Is it better to use copper or aluminum for radiators?


Today's PC common CPU heat sink, whether mainstream or high-end products, almost all use such a structure: the structure of the contact with the CPU/GPU surface is made of copper, that is, we often say copper base, and the heat pipe fin or water-cooled fin is made of aluminum. The so-called "copper aluminum combination" said is such a structure, and the use of copper fin or copper water cooling radiator, that is, we commonly known as the "pure copper radiator" in the PC field can almost be said to be a flash in the pan, only in the earlier years ever appeared.

So why "copper aluminum combination" can become the mainstream design of PC heat dissipation? If you go to online search, gets the most that should be "copper and aluminum combination after comprehensive factors with the best balance", you can be as simple as size, weight, technology, cost, the heat efficiency and other aspects of comprehensive, even say, aluminum radiator efficiency is better than copper, copper heat faster, just Therefore, the combination of copper and aluminum is to combine the advantages of the two together, and the heat dissipation efficiency will be higher than the pure copper structure. All this seems plausible, but is it really the case?

The material of the fin has no effect on heat dissipation

In fact, the answer has been given by the Newton cooling formula Q=hAΔT in thermodynamics, Newton cooling formula is mostly used in the calculation of convective heat transfer, and the CPU heat dissipation is essentially to let the CPU heat transfer to the air through the radiator, the radiator itself will not generate heat, also can not eliminate heat, That IS TO SAY, the radiator needs to bear the "heat" is the CPU "heat", or call it total heat flow. As we have explained in our Super class article "Super Class (302) : What is Heat? (Part 2)", the energy used to drive the CPU will basically be converted into heat in the end, so as long as the CPU power consumption is unchanged, the total heat flow will not change, and the total heat flow is Q in Newton's cooling formula. A in the formula is the contact area between the radiator and the air. As long as the structure and size of the radiator are unchanged, the heat exchange area will not change, and the material of the radiator does not have any relationship; h is the heat transfer coefficient of the fluid, which is basically a constant value as long as the convection mode and the type of gas are constant.

Therefore, for convective heat transfer, if the heat is unchanged, the air parameters are unchanged, and the radiator structure design is unchanged, then the ΔT should also be unchanged. ΔT is the temperature difference between the air and the fin. A constant air temperature means a constant fin temperature, and it has nothing to do with the fin material. In OTHER WORDS, radiator USES WHAT KIND OF MATERIAL TO PLEDGE, TO TOTAL HEAT FLOW NAMELY DISSIPATE HEAT IS TO HAVE NO EFFECT, THE TEMPERATURE ON RADIATOR ALSO WON'T CHANGE.

However, in practice, radiators of the same size and structure but with different materials do have obvious effects on the operating temperature of the CPU, especially the temperature of the full load, which seems to contradict the results shown by Newton's cooling formula. In fact, many students have fallen into a misunderstanding, that the temperature control and heat dissipation confused. So why is temperature control not the same thing as heat dissipation? It starts with "Why do cpus need radiators?"

Why does the CPU need a heat sink?
Why does a CPU on a PC need a heat sink? In fact, this can also be explained by Newton's cooling formula Q=hAΔT. If the CPU directly allows the core to conduct convective heat transfer with air, then the core area and the limit on ΔT (the upper limit on the CPU operating temperature) can be used to quickly calculate the heat dissipation requirements of the CPU, which is equivalent to the upper limit on the heat transfer rate or the total heat flow rate. If the CPU heat at this time does not exceed the upper limit, it is natural that there is no need to install additional radiator, directly with air convection heat dissipation; However, if the actual CPU power consumption is higher than the upper limit, there are basically only two ways to increase the total heat flow rate. Either increase its own temperature for higher ΔT, or increase its own heat transfer area A. Both methods can increase the total heat flow rate Q to meet the actual CPU power consumption level.

