to be a heat transfer mechanism between the diode and the heatsink, they were designed to BE the heatsink for very low power diodes. Brass isn't terrible in terms of thermal conductivity, it is only a little less than half that of Aluminium, but has a much higher density so it functions better as a sink, which is it's 2)original purpose. The copper modules function much better as a transfer medium, though, that is why they are better for our use.

I think some people might be missing what RHD was saying...

1)Thermal conductivity is the measure of how quickly an object can move heat from point A to point B.

Specific Heat is the measure of how much heat an object can hold before it reaches equilibrium with the heat source (ignoring radiative and conductive losses).

If your heatsink reaches equilibrium with your heat source (diode) and it isn't radiating or transfering that heat somewhere else faster than your diode is giving it heat.... your diode will get very hot, very fast.

So, if your HOST isn't able to hold a lot of heat (sinking) or transfer a lot of heat to the air (radiative capability) you need a heatsink which can make up for it by holding a lot more heat. A heatsink that can only move heat quickly will not help as it has no where to go and it will fill up fast. Copper moves heat quickly but doesn't hold nearly as much as other metals do, like steel or Nickel. It still holds a lot more than aluminium because it is much denser, but it doesn't always hold enough to justify it's use over something cheaper and easier to machine (like steel).
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Old 03-21-2012, 03:18 AM #12
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Default Re: I want copper copper copper! (well... maybe not)

If you picture the diode -> module -> heatsink -> host as one single system, there's probably actually a gradient of where you care more about conductivity (towards the center) versus more about heat capacity (towards the edges).

My practical takeaway from all of this, is that for a host that isn't itself going to be a radiator, I want copper for my module, and steel for my heatsink. In practice, nickel would be better, but it's a heck of a lot pricier than even copper.

From a practical perspective, a machinist will need to weigh in on the machinability of the various metals - but I *think* steel is not terrible. In fact I think it's harder to machine than aluminum, but easier than copper?
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Old 03-21-2012, 03:27 AM #13
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Default Re: I want copper copper copper! (well... maybe not)

Aluminum is pretty damn easy to machine.

I'd say in terms of difficulty to machine: (less to more)

aluminium - mild steel - stainless steel - brass - copper - tool steel - titanium - beryllium - sodium ( =P )
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Old 03-21-2012, 03:55 AM #14
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Default Re: I want copper copper copper! (well... maybe not)

RHD, one thing I'm not seeing is ANY mention of nickel and steel as heat sink materials on any searches, everything comes up as copper vs aluminum.

This is an interesting read with copper winning.

Comparing the Impact of Different Heat Sink Materials on Cooling Performance | ECN: Electronic Component News

I also found this and meant to put this here...

Here is an article on heat sinking CPU's

"Cooling - An In-Depth Look" Mike Larsen


Introduction

This article will attempt to present some ideas along with a general engineering analysis of a CPU (or other component such as a graphics chip or chipset) cooling system. The object is to lay out the physics behind such a system in terms that most should easily understand. There will be a series of articles dealing with basic cooling principles and physics, as well as specific design guidelines for successful water and air-cooling.

To give a little background on myself, my degree is a Master's of Engineering with emphasis on structures and heat transfer. I worked during my graduate years designing and building cooling systems for satellites that use extreme cooling methods such as liquid nitrogen and colder methods.

My current occupation is as a process engineer for a large contract manufacturer in the printed circuit assembly business. We basically build the boards that other companies (i.e. HP, Dell, Sun, etc.) put their name on and sell to the public. It is my (along with many other engineers) duty to make sure that all the necessary processes are in place to be able to properly build the boards. If my education, current job, and hobbies are all rolled into one, what to you get? An overclocking freak that has to try everything at least once!


Conduction, Convection and Radiation

There is a lot of general confusion about the different modes of heat transfer and how they relate to system cooling. To fully understand how cooling works, it is imperative to understand the basic modes of heat transfer.

