Heat Sink’s Material - Copper Or aluminum?

As the saying goes: "Copper conducts heat quickly, aluminum dissipates heat quickly." For a long time, the debate between copper and aluminum fins has never stopped.

Are copper fins better or aluminum fins better? Why are notebook heat sinks mostly pure copper, while desktop heat sinks are often aluminum fins?

In this article, I will start with the characteristics of copper and aluminum materials, combined with the design example of the heat sink, and carry out specific advantages and disadvantages analysis.

Material characteristics analysis:

Thermal conductivity

The most prominent feature of copper is that it conducts heat faster. The thermal conductivity of pure copper is as high as 401W/m*K, which is second only to silver (429W/m*K) among common metals, and much higher than the 240W/m*K of aluminum.

Copper is an excellent heat conductor. Therefore, on the base and heat pipe that conduct heat conduction directly on the heat sink, copper with higher thermal conductivity is far better than aluminum, which is no doubt. But on the fins responsible for dissipating heat, does the thermal conductivity parameter still play such a big role?

Density and cost

Copper is much denser than aluminum. The density of pure copper is as high as 8.9g/cm^3, 3.3 times that of aluminum. Therefore, the use of pure copper fins will significantly increase the weight of the heat sink.

Taking Limin's classic U120E as an example, after replacing the aluminum fins of the same heat sink with pure copper, its total weight increased from 780g to 1.9kg! In the background of the increasing volume of air cooling, the excessive heat sink tower will cause a huge burden on the motherboard.

At the same time, the price of copper is up to three times that of aluminum. If the weight added to the motherboard "perception is not strong", then the price of 50% is really "meat pain". This is also an important reason why the heat sink is generally made of aluminum.

Heat Capacity

Aluminum has a greater specific heat capacity. The specific heat capacity of pure aluminum is as high as 0.88×10^3J/(kg·℃), which is 2.25 times that of copper material. But does a greater specific heat capacity necessarily mean a greater heat capacity?

The heat sink is not a product that can infinitely enlarge the volume, and in the actual comparison, the constant is often not the weight of the fins, but the volume of the fins. Contrary to the conclusion based on the specific heat capacity, the copper heat sink can absorb more heat with the same fin volume.

As mentioned earlier, the specific heat capacity of aluminum is 2.25 times that of copper. However, pure copper is 3.3 times denser than aluminum! After some conversion, we can know that the same volume of copper can absorb about 30-40% more heat.

The greater heat capacity means that the heat sink is able to heat up more slowly, effectively reducing the heat accumulation rate of the CPU. When the heat dissipation performance is insufficient, the pure copper heat sink can also delay the overheating time of the processor.

But this does not mean that aluminum heat sinks do not have advantages. Here we need to introduce the concept of "heat convection". The heat dissipation of the heat sink is completed by the convection of the fluid (that is, air), according to the formula used to estimate the heat convection:

Heat flow rate = convective heat transfer coefficient × surface area x (temperature difference between the surface and the fluid)

It can be seen that in the case of the same convective heat transfer coefficient (which can be simply understood as wind), the greater the temperature difference between the surface and the fluid, the greater the heat flow, the better the heat dissipation effect. Although the heat capacity of the aluminum heat sink is smaller, it has an advantage in heat dissipation speed because it heats up faster and the larger temperature gradient brings greater heat flow.

Manufacturing difficulty

An often overlooked point is that welding between different materials has no small difficulty. Welding the copper heat pipe of a heat sink to an aluminum fin often requires a transition layer - usually nickel - to assist in reflow soldering. Nickel itself is a metal with poor thermal conductivity, and the thermal resistance of solder (tin) is also high.

Due to the existence of multiple layers of low thermal conductivity substances, the shortcomings of poor thermal conductivity of aluminum are further amplified. If the Fin process is used, problems such as fin loosening and performance attenuation will occur after long-term use because of the different thermal expansion coefficients of copper and aluminum.

Because they are made of the same material, welding between heat pipes and copper fins is much easier. Without worries, pure copper heat sinks are often reflow welded.

Of course, compared to the soft copper, aluminum will be easier to create fins. But the advantages of copper are stronger welding and the absence of low thermal conductivity interlayers. Therefore, the pure copper heat sink has better thermal conductivity and reliability.

Application example

From the previous article, we already know that pure copper fins warm up more slowly and conduct heat more quickly. But in terms of thermal convection, they're not as fast as aluminum fins.

The heat dissipation characteristics of copper and aluminum are not the same, so there is no absolute advantage or disadvantage. Only by selecting different materials according to the heat dissipation needs of each part can we achieve a balance between cost and weight while maximizing efficiency.

On the notebook

In the space of the notebook, it is often impossible to lay out too many cooling fins. Their fin area tends to be relatively small or even severely inadequate.

On the desktop

On desktop heat sinks, fin area is no longer the biggest constraint. The bottleneck has shifted from "heat conduction" to "heat convection". On this basis, it is divided into tower type heat sink and pressure type heat sink.

Because the tower heat sink is generally large in size, if it is replaced with pure copper fins, the weight and cost will explode. Therefore, considering the cost and ease of use, tower heat sinks are often made of aluminum fins.

Only a handful of high-end heat sinks use pure copper or copper-aluminum hybrid heat sinks, and their performance has not improved to match the significant increase in cost and weight.

In order to maximize the performance of the aluminum fins and make up for the deficiency of their thermal conductivity, the heat pipes of the tower heat sink are often scattered to ensure that the fins are heated evenly as much as possible and improve the heat transfer efficiency of the heat pipes.

At the same time, because aluminum heats up faster and creates a larger temperature difference, aluminum fins have better heat convection when the wind is not enough to quickly carry away the heat of the fins. Therefore, large aluminum fins are often used on large passive heat sinks, and the gap is increased at the same time, and the opening is set up to allow the convection in the chassis to better flow through the heat sink.

On the lower pressure heat sink, it is often faced with a situation similar to that of a notebook: insufficient space and insufficient fin area. Therefore, the design scheme adopted is also relatively similar.

The so-called "secondary welding" is to avoid the problem of uneven heating of the fins while increasing the contact area between the heat pipe and the fins as much as possible. The process is a big boost for heat sinks with aluminum fins.

At the same time, because the body of the lower pressure heat sink is relatively small and lightweight, in order to press the heat dissipation performance in the limited heat dissipation area, there are also pure copper models. Although the weight and price of the body have increased dramatically, it is still within the acceptable range.

Sum up

In short, copper heats up more slowly and conducts heat faster, but the heat convection is not as fast as aluminum. As a result, copper fins require a more powerful fan to quickly remove heat from the fins.

Aluminum heats up faster, creating a larger temperature difference that maximizes heat dissipation when wind cannot penetrate the heat sink.

If the bottleneck is the heat capacity, fin size and thermal conductivity efficiency, copper is superior to aluminum because copper has a large heat capacity and can make full use of the fin area.

If the bottleneck is to carry the heat away with the wind, aluminum is superior to copper because aluminum's heat convection efficiency is higher. As a result, aluminum fins require a larger area, while copper fins rely on greater wind power.

Specific to the heat sink design, when the fan performance is insufficient, aluminum is better than copper. When the heat pipe performance is insufficient or the fin size of the heat sink is not large enough, copper is better than aluminum.

Of course, pure copper is bound to be expensive. However, when pressing the heat dissipation potential, but also to control the weight within an acceptable range, even aluminum needs to pay a huge processing cost.