The top of today's CPU is essentially a small-scale heat sink

As an example, when the power consumption of a CPU with a core area of 200mm2 is 200W, and the air with a temperature of 25℃ and a convective heat transfer coefficient of 200W/m2·Λ is used for direct heat dissipation, what will be its operating temperature? The operating temperature can be obtained by inserting the corresponding values into Newton's cooling formula. Using these conditions, however, we will find that the operating temperature will reach the level of 5000℃, which no CPU can withstand.

However, when we install a radiator to make its contact area with the air equivalent to 5000mm2, it is calculated that the working temperature of the CPU is only 45℃ in the ideal state, which is obviously much more reasonable. This is why the CPU on the PC platform needs to be equipped with a radiator.

Relationship between heat sink material and CPU temperature
At this point, however, we still haven't explained why different heat sinks make different cpus work at different temperatures. In fact, in the previous section, we just idealized the integration of CPU and radiator computation. The calculated 45℃ is actually only the temperature of the radiator and air contact surface, not the real sense of the CPU core temperature. In fact, the whole heat dissipation process of the CPU is the process of core heat conduction to the air through the radiator. There are two heat dissipation systems in this process, one is composed of the radiator and the CPU, the other is composed of the radiator and the air, the latter can be directly used to quickly calculate Newton cooling formula. The former requires the use of heat conduction, heat diffusion and other aspects of the formula sub-calculation, which involves the basis of thermodynamics and heat transfer, but also involved in the material problem of the radiator.

Heat transfer is A science based on experiment, after A large number of experiments have found that if the object is in the form of A rule, cylindrical, for example, the body size of the heat transfer rate and Δ T = T1 - and T2 is proportional to the temperature difference on both ends, with objects is A sectional area and is inversely proportional to the object length L, and the object of different material will follow the above rules, Therefore, we can introduce A coefficient λ for objects of different materials, so as to obtain the formula of Qx= λAδT /L, and the coefficient λ is often mentioned in the field of heat conductivity.

So if we see the CPU radiator as A rule, the contact area with the CPU temperature of T1, and the temperature of the air interface for T2, so it is easy to see that, when the Qx total heat flow, A cross-sectional area and length L, T2 temperature remains unchanged, if the lambda coefficient of thermal conductivity is high, so Δ T will lower T1 temperature will be lower, The opposite is higher. The thermal conductivity of copper is 401W/(m·Λ), while that of aluminum is 238W/(m·Λ). The former is 1.7 times higher than the latter. Accordingly, the temperature difference is 1.7℃ in numerical terms. Similarly, we can further calculate the core temperature of CPU based on this value. It can also be found that when we use copper heat sink, the core temperature of CPU will be lower, but there is no big difference in the value compared with that of aluminum.

What is thermal resistance?
Of course, this also makes a theoretical calculation, and even so, it is not a convenient thing to directly calculate the working temperature of the CPU and compare the influence of the heat sink of copper and aluminum material in the entire heat dissipation system. So we might as well rewrite the formula of heat transfer rate, Qx=ΔT/(L/λA)=(T1-T2)/R. Now we compare this with Ohm's law in circuits, Ohm's law I=(U1-U2)/Re, and do we find that the two are quite similar?

In fact, the process of heat diffusion is similar to the process of charge diffusion, they are under the action of potential difference, and in the process of diffusion will be subjected to resistance, if the resistance of charge diffusion is resistance, then the resistance of heat diffusion is naturally thermal resistance. According to the way we have previously rewritten the heat transfer rate, we can quickly get the thermal resistance in conduction, convection and radiation, respectively, are the following three formulas.

R conduction = L/(λ·A)

R convection = 1 / h·A.

R radiation = 1 / (hr·A)

With the concept of thermal resistance, we can analyze and calculate the heat transfer rate much more easily, and the heat transfer process can be transformed into a simple series and parallel circuit structure, and this structure is generally called hot circuit or hot network, for example, the heat dissipation process of CPU can be simply understood as a similar hot circuit with series circuit.