Conduction is the transfer of energy (heat) from a more energetic particle to that of a less energetic particle by direct interaction. This is what happens when someone sticks his or her finger on a hot iron (or for that matter an overclocked AMD processor, which may actually be hotter than the average iron!) The particles in the iron have more energy than the particles in the finger, and thus there is a net energy transfer from one to the other (ouch!).

Convection is the transfer of energy from a solid surface to that of a moving fluid. In order for heat to be transferred by convection, it must first conduct from the hotter material (either the fluid or the solid) by molecule to molecule interaction, and then the moving fluid displaces the molecules closest to the solid with other molecules as they move along.

Radiation occurs when there are two surfaces at different temperatures that emit electromagnetic waves between each other. This is basically how the sun warms the earth and everything on it. This mode of heat transfer is virtually negligible when talking about cooling your processor and motherboard; unless of course you are in a vacuum (space) or you computer is in a black case sitting out in the sun.

It is absolutely vital to understand the differences between the modes of heat transfer and when they occur in a computer system. I cannot stress this enough! There is an absolutely amazing amount of myth and misinformation circulating the internet overclocking world that could be easily put to rest if everyone could grasp a basic knowledge of the nature of heat transfer.


Heat's Journey Through the System

As power is applied to a CPU, it manifests itself in the form of heat and that heat (energy) must be moved away from the CPU or the CPU will eventually rise to an intolerable temperature and possibly burn out. To compound this problem, we as overclockers will often apply even more heat (energy, in the form of additional voltage) to the CPU in an effort to get maximum performance.

To remove the heat, we use either a heatsink/fan combo, waterblock, or a peltier combined with either of the other two aforementioned methods. Both the waterblock and the heatsink/fan systems act in basically the same way. A solid material conducts the heat away from the CPU and a fluid, either water or air, convects the heat away from the waterblock or heatsink and ***** it outside of the system.

In a peltier cooled system, the cold side of the thermal electric cooler cools the CPU by conduction but in order to do so uses additional energy. This energy must be removed from the hot side of the peltier in the same way as direct heatsink or waterblock cooling.

Remembering the definitions of conduction and convection, let's follow the path of the heat and its transfer methods from the CPU to the outside of the case. I'll only use the terms 'heatsink' and 'air' for this particular discussion, but 'waterblock' and 'water' could both be substituted instead; the physics are nearly identical.

As heat leaves the CPU, it is conducted directly into the heatsink. In order to get good conduction and thus effective heat transfer, the heatsink and the CPU must be in good contact with each other. Whenever there are multiple layers through which heat must conduct, there is what is called a contact thermal resistance. This is defined as such that where the two solids meet (i.e. the CPU slug and the heatsink) there will be a thermal contact resistance that results in an immediate temperature drop across the joint.

There is no such thing as a perfectly smooth surface and thus the two mating surfaces will not fit together flawlessly. The rougher the surfaces and the more imperfect the fit, the higher temperature drop across the interface. This is why it is important to use a quality thermal interface material between the CPU and the heatsink.

Often we overclockers even go to the extreme of lapping, or sanding down, either the CPU or the heatsink, or both, in order to get a more perfect fit. If there is a large gap or imperfect fit between the CPU and the heatsink, heat from the CPU will not be effectively transferred into the heatsink and the CPU will remain at a much higher temperature than the heatsink (bad!).

After the heat has been transferred into the heatsink, the material of the heatsink will then distribute the heat throughout itself. How well a heatsink does this is primarily a function of the thermal conductivity of the material.

Thermal conductivity is defined as the proportionality constant that when multiplied with the ratio of the temperature change to the change in distance from the zero plane, will result in the value of the energy flux (Fourier's Law). This basically means that a higher thermal conductivity constant will result in a material moving heat along its geometry more effectively. For example, the handle of a wooden spoon stuck in boiling water will not get nearly as hot as the handle of a metallic spoon because metal has a much higher thermal conductivity than wood.

There are a few substances that are at the top of the thermal conductivity charts, namely:

Diamond (2300 W/mK)
Pyrolytic Graphite (1950 W/mK)
Silver (429 W/mK),
Pure Copper (401 W/mK), and
Pure Aluminum (237 W/mK).


The first two are cost and geometrically prohibitive and are thus not candidates that will be looked at. Alloys of silver, aluminum and copper will always have a lower thermal conductivity than their pure counterparts, and often have thermal conductivities much lower. Thus it is very important to use pure metals! I.E. 6061 aluminum alloy will perform much worse than pure aluminum.

Aluminum is the most common heatsink material because of its cost, low density, availability, and machinability. Copper is beginning to become more popular but is much more difficult to work with and has a density approximately three and a half times that of aluminum. There has been talk recently of some silver heatsinks and/or waterblocks that may prove to be very interesting should they be able to be produced cost effectively.

Assuming that a system is constantly outputting heat, the material with the higher thermal conductivity will better move the heat away from the heat source. In the case of aluminum vs. copper, assuming identical geometries, copper will more effectively move the heat away from the point of contact with the CPU and into the extremities of the heat sink. This will give the heatsink a higher average temperature overall.

Aluminum will have a higher temperature difference between the point of contact and the heatsink extremities. This becomes and important factor because the heat must be removed from the heatsink along its entire surface through convection. Also recall that anytime there is an interface between two solids or materials, there is an associated drop in temperature across that interface. Thus if a heatsink is made of multiple pieces that are not properly bonded together, the heatsink will lose efficiency.

Now that the heat has been moved throughout the heatsink, it must be removed by forced convection. Air that is cooler than the heatsink is blown over the surface and the individual air molecules pick up energy from the heatsink and are (hopefully) ejected out of the case. There are only two factors that determine how much energy can be transferred from an individual air molecule that is in contact with the solid surface at a given temperature: time of contact and the lower of the two material's thermal conductivities.

In this case, air has a much lower thermal conductivity than the metallic heatsink and is thus the limiting factor. NOTE: There is no such physical phenomenon as to how well a material 'gives up heat'. This is an internet-overclocking myth that has propagated for far too long and will now be laid to rest!

Aluminum does not 'give up its heat' better than copper! Let me repeat this once more; aluminum does NOT 'give up its heat' better than copper. It is true that, in general, aluminum will radiate heat better than copper but radiation is such a miniscule part of heat transfer in a computer system as to be deemed completely inapplicable.

The physical action of conduction/convection relies solely on the two material's individual thermal conductivities, their proximity to each other, and their time in contact with each other. Thus, a pure copper heatsink will always outperform a heatsink of the exact same geometry of a pure aluminum heatsink assuming that both have the same contact with the heat source and the same rate of airflow over the surface.

So why don't the current copper heatsinks far outperform (all tests I have seen show that copper heatsinks do outperform aluminum, just not by much) their aluminum counterparts? In my opinion it is because of a few things; namely poor design, multiple piece heatsinks (remember thermal contact resistance!), impure copper, and difficulty of producing a copper heatsink in the desired form.

Once the air has picked up the heat from the heatsink, it is simply ejected from the case and replaced with cooler ambient air. Thus our look at the journey of heat through a system comes to a close!
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Old 03-21-2012, 04:01 AM #15
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Default Re: I want copper copper copper! (well... maybe not)

The above article for cpu heatsinks doesn't really apply well to handheld laser heatsinks. It states one main factor is airflow over the heatsink, and in comparisson to that the radiative losses are miniscule. Well we do not (often) use forced air active cooling on handheld laser builds, so convectional heat transfer is the minimal factor, not radiative cooling.
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Old 03-21-2012, 04:11 AM #16
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Default Re: I want copper copper copper! (well... maybe not)

It sounds like we nead a head with fins and threads that has a place for the diode to be pressed into. That way we have fins to radiate the heat and threads to connect it to the body. It would be nice to something like a copper core that the diode resides in with aluminum fins outside. The tolerances would have to be real small to ensure maximum thermal transfer. That would mean it would most likely have to be press fit with no way of seperating the two easily.
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