As we had talked about, of course, we here is more on the analysis of an idealized model, the real CPU will not only through the radiator to heat the air, as well as through the CPU board, base, main board PCB and other path into the air, in addition, silicon grease exist in the whole cooling system of the cooling medium, such as Therefore, the thermal circuit of CPU heat dissipation is actually more complex, which should be a series and parallel circuit. Here we make an idealized model for the convenience of understanding. Students who are interested in it can make further study and understanding by themselves.

Through the above thermal path, we can see that the heat of the CPU originates from the core and conducts to the radiator through the top cover, and then the radiator and the air undergo convective heat transfer. If the thermal resistance inside the core and the contact thermal resistance between the core and the top cover and the top cover and the heat sink are not considered, then the thermal resistance in the whole thermal path is composed of the thermal resistance of the top cover, the heat sink and the air, so as to obtain the following formula.

Qx = (T1-T2)/(R cap + R radiator + R air)

In this formula, head cover and the radiator thermal resistance can use formula of conduction heat transfer of thermal resistance, while the air using the formula for calculating thermal resistance, heat convection for CPU temperature T1, T2 air temperature, so if we only change the material of radiator, such as the aluminum radiator replacement for copper radiator, without changing the structure size, volume, The higher the thermal conductivity of the material used in the radiator, the lower the thermal resistance it will show. Since the thermal resistance between the top cover and the air is also constant, the CPU core temperature can be calculated according to the following formula:

T1 = T2 + Qx · (R cap + R radiator + R air)

Therefore, the higher the thermal resistance of the radiator, the higher the working temperature of the CPU will be, so that there is enough temperature difference between it and the air to make up for the influence of the higher thermal resistance. Therefore, when we test the heat sink, it is essentially to measure its thermal resistance. In order to accurately display the thermal resistance, we need to control the variables in the test environment. It is especially important that the total heat flow rate of Qx cannot be changed. This is what we think, in the use of practical platform for the cooling test, only to lock the CPU power consumption and guarantee under the condition of room temperature is the same, the CPU load working temperature can be as different performance of radiator, to strictly is compared on the basis of the cause of the temperature control performance, and we are to switch to fixed power heating radiator platform test, And according to the temperature difference to evaluate the cause of the radiator.

Temperature conductivity coefficient: represents the ability of temperature transfer
According to the above discussion, it is not difficult to draw the following conclusion, that is, under the condition of variable CPU temperature and constant total heat flow rate, as long as the structure, size, air temperature and convection mode of the radiator do not change, then the "heat dissipation efficiency" of the radiator will not change with the change of material. But the ability to control CPU temperature is really with material has a great relationship, that is to say, the heat sink of copper material in the performance of CPU temperature will not be worse than the aluminum material under the same conditions, the so-called "aluminum material is more conducive to heat dissipation" is just a kind of not rigorous, incorrect statement.

Now it is almost impossible to see the trace of pure copper radiator

So why now the radiator almost all won't use pure copper material? First of all, we can see from the previous calculation, in fact, the copper radiator and aluminum radiator in the actual use of the CPU temperature gap is not very big, basically only the pursuit of extreme heat dissipation effect need to use pure copper structure, in most cases the same structure of aluminum radiator can also meet the demand. Next IT IS THE RADIATOR OF TWO KINDS OF QUALITATIVE MATERIAL, IN ACTUAL USE, FROM THE BEGINNING OF WORK TO ENTER THE TIME OF TEMPERATURE stability IS DIFFER LITTLE. In heat transfer, when the temperature of the whole system is stable and does not change, it is generally called steady-state heat transfer, which is equivalent to what we often say "maximum heat dissipation efficiency". Therefore, for the heat dissipation system, the faster it enters the steady-state heat transfer, the more favorable it is for heat dissipation.

While thermal conductivity is just the ability of a material to transfer heat, temperature is related to its heat capacity, while heat capacity is related to its specific heat capacity and mass. However, in the heat dissipation system, the radiator is more with the volume comparison rather than with the weight comparison, so here we can introduce a coefficient, called the temperature conductivity coefficient, also known as the thermal diffusion coefficient. If thermal conductivity shows the ability of different materials to transfer heat, temperature conductivity shows the ability of different materials to transfer temperature.

The formula for the coefficient of temperature conductivity is α=λ/ρc, where λ is the coefficient of thermal conductivity, ρ is the density, c is the specific heat capacity, ρc product represents the unit volume of the object temperature rise 1℃ required heat. These parameters are actually known values, so we can calculate the temperature conductivity coefficient of copper and aluminum, the former is 115mm2/s, the latter is 100mm2/s, that is to say, in the case of the same structure, the speed of copper heat sink to reach temperature stability is only about 15% faster than aluminum, and the former thermal conductivity is 1.7 times of the latter. That is to say, the heat sink of copper material is in the time of entering the best heat dissipation efficiency, in fact, there is no more than the lead of aluminum material, with the common CPU heat sink at present, it is the gap of 1 minute or so, in actual use can be ignored.

Conclusion: It is not only the heat dissipation ability that determines the radiator structure
In fact, this time we just discuss more thermodynamics and heat transfer in the steady state heat conduction, PC heat will actually is a more complicated process and actual calculation need to consider more parameters, such as we the radiator in this discussion, and is designed according to the idealized model, the actual design of the radiator structure will be more complicated, Thermal resistance of the composition is also more of the project, even the convective heat transfer coefficient of air, actually also is not a fixed value, but in a different position to do the corresponding processing, which involves a fairly wide range of knowledge is not we can fully explain what a beast class, we can only make the ideal words of explanation.

The actual structure of the radiator is far more complicated than the theoretical calculation

Return to the CPU radiator in the end with copper or aluminum is better, in fact, many times students subconsciously think that the CPU full load when the temperature is higher, the heat will be greater. In fact, as long as the power consumption of the CPU is constant, the "calorific value" or the total heat flow rate is virtually the same when the temperature is stable. If explained in the way of circuit, that is the whole composition of the CPU and its heat dissipation system, in fact, is equivalent to the "variable voltage constant current circuit", the current is equivalent to the total heat flow, the temperature difference between the CPU and room temperature is equivalent to the voltage or potential difference, thermal resistance is naturally the resistance in the circuit. When the thermal resistance increases, because the heat is unchanged, the temperature difference naturally needs to increase, that is to say, if the room temperature does not decrease at this time, the temperature of the chip can only rise.

If you want to reduce the temperature of the chip, you can only reduce the thermal resistance of the heat dissipation system without changing the heat output of the chip and the room temperature. If restrictions are added to this premise, such as not changing the size and structure of the heat sink, then the only option is to switch to a material with higher thermal conductivity, such as switching from aluminum to copper. In this way, the heat gained and lost in the whole system remain the same, but the temperature of the chip can indeed be lowered. Therefore, the statement that "aluminum is better than copper heat dissipation" is not correct in the strict sense. The radiator uses copper or aluminum, or even uses other materials, which only affects the temperature of the heat source more, and does not change for the "heat dissipation". Of course, if you're just looking at how cool the CPU is, copper does keep the heat cooler than aluminum for the same size and construction of the heat sink.

So why do most radiators on PCS today use a "copper-aluminum combination" instead of the pure copper structure that is more ideal for temperature control? According to the formula of thermal diffusivity, we can also know that there is no big difference in thermal diffusion rate between aluminum and copper, that is to say, there is little difference in the time between the two to enter the temperature equilibrium state. Therefore, the radiator of pure copper MATERIAL HAS a better performance on CPU temperature indeed, but after cost comprehensive consideration includes many factors such as cost, weight, processing difficulty, the radiator OF its comparison with aluminum material has a little input and income is not proportional to the meaning.

And pure aluminum radiator is indeed on the temperature performance is not dominant, so after comprehensive many factors, in the radiator of bulk volume, it is mainly used for extended fins used aluminium heat dissipation area, the structure of the direct contact with heat source, such as base, such as heat pipe using copper mass, this structure is formed a sweet spot, It will gradually evolve into the mainstream and even the basic structure of the high-end radiator.